che 451 - pollution prevention projectinfohouse.p2ric.org/ref/35/34431.pdf · prevention jenny...

-34-4 3 I

Article System Design Parameters Comparison of No. Information (Flow, T, P) Processes 1

CHE! 451 Literature Group 23: Pollution Cited Table Stacey Martin Prevention Jenny Morgan Project Angela Russell

Activated Flow Solvent Carbon Factors Diagram Regeneration

PS 1 2 3 4

- . . Pg. 103-116 Pg. 1 1 1 Pg. 94-95

Pg. 556 Pe 1-2

5 6 7 8

-

Pg. E-22 Pg. 275-276 Pg. 277

Pg. 864 Pg. 870 Pg. 865 Pe. 16-77

Reference List:

“Carbon Adsorption Solvent Recovery System Pays for Itself in 2-1/2 Years.” On-line Internet. 11 September 1995. Available World Wide Web: NCDNR Case Study CS109.

Cheremisinoff, Nicholas, and Cheremisinoff, Paul N. (1993). Carbon Adsorption for Pollution Control. Englewood Cliffs: Prentice Hall, pp. 94-95, 103-1 16.

Cheremisinoff, Nicholas, and Cheremisinoff, Paul N. (November 1994). Carbon Adsorption for Pollution Control. Waste Management. 14,556-557.

Coconut Shell Activated Carbons for Vapor Phase Applications. Columbus: Bamebey & Sutcliffe Corp., 1996.

Crompton, David. (March 1990). Carbon Adsorption for VOC Emission Control-, E-20-E-22.

Fang, C.S. and Khor, Sok-Leng. (November 1989). Reduction of Volatile Organic Compounds in Aqueous Solutions Through Air Stripping and Gas-Phase Carbon Adsorption. Environmental Progress. 8f4), 270-278.

Gong, Runzhi, and Keener, Tim C. (June 1993). A Qualitative Analysis of the Effects of Water Vapor on Multi-Component Vapor-Phase Carbon Adsorption, Journal of Air and Waste Management Association. 43,864-872.

Hairston, Deborah. (November 1995). Activated Carbon Gets Rewed Up Chemical Engineering, 75-78.

Hunter, Marie. (June 1990). Vapor Recovery System Protects Environment. PollutionEnaineerinn, 27-29.

Introduction to Activated Carbons. Columbus: Bamebey & Sutcliffe Corporation, 1996.

Koe, Lawrence C. and Tan, N.C. (July/August 1990). Field Performance of Activated Carbon Adsorption for Sewage Air. Journal of Environmental Engineering. 116(4), 721-734.

Logsdon, P.B. and Basu, R.S. (MarcWApril 1993). Recovery and Recycle of HCFCs by Activated Carbon Adsorption. Journal of the IES. 36,33-36.

[ 131 McLaughlin, Hugh. “Solvent Regeneration of Activated Carbons.” On-line Internet. 17 February 1997. Available World Wide Web: http://www.eba- nys.org/water/carb. html.

Methvlene Chloride Adsorption on Tvpe BPL Carbon. Pittsburgh: Calgon Carbon Corporation, February 25,1997.

[I41

[I51 ‘Norit Activated Carbon and Gas/Air Purification.” On-line Internet. 17 February 1997. Available World Wide Web: http://www.norit.codcarbon/l-O- 4.htm.

[I61 Ong, S.L. (December 1984). Simplified Approach for Designing Carbon Adsorption Column. Journal of Environmental Engineering. 110, 1184-1 188

“Index Will Decline.” (December 1996). Purchasing. On-line Internet. Available World Wide Web: httu://www.manufacturing.net/magnazine/purchasina/al996/12/issues/l/l 2ltrau.htm.

Removal Capacity of Standard Activated Carbon for Various Contaminant Vapors. Columbus: Barnebey & Sutcliffe Corporation, November 1995.

Solvent Recovery Beneficial for Hayworth. (November 1983). Industrial

[ 171

[I8 ]

[ 191 Finishing, 28-29.

[20] Sorrento, Louis. (July 1994). The Proven Process of Carbon Adsorption 94-95.

[21] Tsai, Wen-Tien, and Chang, Ching-Yuan. (April 1994). Adsorption of Methylene Chloride Vapor on Activated Carbon. Chemical Technology and Biotechnology. 61, 145-151.

[22] “VOC Separation Systems for Gaseous Wastes.” On-line Internet. 17 February 1997. Available World Wide Web: httu://www.owr.ehnr. state.nc.us/ref/OO034d. htm.

Werner, Martin D., and Winters, Nancy L. A Review of Models Developed to Predict Caseous Phase Activated Carbon Adsorption of Organic Compounds. CRC Reviews in Environmental Control, 16(4), 327-356.

[23]

http://www.eba

http://www.norit.codcarbon/l-O

Carbon Adsorption Solvent Recovery System Pays for Itself in 2 4 2 Years Case Study: CS109

***** DOCNO:

034-0 10-A- 108 *****

INDUSTRY/SIC CODE:

Flexible Packaging/275

NAMEKONTACT:

North Carolina Department of Natural Resources, P . O . Box 27687, Raleigh, North Carolina 27611-7687

TECHNOLOGY DESCRIPTION:

The process utilizes a carbon adsorption system. RJR Archer installed a carbon adsorption system to capture and recover 98 percent of the solvents that were lost to the atmosphere from printing presses. A n average of 2,655 gallons per day of solvent are recovered and either reused in the printing process or sold. To make the system cost effective, two process modifications were initiated prior to installation. Hot exhaust air from the press drying ovens was recirculated in order to minimize the total exhaust volume. The recirculation enabled the size of the recovery system to be scaled down. Additionally, all inks and coatings which normally contain up to five different solvents, were reformulated using only two solvents in each blend. This allowed the adsorption system to operate efficiently. Steam is used to regenerate the carbon beds and recover the captured solvents.

The recovery system saves RJR Archer over $6,300 per day in solvent costs. Such savings enable the company to recover the $ 4 . 3 million cost of the recovery system in 2.5 years. After pay back, RJR Archer estimates about $1.7 million each year will be saved on operating costs.

FEEDSTOCKS:

Solvents

L

WASTES:

Solvents

MEDIUM:

Inks, coatings

COST

CAPITAL COST:

$4.3 million

OPERATIONMAINTENANCE:

MONTHS TO RECOVER:

30

SAVINGS:

DIRECT COST: FEEDSTOCK REDUCTION: $6,30O/day in solvent costs WASTE PRODUCTION:

IMPACT:

Solvent recovery system pays for itself in 2-1/2 years

CITATIONFAGE:

Accomplishments of North Carolina Industries - Case Studies, G. Hunt, et al.

KEY WORDS:

NORTH CAROLINA, CARBON ADSORPTION, PRINTING, SOLVENT, REBLENDING, MATERIAL SUBSTITUTION, RECOVERY, SIC=27

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Last Updated: September 11,1995

94 Cas-Phase Adsorption and Air Pollution Control Chap. 4

opcrztion, an MTZ curve (A) is obtained. The continued circulation of the carrier gas in the absence of contaminant causes the adsorbate to diffuse through the bed by the process of desorption into the carrier and readsorption until the low concentration causes elongation of the MTZ, represented by curve B (dashed line). Short periods of intermittent operation d o not affect greatly the overall capacity of an adsorption system if the bed depth equals several MTZ lengths, but long periods of intermittent operation, particularly in a n undersized system, will cause a serious capacity drop.

Regeneration

In regeneration of a system, the main factor-economics with in-place regeneration-is or is not preferred to the replacement of the entire adsorbent charge. It is also important to establish that the recovery of the contaminant is worthwhile, or if only the generation of the adsorbent is required. If recovery is the principal objective, the best design can be based on a prior experimental test to establish the ratio of the sorbent fluid to the recoverable adsorbent at the different working capacities of the adsorbent. A typical plant, for example, will have a steam consumption in the region of 1-4 Ib of s team per Ib of recovered solvent. Description of the contaminant can be achieved by several of the following different methods:

Percentage of CharEe Expelled

Heating at 100°C (212°F) far 20 min Vacuum 50 mm Hg at 20°C (68°F) for 20 min Gas "rculation at 130°C (266°F) for 20 min Direct steam at 100°C (212-F) for 20 min

15 25 45 98

Under most conditions, direct steam regeneration is the most efficient. The steam entering the adsorbent bed not only introduces heat, but adsorption and capillary condensation of the water will take place, which will supply additional heat and displacement for the desorption process. The following factors should be considered when designing the stripping process:

* Length of time required for the regeneration should be as short as possible. If continuous adsorption and recovery are required, multiple systems have to be installed.

* Short regeneration time requires a higher steaming rate, thus increasing the heat duty of the condenser system. - Steaming direction should be in the opposite direction to the adsorption to prevent possible accumulation of polymerizable substances, and also to permit the shortest route for the desorbed contaminant.

* To enable a fast stripping and efficient heat transfer, it is necessary to sweep out the carrier gas from the adsorber and condenser systems a s fast as possible.

L

Chap. 4

)n of the J diffuse lsorption ented by iot affect th equals rticularly

-place re- idsorbent Itaminant red. If re- m a prior roverable 'ical plant, 3 of steam I achieved

:e Expelled

st efficient. adsorption will supply e following ISS:

iort as pos- .ultiple sys-

j increasing

adsorption es, and also

iecessary to . systems as

Air Pollution Control 95

1 A larger fraction of the heat content of the steam is used up to heat the adsorber vessel and the adsorbent; thus, it is essential that the steam condenses quickly in the bed. The steam should contain only a slight superheat to allow condensation.

* It is advantageous to use a low-retentivity carbon to enable the adsorbate to be stripped out easily. When empirical data are not available, the following heat requirements have to be taken into consideration: - heat to the adsorbent and vessel - heat of adsorption and specific heat of adsorbate leaving the adsorbent - latent and specific heat of water vapor accompanying the adsorbate - heat in condensed, indirect steam - radiation and convection heat loss

Since the adsorbent bed must be heated in a relatively short time to reactivation temperature, it is necessary that the reactivation steam rate calculation is increased by some factor that will correct for the nonsteady-state heat transfer. During the steaming period, condensation and adsorption will take place in the adsorbent bed, increasing the moisture content of the adsorbent. A certain portion of the adsorbate will remain on the carbon. This fraction is generally referred to as heel. To achieve the minimum efficiency drop for the successive adsorbent cycles, the adsorbent bed should be dried and cooled before being returned to the adsorption cycle. The desired state of dryness will depend on the physical properties of the adsorbate and on the concentration of the adsorbate in the carrier stream. When using high- adsorbate concentrations, it may be desirable to leave some moisture in the adsorbent so that the heat of adsorption may be used in evaporating the moisture from the adsorbent, thus preventing any undue temperature rise of the adsorbent bed.

It is also necessary to establish the materials of construction on the basis that several compounds, especially chlorinated hydrocarbons, will undergo

L

a partial decomposition during regeneration, forming hydrochloric acid. Safety factors have to be considered in designing a regeneration system,

assuring that the adsorber is not being used at temperatures higher than the self-ignition point of the contaminant. Carbon does not lower the ignition temperature of solvents and , as an example, solvent adsorbed on carbon ignites at the same temperature as the solvent vapor alone.

AIR POLLUTION CONTROL

Air pollution control is a broad and far-reaching subject that covers many complex unit operations, some consisting of filtration of fluid-solid particles, heat transfer and condensation, adsorption in a liquid state, and adsorption on a solid surface. When considering the problems of odor control and solvent recovery, which have become increasingly important in recent years d u e to the escalating costs, there are only two viable mechanisms: (1) oxidation by

Chap. 4

described ents were ier uses of , masks. It ?d marked

:ation that sir. Exper- r on chars 3n process irptive ca- neous dis- not show

rather will molecular in a water- ted carbon ications to n are gen- typical set

by Baron : structure

-

-

S ) ~

111

I?

E I 1

I:

Engineering Design 103

endorsing cavities occupied by large ions and water molecules, both of which have considerable freedom of movement, permitting ion exchange a n d reversible dehydration. Activation of zeolites is a dehydration process accomplished by the application of heat in a high vacuum. Some zeolite crystals show behavior opposite to that of activated carbon in that they selectively adsorb water in the presence of nonpolar solvents. Zeolites can be made to have specific pore sizes that will increase their selective nature due to the size and orientation of the molecules to be adsorbed. Molecules above a specific size could not enter the pores and therefore would not be adsorbed.

ENGINEERING DESIGN

To effect the good engineering design of an activated carbon adsorption system, it is first necessary to obtain the following data:

* The actual cubic feet per minute (ACFM) of air to be processed by the adsorber.

* The temperature of gas stream. The material(s) to be absorbed. - The concentration of the material to be adsorbed.

* The odor threshold of the material to be adsorbed (Table 4.3).

TABLE 4.3 ODOR THRESHOLD CONCENTRATIONS

Substance

Carbon Tetrachloride Ammonia Phosgene Chlorine Acrolein Amyl Acetate Pyridine Hydrogen Sulfide Oil ui Wintergreen

Benzyl Sulfide Diphenyl Ether

Ethyl Mercaptan Vanillin Butyric Acid Artificial Musk

Crotonaldehyde

Isuamyl ,Mercaptan

i1.8 53.0 5.6 3.5 1.8 1.0 0.23 0.18 0.066 0.062 0.006 0.0012 0.00043 0.00026 0.000079 0.000065 0.0000034

1 04 Cas-Phase Adsorption and Air Pollution Control Chap. 4

The presence of other pollutants in the air stream - Is solvent recovery required or justified?

After this information has been obtained, the cyclic time of the system must be determined. This is primarily an economic consideration, and should be reviewed and reevaluated after the initial sizing of the system. Should the initial capital cost of the adsorber be too high, the cyclic time may be reduced to enable use of a smaller system. In general, the larger the system, the greater the overall efficiency, and the less energy that will be spent per pound of material adsorbed. The normal starting point would be to choose a half working shift cyclic time so that the unit changeover could be made during a working break.

The weight of adsorbent required is then determined using the following equation:

t e Q,M C, W = 6.43(10)6 S

(4-11)

where

t = duration of adsorbent service before saturation (hr) e = sorption efficiency (fractional)

Qr = air flow rate through the sorbent bed (ACIM) M = average molecular weight of the sorbed vapor C, = entering vapor concentration (ppm by volume) S = proportionate saturation of sorbent (fractional)

Refer to Table 4.4 for typical maximum values (retentivities). The sorption efficiency e is a variable determined by the characteristics

of the particular system, including concentration and temperature. For the purposes of engineering design calculations, it is normally assumed to be 1.0. The design engineer must also control the inlet temperature to be less than 100°F at the inlet to the unit.

The next step is to calculate the volume of carbon required based on the bulk density of the carbon (Dc):

i

F

(4-12)

An equation for the overall pressure drop of the system is determined as follows:

(4-13) QI At, = - v s

Area of bed

n Control Chap. 4

: time of the system eration, and should system. Should the me may be reduced system, the greater

;pent per pound of choose a half work- be made during a

using the following

(4-11)

tivi ties). the characteristics iperature. For the issumed to be 1.0. !re to be less than

tired based on the

(4-12)

:em is determined

(4-13)

Enginecmg Design 105

TABLE 4.4 RETENTIVITY OF VAPORS BY ACTIVATED CARBON (percent retained in a dry air stream at 20°C. 760 m by weight)

Substance Retentivity (%) Remarks

Acetaldehyde Acetic Acid Acetone Acetylene Acryaldehyde Acrylic Acid Ammonia Amyl Acetate Amyl Alcohol Benzene Body odors Bromine Butane Butyl Acetate Butyl Alcohol Butyl Chloride Butyl Ether Butylene

Butyraldehyde

Butyric Acid Camphor Caprylic Acid Carbon Disulfide Carbon Tetrachloride Chlorine Chloroform Cooking Odors Cresol Crotonaldehyde Decane Diethyl Ketone Essential Oils Ethyl Acetate Ethyl Alcohol Ethyl Chloride Ethyl Ether Ethyl Mercaptan Ethylene

Butyne

7 30 15 2

15 20

Negligible 34 33 24

High -10 (dry)

n 28 30 25 20 8 n

21

35 20 35 I 5 45 15 (dry) 40

High 30 30 25 30

High 19 21 12 15 23 3

Reagent Reagent. sour vinegar Solvent Welding and cutting Acrolein, burning fats

Lacquer solvent Fuel oil Benzol, paint solvent, and remover

Heating gas Lacquer solvent Solvent Solvent Solvent

Present in internal combustion

Sweat. body odor

Animal odor

exhaust. Le., diesel

Solvent, cleaning fluid

Sulvent, amsthetic

Wood preservative Solvent, tear gas Ingredient oi krrvsene Solvent

Lacquer solvent Grain alcohol Refrigerant, anesthetic Medical ether, reagent Garlic. onion, sewer More retentivity by reaction

iconrinuedi

. ~~

L

A

BARNEBEY & SUTCLIFFE CORP P . 0 Box2526 Columbus, OH 43216 Phone (614) 258-9501 Fax (61 4) 258-3464

Q Product Information

Coconut Shell Activated Carbons for Vapor Phase Applications

DESCRIPTION

Granular activated carbons for use in vapor phase applications fall under two main headings, BASE ACTIVATED CARBON relying on physical adsorption, and IMPREGNATED ACTI- VATED CARBON using chemisorption. Certain physical features of activated carbon are of prime importance in its effective performance in a particular application, especially PORE SIZE DISTRIBUTION, HARDNESS and GRANULAR SIZE. Airfiltra- tion is invariably applied to very low levels of vapor phase contaminant such that a predominance of the finest pores is beneficial in trapping individual molecules. COCONUT SHELL more than any other raw material produces a range of activated sarbons complying with this requirement, a n d accordingly satisfying the needs of the major part of the air filtration market. In more specialized instances, certain individual contaminants 3re able to avoid physical adsorption due to very small molecu- ar size. adverse shape or polarity. Chemisorption of such onto mpregnated carbon is recommended in this case. Further jetails on our impregnated carbons can be found in our SPECIALTY CARBON brochure (T-1168).

FEATURES & BENEFITS OF COCONUT SHELL ACTIVATED CARBON

Various granular sizes Competitively priced

Effective pore size distribution (95% microporous) High activity (surface area) & retentivity Superior hardness & handling characteristics

TECHNICAL CAPABILITIES

Barnebey & Sutcliffe provides a comprehensive R&D and laboratory service using state-of-the-art equipment to evaluate the following:

carbon life remaining analysis

pilot plant testing 9 corrosion rate coupon services

performance evaluation

COCONUT SHELL ACTIVATED CARBONS

TYPICAL PROPERTIES A€ AT BT AB AC KE VG

Base Material .............................................................. Coconut Shell ..................................

Particle Shape ................................................................. Granular ......................................

Density g/cc 0.50 0.50 0.50 0.55 0.50 0.50 0.47

%Moisture ................................................................... 5% as packed ..................................

Hardness .......................................................................... 97 Min ........................................

CTC% 55-65 50-60 50-60 35-45 55-65 50-60 70-80

Size (ASTM) 4x6 4x8 4x10 6x12 6x12 8x14 6x12

. ~~

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F

Custom made products are available in addition to the above.

Technical Performance

Activated carbon has the ability to adsorb a wide range of contaminant vapors. Here is a typical list of odors that are well adsorbed onto coconut shell activated carbon.

Amyl alcohol Benzene Carbon tetrachloride Dichlorobenzene Ethyl acetate Floral scents Gasoline Heptane iodine Jet fuel Kitchen exhausts Lactic acid

Methylene chloride Naphtha Onions Perchloroethylene Rubber Styrene monomer Toluene Valeric acid White spirit Xylene Yeast

... , 7h,,dlll< ,,,,, k,% ~",a,n~u,,"r',nx,~r~~~,"",Tlran',ndBEKLfNZ"lmrr.,~n"ir~,~~",",.

PACKAGING rhese carbons are packaged for shipment in 50 Ib. kraft bags, 50 Ib. boxes, 180 Ib. drums, 1,000 Ib. bulk bags or customized or your special needs.

.h!s data and infarmarion is presented to assist a technically knowledgeable Nstomer in he evaluation Of carbons produced by Bamebey8 Sutcliffe Corporation. However. dueto ariations in the contenl of specilic gas or liquid streams. and the fact that the use 01 the arbon is beyond the conlrol of Barnebey B Sutcliffe. no guarantees or warranty. '%pressed or implied, is made as IO such use. any effects incidental lo such use. or the Dsults to be obtained. Barnebey 8 Sutcliffe expressly disclaims responsibilily therefore nd Ihe userac~epls full responsibilily lorpedormance of systems using carbon based on l is dala. Please contact Barnebey 8 Sutcliffe lor a more delailed review of your Pplicalion, belore proceeding.

T-1187

PORE SIZE DlSTRlBUTlON FOR ACTIVATED CARBON COCONUT SHELL63 CTC

i

PORE RADIUS RANGE (Ang)

uss SIEVE NO mm

1.4" 6.3 4 4.75 6 3.35 7 2.80 8 2.36

10 2.00 12 1.70 14 1.40 16 1.18 18 1 .oo 20 0.85 25 0.71 30 0.60 35 0.50 40 0.425 50 0.30 60 0.250 70 0.21 2

100 0.15 200 0.075 325 0.045

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Puzzle Piece No. 3 TREATMENT

Jane Bailey Associate Editor

Carbon Adsorption for Control

David Crompton Environmental Systems Div.

Dun Industries lnc. Plymouth. MI

ue to increased regulatory pressure to curb emissions. carbon adsorption has D bewme a popular method

for reducing VOC emissions. The technology is not new. Activated carbon has been used for years to solve pollution problems, including liquid and gaseous waste streams. In the area of water treatment, it has been used to remove trace organics, odors and chemicals that affect taste. In gas treatment activated c a m has been used to capture and retain volatile organic compounds (VOCs). The organics captured on the carbon can be either destroyed by reactivating the carbon or recovered through steam regeneration. Solvent recovery has been one of the most popular applications of E-20-INDUSTRIAL FINISHING-3/90

carbon adsorption in the past. Coatings application presents sev-

eral new challenges for carbon adsorp tion. The exhaust air that needs to be treated typically occurs at large flow rates and with low VOC concentrations. Also, the exhaust air typically contains a blend of solvents instead of a single solvent. The last several years have seen numerous Improvements to carbon adsorption technology to handle these problems.

At the heart of any carbon adsorp tion system is the activated carbon. AG tivated carbon is derived I" carbonaceous raw material that has been thermally treated to remove most of the volatile noncarbon content. This leaves a material almost completely made of carbon. When viewed under a microscope. the carbon has a spongelike structure contalning many openings, thus providing a large surface area. Commercial grades of activated carbon are normally produced from m n u t shells, bituminous and lignite coals. petroleum coke, sawdust and other wood

-

products. Most commercial carbons are manufactured by heating raw material to very high temperature under controlled conditions.

The adsorption process can be quite complicated. In simple terms, ads- tion occurs when a molecule moves out of the gas phase and deposits on the surface of the carbon. Eventually, an equilibrium will be established between the adsorbed molecules and the molecules that remain in the gas phase. Thus, adsorption is a surface-related phenomenon and depends heavily upon the "available" surface area of the carbon. Good adsorption effiuency is therelore characterized by a great amount of surface area packed into a very small volume and unit weight. As an example, typical granular activated carbon (GAC) has 3 to 8 million sq ft of surface area per Ib of carbon. With the typical GAC. much of this area occurs in miuopores in the inner reaches of the granule and is not easily accessible to the adsorption process. However, the "available" surface area of the

carbon can be increased. One technique uses a carbon fiber with micro. pores exposed directly at the carbon surface. This results in a carbon with superior adsorption/desorption properties compared to GAC.

Designing a system A typical system to remove VOCs

from spray booth exhaust would con-' sist of secondary filtration. carban adsorption with on-site regeneration and VOC destruction.

Several factors are important in the system design. The first is paint patti- culates in the exhaust. High-efficiency waterwash spray m t h s can remove particulates larger than 5 microns in diam. However, much of the material smaller than 5 microns will move through the scrubber and cw!d possibly bewme deposited in the carbon adsorption system downstream. This would plug the'carbon bed and decrease the available surface area. Therefore, it is important to filter particulates and prevent them from reaching the carbon bed. Several different methods are available, ranging from dry filters to wet SCNbberS to wet electro. static precipitation. Each has advantages and -disadvantages, depending on the application.

The two most important factors in the design of an adsorption system are the relative humidity of the incoming gas stream and the handling of the blend of solvents that must be removed from the air stream. The water content of the air stream wiii tend to m u p y adsorption sites on the carbon surface. Water is easily displaced from the carbon by high-molecuiar-weight organics but is not easily displaced by lowmolecular-weight compounds. If the relative humidity of the incoming air is too high, removal of bw-molecular- weight compounds that are immiscible will be very difficult.

If a solvent blend is present in the air stream, the effects of competitive adsorption need to be considered. Each organic compound will compete for an adsorption site on the carbon surface. Adsorption of the various compounds will not be uniform, and the components are generally retained in an a p proximately inverse relationship to their volatilities. For example, high boiling compounds are retained in preference to solvents with low molecular weights. When air with a mixture of solvents is passed through a carbon device, all vapors are at first equally adsorbed. Over time, the more easily adsorbed high boilers wiii force the low-molecular-weight compounds out of the bed. The adsorption system must accom- modate this phenomenon.

The removal of certain organic compounds such as ketones requires special attention. Ketones possess the

.

E-22-INDUSTRIAL FINISHING-3/90

ability to undergo exothermic chemical reactions while they are attached to the carbon surfaces. Since the reactions could increase bed temperature, the system needs to control the rise in temperature.

The two most popular designs of carbon adsorption systems for VOC removal are the static bed and the rotary wheel.

Static bed The typical static bed system has at

least two adsorbers filled with GAC. Multiple adsorbers may be used depending on the exhaust flow to be treated. VOGladen air is passed through one of the adsorbers. where the carbon removes the VOCs, and the dean air is discharged to the atmos-

redirected to the k d adsorber while the first adsorber is reactivated.

To reactivate a carbon bed, hot air or inert gas is passed through the carbon bed in a direction counter to the flow of contaminated air. The hot air desorbs the VOCs from the carbon and trans- fers them to the hot air. After leaving the carbon bed, the hot VOUaden air is passed to an incinerator that destroys the VOCs by thermal oxidation.

Several variations are possible on the design of the static bed system. Saturated steam is sometimes used to regererate the carbon beds. After de- sorbing the VOCs from the carbon. the steam is passed through a condensor,

~ ~~~~~ ~ ~ ~~. deanair passes up the exhaust stack. At the same time, a section of the rotor is continuously desorbed with hot air (Figure I). m e desorbed solvents are carried in the desorption air stream to an incineration device for deStNCtiOn.

The rotary system is designed to concentrate dilute solvent emissions into a small, manageable air stream. The flow rate for deswption is usually 10% or less of the raw VOUaden PO- cess exhaust. This lowers the capital and operating costs of the final incine!- ation system. The rotary system IS more efficient than the static bed system, since adsorption and desorp tion are performed continuously. A

:

106 Cas-Phase Adsorption and Air Pollution Control Chap. 4

TABLE 4.4 RETENTIVITY OF VAPORS BY ACTIVATED CARBON (percent retained in a dry air stream at 20°C. 760 m by weight) (continued)

Substance Retentivitv (W) Remarks

Eucalyptole Food (raw) odors Formaldehyde Formic Acid Heptane Hexane Hydrogen Bromide Hydrogen Chloride Hydrogen Fluoride Hydrogen Iodine Hydrogen Sulfide

lndole Iodine Iodoform Isopropyl Acetate Isopropyl Alcohol Isopropyl Chloride Isopropyl Ether Menthol Methyl Acetate Methyl Alcohol Methyl Chloride Methyl Ether Methyl Ethyl Ketone Methyl Isobutyl Ketone Methyl Mercaptan Methylene Chloride Naphthalene Nicotine Nitric Acid Nitro Benzene

Nitrogen Dioxide No"a"e Octane Oz""e Packing-House Odors Palmitic Acid Pentane Pentylent

20 High

Negligible 7

23 16 12 12 10 15 3

25 40 30 23 26 20 18 20 16 10 5

10

25 30 20 25 30 25 20 20

10 25 25

Decomposes to oxygen Good

35 12 12

Disinfectant, plastic ingredient Reagent lngredient of gasoline Ingredient ok gasoline

Oxidizes to increase retentivity

In excreta

considerably

Antiseptic

Sol\,e"t Lacquer S0Ive"t

Sulvent

Solvent Wood alcohol Refrigerant

Solvent Solvent

Reagent, moth balls Tobacco

Oil of bitter almonds Oil of mirbane Hydrolyzes to increase retentivity lngredient of kerosene Ingredient of gasoline Generated by electrical discharge

Palm oil Light naphtha

ntrol Chap. 4

ercent retained

larks

:tic ingredient

oline dine

3se retentivity

707 Engineering Design

TABLE 4.4 RETENTIVITY OF VAPORS BY ACTIVATED CARBON (percent retained

Substance Retentivity (%)

in a dry air stream at 20°C. 760 m by weight) lcononued)

Remarks

Phenol 30 Carbolic acid, plastic ingredient Propane 5 Heating gas Propionic Acid 30

Propyl Mercaptan 25

Putrascine 25 Decaying flesh Pyridine 25 Burning tobacco Sewer odors Skatole Sulfur Dioxide (dry) 10 Oxidizes to sulfur trioxide,

13 Hydrolyzes to sulfuric acid Sulfur Trioxide

Propylene 5 Coal gas

Propyne 5

High 25 In excreta

common in city atmospheres

Sulfuric Acid 30 High Toilet Odors

Toluene 29 Manufacture of TNT Turpentine Valeric Acid Water None Xylene

32 Solvent 35 Sweat, body odor, cheese

34 Solvent

where V, is the superficial linear velocity through the bed.

vbed Height of bed H,, = - Ao

Total pressure drop APT = HE, ( A p/ft)

where p/ft is obtained from Figure 4.10. Substituting Equations 4-13, 4-14, and 4-15 and rearranging yields:

(4-14)

(4-15)

The final system pressure drop is then determined by an economic balance between the size tank required for a specific velocity and the power requirements for the pressure drop. Once the pressure drop has been determined, the height and area of the bed are calculated using Equations 4-5 and 4-6, and a cylindrical tank or pressure vessel of the appropriate size is selected (Figure 4.11). If the system is to be used for solvent recovery, the

Chap. 4

u

>proximate entivity in b at 20°C 760 mm

14 21 28 15 30 7

40 40 40 40

25 25 15 16 19 23 28 34 15 25 30 35

15 18 20 20

High High

20 23 25 20 20

High 15

Engineering Design 1 09

PROPERTIES OF CASE5 AND VAPORS AND THEIR RETENTION TABLE 4.5 BY ACTIVATED CARBON (continued)

Approximate Boiling Critical retentivity in

Molecular point temperature % at 20°C Substance Formula weight 760 mm C "C 760 mm

Ethyl Chloride C2H5CI Propyl Chloride C,H,CI Butyl Chloride C4H9CI Methylene Chloride CH2C12 Chloroform CH.CI3 Carbon Tetrachloride CClr Iodoform CHI, Phosgene COC12 Pyridine C5HjN Indole CsH7N Skatole C&M'J Nicotine CiuHirN Nitrobenzene CsHsNOz Urea CO(N02) Uric Acid C&WL03 Putrescine (CH2)dNH2)2 Packing House Odors Cooking Odors

Sewer Odors

Chlorine CI2 Bromine Br2

Hydrogen Fluoride HF (Hydrofluoric Acid)

Hydrogen Chloride HCI (Hydrochloric Acid)

Hydrogen Bromide HBr Hydrogen Iodide HI Nitrogen Dioxide NO2 (Nitrogen Tetraoxide) (N,O,) Nitric Acid HNOi Sulfur Dioxide SO2 Sulfur Trioxide 5 0 3 Sulfuric Acid H 3 O r Hydrogen Sulphite H2S Water H,O

12

64.52 12.2 187.2 78.54 47.2 230.0 92.57 78.0 84.94 40.1

119.39 61.26 263.0 153.84 76.0 283.1 393.78 -

98.92 8.3 183.0 79.10 115.3 344.0

117.14 254.0 131.17 266.2 162.23 247.3 123.11 210.9 60.06 Decomposes

168.11 Decomposes 88.15 158.0

Nitrogen compounds Nitrogen and Sulphur

Nitrogen and Sulphur compounds

compounds 70.91 -33.7 144.0

159.83 58.78 302.0 253.84 183.0 553.0 20.01 19.4 4

36.47 -83.7 51.4

80.92 127.93 46.01

(92.02) 63.02 64.06 80.06 98.08 34.08

-67.0 90.0 -35.38 151.0

21.3 158.0

5 - 10.0 157.2

44.8 218.3 330.0 20

-61.8 100.4

20 25 30 30 40 45 30 20 32 25 25 25 + 20 15 20 25

15 40 40 +

5

8 10 5

10 dry 15 dry

3 dry 18.02 100.0 374.0 None

.~

L

Figure 4.12 Typical activated carbon-bed adsorber.

A - S O L V E N T L A D E N AIR INLET B - S T R I P P E D AIR O U T L E T C - S T E A M I N L E T G - E F f L U E N T TO DRAIN D - C O O L I N G WATER SUPPLY

E - C O O L I N G WATER R E T U R N F - R E C O V E R E D S O L V E N T O U T L E T

H - C O N D E N S A T E TO D R A I N

Figure 4.13 Preliminary flow sheet for solvent recovery plant.

SLA INLET

Figure 4.14 Adsorption system.

CONDEM8ER

PROD. COOLER

1 I I

Solvent Recovery 7 13

TABLE 4.6 TYPICAL APPLICATIONS AND EFFICIENCY OF CARBON ADSORPTION SYSTEtMS

Control efiiciency Comoound I7e) Comments

~~

Acetonelphenol

Dimethyl terephrhalate Maleic anhydride Methvlene chloride

Perchloroethylene

92

83.4

99 80 97

85

>90

96, 99

96, 97

Overdl Ihydrocarbon removal

Efficiency calculated trum design

Efficiency including condenser VOC removal efficiency p-Xylene removal efficiency System control efficiency Reported efficiency for controlling

emission from pharmaceutical manufacturing

eificiency

industry

efiiciency

data

Perchloroethylene control

Test data from dry-cleaning

amount of solvent retained in the bed must be calculated to determine how much steam to use for stripping, where:

Ib of solvent = 5 W

Solvent recovery systems would also necessitate the specification of condenser duties, distillation tower sizes, holding tanks, piping, and valves.

In summary, engineering design of a n adsorption system should be based on pilot data for the particular system. Information can usually be obtained directly from the adsorbent manufacturer. The overall size of the unit is determined primarily by economic considerations, balancing the operating costs against the capital costs. Adsorption, as can readily be seen, is not a n exact science, but rather a n art that draws on the experience of the design engineer. Figures 4.12, 4.13, and 4.14 are schematic diagrams of air pollution/solvent recovery systems.

SOLVENT RECOVERY

Volatile solvents vaporized during a manufacturing process may be recovered and used again. From the mixture of air and vapor, which is generally the form in which the solvent must be sought, the latter may be condensed to a liquid and trapped by the application of cold and moderate pressure; the vapor-laden air may be passed through a liquid adsorbent such as water; or finally, the mixture may be passed through a sufficiently thick bed of a solid adsorbent such as activated carbon and later driven off by steam. There are

L

556

environmental and industrial pollution control technology. It is a must for the consultant’s, educator’s, and practicing engineer’s bookshelf.

S . A. Churchill Departments of Biological Sciences

and Civil Engineering The University of Alabama Tuscaloosa, AL 35487, U.S.A.

Carbon Adsorption for Pollution Control. Nicholas P. Cheremisinoff and Paul N . Cheremisinoff. En- glewood, NJ: P T R Prentice Hall, 1993. (216 pp., ISBN 0-13-393331-8) $52.00 hardcover.

The text is designed to provide a working knowledge and description of adsorption, process equipment, and materials, in particular, activated carbon. The book is divided into seven chapters, which include a discussion of adsorption principles, applications, treatment methods, activated-carbon regeneration, and materials safety information. Ad- sorption is a separation process in which certain components (adsorbates) of a fluid phase are selectively transferred and concentrated onto the solid surfaces of specific materials (adsorbents). Acti- vated carbon is the most widely applied adsorbent material currently in use for pollution control and is used in three forms: granular, pelleted, o r powdered. Adsorption efficacy depends on’ solid-fluid equilibria and mass transfer rates. Regeneration of the adsorbent yields the adsorbate in concentrated form. Hence, the adsorbent is carried through a cycle of contact, regeneration, stripping, and recovery, each of which may be achieved by a number of methods. Type application of carbon adsorption operations depends on the nature of the fluid phase. Gas phase applications include solvent vapor recovery from surface cleaning, film casting, or coating operations. Liquid phase applications often focus on process stream purification or decolorization.

The introductory materials discuss the basis of liquid phase adsorption and the equipment designs commonly employed that make use of either powdered or granular activated carbon. Slurry is applied in a contact-batch operation. Granular carbons are used in fixed-bed arrangements, while pulsed-bed designs operate on a semicontinuous, countercurrent basis. Discussions of regeneration basics, labor requirements, and industrial applications of liquid phase operations follow. Gas phase adsorption may also employ either fixed- or moving-bed equipment. Regeneration methods are de- scr ibed. Equipment configurat ions, operat ion,

BOOK REVIEWS

maintenance, and industrial application discussions are featured. A nontechnical description of adsorption theory, regeneration, and activated carbon production and brief descriptions of two other adsorbents (silica gel and activated alumina) finish the chapter.

Gas phase adsorption for removal of dilute contaminant concentrations is discussed in the second and fourth chapters. Chapter Two, entitled “Car- bon Systems,” includes basic discussion of the adsorption process and the physical properties that impact the effectiveness of activated carbon for removal of organic compounds: total surface area, which affects adsorptive capacity, and carbon particle size, which influences mass transfer rates. Dis- cussion of water and wastewater purification operations utilizing activated carbon are accompanied by process schematics and cost and regeneration cycle specifics. A similar treatment of material describing solvent recovery concludes the chapter. The first half of Chapter Four, “Gas-Phase Adsorp- tion and Air Pollution Control,” is devoted to build- ing the technical basis for high-efficiency gas- adsorption operations. Adsorption is discussed in thermodynamic terms, as surface resistance and adsorption dynamics, which includes mass transfer zone characterization and effects. The section pro- ceeds to discuss design criteria, which include adsorbent selection, concentration, regeneration, bed depth, temperature, and particle-size effects. The chapter focuses on two important pollution control applications: odor control and solvent recovery. Chapter sections march the reader through adsorption system requirements and engineering design and include a helpful glossary of terminology.

Liquid phase adsorption is presented in Chapter Three. Rationales for the increasing preference of the granulated form of activated carbon over the powdered form are presented. Laboratory procedures for evaluating granular act ivated-carbon treatment are enumerated. These explore static experimental conditions and variables through isotherm plots. Once optimal static conditions are selected, dynamic column tests provide d a t a on operation performance. System selection criteria and options are accompanied by sections that describe adsorption systems equipment type and con- figuration, materials of construction, and adsorbent storage considerations. Chapter Five, “Carbon Ad- sorption Treatment of Hazardous Wastes,” targets liquid streams having relatively dilute hazardous or toxic contaminant concentrations. Applications, economics, energy, and environmental impacts are presented. Special emphases is placed upon adsorption treatment of pesticide by-products and wastes. Chapter Six presents an in-depth discussion of activated-carbon regeneration methods, including

BOOK REVIEWS

equipment, economics, and operation design variables. The last chapter, “Exposure, Controls, and Safety,” pertains to employee exposure to the carbon adsorption unit operation and safety considerations. The text concludes with an appendix of physical constants and conversion factors, subject index, and bibliographical references.

This volume is an excellent practical handbook and basic adsorption operations technical reference

557

and will be readily comprehensible to engineers, scientists, and plant managers, irrespective of technical background.

S. A. Churchill Deparfments of Civil Engineering

and Biological Sciences The Universify of Alabama Tuscaloosa, A L 35487, U .S .A .

Reduction of Volatile Organic Compounds in Aqueous Solutions Through Air Stripping

and Gas-Phase Carbon Adsorption _- * <* C. S. Fang and Sok-Leng Khor

Department of Chemical Engineering, University of Southwestern Louisiana, #d Lafayette, LA 70504 94 $j. _L

INTRODUCTION .''

Dilute aqueous solution of volatile organic compounds (VOC's) is a.waste stream of many chemical process industries, and often is a serious environmental concern. Conventionally,,it:is treated with a liquid-phase activated carbon adsorption'process. The process is reportedly effective in removing VOC's from water [ l ] , but its cost is high. The combination of air stripping and~gas-phase activated carbon is an alternative process, and it appears to be efficient and cost-effective. Before any pilot-plant test, a preliminaiy design study w a s conducted to examine the technologyand its cost. The results ofthe study are presented here. . .' ~~

~. .:. : i . , . ,.... i-

'VOLATILE oR~&it .. ~ COMPOUNDS

ound, which has relative volatility of om temperature, relative to water, is olatile organic compound (VOC). ous wastes generated in Louisiana in

d in 1985 Iby Jacobus [Z]. I t identified 1983 was

.... ;..

...

tries. Fourteen of these sixteen VOC's listed in th are EPA priority pollutants.

HENRY'S LAW CONSTANT

Henry's law describes the distribution of a co

where

. , ,Environmental Progress.

TonsNear Reported in ~ n . r a

VOC's identificd in La.:

Vinyl chloride' 1,2-Dichloroethane* C;wbon tetmchioride' Trichloroethylene' I .I-Dichloroethane* T<,l"e"e* \Icthyl ethyl ketone' I :I,lorafi,rm* \cetane ,\crulein* 1,1,1-Trichlarocthane'

Others:

Ile,,Ze"t2* l l e t h y l chloride'

It nlso can be written in other terms. For example. i T 8 1 1 1 1 1 1 rV.8, I;,,,, is /l\l.ll.

\YIie:Te

v, = mole fraction of the component i in gas-pli:ise x, = mole fraction of the component in 1iquid-ph;ise m = [Henry's law constant, dimensionless Ilcnry's liiw constant is : I good indication whetl.2- r :!

VOC ciiii be removed from an aqiieous solution 11:;. air stripping. i3enry.s law constants of eighteen VOC'! are shown in T h l e 2. These values were reported I)] l l t i - Im1e:uax [.3]. Uilling [41. and Macknv c.51. . . ,

I [<!nry's iaw r O I I S t i l l l l l;,r ""!11,).1 <.111y1 k < ! t < , , , < . \\,, iw ted to be 2.97 x 10P mJ atinlg-mole, which ;Ipp&ld;tu he too low. When this value is used in design calculn- tions, the results are less than reliable.

yd = in xi (2 )

I

:.

15,513 13,356 2.396 1,775 H23 67 61 5.5 3.8 3.0

Relative Valatility

25°C

125.0 3.353 4.843 3.150 9.650 1207 3.825 8.257 9.751

4.037

18.15 4 . W 4.138

17'3.3

PROCESS DESCRIPTION e. -. / Figure I shows the process liow dingrani o f i h e procsrs

to lie studied. The process includes two phF (1) iiir stripping operation, and (2) gas-phase activ:ited carbon adsorption, including spent carbon regeneration. Equip- ment used in the air stripping operation includes a feed pump. air stripping column, Mower a n d a ir filter.

Tllc VOC I;&n ai: from the air stripping coliiiiin is led to one of two carbon adsorption columns to sepnrxte VOC's from air. Air after adsorption is vented to the nt- mospliere. while the VOC's are c:tptured and collected i n tlw ;idsorption column. When the concentration of VOC's i i i : b i t r v a c l ~ t ~ s IIIC I ~ n ~ ~ t k - t l ~ r m ~ ~ I ~ l x j t n ~ titc lid i \ switched to the other column which h a s regenerated carbon. Spent carbon is regenerated either by hot air, electrical-resistance heater, or microwave radiation.

TABLE 2. HENHYS L~WCONSTANT. 25°C

Henrv's Law Constant

Vinyl Chloride 12-Dichloroethane ;, :,, , Carbon Tetrachloride :

0.199 0.00123 0.0286 0.0117 0,00577

Trichloroethylene :: 1.1-Dichloraethane 1

. 0.00688 Tolriene Methyl ethyl ketone Chloroform : ' ' 0.00385 Acetone : 1.16 x 10.' Acrolein .I. ' 6.24 X

? (2.97 x 10.5)

4,680 68.3

944.8 649.5 320.3 370.8 (1.64)

213.7 6.4 1.1

,.I. 50.6 * 304.7

.. . 292.5

.73&

.&, . .I , .

. . 991.1

,'.?k'17,7

..368.6 -!e.:.-,.: .... *. ~.. I

;s.h

: ,

.-. +..~

3,175

185 12.4

88.8 58.1 72.3 (0.409) 32.2 2.0 3.5

6.8 70.1

224

November, 1989 271

L

. .

...

Figure 1. Process flow diogram

AIR STRIPPING

Air stripping of an aqueous solution is a well e s t a b lished process. Recently, it hils been used to remove vnri- ous VOC's from the drinking wiiter supply in a number of communities [6, 7, 81. Basically, it is an operation providing inter-phase mass transfer o t VOC from water to air. It can be accomplished with a tank and spargers, tray column, or packed column. In this work, both types of columns were studied; however, only the results of packed columns are reported here. The diagram given by Knvanaugh and Trussell [91 shows a comparison of tray and p;Icketl coliimns in the stripping operation. When ! I ~ . ~ , N ) ' , I.,\(. i c ) i i \ ~ . t t i ~ > zi<<.r, ~ i h ' ! ' . c i ) / i : 3 , \ ~ < ~ a ~ ~ ~ > I i c J I C , iIii>

, . dingram, i t is found that the diagram recommends i~

packed column for air stripping of most VOC's. In ;iir stripping ofVOC's from drinking water supplies,

the contaminated air is vented to the atmosphere. The more air is used in stripping, the VOC concentration in the effluent air of the stripping column is lower, which nukes the r,l,e:rntion look better. However, the VOC c m - centration i n the elfluent air from the strippin$column in industrial wastewater treatment is too high to be discharged directly to the atmosphere. Dilution is not considered :IS a sohition in indl1stri;d operations.

111 Illis s ~ t ~ l y , [lie ~ , , , ~ l ~ , i i i i , ~ ~ , l ~ ~ ~ l a ir will I,e tre:iled Therelore, the less air is used ia stripping, the cost of treatment of contaminated air will be less. The minimum gas-to-liquid ratio is equal to the reciprocal of Henry's law constant in [Jill:

Acetone Acrolein

$$$:~. 272 .November; 1989 . .. ,ILL

The achiai galue of 1.5 to 6 times the (GIL),,,;,, = nim, where liquid ratio of Various stripping of trichloroethylene and perc the drinking water supply of the city o CA, the gas-to-liquid ratio is more than of order ereater than the minimum 17l. . _ ... .

Also shown in Table 3 is liquid loading, which s&z, flow rnte of liquid fed to the air stripping column diAdPa.. by the cross-sectional area of the column. It varies fm:~:: 258 litersiininhn' [7.1 gpmif?) for vinyl chloride to.6710 literlminlm' (165 gpmlf?) for methyl ethyl ketone.. Th$. high liquid loadings for several VOC's are due to their low Henry's law constant.

T h e height of the packed bed is determined as fol&T .. .

[JJ]: A>.,+. 'y.-. .. 44):. : ,.: 2 = N,,,L ' H<.L

where % ..:.

2 = height of packed bed w* : N,(,,. = number of overall transfer units (NTU), base

/~f,$,,, = height of overall transfer units (IITU), bh& on liquid phase ,.i-.a,y *'

on liquid phase H rr'.

The values of NTU and HTU can he calculated froG? p . 4 , . ..?

tlir followins tqt'ntion I//. /?I:

_.I

i : ,

i Liq. Loading Prebsure Drop ( t

TABLE% GAS-LIQUIU %TI0 ANU LUAUINC OF STRlPPlNC COLUMN

literlmin/m2 ,. cm HID I I - 7

0.298 0.554

5.461.6 6,263.1

Car-Liq. Ratio voc's SCMlliter

Vinyl Chloride 1.61 x 10-3 288.48 1.2-Dichloroethane' 0.0749 3.058.8 Carbon Tetrachloride 5.39 x lo-' 724.47 Trichloroethylene 1.1-Dichloroethane Toluene 0.0138 860.15 Methyl ethyl ketone 1.167 Chloroform

~~

l.l,l-Trichlaroeth~n~ 5.16 x 686.98 1.1.2-Trichlaroethane 0.101 3,587.7 - . Benzene .i 0.0168 1.030.5 Acrylonitrile

- ' o-Xylene .. .

'' p-Xylene ' , 0.0138 0.0159 980.35 ., . . . .

Tucrmvert SCMllitrr to SCFIml. mullipl~ by 131.EWX4. T u r ~ ~ n v m l i t d n ~ i d m * t o r"@. n ~ l i i p l y Ihy 0.02451% . , Tu mnrrrt 'w llto 80 in. HP,mvltiply hy0.3W7.

' '

.-'-.-+< T A ~ L E ~ . A $ ~ H I P P I N C COLUMN .r + waste doutinn: 500 to I mgll. Operating Temp. = 25°C

NTU '' N . , L voc's , . . ..'*

Vinyl Chloride 2.96 7.24 1,2-Dichlaroethane ,: 1.96 7.90 Carbon Tehachlaride 2.56 7.90 Trichloroethylene 3.36 7.90 1.1-Dichloraethane 2.12 7.90 Toluene 2.43 7.90 Methyl ethyl ketone 0.192 15.36 Chloroform 2.05 ; 7.90 Acetone 1.29 15.36 Acrolein 0.576 15.36

. . ... . . . - -

1,l.l-Trichlarot.thane 2.68 1.90 1.1.2-Trichlorcethane .2.01 1.90 Benzene 2.15 7.90 Acrylonitrile 1.92 11.05 0-Xylene 2.43 7.90 m-Xylene 2.49 7.90 p-Xylene 2.59 7.90

ccxi js L r LYHK swmd $~aunn. Tv ~ ~ u i v e ~ ni Io le. ~nwltiply lhp 3.28ux4.

.,. .- . .. . . .

Hc = 0.011 Gh (SJC'" . , .

,,., - ' - L*,, = liquid loading based on mass How rate, Kg/m2S , ,. . . . - . -.

i / ;, :;<. , ~. . . I . ~. .. . .,, . . Ji = ( W C J P ~ ) ' . ' ~ I, ,. . . , . . ...

. ., ~ . ~ , f2 = (P,JPJO'~ . .

fJ = (u,Ju4'-'

Values of +,,, SI,, and & were given by Sinnott graplii- cally [12]. They are converted to mathematical equations.

. . . , ..

Stripping Column Height Diameter

m 111 - 21.40 4.09 15.45 1.26 20.21 2.58 26.61 3.14 16.75 2.21 19.05 2.37

16.21 1.86 19.75 0.94

21.20 2.65 15.90 1.16 17.00 2. IR

22.21 0.85

x.85 0.m

~ ~. 21.20 1.07 19.25 2.12 19.71 2.22 20.50 2.36

'14,900

277,YOO 110,000 191.G00 117.500 206.500 221.800 238.200

42.800

so that they can be used in computer programs in this work. These mathematical equations will be presented later.

till: cross-sectional area u t tile column ;it its loading u r i i -

clition. The Hooding condition is given by Peter : ~ ~ i d Tin- inerhaus [13] graphically. Again, the graph is converted to a mathematical equation for computer calculations.

Using these equations, the diameter and height of the air strip ing column for various VOC's w+rc calciilaterl and are 3:Yted in Ttble 4.

Press, :k drop across the column w a s ;ilso cdcul;ited. I t is smal1,iless tliiln 4.6 m of water (15 psi). A low pressure blower is sufficient to deliver the needed air in the columq. &,

. . , , .4. :I I . ,.>I . - . '.,

'-

'.- '

100 I

'ov/ Berl 5aM1es

V I I I 0 10 20 30 10 SO 60 IO 80 90

I . :. Li , . I ! .

Figure 3. Factor far gor-phare HTU for Bed Sad,dles.,(Cm'er are"fr0m :. , .? , ,a , . Zi. I. Sinnott [f211" .. .... . c .< *. November, 1989 273

,

TABLEG. Co~OFAIRSTRIPPlNC(1988, SmondQimrtter)

Wastewater Flow Rate = 1000 mm. VOC Reduction: 500 to I mg/l, Operating Temp. = 25°C

Total Stripping Cast Total Equipment Ut i l i@ Cost Labor Cast VOC'S cosr", $ a/,enl- $/year $/year $11000 liters -

428.800 373,400 3 16,400 445.JOO 274,600 .303.700 2 I4.70O 33i.xoo 32%400 !?76.4Oll 421.700 293.200 304.300 315.700

COST OF AIR STRIPPING

Using the equations and computation procedures described above, equipment sizing and cost estimates were performed for an air stripping fiicility of 1000 ppm wistewater capacity and 99.8% VOC reinoval, from 500 mg/l to I iiiglJ. Tlie operating temperature is 25°C (77°F). and

' I ' l i $ , ' s u ~ s ;wd costs ,,i stripping C ~ U ~ W for w w u s clilorinntcd ~ y & ~ c : ~ r i x ~ ' i s , toluene ;ind I I ( J I I Z U I I L . :ire shown in Table 4. Cost data given b y Gathrie [ I d ] and Peters-Timmerhaos [I31 are used. The costs are Ibr the second-qunrter of 1988. The column costs include the costs of a shell, support and pricking msterinl. T;il>lo S

,111.1 4..:-.: ! i l l : , , . 1.

9.800 38.300 IO.800 1.7.600 12.270 10.600 I8,4l10 l O . ~ ~ I l 8l.620 1i.I(10

I 16.10(1 l8,.7(1(1 Ii.000 1.5.700

v o c s

Vinyl Chloride Carbon Tetrachloride Trichloroethylene 1,l-Dichloroethane Toluene Chl"rof"m l.l,l-Tiicl~laroethane Benzen.: o-Xylene in-Xylene p-Xylene

57.500 57,500 57.500 57,500 57,500 57.500 57.500 57,500 57.500 57,500 57.500 57.5lX1 57.500 57,500

153,100 170.500 131,600 159,300 124,700 128,800 118,800 134.700 203,600 129,900 257,900 134,300 135,400 136,300

0.141 0.156 0.121 0.146 0.114 0.118 0.109 0.124

0.119 0.237 0.123 0.124 0.125

0.187

sliows the sizes and costs of the supporting facilities which include a feed pump, blower and air filter.

Table 6 shows the required capital investment, or the total installed equipment cost, for the a i r stripping ofwri- 011s VOC's. I t includes the costs of air stripping, pumping and air supply. Tlie utility cost is calculated, based on IO$/K\V-hr. The labor cost is lxised on two semi-skilled 9;,<.:;t:<,r5 , i i 5?,000 j w ~ :~ t . . i ; - . j;!iiw 1.5';;. ?rii,q2. ill oL,;il.\ (total $28.750 per yenr). It is assiimed that the facility is operated 16 lioiirs per day, 300 days per yenr, and tlie service time of the facility is 5 years. It varies from $118,800 tO 5257,900 per year, or from $0.109 to $0.237 per 1000 liters ($0.39 to $0.90 per 1000 aallons) OfVOC a(lueous so-

Influent Efluent

98,434 13.257 10.697 7,030 8,726 3,901

15,992 8,461 5,991 6,590 7.537

Carlion Bed Diameter, D Lengtil, n

m m

0.93 1.62 1.95 2.78 2.58. 3.39 1.58 2.85 2.90 2.77 2.59

13.96 5.43 3.81 1.95 2.22, 1.37 5.60 1.83 1.81 1.97 2.23

. . .,. . ..:_ . . Carbon' tehachlolide [26 ]~ . ,

Benzene [19] n-Hexane

.. , :

* ..

g organic (gclrbon) ( m m Hg)"

k

,," I- 0.2928 0.1114 0.2367 0.134'

n

0.20999 , 0.36462

0.1780 0.06032'

37.8

33.3 25.0

100

Temp., 'C

November, 1989 275

_- T A B L E ~ . COSTOFVOC AUSORPTION ' . I

Capital Investment Vessel VOC'S Type"" 2 COI"""5, $ C~rl,on"". $ Total. $ - Vinyl Chloride v 111.000 Ciwbcrn Tetrachloride v 61.000 Trichloroethylene \' 80.400 1,l-Dichkimcth;ine H 40.noo Tcrliiene 1-1 39,600 C111"1"I"llll H 58,000 I . I,l-Trichlorocth;inr \' 07.000 Benzene H 39.600 <,-Xylene tl 39.600 "ylene t I : 40,800 p-Xylene H 39,600

21,000 24,800 25,200 26.000 25.600 27,200 24,400 25.600 26.400 26.200 25,noo

Based on the concept of fixed-bed mass-transfer and Freondlich isotherm equation, the depth of the.% quired carbon bed is obtained as follows: 32

CARBON ADSORPTION

Gas-phase activated carbon adsorption has been used for two or three decades in the solvent recovery and odor control, removing alcohols, benzene, paraffins. trichloroethylene, chloroform and carbon tetrachloride from air streams (15,1G, 17). A recent field study hy Nelson, e t a]., repoited a high removal of VOC's from air using gas- phase carbon adsorption [ l e ] .

A i r cnrrying VOC? stripprd frnm aqiieons snliitinn is iud tu a i l :~cii(.;~~cii ., ~-s:a ,d,oipliosi cciiiiiiiib. ' l . i i c . tioii-

centration of VOC id air varies from.~,OOl ppm to 98,434 ppm by volume, as shown in 'Cable 7. The concentration of VOC's in efluent air after adsorption is determined by the adsorption equilibrium. It is approximately 3 ppm for carbon tetrachloride, according to Parmele [lG].

The adsorption equilibrium data of henzene and car- Ixm tetrachloride were reported by Coolidge [191 and Par- mele [lG], respectively. These data were used to size the adsorption column.

The Freundlich isotherm equation is used to express the adsorption eqnililxiwn. The equatinn is:

,y = ,,.a (17)

where X

p k,n = Freundlich-equation constants

= weight ratio, grams ofVOC adsorbed per gram

= partial pressure of VOC in gas-phase, mm Hg of activated carbon

The values of k and n for carbon tetrachloride, benzene and n-hexane are shown in Table 8. The values of k and n of n-hexane are estimated from k's a.nd n's of ethane, propane and butane.

Adsorption equilibium data of other VOC's are not available. Therefore, k and n of n-hexane are used, as an approximation, in sizing adsorption columns for these VOC's. There are two reasons to use n-hexane: they are:

. a conservative design.

132,000 85,800

105,600 Gfi,800 65,200 85,200

121,400 65,200 66,000 67,000 65,400

H -- r-. B PO

..,

to be 0.17 m/s (0.55 ftlsec), which is

tion (18). j I-. ;.;:

' ;p>?$ . ,.. ... . .. ...:. :", ..

REGE

Re whic nn 01

SP trim d s o rege inatc

IC[: elect .It 1: ;!,itti I ege t i d tion! SllOV

and $0.7. \ Y R S l

MICF

TI iite 5 [23] pate Unit tion I l l h

T.

3,f a The

COST OF ADSORPTION

activated carbon is $1.00 per pou

of various VOC's are calculated.

and make-up carbon.

TABLE 10. COSTOF SPENT CARBON REGENERATION

Cost of Regeneration Cost of Make-up Carbon

Carbon tetrachloride 174,500 0.160 Benzene 316,400 0.291 Others 400,600 0.367

r276 'November,l989 -Environmental Prog

pea 6. 'I of i

ace1 per; hex

1.1.. Ben o=X! m-X PX!

Env

REGENERATION

Regeneration of spent carbon is a supporting operation, which can be accomplished as a part of the process, or b y tin off-site commercial operator through a contract.

Spent carbon can be regenerated by hot air [ZO] or electrical-resistance heater [Zl]. Microwave regeneration is nlso a possibility [ZZ]. However, the conventional steam regeneration is not considered, since it produces contaminated water:

Marquess and Nell, Inc. (New York) reported that their electrical-resistance heater can regenerate spent carbon ;It 13$/Kg (6Ylb) with only 4% carbon loss. Without com- iniitting to any specific regeneration pethod, the cost of regeneration and carbon loss are assumed to be, respectively, 22$/kg (10$/1b) and 10%. Based on these assumptions, the cost of regeneration operation is calculated and shown in Table 10. For carbon tetrachloride. beiizene and other VOC's, it is, respectively, $0.320, $0.582, and $0.734 per 1000 liter ($1.21, $2.20, and $2.78/1000 gal) ot wastewater.

MICROWAVE REGENERATION

The concept of using microwave radiation to regenerate granular spent carbon was first suggested by Schulin [23] in 1971. It w a s followed by more than a halfdozen of patent awards in Japan, the United Kingdom, and the United States. However, there is no commercial apglica- tion renorted. and the I i tAture in the ooblic doni;iin on

popular in industrial' heating :doni;stic cooking. When microwave radiation is applied to polar molecules, which includes all VOC's, molecules are heated by the induced molecular rotations, and separated from the carbon.

In laboratory tests [ Z Z ] , samples o€ 100 grams granular carbon loaded with acetone were placed in :I domestic (Kenmore) microwave oven, and received microwave radiation for 48 minutes. The weight.loss was monitored during the radiation. The weiEht loss represented the re- ,11,1Vill 01 3CCtl>l,,! l w m spcn1 c:,rl><l,l~ 'l'hc lCS1l l tS :,sc shown in Figure 6. All three tests showed the desorption of acetone, as shown by the three curves in the figure. The same microwave regeneration experiment was repeated for n-hexane. The results are also shown in Figure 6. The rate of desorption for n-hexane is slower than that of acetone. The dieleclric constants of n-hexane and acetone are, respectively, 1.89 and 20.7 at the room temperature [24]. Therefore, the slower desorption rate of n- hexane is expected.

voc Vinyl Chloride Carbon tetrachloride Trichloroethylene 1.1-Dichloroethane Toluene Chloroform 1,l.l-Trichloroethnne Benzene o-Xylene m-Xylene PXylene

G i a n u l ~ wtivated c u t o n

I I I LO 20 10 40

Tim Of 14icr-v~ Radiation.

Figure 6. Microwore desorption.

The experiments show that microwave mdi, 'I t. Lon can remove VOC's from spent carbon with very little purge gas, if any. This is the advantage 01 microwave regeneration over other types of carbon regeneration. The energy con-

(11,230 Utll/lb) 0 1 acetnne. .- ,, I , , . ,,.<,.!,,.:,t:;,,.> i \ 7:; h z

CONCLUSIONS

Neither air stripping nor gas-ph;ise carhon adsorption of volatile organic compounds is new, \ r o t thei coml,inn- tion shows :I pntential to remove VOC's Siom :t:~:.~eous solutions economically, particularly from dihiteisol;ti~,ns. Water involved in activated carbon is much ILss in gas- phase adsowtion than that in liquid-phase :I s rption It makes repenerntion o f spent c:irI)nn eiisier a$I'%,e ieciv- c r y of VOC's S n > r ~ t a ~ o c c ~ o ~ s o l i i l i i i i i s ~ x > s ~ i l , l . : .

The case of3.785 literdinin. (1000 gpm) otFaqtieous solution containing 500mglI of VOC was shidied. The 99.8% of VOC can be removed at costs ranging from

the VOC involved, as shown in Table 11. The cost can be reduced, if the credit for the reuse of treated water is considered. The recovered VOC's are in high concentrations, which can be recycled. The required capital investment is low, in the range of $580,000 to $840,000.

1 .

*

$0.457 to $0.899 per 1000 liters of solution, depending on . ~~ ~~~

-

TABLE 11. TOTAL COST OF VOC SrniPriNc A N U RECOVERY

Cost ofOpention, $/lo00 liters

Stripping Adsorptinn Make-up Carbon Totd

0.141 0.0242 (0.734) 0.899 0.121 0.0157 0.320 0.457 0.146 0.0194 (0.734) 0.899 0.114 0.0123 (0.734) 0.860 0.118 0.0120 (0.734) 0.864 0.1W~ 0.0156 ' (0.734) " . 0.859 0.124 0.0223 (0.734) : 0.880 0.119 0.0120 . 0.582 T 0.713 0.123 0.0121 (0.734) ;;. 0.869

0.125 0.0120 . ' (0.734) ;. 0.871

Air Regeneration +nd -

0.124 0.0123 (0.734) ...e; 0.870

Environmental Progress (Vol. 8, No. 4) November, 1989.;" .:2n . . . .

.,. ..' Air stripping can be accomplished with much less air-

to-liquid ratio than that in the air shipping of drinking water supplies. This means that equipment size and the cost for the treaheni of contaminated air can be signifi. cantly reduced.

The cost of regeneration is the major cost of the process. Microwave regeneration hiis ii potential to make the re-

IL,~ = viscosity of water. k g h " or l l ~ m / & ~ ~ ~ ~ * PI. = bulk density ofcnrbon bed, k g h 3 P.r = density of water. kdm' V L = surfilce tellsion ofliquid. dynksn ~ . v,, = surfice tension of water, dyn/c,,, eH = time ti) reiicli breakthrough. ,,,in

LITERATURE CITED covery of VOC's from spent carbon ensier and less ex-

pensive. . . . ~>,I ..,. .:*,:

I . Giristi, D. hi.. R. A. Ccmwity. ;d c. T. L ~ , ~ ~ ~ ~ , ,-Activatd

(4 C d m n Adsnrption of Pctmche~nicnls;' 1. WPCE, No. 5. n. 947-965 (Ma" 19x41. ACKNOWLEGMENT .-.. . .

7. Jac~I~os . J.. A. J. Englande, J r . , and C. DAmuur , youfri-

nient of Environmental Quality, the State'of Louisi;in;i. Survey xnd Disposal Tedinolrigtcs," Repont for ~ ~ ~ f ~ i ~ The stipport is greatly appreciated. Hoard of Hegentr. Fel,. 1985.

3. Thiliorleaur. L. J., Chesio~l!,rinr,iic.s, John Wiley (197~). 4. Villing, W. L.. "lnterphiise Tr:insf& Processes [I." ~ , , ~ i , , , ~ ,

NOTATION Sci. 6 Tech., Vol. 11, No. 4. p. 405409 (Apr. 1877). 5. blackey, D. iind P. J . Leinonrn. "Rete OF E v ~ ~ , , ~ ~ R , , ~ of

Low-Solubility Contaminants Ihm Water Bodies to ~h~ phere." E n u i m n Sci . 6 Tech.. V d 9, No. 13, p. 1178.1180 A

c, (Dec. 1975). 6. Willey, B. R. and R. B. \Villimms, "Ways to Txckle the \" D

Prol,lcm." IV~,tcrlE,igirieerii,B 6 ~AlanoKerrzer~t, 1,. 20-45 D,, D, (May 1986).

7. Norwood. S. G., "Air Stripper Clems U p Chlorinated H.lbi. fa J2 = liquid density correction factor drocarbons." Pollution Eqrripment News, 1988. .&t&. Del

8. IVuferlEwince~ng 6 Motiogurrtolt. p. 22-23 (Jan. 1985). r. $ $1. Kavnnaugh. M. C. and 11. R. T~.~isscll. '',\ir Stripping s . H,: Treatment Pmcesr." paper presented ;it the A W V A A n n d H I . Conference. St. Louis. MO (1981).

IO. Eckenfelder, Jr.. W. W.. Prirwip/e.s oJ Il'nter Q w d i i y H,,.,. , , , t , . f ,> , . ? l , . c n r Pl l l l 11<15~1~

K , 1 I. ' l 'tcshi, ii. E., , l l~,~.,- '~,, , , , . , j~, O ~ J ~ . , , I ~ W I . .;td c d , hie<; L = liquid flow rate, kdhr-m? o r l lm l l&- l tL "i - Hill (I1IRO). L 12. Sinnott, 11. K.. Chceticol EuCir i eer iu~ . Vol. 6, J. XI. Cod L.,. ;md J. F. Hichardson, Pcrpnmon (1883).

13. Peters. M. S. and K. D. Timinerh:nq Plnnt Desig r n Economies for Clicriricrrl Enairiccrr. 3rd ed.. McGra

n = F'reundlich cqnation cnnst;uit

JI = p;wtial pressitre of VOC in gas-phrse. rmn l l p 11, = partial pressure of component i in gas-phnsr. iatni P,,,,,,, = total pressure i n adsorption column. ninl IHg

S = stripping hctor por-plnsr Adsovticm Cuts I'olliitiiin. Ilc~coverr S (Scj,. = liquid Schmidt n w > t I x ~ r = (~~,hf>,.i (Sd. = "i,l>,,l SCl,l,li<lt I I , I I I , I > c r = (p,I,,J>,.) V,, = flow rate of contaminated air. Ib-mole ofairltnin

This work was supported by a grnnt horn the Vepnrt- In~lustl-y-Resource RrcDverv ;I"d By-Pro<luct convenion

= cross-sectional nren of adsorption C ~ I U I I I I I . 111' or It' = concentration ofcomponent i in liquid phase. cmgll = column dinmeter. n o r ft = diffusion coefficient ofliqtiid, m% or ft'k = dillusion coefficient of viipor, m% o r ftYs = liquid viscosity correction tictor

= liquid surface tension correction factor = gas tlow rate, kglhr-m' or Ibmlhr-ft' = height of;, gas-phase tnnrfcr unit, m 01 ft = height afn liquid-phase transfer unit. m or ft = height ofan overall liquid-phnse t .m&r \ init . 111 or It

- I , , . I (.t . , I , ;'E" il,,,nli,,~:',,,,l.,llllll l,/l'l_~~ 'z = depth ofcnrljnn adsorptirm bed. 111 or It = liquid mass flow rate per unit coluinii cross-sectiimiil

= Henry's Law constant. dimcnsionlers

P

k = Frciinrllich equation comsliint 4 . .-3 J

ue;l, kglm'r

M,. = mole~ular weight ofVOC, glg-mole o r l l ~ t ~ ~ / l l ~ - ~ ~ ~ o l c (1980).

~ N#,,,, = numlxr of overall liquid-phwc tr;insfcr i ini t r Conlrol. Craftsman I3uok Co. (lCJ74). 15. niescnlcld. I;. C. ;md ,\. L. &hI, Car Pirrijcation,

Gulf Pub. (1974). 16. Ptrmele. C. S., W. L. O'Cont~cll, ;and 1-1. S. Biisrleki

>l (.'Iwut. I,;ng., 1). 53-XI (1)c.c. t i l . I!G!J). . . I 17. Decker, M., 1'. Naik. end bl. W o d l , "l'ollution Cont'of.

Y,,,,, = mole fraction of solute in gas-phase at leaving condition,

sorption I d Z = height of stripping column, m or It

23. Srvicik, J., "Thermal Desorption of Enviro Samples,"American Lab., p. 48-57 (July 1984). .;

~~. ~ ,.. .

. . . .

,

P

A Qualitative Analysis of the Effects of Water Vapor on Multi-ComDonent

.i 5 %

Vapor-Phase Carbon Adsorition i i Runzhi Gong and Tim C. Keener

Civil and Environmental Engineering Dept. University of Cincinnati

Cincinnati, Ohio

The effectsofwatervappron binaryvaporadsorption oftolueneand methylenechloride by activated carbon were investigated on a bench-scale experimental system. Three levels of relative humidity(l5,65 and 90 percent) in conjunction with differentconcentrationsof individual adsorbates (from 400 to 1200 ppmv) were tested by tracing the breakthrough curves of each adsorbate eluted from afixed-bed adsorber. The adsorption capacnies of the activated carbon tested for each adsorbate under the various conditions wereobtalnedfrom calculations based on area integration of the breakthrough curves. It was found that with increasing relative humidity, the shape of breakthrough curves was asymmetrically distorted and the width of the breakthrough curves was broadened for toluene and steepened formethylene chloride. Theadsorption capacitiesfor bothtolueneand methylene chloride were decreased with the increase of relative humidity. The magnitude of the effect of water vapor is greater at the lower toluene concentration and at the higher concentration of methylene chloride. The mechanisms of water vapor influence on the process of multicomponent vapor adsorption are discussed.

In vapor phase activated carbon adsorption .for air pollution control of organics, the eff+ of water vapor may detract fromtheoveraladsorptioncipacity. For example, austripping has been shown to 6i + emnomid method for &moving volatiIe.or@ic compounds (vOCs) h m contaminated groundwater and for the pretreahnent 'of some types of aqueous industrial wastes. However, the stripping units' exhaust gases may cause air pollution problems ifnocontrolmethodsareemployed xfore discharge. Another example is the activated sludge process which is widely

used as a secondary treatment method for wastewater. The VOCremoval efficiencies in the activated sludge process have been reported tobegreater than 90 percent, buta considerable portion of that removal is achieved by volatilization, especiaUy for the biorefractory components.TheU.S.EPA estimates that publicly owned sewage treatment facilities (POTWs) emit more than 26 million Ibdyear of VOCs into the atmosphere.' The emissions of VOCs may produce potential health problems for treatment facility personnel and thegeneral public in the surrounding areas. Vapor

-

lmpllcatlons Tiile I11 of the Wean AlrAct Amendments of 1990 will -Wish maxlmum availaMemntml technology requlremenls (MACT) for a wide range of toxic air pollutants. Vapor phase carbon adsorption will undoubtedly be required for controlling some of the Volatib organlc compounds (VOCs) that are listed as toxic. This paper ralales qualiilivaly the elfedof watervapor on the efficiency of vapor phase activated carbon systems operating on multiamponem gas streams. Mulllcomponenl adsorption may resun in a'roll over.' or dromatographic effect in which the less favorably adsorbed component is desorbed lo the extentlhatlhe outlelco.lcentration may exceed the InletconcentraUonforaperbd oftime. Theeffectofwatervaporonthese brenMhmughcurvesdapendsontheeiu(ion order of the adsorbales. the concenlration of tho adsorbates and the level of relalive humidily.These relationships should be consldored when activated carbon syslemsare belng designed for conlrolling toxic air pollutants.

adsorption by activated carbon is one of the alternative techniques for the removal. ofVOCs. However, fortbeexampleilisted j earlier, the relative humidity of the air streams are high, sometimes approaching the water vapor saturition'conditions. < Inordertodesignanefficient adsorption system using activated carbon, the effects of relative humidity (RH) on the vapor adsorption process, in addition to other factors, must be taken into auount. It has been knownthatboth therelativehumidity in the air stream from which adsorption lakes place and the amount of water vapor that ha.$ already been adsorbed:by the adsorbent can considerably influence fhe efficiency of organic adsorption : from flowing air by activated carb$Studies have been conducted for three airretent.

adsorbate tests (or the use

introduced concurrently into bed. In. the wet adsorbent preconditioning tests)' the d is introduced into a wet carbon has been equilibrated to a certain level&; relative humidity before the beginning of adsorption. The third type is the wet adsorbate and wet adsorbent test which mmbinesthe first two situationstatestthe wet adsorbate on the wet adsorbent. Most-,: 3f the investigations which have been reported havebeen mnductedwith asingk adsorbate organic compound.@ general, In increase of relative humidity reduces :he adsorption capacity of activatedcarbon tor many volatileorganic compounds'and .esults in an earlier breakthrough and madened breakthrough curve.''>e :xtentto which relative humidityaffethe iaporphascadsorption processdependson

864 -June 1993 - Vol. 43 * AIR & WASTE

.Experimental System.

%of adsorbates, their partial pressures he level of relative humidity.6 nation related to the effect of water on multiwmponentvapor adsorption een reported by Grant? The wet ,ateandwetadsorbenttests(RH=80 nt) for two ternary mixtures of ins, aromatics and halocarbons were icted in their study. It was postulated he adsorbed water reduces the pore available for adsorption on a one to olume basis. Theypresented a model on the Polanyi adsorption potential which provedadqualetpredicthe ofmoishueonbreaklhmughcapcitis iixture cdntaininglow concintrations hlomlhylene, n-hexane and toluene, dnot predict h e b r e a k t c a p a c i ~ relatively soluble methylene chloride ternary mixlure which included n- ne and toluene. he investigation of single vapor .ption under the influence of water r provides a basic understanding of tfect of water vapor on the adsorption lividual adsorbates. For more precise ;n purposes, more information is ed on the effects of water vapor on icomponent adsorption. With this in I , this project focusedon theeffectsof ive humidity on the dynamic process nary organic vapor adsorption.

!rimental Methodology lynamic adsorption ofvapor mixtures iluene and methylene chloride with ,usconcentrationsofwatervaporwas stigated by lracing the complete

)reakthroughcurvesgenemted by charging I fixed bed of pelletized activated carbon n a bench-scale experimentalsystem. The Japan were analyzed by means of gas :hromatography in order to distinguish iehveen thebreakthrough periods for each

compound. The wet adsorbate tests wet conducted under conditions which wet considered tobeofpracticalintemt.~~ included the adsorption temperature, th amount of adsorbent used, and th regeneration cdnditions. ,

mure 2. Sinole moor test results. .. . .: .... - . ~ ~ ~~ ~ .

AIR 6 WASTE * VOl. 43 -June 1993 * 865

polarity. adsorbability and solubility) ant because they are widely used in indust? and listed among 129 wastewater priorit! pollutants designated by the U.S Environmental Protection Agency.' Thl bench-scale experimental system wa comprised of five sections: (1) carrier ga pretreatment, (2) humidity generation, (3 adsorbate vaporgeneralion, (4)adsorption and (5 ) adsorbent regeneration. A flou chart of the experimental system i illustrated in Figure 1. The compressed ai (80 psig) from the laboratory air suppl! system was pretreated to control thi pressure and remove impurities such a: oil, water vapor and particles. Aregulato was used to reduce the air pressure fron 80 psig lo 40 psig. at which point the ai flow was purified by an oil removal filte (Model 42036, Dayton Electri( Manufacturing Co.) and a dryer (Mode XO3-02-OOO 1/4 desiccant dryer, Fauvei Company, Inc.), which could remove al oil and water and all submicron particle: down to 0.3 micron size. The various ail humidities were achieved by diverting i portion of the total air stream through thi humidifier, then combining it with tht main stream. The humidifier consisted oi two half-filled water tanks containin! distilled water wrapped with electrh heating tapes to produce the water vapor The designed relative humidity level wa!

I adjustedbytheflowrateofthehumidifi air Stream and/or the water temperature

- . -Gl&,ByTII

a Mhy*nuhwEm.l

- 0 Llehy(aaaJ&,BM*3 Liquid mixtures of toluene I ] Td-.Bn*3 methylene chloride were pumped

. Td-,BnNl

0 Td-ByT12 I40 -

L.T.Y.IIDlT..- F-asrrhs"..m

120 -

form a humidified, adsorba e challenge ,stream. The d e

air stream were obtaine flowrate of the adsorba

maintained constant. Theadsorption processwascon

Adsorption Time, mins.

~ g u n 3 . B i n a r y v a ~ o r t e s t r e s u ~ a t t 5 p e r c e n t r e b ~ e humidify.

Trials were performed under three different relative humidities(RH= 15,65. and 90 percent, with 15 percent RH as the control) inconjunctionwiththreedifferent adso~atesinfluentconcentrations, ranging from 400 to 1200ppm. In addition, four single vapor adsorption tests were conducted to' provide information for comparisons. The amount of adsorbent 'usedineach trial was lmgrams, resulting in a bed depth of approximately 32.5 cm (12.78 inches). Air flow through the adsorberwas0.51i0.01m3/min.(18.15i 0.5 scfm), corresponding to'a superficial airvelocityof65m/min.(195fpm)andan air retention time in the carbon bed of 1.3 sec. The feed air pressure was 20.22 psig and the temperature was 37.5OC + 2°C.

The adsorbent used in this study was pelletized grade JXC activated carbon (4x6 Mesh) manufactured by the Witco Chemical Corporation. The carbon was found to have a BET surface area of approximately 1040mVgby analysis. The activated carbon was precodditioned at 105°C for 24 hours to remove impurities which may have been adsorbed. stored in a desiccator. and packed into the adsorbers.

Toluene and methylene chloride (Certified, Fisher Scientific) were chosen as the test.adsorbates because of the differences in their physical and chemical properties (Le., molecular weights,

thesame as that in the identical adsorbers

..

(sahhedvapor) a;the rateof three of steam per pound of organic adsorbats? After steam regeneration. prior to ttie initiation of a new adsorption cycle, *e hot. wet adsorber was dried and cooled lo the operational temperature by blowing it with cleaned and dried air.

Grab samples of air were manually, takenbyinserting5or lOmlpressure-locK gassyringes(CA-2, Supelco, IC.) through theseptaofsamplingportsat the inlet and the outlet of the adsorbers which were a swagelock Tee connection containing a themogreen septa from Supelco Gorp. The samples were injected into the gas chromatograph as smn as possible (less than one minute). A model Sigma 1 gas chromatograph (GC) with a flame ionization detector (FID) was used I O

. .

,

., .. 866 *June 1993 - Vol. 43 - AIR & WASTE

line the adsorbate concentrations. iC column (MR 51539 Supelco, 3 Inc.) was 8' x 118" stainless steel d with 100/120 chromosorb PAW I with IOpercentTCEP. The carrier r the GC was nitrogen at 30 ml per e. The injector temperature was : and the detector temperalure was :. The GC operated at an initial :rature of 7 0 T and increased 10°C inute up to 115°C. Liquid standards prepared by dissolving different

nts of toluene and methylenechloride ified, Fisher Scientific) in a solvent lrbon disulfide (Technical GRA, ,.).The model Sigma 1 GC contains xoprocessor-controlled console to ilish analyzer chromatographic itions, collect and reduce data from natogaphy, and print out the analysis *i. Thus, each analysis was executed :ding to an analysis method whikh generated to determine the analyzer litions and data handling. The matogram and data report, including areas and peak retention times, were edout automaticallyfromtheintemal er/plotter.

:ussion tom the data collected in this study, ktbrough curves were plotted based the ratio of the adsorbate outlet xntration to the inlet concentration :o percent) versus the adsorption time nin.). Figure 2 through Figure 7 ;$rate these breakthrough curves under ous test conditions. Figure 2 illustrates effect of relative humidity on CYCl,

C, H, respectively, for single lponent. adsorption at relative iiditiesof 15 andwpercent. Figures3, id 5 show the results of binary vapor eriments at relative humidities of 15, md 90 percent, respectively. f ie adsorptioncapacitywas calculated m the areas determined by the akthrough curves as determined by ynomial curve fitting? As shown in ure 6, the hatched area corresponds to amount of toluene adsorbed during the ary adsorption process. The adsorption iacity for toluene can be calculated by following relationship:

q, = C, A. Q M424.5 x l0"W) (1)

where: q, = adsorptioncapacity,gramstoluene

adsorbedlgram of activated carbon.

C, =inlet concentration of toluene, PPmv.

Aa=area integrated from the co- ordinate to the breakthrough

curve, representing the amount ofthetolueneadsorbed, inunitof time, min.

Q =carrier gas flowrate at room temperature and pressure, Vmin.

W =weight of activated carbon in the fixed bed, gm.

M =molar weight of the adsorbate. Due to the rollover phenomenon (a

pica1 phenomenon observed in multi- imponent adsorption, which will be plained later), the net adsorption of ethylene chloride in the fixed-bed is pal to the difference between the lsorbed methylene chloride and the :sorbed methylene chloride. As shown

Figure 7, the adsorbed amount is presented by the hatched rectangular ea (Aa) and the desorbed amount by the ea under the breakthrough curve (Ad). 'le calculation formula for methylene iloride is given by the following:

qm = (A,-Ad) C, Q M /(24.5 X IF W) (2)

where: qm= adsorption capacity, gm

methylene chloride/em activated - carbdn

chloride, ppmv. C, = inlet concentration of methylene

The values of A, and A were obtained

thecalculation forthe adsorptioncapacity of each component are listed in Table I.

by numerical integration. k The results of

Water Vapor Effect on Breakthrough CUNe

Theeffectsofwatervaporontheshapes of breakthrough curves for the adsorbates studied are shown in Figures 8 and 9. The breakthrough curves of both methylene chlorideandtoluene wereshifledand were distorted with increasing water vapor. 'Illese breakthrough curves possess an asymmetric character, with the curve's front portion beingsteeperthan that of the rear. This indicates that a1 high relative humidity, the method to calculate adsorption capacity based on 50 percent breakthrough time'.'O may result in error. Therefore, area integrationofthe oomplete breakthrough curve is recommended in order to obtain more aocurate adsorption capacity for these conditions.

The width of the breakthrough curves may bemeasured by thed~erencebehvecn thesaturationtime(tJ andthebreakthrough time(tJ, asshowninFigure6.Therelative positions of ts and $ are arbitrary. In this study, the value of.$ and $were taken as 90 percent and 10 percent, respectively.

Igun 4. Binary vapor tests resuns at 65 percant rehtVe humldw.

AIR & WASTE Vol. 43 -June 1993 - 867

It isofimportancetonotethat when the elative humidity increased from 15 jercent to 90 percent, the width of the Breakthrough curves of toluene become vider and flatter, whereas those of nethylene chloride are narrowed and teepened. Forexample, when the relative iumidity increases from 15 percent to 90 arcent,thevalueoft,-ifortolueneat the ligher concentration increases from 80 to 10 minutes, and at the lower oncentration, from 170 to 390 minutes. brmethylenechloridethewid1hdecreas.es rom 90 to 40 minutes at the higher oncentrationand from 1101065 minutes t the lower concentration. This indicates iat !he effects of relative humidity on the reakthrough curves of toluene are ifferentfromthoseof methylene chloride.

To explain the influence ofwater vapor nthebreakthroughcur,it isnecessary 1 understand the dynapic process of idticomponent adsorption ina fixedbed.

Breakthrough C U N ~ S observed from a ynamic adsorption process are actually a :flection of a moving mass transfer zone MTZ) in a fixed bed. The width of the reakthrough curve is proportional to the iickness of the MTZ. The formation of le MlZ is due to the distribution of korption velocities among the adsorbate ~olecules. In a microscopic view, the Isorptionvelocity of individual adsorbate

80.00

40.00

0.00

0.00 m.00 Adsorption Time (mins.)

FI~uro 5. llluslratlve dra!+lnp lome amount Of toluene adsorbed In binary vaportsts.

0 200 400 Adsorption Time, mins.

iura 5. BlnarJ vapor tests fesults a190 perm1 relalive humldlly.

moleculescanbeaffected by manyrandom factors: the kinetic energy distribution among the adsorbate molecules, the irregular shape, s u e and pore path of the adsorbent, and the inhomogeneou channels around the adsorbent particles etc. As a result, some adsorbate molecule: move relatively quickly and some mow relatively slowly due to the differences ii their kinetic energy and encounterec resistance. Also,someadsorbatemolecule can be adsorbed on the adsorbent surfau faster than others, depending on the probability of an effective collision witt the active adsorption site. As a result, the adsorbed molecules entering the adsorbel at the same time may be adsorbed ai different depthsof theadsorber. Therefore there is a finite layer (Le., a MTZ) in an adsorberonwhichtheadsorbed molecules distributeovera certain thickness.Thus, il can be concluded that the thickness of a MTZ, and in turn, the width of a breakthrough curve, is affected by the same factors that cause the variation of the adsorption velocity among the adsorbate molecules.

For a specified adsorber and gas flowrate, such as in this study. the only factors that affect the thickness of the MTZ are the change of the adsorption capacity for the adsorbate. and the

868.June 1993 .Val. 43 - AIR ti WASTE

x and driving forces for mass , The thickness of the MTZ will :with a decrease in the resistance, :ase in the driving force, and an : in adsorption capacity. he case of multicomponent ion, more than one MTZ exists in d bed due to the differences in the sn rates and the adsorbabilities of Irbates. These MTZs may overlap ,ate, depending on the components' ies. In general, the MTZ of the adsorbed component locates ahead of the more strongly adsorbed

lent. This is due to two reasons. that the less adsorbed components ly have smaller molecular size and weight compared to the more y adsorbed components so their mvelocitiesaregreaterthanthatof er. The second reason is that the trongly adsorbed components ca& e the less adsorbed component from sorption site they have already ed and thus, force them to desorb ove down the bed to find new tion sites. The replacement of the f adsorbed component results in a itration increase of that component gas phase because this increased itration at any time is equal to its :oncentration plus its desorbed itration. Therefore, the breakthrough of the less strongly adsorbed

ment elutes earlier than that of the strongly adsorbed component, and rs with a hump at which the outlet ntration is higher than the inlet ntration. This has been called the ver" phenomenon?' sed on the previous discussion, it e inferred that in multicomponent ,tion, the~thickness of the MTZ or dth of the breakthrough curve tends :ease for the more strongly adsorbed onent and decrease for the less bed component. Since the MTZ of a jsorbed component locates ahead of of a more strongly adsorbed onent, the mass transfer resistance more strongly adsorbed component

ses, due to counter diffusion from ption of the less strongly adsorbed onent, and the change of the surface ore path of the adsorbent which is a I of the less strongly adsorbed ionent already adsorbed. Thus the nessof the MTZofthemorestrongly .bed component increases. Also, the xtiation of a less strongly adsorbed ,onen1 entering the MTZ is higher it$ inlet concentration, due to the

iver phenomenon. The higher entration implies a greater driving : of mass transfer and a higher

becomes more narrow compared to the single component adsorption case.

Therefore, the effect of water vapor on the shapeofbreakthroughcurvesoforganic vaporsmay dependon theelutingorderof the water vapor from the adsorbent bed if water is considered as an adsorbate component. Increasing the water vapor concentration will result in wider breakthrough curves for the components which elute from the adsorbent bed later than water, and narrower breakthrough curves for those eluting morequickly than water.

Theelution order is mainly determined by the adsorbability of the components. Sincewater isapolar molecule, itsaffinity to a non-polar surface adsorbent, such as activated carbon, is much smaller than thatoftheorganiccompnents. Inaddition, the molecular size and weight of water is generally smallerthanthatofmostorganic components. Thus. in the concurrent adsorption of water and organic components, water is likely to elute much earlier than other organic components.

In this study, toluene is the more strongly adsorbed component with ahigher boiling point and lower polarity when compared to both methylene chloride and water.Therefore, itisapparent that toluene should he the last component (after methylene chloride and water) to elute

from the bed. As a result, in addition to the influence of methylene chloride, the breakthroughcurveoftoluene was furthel broadened due to the presence of watei vapor.Thisbroadeningofthe breakthrough curve is caused by the reduction of the mass transfer rate and the availability 01 active adsorption sites. The decrease ol the mass transfer rate of toluene can be attributed by two factors. The counter diffusion of the desorbed molecules in the gas phaseobviously restrictsthemigration of toluene to the surface of the adsorbent In addition, the condensation of water in the micropores also produces an extra mass transfer resistance for a hydrophobic adsorbate, if a water film is formed.

The effect of water vapor on the breakthrough of methylene chloride in this study may be explained by the assumption that methylene chloride elutes before water due to its weaker adsorbability and its similar polarity to that of water Although the molecular weight of wale] (18) is smaller than that of methylent chloride (85), the adsorbability of watei onto the sorbent (activated carbon) at the higher relative humidity levels (RH > 65 percent) becomessignifcant [the relative pressures of methylene chloride undei these test conditions are much smallei (< 0.0032) than that of water (> 0.65)] Therefore, it seems possible for the watei

0 m 400 Adsorption lime (mins.)

~ ~ ~~~

AIR & WASTE - Vol. 43 *June 1993 - 869

Table I. Experimental adsorption capacity (AC).

vapor to competewith methylenechloride and drive the latter ahead. The MTZ of water and methylenechloridemay bevery close to each other or even overlapped. A t thelower relative humidity, water islikely to elute out before methylene chloride. As relative humidity increases, i t may elute out after methylene chloride. Therefore, the width of the breakthrough curves of methylene chloride could be narrowed with an increase of relative humidity according to the previous discussion. Further investigations on the elution order of water and organic components as a function of relative humidity and organic concentration is recommended.

Magnltude of the Water Vapor Effect An examination of these results

indicates that the effect of water vapor i s stronger with an increase of the methylene chloride concentration and weaker with an increase of the toluene concentration. This wasobserved from the effict of water vapor on the adsorption capacity.

From Table I, the adsorption capacity for both toluene and methylene chloride decreases as the relative humidity increases. The largest decrement o f adsorptioncapacity (0.0377g/g)occurs at the lowest concentration for toluene (435 ppmv)and at the highestconcentrationfor methylene chloride (1200 ppmv). Apparently, the adsorption capacity is subject to a larger influence ofwater vapor at the lower concentralion for toluene and at the higher concentration for methylene chloride.

A consideration of the fundamental physical differences between the adsorbates may be used to explain these differences. One significant difference between methylene chloride and toluene i s their molecular polarity. The dipole moment of methylene chloride (1.60) is larger than that of toluene (0.36) and approaches that of water (1.85).

In physical vapor adsorption. the forces bonding the adsorbate molecules to the adsorbent surface are electrostatic in nature. Therefore, adsorption can be

hindered by the intermolecular forces 01 attraction in thegas phase.'zTheattractivr forces among gas molecules result from three different effects: orientation effects dispersion effects, and induction effects depending on the polarity o f the relevani molecules. The dispersion effect act! between al l the molecules while the induction effect exists between polar and non-polarmolecules. The orientation effea occursonly between polarmolecules.The orientation effect is the greatest i n magnitude. while the induction effed i s the smallest. Since water and methylene chloride are both polar molecules, the attractive force between them, caused by theorientation effect, i s muchgreater than that between waterand toluene, caused by the induction effect.

Activated carbon is considered a non- polar adsorbent. Tke adsorption onto a non-polar surface i s accounted for by dispersion effects for non-polar adsorbate molecules, and by induction effects for polar adsorbate molecules. The energy of the orientation effects between water and methylene chloride in the gas phase i s greater than the energy of the induction effects between methylene chloride and the activated carbon surface. The intermolecular forces of attraction are a sum of the forces exerted by the surrounding molecules, which are proportional to the numberofmoleculesperunit volume; i.e.. the higher the concentration of the adsorbate and water, the greater is the attractive force. Therefore, the effect of water vaporon theadsorption of methylene chloride increases wi th increasing concentration of methylene chloride and water.

For toluene, however, the intermolecular forces in the gas phase caused by induction effects between water and toluene molecules are smaller than the dispersion forces on the surface of the activated carbon between the solid molecules and toluene. Hence, the adsorption of toluene is unlikely to be affected by the reason explained for methylene chloride.

Difference between Single Adsorption and Multicomponent Adsorption

Additionally. the data from Table I indicates that the decrement o f adsorption capacity for both methylene chloride and toluene is more significant for the single component adsorption case than for the binary adsorption case when relative humidity was increased from 15 percent to 90 percent and the adsorbate concentration was 800 ppmv. I t appears that the adverse effects of water vapor on organic vapor adsorption are reduced for

I

870. June 1993 * 8 Vol. 43 - AIR & WASTE

nponent mixtures. An important mobtainedfrom thisstudy isthat ct of water vapor on multi- :nt vapor adsorption could be ifferent than that of the single e case.

ions iresenceof high concentrations of. vapor results in an earlier ?ugh for thetwoadsorbatestested, and methylene chloride, and an etrical distortion of the bugh curves. The method which ly one point on the breakthrough io percent breakthrough poinl) to te the adsorption capacity of ent, may not be suitable for iontestsdesigned tostudytheeffect :r vapor. In order to obtain an e measurement of the adsorption y of carbon, an area integration of mplete breakthrough curve is nended to calculate the capacity of orbate adsorbed for conditions of lative humidity. h an increase in relative humidity, eakthrough curve of toluene is nedwhilethatof methylenechlonde ,pened. From an analysis of this ation, it is-inferred that the effecects rvapdron thewidthofbreakthrough

0 200 4M) 6w Adsorption Time, mins.

,sure 9. Effecl of relative humldiiy on breakthrouah: lau adsorbate conmnlration testf (C, > 550 ppm).

0 200 400 600 Adsorption Time, mins.

8. Enect humidlly on breakthrough high adsorbate concentration tests (C, > wm).

:urves in multicomponent vapor ac iorption will depend on the elution ordi )f the adsorbates from the bed. The wid1 )f breakthrough curves of those elute :arlier than the water tends to be reduce while that of those components elute later than water may be increased.

?he adsorption capacity of activatc arbon for tolueneand methylenechloni is reduced by thepresence of water vapc But themagnitudeof thiseffectisdifterer dependingon h e adsorbateconcentration Toluene is subject to a strong water vap effect at its lower concentration, whi methylene chloride is subject to a StrOl effect at its higher concentration. TI tendency of the water vapor effe increasing with increasing adsorba cbncentration may he possible for tho organicadsorbates which have a molecul polarity close to that of water.

The effect of water vapor i

multicomponent adsorption is mo complicatedthanthatforsinglecompone adsorption. ?he experimental data shc that for multicomponent adsorption, 1 adverse effect of water vapor on t adsorptioncapacity is not as significant that for single component adsorplk Therefore, any attempt lo predi multicomponent adsorption based on I dataofsinglecomponent adsorption unc

AIR &WASTE. Vol. 43 -June 1993 - 871

c

the influence of high humidity may not prove successful.

Additional experimental work is suggested with various types of organic adsorbates to model the water effect, especiallyon thoseorganics whichcontain functional groups possessing some special properties, such as higher molecular polarity and greater water solubility. This information would be very useful in the design of adsorption systems which could take into consideration the effect of water vapor on multicomponent adsorption.

Acknowledgment The work presented in this paper was

partially supported by the US. Environmental Protection Agency through contract number 68-03-4038 to the University of Cincinnati. This support is greatly acknowledged.This article hasnot been subject to the agency's qeview and therefore does not necessarily reflect the views of the agency, and no official endorsement should be inferred.

References 1. SA. Hannah, 'Report to Congrcss on the

DischargeofHazardous Wastes10 Publicly Owned Tnatmcnt Works." EPNS30-SW- 86-004. February 1986.

2. L.A. Jonas. et al.. 'Tho effect of moisture onthcadsorptionoffhlorofarm byactivated carbon." Am. Ind. Hyg. Asroe. 1. 4620 (1985).

3. G.O. Nelson, et al., "Respirator canridge efficiency studies: vii. Effect of relative humidity and temperature,"Am. Ind. Hyg. Assoc. 1.37: (1976).

4. M.D. Werner, "The cffccts of relatiw humidity on the vapor phase adsorption of trichlarocthylenebyacti"~t~dca~","A~. h d . Hyg, Asroe. J. 46585 (1985).

5. Y . Takeuchi. E. Furuya, "Prediction of the Breakthrough in Solvent Recovery in an Activated Carbon Bed,"papcrpresentedat the Engineering Foundation Conference, hdamentals of Adsorption, Bavaria, W. Germany, May 1983, pp. 629-636.

6. M.D. Weme1,N.L Wintea."AReview of ModclsDevelopedtoPrcdictOawousPhaw Activated Carbon Adsorption of Organic Compounds," CRC Critical Reviews in Environmental Control. Vol. 16, Issue 4, 1988, pp. 327-356.

1. R.J. Grant, et al., "The Effca of Relative Humidity on the Adsorption of Water- Immiscible Organic Vapors on Activated Carbon,"papcrprescntedatthe~~i~~nng Foundation Conference, Fundamentals of Adsorption, Bavaria, W. Germany, May 1983, pp. 219-227.

8. M. Callahan. et al., 'Water-Related Environmental Fate of 129 Priority Pollutants," EPA-44014-79-029a.b. US. EPA, Washington, D.C., 1979.

An Intemational Symposium

9. R.Gong,"Effcctsof RelativcHu?idilyo Multicomponent Vapor Ad&arption c Toluene and Mcthylcne Chloride b Activated Carbon," Masters' Thcsir University of Cincinna:i, 1989.

10. C. Nunre, M. Korusko. B. Daniel, "Effu of Humidity on Carbon Adsarptio Performance in Removing Organics fro, Contaminated Air Streams." Paper No. 81 29.2. presented at the 82nd Annual Mectio & Exhibition of the A i r and Was1 Management Association. Anaheim, W June Z-30,1989.

11. R.T. Yang, CorSeporefion by Adsorpfio Prcxesses, Bullcrrionh Publishers. 1907

12. J.Jowph.D.S.&achler,M.Leslic,"ConV1 of Gawous Emissions." EPA 45Of2-81 006,May 1981.

About the Authors The authors are with the Civil ani

Environmental Engineering Dept.. 74 Baldwin Hall (ML 071). University a Cincinnati,Cincinnati,OH45221-0071.Thi manuscriptwassubmid for peerreviev on July 24,1990. The revised manuscrip was received on March 4,1993.

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872 *June 1993 - Vol. 43 * AIR & WASTE

ACTIVATED CARBON hen it comes to adsorption, few products can match the low cost and efficient performance

ofactivated carbon. That’s why the material is so widely used for wastewater treatment. But with processors looking for economical ways to achieve envimn- mental compliance in other operations, the market is taking off in different di- rections, with much attention focused on air purification, process water treat-

It is the opportunitylthat manufac- turers and marketers of activated carbon have been waiting for. All rewed up, they are generating new products and regenerating spent carbon, and rounding out their offerings with equip ment and technical service. ‘The Clean Air Act is generating a lot of activity,” says William Gdlece, vice president of Barnebey & Sutdiffe Corp. (Columbus, Ohio), which supplies about 100 different types of activated carbon. ‘That’s where the interest is now.”

Under the U.S. Clean Act Air, strict regulations on emissions of volatile organic compounds (VOCs) have prow- sors turning to activated carbon and other adsorption technologies (see p. 92), say market analysts a t The Free- donia Group (Cleveland, Ohio). In those applications, granular activated carbon, which can be reactivated, is

and solvent recovery.

HOW REACllVAlED CARBON MEASURES UP

Went carbon

at putting in a lot of new installations over the next 5-10 years,” says Tom Stocker, business manager for performance products at Elf Atochem North America (Philadelphia, Pa.).

To keep pace wlth growing demand, producers in China, Sri Lanka, Philip- pines, and other countries where production is plentiful and inexpensive, are increasing exports and forming global alliances. Elsewhere, producers are are coming up with new ways to embellish carbon to handle more-de- manding tashs, and expanding their capabilities to regenerate spent material, both hazardous and nonhazardous.

New products and applications In Japan, Takuma Co., Kuraray Chem- ical Co. and Suntory Ltd., all of Osaka, have jointly developed an antibacterial activated carbon for water sterilization. T 1 ..I .. . ,. unpregnaceu mm suver ions, me new material is mixed with four parts conventional activated carbon to provide the right balance of adsorptive and sterilizing properties. The antibacterial carbon, which sells for $1,650 in 15-kg hags, is being used to purify water for food processing.

In the Asia and Europe, demand for granular activated carbon is being driven by the need for lower levels of organic contaminants and the use of

ozone as an ----r---a “..- alternative disiiectant

F : 2 2 i c h G E T S 1s lnmted generally to REWED Up to says chlorine, Elfs one-time use. Industrial wastewater treatment, driven by the U.S. Clean Water Ad, continues to have the strongest demand for activated carbon. Farther down the road, the U.S. Safe Water Drinking Water Act, which determines allowable levels of various pollutants in drinking water, will propel demand in municipal potable water treatment.

Already in Europe and Japan, where supply of drinking water is largely managed by the private sector, there are signs of an uptick in demand for specialized carbons. “Japan is looking

Impregnated products and regeneration services are

changing market dynamics

Stocker. Last year, Calgon Carbon Corp.

(Pittsburgh, Pa.) introduced Centaur, a catalytically active carbon that in terms of applications, is Tust beginning to scratch the surface,” says Anthony Mazzoni, the company’s marketing di- redor. The material can be used to capture emissions of hydmgen sulfide, sulfur dioxide and phosphine, as well as remove chlorine ti“ water.

Still in development is an activated carbon designed for solvent recovery. The manufacturer, Cameron Yakima, Inc. (Yakima, Wash.), won’t say when

\ CHEMICAL ENGINEERING /NOVEMBER 1995 75

CHEMICALS

the new material will be available, but Robert Hanson, president, assures that it will have ”distinctive capabilities.”

More regeneration capac i ty Recycling mandates and escalating disposal costs are encouraging suppliers to invest in regeneration capabilities. Re- cently Elf Atochem got the go-ahead

from the Oklahoma Department of En- vironmental Quality to reactivate hazardous spent carbon a t its regeneration facility in Pryor, Okla. The company has been regenerating nonhazardous carbon there since 1991.

Elf, which regenerates carbon as a service to its customers, is very ‘conservative” about the material it takes back

to process, Stocker acknowledges. “Some say our service is more expensive, but customers can be sure they re- ceive high-quality reactivated carbon.”

For customers who buy carbon from suppliers that do not have reactivation capabilities, there are a number of merchant service providers and more are on the way. Eight months ago, ThermTech, Inc. (Houston, Tex.) began traveling to remediation sites with its portable regeneration service. Don Hedges, carbon division manager, be- lieves the company is the first in the U.S. to offer this service. And Michael

-

scheduled for late summer 1996. 1

Undaunted by the

lmkingto recycle product,

L

sumption of carbon is steadily in ing, open market prices have meted over the last few years.

Through the 1980s, the price steady at about $Mb. Around ,

says Hugh McLaughlii, presid Waste Min Inc. (Groton, Mass.).

“Certainly it’s not practical to re vate carbon when new material bought at almost the same price,” s

For Mare Information, Circle 70 76 CHEMICAL ENGINEERING I NOVEMBER 1995

. .

Bamebey’s Gillece. Wowever, with dress wastewater treatment and air bon.. An altemative to thermal meth- high-activity and pelletized carbons, it pollution control,” he predicts. “Com- ods, solvent regeneration uses solvent pays to reactivate.” pared with what it Costs to send mater- to dissolve adsorbed material out of the

The economies of reactivated carbon ial out to be regenerated, it will always pores of activated carbon. The solvent are expected to improve as more cus- be more cost effective to have someone is. then removed by steam, using a tomers opt for onsite regeneration, came in to service a purifier.” process similar to steam regeneration says Waste Min’s McLaughlin. “Just Waste Min is in the process of com- of carbon used for solvent recovery. Wre steam and cooling, carbon will be a .mercializiig a standalone facility for “Costwise, solvent regeneration is utility a t manufacturing sites, to ad- solvent regeneration of activated ear- .only a few cents per pound more expen-

sive than steam regeneration, and an I order of magnitude less expensive than

tion. about 10% of the carbon is burned Let us - help you m

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The ACOUSTICAIR” blower hlizes a new. patent- pend design to drastically reduc3ke noise en ed whh a positive d y s - blower. Even vdlmtslenrrvs. h? &era 6 asf&l

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POLLUTION ENGINEERING

An environment-protecting vapor recovery system handles methylene chloride emissions both eEciently and economically at Michigan-based Chem-Trend, Inc. Methylene chlo- nde is a highly volatile solvent carrier used in the company’s plastic and rubber mold release agents to accelerate drying. Because of their volatility, methylene chloride vapors collect above the mixture in the compounding vessel during the production of the release agents. As ingredients are added to the vessel, the vapors are displaced. The new vapor recov- ely system collects these displaced vapors and virtually elimi- nates them from the plant exhaust.

The Michigan Department of Natural Resources (DNR) regulates ‘smokestack‘ emissions, including those of methylene chloride, throughout the state. DNR emissions regula- tmns, based on values assigned by the Environmental Protec- tion Agency (EPA) risk guide, must be met before plant operating permits can be issued. Chem-Trend had to devise a method of controlling the methylene chloride emissions before beginning manufacturing operations in its new com-

casebook I

by Marie Hunter

Yiirpor Recovery System Protects Environment pounding plant in Howell, MI. It was necessary for the company to remove as much of the methylene chloride as possible from the process vapors, so any remaining traces of Gapor vented to the atmosphere would be within DNR limits.

Company engineers evaluated three basic methods for volatile organic compound extraction, including activated carbon adsorption, refrigerated condensation, and pressurized condensation. With any one of the methods, vapors are drawn off the top of the compounding vessel and piped to the extraction equipment where they are further processed.

The equipment for processing the vapors through activated carbon is not expensive. However, the methylene chloride adsorption rate for activated carbon is low. In addition, the disposal costs for spent activated carbon can be extremely high, because it requires expensive catalytic incineration by an outside vendor.

The refrigerated condensation method of processing the vapors would have required cooling to temperatures below -70°F. To reach these temperatures, a cascade refrigeration system with a defrosting device would have been necessary. Installation, operating and maintenance costs for such a sys-

For information circle 45 JUNe l%Ll POLLUTION ENGINBERING 27

in the industry.

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tem would have been very high. Pressurized condensation presented the most p

method for extracting the methylene chloride vapor, a was the system selected by Chem-Trend. With this the vapor is collected and compressed, in two stages, t psi. At 250 psi, the methylene chloride is easily extract&>

holding tank. In addition, the equipment tion and maintenance costs are considerab for the refrigerated condensation method.

The company’s vapor recovery system is indepe connected to all vessels that are used to compound produ containing methylene chloride. A system lockout proc prevents the inadvertent use of methylene chloride, ou of the recovery system.

Recovery of the methylene chloride by pressuri sation approaches 99 percent. The removed picked up by a licensed waste hauler, and the b collected vapor is slowly dispensed to the at rate designed to keep the methylene chloride emmi the limits set by the DNR.

Chem-Trend‘s maintenance people conducted preve maintenance checks on the system several times durin first six months of operation to check for corrosion an other problems that could adversly affect the operation. minor modifications and maintenance were required.

ride re, at about 40°F, with a chiller. The remain I

Them in the e with it that tho increas

: are not f from ti ! porting

danger( ! waste c ’ even th

waste t, An effi be mo\ dispose nomica 1 awardei

I

The vapor recovery system is designed so that if emission liiilits become more strict in the future, additional stages of ixatment can he performed on the vapor that is presently released to the atmosphere. Activated car6on adsorption could he practical in this case, since so little methylene chloride remains in the emitted vapor.

For FREE INFO circle 353 on post card

Thermal Oxidizer In the early 1980s Bennett Environmental Consultants, along with its manufacturing division, Aqua-Guard, recognized that the problem of disposing of oily waste materials was increasing along with industrial growth. Oily waste materials are not welcome at landfill disposal sites which are often far from the location where the wastes were generated. Trans- porting inflammable waste materials over long distances is dangerous and expensive. Because of these considerations, waste disposal near the waste generation site is desirable, even though few generators produce a sufficient volume of waste to justify permanently established disposal facilities. An efficient transportable disposal system must he able to be moved easily and quickly from location to location to dispose of oily waste materials effectively, safely and economically. In 1982, Aqua-Guard Technologies, Inc. was awarded a contract by the Canadian Government to design,

ATTENTION

MONTEDORO-WH ITNEY SYSTEM Q

FLOWMETER OWNERS

IMPORTANT NOTICE

Effective April 1990, Marsh-McBirney, Inc will begin servicing System 0 Flowmeters which Incorporate t h e Soniflow probe.

As you may b e aware, System Q Flowmeters which incorporate the Soniflow probe can no longer b e senriced by its original manufacturer, MontedoreWhitney Corp.

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Introduction to Activated Carbons

Carbon can exist in a number of forms with either crystalline or amorphous structures. The most well known crystalline forms are diamonds and graphite, the uses of which are widespread and well documented. The amorphous forms include carbon black, carbon fibers, and porous carbons, all of which are obtained by heating or burning under controlled conditions such carbonaceous materials as coal, coconut shells, wood, peat, lignite, and petroleum. The carbonaceous material is usually solid and naturally occurring.

Porous carbons are obtained as a residue after the volatile components of the carbonaceous material are removed by a thermal process in the absence of air. The most important products are cokes and charcoals, which are used in very large quantities in the iron and steel industry. Charcoal is the product that provides the raw material for Activated Carbon.

Charcoal has to be treated further in cr ier to develop the extensive internal pore structure that cate- gorizes activated carbon. Adsorption capacity is determined to a great extent by the degree of development of this internal pore structure, and also by the nature of the carbon's surface chemistry (acidic or alkaline).

Although in the laboratory activated carbon can be prepared from a large number of materials, those most commonly used in commercial manufacturing are coconut, coal shell, peat, and wood.

The most common method of activating carbon is the steam activation process, which is accomplished in two stages. The material is first carbonized to an intermediate product, the pores of which are either too small or too constricted for it to be a useful adsorbent. Enlarging the pore

structure to produce a more acces sible internal surface area is then achieved by reacting the carbonized product with steam at a temperature between 800°C and 1,000"C.

Adiunled Con1

Carbonized Cocoitut 5hrU

Stereo scan electronmicrographs showing yore stnrctiires of activated carbon

. ~~~

L

The reaction takes place on all of :he internal surfaces of the car- >on, thereby enlarging the pore size. Control of temperature is xitical. If the temperature is gelow 800"C, the rate of reaction IS too slow to be economical (the mergy cost to open up the pore Structure increases while the yield iecreases). Above 1,00O"C, the :extion becomes erosive, concen- :rating on the outer layer of the iarbon particles, reducing each >article in size, and leaving the nterior unactivated.

lareful control of the steam ictivation process allows the 3ore size to be readily altered to

~

suit a wide range of specific applications. For the adsorption of smaller molecules from solution, i.e., water purification, the pore structure obviously does not have to be opened up to the same extent as for the adsorption of larger molecules.

Activated carbon can be manufac tured in powder, granular, pellet, and spherical forms. Rotary, vertical, and multiple hearth kilns are all used, depending on the individual preferences of each manufacturer. Activated carbon that has been determined through laboratory testing to be spent, may often be reactivated in a kiln and reused.

Three groups of pores can be dis- tinguished in an activated carbon:

1. Micropores (0-20 Ang) 2. Transitional pores (20-500 Ang) 3. Macropores (> 500 Ang)

The major portion of the surface area is derived from the small diameter micropore and the

Macro- pores avarlable to both fldsorbate and solvent T,WW- tXWI"1

pores nvnilnbk

soluent arid maller adsorbate

nunilable

50lW,lt

0,dy to

MlCKJpreS

only to

lriferrial pore size distribrrfiorz

Pore Size Distribution of Activated Carbon Raw Materials

Peat

Wood

Coal

Coconut Shell

0 5 10 15 20 25 30 35 40 45 50 55 60 65 Percent of Total Pore Volume

Micropores (0-20 Ang) Transitional Pores (20-500 Ang) 1 Macropores (>500 Ang)

edium diameter transitional )re regions. Micropores have en found to be the most effec- re in trapping small molecules gas and liquid phase applica- m. The transitional pore 3ion is most suitable for adsorb- 3 large molecular species such color molecules.

le raw material for an activated rbon plays a major part in termining the ability of the tal product to adsorb certain Aecular species. Activated car- ns produced from coconut 211s exhibit a predominance of cropores, while coal based car- ns have a wider range of transi- nal pores. The development of extensive macropore structure 'ound when either peat or jod is used as the raw material. e above graph shows the fering pore size distribution of 'se materials.

carbons with a predominance nicropores, the internal surface

area is incredibly large. Many activated carbons have internal areas in the region of 500 to 1,500 square meters per gram, and it is this enormous area which makes them effective adsorbents. Viewed another way, just one pound of activated carbon at 950 mz/g has the equivalent surface area of 100 football fields. All

tively adsorbed in the activated carbon pores dependent on their size.

organic compounds will be selec- L

4dsorption is the process by Nhich fluid molecules become 3ttached to a surface by physical )r chemical forces (or a combina- ion of both). In physical adsorp- ion the impurities are held on the iurface of the carbon by low level ran der Waals forces. In chemi- iorption using impregnated :arbom, the forces are relatively

Rota y renctiuntioiz kiln

strong and occur on the impregnated sites on the surface. Physical adsorption is predominant when using activated carbon in water purification, and the efficiency of the carbon will depend upon its accessible surface.

Adsorption from solution is complex, because in many cases both the impurity and the solution have an affinity for the carbon surface. The most widely applicable means of representing the adsorption process is the empirical Freundlich equation:

Where: x = weight of impurity

m = weight of carbon. c = concentration of

residual impurity k and l / n are constants.

adsorbed.

X When 15 versus c is plotted on log/log paper, the plot is often linear in nature and gives an adsorption isotherm.

A number of factors can affect adsorption such as pore size distribution, molecular size of the impurity, particle size of the carbon, temperature of the carbon treatment, and the pH of the solution. The following relationships, however, generally apply when other variables are held constant:

Adsorption efficiency increases as the particle size of the carbon decreases.

Adsorption efficiency increases as the temperature decreases.

Adsorption efficiency increases as the solubility of the contaminate decreases.

Adsorption efficiency increases as the contact time increases.

Activated carbons which have been chemically coated or treated are referred to as impregnated carbons. These specialized adsorbents are available in both granular and pelletized forms, and provide advanced treatment technology for many municipal and industrial applications.

Impregnated activated carbon adsorbs and retains specific gases long enough for the chemical impregnant to react with the contaminant and form a stable or fixed compound within the carbon, thus elimi- nating the contaminant from the stream.

Impregnated carbons have been specifically formulated for many chemical compounds which have proven to be difficult to control with standard activated carbons. Examples of these compounds include: ammonia, mercury, sulfur diox-

hydrogen chloride, chlorine, methyl iodide, formaldehyde, and hydrogen cyanide. Specialty carbons have also been designed for specific industries, such as the silver impregnated carbons used for the control of bacteria growth in drinking water filters.

-

.~ ~ ~

ide, hydrogen sulfide, ethylene, L

-

T h e n u m b e r o f appl icat ions f o r act ivated carbon is v i r tua l l y endless. T h e table b e l o w l is ts a representative sampl ing:

IndustrylApplication Contaminant Removed

Adhesives Toluene, Acetates, Alcohols

Battery Production Mercury

Cellophane Acetone

Computer Systems

Dry Cleaning Perchlorethylene

Electronic Component Protection

Foam Furniturs Formaldehyde

Fume Hoods Ammonia, Mercury, Formaldehyde,

Mining Mercury

Hospitals Ethylene Oxide, Formaldehyde

Hydrogenation Mercury

Industrial Respirators

In-Home Point-of-Use Water Treatment

Laboratories Acid Gases

Military Respirators War Gases

Museums, Air Purification Sulfur Dioxide

Nuclear Power Plants Radioactive Iodine

Petrochemical Ammonia, Acid Gases

Printing & Packaging

Potable Water Treatment

Poultry Farms, Animal Waste Ammonia

Remediation Organics including: Chlorinated Hydrocarbons

Rubber

Semiconductor Industry Arsine & Phosphine

Sewage Treatment Plants Hydrogen Sulfide

Sporting Goods Hexane

Welding Fumes

Hydrogen Sulfide, Acid Gases

Hydrogen Sulfide, Chlorine

Radioactive Iodine, Arsine & Phosphine

Acid Gases, Ammonia, Mercury, Radioactive Iodine

Bacteriostatic (reducing bacterial growth on the carbon)

Toluene, Xylene, Acetates, Alcohols

THMs, VOCs, Taste & Odors, Chlorine, Chloramine

Methyl Ethyl Ketone, Toluene, Hexane

Sulfur Dioxide, Nitrogen Dioxide

L

1

...

The inlet works of a wistewater treatment plant are usually a major source of sewage odor emissions, largely because anaerobic decompositions often occur in the filled sewer pipelines conveying wastewater to the treatment plant. It is, therefore, not uncommon to detect exceptionally high concentrations of sulfur compounds, such as H,S, at this location (Pomeroy and Parkhurst 1976). The effective removal of such H,S-dominated odor is therefore of concem to treatment plant operators.

Various measures are available to control odor emissions at sewerage facilities. They include the addition of chemicals to the wastewater, adjustment and alteration of the sewage flow regime, and the enclosure and subsequent treatment of the odorous headspace air by either catalytic oxidation, combustion, wet scrubbing with chemicals, dilution with clean air, or activated carbon adsorption. The efficiency and the cost related to each of these methods can vary widely from plant to plant depending on wastewafer characteristics. local topography, meteorological conditions, and existing plant design.

Activated carbon has been used widely as an adsorbent for odorous air treatment at sewerage facilities. It has a low investment cost, incurs moderate operating costs, and has the ability to adsorb a large variety of odorous compounds. Considering that sewage odor is often dominated by H,S, the activated carbon commonly used for sewage-odor removal is usually impregnated with appropriate chemicals, such as sodium hydroxide, to enhance its ability to remove sulfur compounds that include H,S. The internal surfaces of such alkali-impregnated activated carbon are used as catalyst sites for efficient chemical conversion of H,S to elemental sulfur in the presence of air. While activated carbon with such chemical impregnation might be very effective in removing H2S and the associated rotten-egg odor, it might not be suitable when used in an environment where odors are caused by groups of non-H,S compounds. Since sewage air does not consist of only one group of odorous compound, the performance of such alkali-impregnated activated carbon in removing sewage odors warrants further investigation. It should be interesting to compare the performance of an alkali- impregnated carbon with that of a nonalkali-impregnated carbon, not only for H,S removal, but also for sewage-odor removal.

EXPERIMENTAL METHODS

In a preliminaIy survey, the inlet works of a local treatment plant receiving 286,000,000 L of municipal wastewater per day were identified as a major source of odor emission. The inlet channel design was such that the influent wastewater flows from the screening channel into another downstream channel, over a weir with a drop of about 1.5 m. The turbulence of the wastewater discharge into the downstream channel resulted in the proliferation of an offensive rotten-egg smell typical of H,S. Significant levels of H2S at this location were confumed by separate analysis with a flame photomeUic d e tector (FPD).

Two columns, one containing an alkali-impregnated activated carbon (carbon A) and the other containing a nonalkali-impregnated activated carbon (carbon B), were set up (see Fig. 1) to evaluate their respective performances in treating the sewage air at this inlet works location. The carbons were

722

V

I, I;"'

FIG. 1. Schematic Dlagram of Field Setup

obtained commercially from local suppliers and were used without any mod- ification to their physical or chemical propelties. Carbon A was manufac- t u r d from bituminous coal and impregnated with a proprietary quantity of sodium hydroxide, while carbon B was manufactured from peat and steam- activated. Both carbons were granular in nature: Carbon A had a larger mean pellet diameter of 3.6 m than carbon B with diameter of 2 mm. Although the kinetics of carbon adsorption is such that the adsorption capacity tends 10 be inversely proportional to particle size, largely because the available surface area for adsorption increases for smaller diameter particles, no attempt was made to make the carbon size similar. Of interest in this study was the performance of commercial carbons used in the forms that they wsre supplied. Additional properties of the activated carbons are summarized in Table 1.

Fig. 1 shows the schematic diagram of the pilot facility, which was sited at the inlet works of the treatment plant. To provide the odorous air for the study, the wastewater channel immediately downstream of the weir was cov-' ered and the headspace air directly above the wastewater surface was pumped at a flow rate of 35 L/min through each of the carbon columns. Each column consisted of four separate cylindrical segments made of a transparent acryiic material (perspex), mounted in series. Each segment was 5.4 cm in diameter and was closely packed with 25 cm of the selected type of activated carbon to yield a total effective column bed height of 1 m. The flow rate through

723

!

1.1'.

Adsorbents (1)

Base material

Impregnation

Pellet diameter (mm) Total ash content (weight

Manufacturing process

Apparent bulk density (g/L)

percentage) Total pore volume (cc/g) Total internal surface area

Moisturc conieni (maximum)

Unit price of adsorbent (SS/

(BET) (m2/g)

(percentage)

W'

Carbon A

Bituminous coal

NaOH 550 3.6

(2)

-

- -

1,050-1.150

15

13.00

ents

Carbon B (3)

Peat Steam activation N0"C

520 2

4 0.65

800

L

8.50

each carbon segment was maintained to yield a carbon/gas contact time of about 1 sec per section, thus resulting in a total column retention time of 4 sec. A sampling port was provided at each segment hence enabling air samples to be collected at column bed heights of 0, 25, 50, 75, and 100 cm (corresponding in Fig. 1 to sampling ports labeled 1, 2, 3, 4, and 5 , respectively). Flow of sewage air through each activated carbon column was measured continuously with a buk gas meter (for total flow volumes) and a flow meter for instantaneous flow rates. Each of the four sections of column A was packed with 340 g of the alkali-impregnated activated carbon A, while that of column B was packed with 320 g of nonalkali impregnated activated carbon B. During the mn, regular sniffing by a panel of three odor detectors were

carried out at sampling ports 2, 3, 4, and 5 to trace the odor breakthrough in each segment of the I-m column. Odor breakthrough in this case referred to the occasion when a slight odor was detected, recognized, and confirmed by all panel members. Since assessments of odor breakthrough at each sampling port were carried out on-site in the ambient sewage environment, panel members were required to refresh themsaes after each odor assessment with deodorized air provided by fresh activated carbon columns installed next to the two main columns. A teflon-coated diaphragm pump operated at 10 L/min was used to sample the effluent air at the column ports for odor detectability, quality (offensiveness of odor), and characteristic (what the odor smells like). The frequency of odor sniffing on-site was about once per day, although more frequent assessments were canied out at upper sampling ports after odor breakthrough had occurred at the lower carbon sections. Flow-meter readings and carbon-test run times were noted for each odor breakthrough at the various sampling pow. Air samples. were also collected regularly, in 10-L Tedlar bags, at each sampling port and transported immediately to the laboratory for the determination of sample odor concentra-

724 a. 1

tion and H2S concentration. Odor measurements were carried out with a dynamic olfactometer, and H2S concentrations were detennined with the flame photometric detector (FF'D).

Odor-concentration values are reported in standard odor units per cubic meter (SOU/m') as recommended by Koe and Brady (1986), where one SOU is the amount of odorous material, which, when dispersed in 1 m' of odor-free air, would produce an odor that is just detectable by the average of an odor panel. All analyses were done within an hour of sample collection to minimize errors from possible sample deterioration in the Tedlar bags.

RESULTS

Gaseous H2S and Odor Concentration of Sewage Air A total of 101 grab samples of the influent sewage air to the activated

carbon columns were randomly collected during the &month study period and analyzed for their HIS and odor concentrations. The HIS concentration of the influent sewage air varies from 0.1 ppm to 28 ppm with an average value of 5.3 ppm, while the odor concentration varies from 34 SOU/m' to 280 SOU/m' with an average of 120 SOU/m'; and with standard deviation of 5.4 ppm and 40 SOU/m', respectively. The existence of great fluctuations in the influent H,S and odor concentrations can be clearly seen from Figs. 2 and 3, which show the H2S and odor concentrations, respectively, in the sewage air with respect to the operation time of the carbon columns. It is believed that the fluctuation in concentration of these two parameters in the sewage air could be a result of variation in factors such as rainfall, wastewater flow, and organic strength of the sewage handled at the treatment plant, as reflected by the variation of some general characteristlcs such as five-day biochemical oxygen demand (BODI), chemical oxygen demand (COD), total solids (TS), and suspended solids (SS) of the influent wastewater, as shown in Table 2. The means and standard deviations of these and some other wastewater parameters, obtained from grab sampling of influent wastewater during the study period, are also shown.

Sensory analyses of the sewage air sampled at the various sampling ports along each carbon column not only yielded information on the time to odor breakthrough, but also on the quality and characteristics of the influent and effluent sewage air. The description of odor quality was based on a commonly adopted, subjective six-point scale: namely, 0, no odor detected; 1, faintly recognizable; 2, moderate; 3, moderately strong; 4, strong; and 5 . very strong.

The influent sewage air samples (monitored at column ports A I and B1) were very odorous, consistently registering subjective values between 3 and 5 on the six-point scale. The characteristic rotten-egg odor of H2S was dis- tinctly observed when the H,S concentrations exceeded I ppm. However, for air samples with H,S concentrations below 1 ppm, a sour odor was observed. Some of these influent air samples with H2S levels below 1 ppm were analyzed for total hydrocarbon concentration (THC) using a Beckman Model 4M)A THC analyzer and were found to have THC of as high as 800 ppm (expressed in parts per million of CHJ. This indicates the possible exertion of smell by other groups of compounds for sewage air containing low HIS concentration. Similar observations were also cited by Cederlof et al. (1966) in a study of H,S and odor emission from two pulp mills.

725

..

Odor ronrenlrali~n iSOU/mll - - - n P Hydrogen sulfide concentration loomi

Operation SS TS Dale (hr) (mg/L) (mg/L) (1) (2) (3) (4)

January 27, 1988 0 388 1.576 Feblvary 2, 1988 384 320 748 March 14, 1988 728 324 736

June 27. 1988 2,480 376 836

July 8. 1988 2,739 240 756 July 29. I988 3.245 295 808 August 31. 1988 3 9 6 440 1.440 October IO, I988 4,741 472 1.060 November 23, I988 ~ 5,167 532 1,736 November 28, I988 ' 5,178 468 1.748 December 12, 1988 5.246 420 1.400

$ May 11, 1988 1,666 204 744

July 6. 1988 2,691 3M) 1,204

" .

2. General Wastewater Characterlstlcs

516

614

333 45 235 250 41. 281

Total Kjeldah (mg/Ll (8)

47 64

44 - -

485 191 37 39 522 201 37 - 517 - 48 - 639 - 28 -

Alkalinity

6.6

7.0

6.7

- -

PO. :mg/L _o 7.3 5.7

5.9

4.6

4.2 5.4 8.6 6.3 6.3 7.5

- -

-

- -

Linear alkylbenzene

sulfonate (mg/L) (13) 5.8 5.4

4.1

3.3

-

-

- - 3.7 4.9 3.0 3.7 3.3

.._____ .I Vu..VU~~y ~ausca oy non-H,S compounds and could possibly be exerted by odoriferous aliphatic:,and aromatic hydrocarbons. The sourish odor in the effluent gas stream gradually changed io the characteristic rotten-egg smell, as HIS breakthrough followed. Even though odor and HIS break- throughs from carbon column A occurred later than those from carbon column B, the characteristic and sequence of breakthrough of odor from both columns were similar.

Table 3 shows some of the results for odor and H2S analyses carried out on the influent and effluent air samples collected from the various column ports. Some observations that can be made follow. First, the characteristic of the sewage gas changed as it was scrubbed through the activated carbon column; typically from rotten-egg smell at the influent to sourish smell at the effluent. Second, the effluent odor quality gradually became stronger and more odorous as a result of the gradual breakthrough of odorous compounds through the activated carbon columns. Finally, the characteristic sour odor exerted by effluent air samples with HIS concentrations at or below 1 ppm can have an odor concentration as high as 120 SOU/m3 and a corresponding odor-quality assessment value of 4 on the six-point scale; such high values are more typical of sewage air having about 3-5 ppm of HIS. This confms that there are some very odorous non-Ha compounds present .in sewage air that are not as effectively removed by activated carbon, hence resulting in an earlier breakthrough compared to H2S compound.

Odor and H2S Adsorption Wkieneies oP Activated Carbons Figs. 4 and 5 show the H$ and odor breakthrough curvess. respectively,

of the two activated carbon columns for the same bed height of 1 m. The asterisks denote gaseous odor or H I S concentrations higher than that in the influent sewage air. For most of the time, both the HIS and odor concentrations io the effluent of column A were lower than those in column B. This is especially so for H I S , which was lower than 1 ppm for all times when the influent HJ concenmtions were not more than 10 ppm.

Odor breakthrough occurred and 0.,1 ppm and 1 ppm H2S concentrations were detected later for the alkali-impregnated activated carbon A than for the nonalkali-impregnated cabon B. C a r b n A continued to remove significant amounts of HIS even after odor breakthrough. It is believed that the HIS was not only adsorbed but was also catalytically and chemically reduced to elemental sulfur. Hence, complete saturation of the H,S removal capacity of the alkali-impregnated activated carbon column might never be achieved. It therefore appears that the impregnation alkali for activated carbon A has significantly enhanced the carbon's ability to remove H2S. However, even though the effluent H,S levels from column A were much lower than those from column B, the odor 'concenhations exerted by the effluent air from column A were comparable to those exerted by the effluent from column B. Hence, the effective removal of H2S by the alkali-impregnated activated carbon does not necessarily correspond with a better odor-refnoval efficiency.

The amount of odorous material (expressed in SOU) .removed by the 1- m carbon bed column is given by the product of the average odor concen- Eation of the influent odorous air and the total volume of sewage gas treated prior to odor breakthrough at the effluent port. Similarly, the amount of H2S adsorbed (expressed in grams of H I S ) before a predetermined breakthrough concentration (0.1 ppm or 1 ppm in this case) can also be calculated. Di-

728

. . .~ .. ~. . . .

II I

/I I1 ,

I

729

0005 0001 0 0 0 ~ 0001 0001 0 I

TABLE 4. Comparlson of Activated (

Adsorbent

Weight of activated carbon for I-m bsd height (kg) Average air-flow rate through column &/mi”) Total gas flow to odor b d t h r o u g h at I-m bed

height (m’) Amount of odorous material removed by I-m bed

height (SOU)’ Total gas flaw to 0.1 ppm 1.0 ppm

H,S brcaltlhrough at 1-m bed height (m’) Amount of H,S removed at

0.1 ppm 1.0 ppm

H d brrakthmugh by I-m bed height (9) Working capacity of adsorbent for I-m bod height

based on odor bxaltthrough m’/ke. SOUjkg

Based an 0.1 opm H S breakthrough

Based on 1.0 ppm H,S breakthrough m’/kg g H S / k

Cost of s w a g e gas treatment (S%/m’) Based on odor breakthrough Based on 0.1 ppm Hg breakthrough Based on 1.0 ppm H,S breaklhrough

pliers at the time of the study), the treatment cost using nonalkali-impregnated activated carbon B for odor and HzS removal can be lower than that obtained for alkali-impregnated activated carbon A. For example, by taking a breakthrough HzS level of 0.1 ppm, the-cost of using nonalkali-impregnated carbon is about two-thirds the cost of using the alkali-impregnated carbon. For odor breakthrough and 1 ppm HIS breakkthrough, the costs of using the two carbon types are comparable. This shows that although the alkali-impregnated carbon, which is specially designed for the treatment of H,S dominant air, is capable of removing a much larger quaytity of HIS than the nonalkali-impregnated activated carbon, its cost effectiveness in removing other odorous but non-H,S gaseous compounds may not be significantly increased by impregnation with an alkali.

A general practice for most sewage treatment plants is the use of H,S breakthrough as an indicator when deciding to replace or regenerate an existing activated carbon column. This study has shown that the odor from the

732

bon Perform

Carbon A alkali

impregnated (2) 1.36 35

3,773

167,890

4.748 10.076

35.7 71.8

2.774 144,030

3.491 26.3

7,409 52.8

0.0047 0.0037 0.0018

:es

Carbon B nonalkaii

impregnated (3) 1.28 35

2,588

923,460

4,550 6,306

34.1 49.3

2,022 l52,700

3.555 26.6

4.927 38.5

0.0042 0,0024 0.0017

i

effluent of the carbon column can have a high odor concentration and be objectionable even before a breakthrough HzS level as low as 0.1 ppm is detected. A carbon column’s useful life, based on odor breakthrough, can be influenced by the combined effects of reduced removal efficiency of an odorous constituent, such as sulfide. and t‘le continued removal of antago- nistic or inhibiting compounds in the sewage air. It is the odor breakthrough carbon life that should govern the practical design of an activated carbon adsorption column for control of odors in the sewage environment. As observed by Huang et al. (1979), in actual practice, the carbon life based on cdor breakthrough might be close to or equivalent to the carbon life based on hydrocarbon satuation.

Since the treatment capacity of each carbon column was based on a breakthrough odor level corresponding to the recognition odor threshold, one can expect to obtain a greater treatment capacity if the carbon column is allowed to operate to a higher and less stringent odor level. for example, the nuisance cdor threshold. Consequently, the cost figures obtained will be lower than those reported here. Also, the cost analysis canied out in this study has been based on the performances of fresh virgin activated carbon. Additional studies should be conducted to evaluate the performances of these carbons after regeneration. Nevertheless, the results presented here should provide a useful guide for wastewater authorities faced with the task of selecting activated carbon for the control of odorous sewage air at the inlet works of a wastewater treatment plant.

CONCLUSIONS

The study shows the following.

1. The sewage air at the inlet works of the municipal wastewater treatment plant studied here has an odor that is dominated by the presence of H,S, resulting in a characteristic rotten-egg smell. However, a sourish odor is observed when HIS levels in the sewage air are below 1 ppm. It is likely that this sourish odor is caused by the presence of odoriferous hydrocarbons.

2. The odor and HIS concentration of the headspace air directly above the sewage channel varied significantly throughout the 8-month study. This study gives an average odor and H2S concentration of 120 SOU/m3 and 5.3 ppm, respectively.

3. The use of HJ as an indicator for the evaluation of performances of sewage-odor-control devices, such as an activated carbon column, may not be appropriate due to the presence of other non-H,S compounds of high odor stimuli,

4. Treakent cost using nonalkali-impregnated activated carbon for odor and H2S removal can be lower than that obtained for alkali-impregnated activated carbon. Although the alkali-impregnated carbon that is specially designed f q the Watment of HIS dominant air is capable of removing a much larger quantity of H I S than the nonalkali-impregnated activated carbon, its effectiveness’\.in removing other odorous non-HzS gaseous compounds is not increased by impregnation with an alkali. This comparison, however, ignores the advantage of regeneration of the alkali-impregnated carbon.

which can break through much earlier than H,S. ..

733

Cederlof, R.. Friberg. L.. and Lindvall, T. (1966). "The organoleptic evaluation of odors with spccial reference to the kraft pulp industry." Proc.. Inr. Conf. on Ar- mospkeric Emissions from Sulphare Pulping, E. R. Hendrickson, ed.. Univ. of Florida Press. Gainesville, Fla., 111-122.

Dague. R. (1972). "Fundamentals of odor control." 1. Wafer. Pollur. Conrrol Fed.. 44(4), 583494.

H e w , J., and Gehr. R. (1980). "Odor control: An operator's guide." J. Water Pollur. Conrrol Fed., 51(10), 2523-2537.

Huang. J., Wilson, G., and Schmepfer, T. (1979). "Evaluation of activated carbon adsorption for sewer odor control." J . Warer Pollur. Control Fed.. 51(5), 1054- 1063.

Koe, L. C. C., and Brady, D. (1986). 'Olfactory quantification of sewage odors." J. Envir. Engrg. Div., ASCE, 112(2), 223-229.

Pomeroy, R., and Parkhurst. J. (1976). "The forecasting of sulfide build-up mtcs in sewers." Proc.. 8th Inr. Cog. on Water PoIIut. Res.. S. H. Jenkins, ed., Inter- national Association on Water Pollution Research and Control, 621-628.

Stone. R. (1970): 'Sewage treatment system odors and air pollutants." J . Sanitary Engrg. Div.. ASCE, 96(4), 905-909.

Van hgenhove, H., et al. (1985). 'GC-MS identification of odorous volatiles in

Young, P. J. (1984). 'Odoun f" effluent and waste wattnent: Effluenr and Wafer wastewater." Warer Res.. 19(5), 597-603.

Trearmenr J . , 24(5). 189-195.

,

i

734

u r r n ~ i ur r&KU IL~'KE AND CLASSIFIER HEIGHT ON AIR CLASSIFICATION

By Jw Everett'and J. JeNrey Peirce'

AESTRAn: The effects of varying the height and numbcr of sDgeS and feed rate for nonpulsing, passive-pulsing. and active-pulsing air classifiers is studied for the. sepamtion of complicated particle mixtures. All of the air classifier cantipations tested achieve high (>94?b) maximum repmion cfficicncy. Increasing height is found to increase the mgc of air flow over which separation efficiencies greater than 90% are achieved. Decreasing feed rate has a similar effect. For cach of the h ClasSificrS sNdied--nonpulShg. paCSiVe puking, and active pulring-thc tallest classifim at the lowest fwd rates achieved thc broaddcst efficient reparation m g e . Results indicate that the passive-pulsing air classifier performs tetter than the PC- tivc-pulsing a i classifier. which in Nm performs tetter than the nonpulring air classifiu.

INTRODUCTION

Waste-to-energy production facilities have met with mixed success in communities throughout the world. Operating difficulties and unanticipated expenses have acted to preclude the use of a range of technologies as de- cision makers face costly waste disposal issues. Research conducted at Duke University over the past 10 yr has, however, led to the development of a promising altemative me,pod for processing waste prior to energy production: pulsed-flow air classification. This concept is successfully demonstrated in the laboratory in its ability to separate the combustible from the noncombustible components of municipal solid waste (MSW). Air classifiers use a vertical constant air stream to separate particles of

different characteristics. Aerodynamic differences can he expressed using terminal velocity. To separate particles with different terminal velocities, the vertical fluid stream velocity is adjusted to an intermediate value, sending particles with lower terminal velocities up and particles with higher terminal velocities down. However, the combustible and noncombustible fractions of municipal solid waste are not cleanly differentiated by their terminal velocity (McNabb 1983). Density, not terminal velocity, is the significant parameter in the process of separating the combustible and noncombustible fraction of MSW (Stessel and Peirce 1985, 1986). An air classifier that can separate on the basis of density is needed. The value of such air classification is not limited to resource recovery plants, but applies to any complicated separation based on density differentiation.

Research has shown that active-pulsed flow air classifiers, using periodic time-varying air flow, can achieve density separation of particle pairs with reversed terminal velocities; that is, where the denser particle .has a lower terminal velocity .than the less-dense particle (Stessel1983; Peirce and Stes- sel 1983; Stessel and Peirce 1986, 1987). Density separatiomis defined as

'Res. Asst.. Dept. of Civ. and Envir. Engrg., Duke Univ.. Durham. NC'27705. Assoc. Prof., Dept. of Civ. Envk. Engrg., Duke Univ., Durham, NC

Note. Discussion open until Janua~y 1. 1991. To extend the closing date one ,month, a written request must be filed with the ASCE Manager of Journals. The manuscript for.this paper was submitted for review and possible publication on June 26, 1989. This paper is part of the Journal of Environmenrnl Engineering, Vol. 116, No. 4. July/August, 1990. BASCE. ISSN 0733-9372/w/ooo4a735/~1.~ + $.I5 per page. Paper No. 24921.

735

2 .

Recovery and Recycle of HCFCs by - Activated Carbon Adsorption

By: P.B. Logsdon and R.S. Basu Allied Signal Inc.

Buffalo, New York

ABSTRACT Various chemicals, such as chlorofluorocarbons (CFCs)

methyl chloroform. carbon tetrachloride, and halons will be phasedoutglobaUy by 1995becauseoftheirpotentialcontri- bution to stratospheric ozone depletion. Hydrochloro- fluorocarbons (HCFCs) are. considered as replacements for the CFCs in cerlah applications (e.g., solvents and foam expansion agents). In a number of applications, CFCs are recovered usingadsorptiononactivatedcarbonandreusedln this paper, we have described how to use. activated carbon to adsorb IJ-dichlom-1-fluomethane (HCFC-14lb) from an airstream and later recover and recycle using steam desorp tion. The capacity of commercially available carbons to adsorb HCFC-14lb and conditions to be used in the process are described.

INTRODUCTION C h l o r o f l u o ~ n s (CFCs) are. presently considered IO

be the prime contributor to smmpheric ozone depletion. Recent findings by NASA scientists havedeterminedthat the depletion oftheaonelayerovertheU.S. isoccurring atarate faster than anticipated. These new data suggest a depletion of four to five percent since 1978, twice the original estimate. Under presidential pmlamqhon, CFCs and methyl chloroform will be phased out in the U.S. by the year 1995 with a sharp decrease in production ineffect in the years prior to that. The Montreal F”t0col has put restrictions on the production of hydrochlorofluonxahns (HCFCS) beginning in 1996. As a result of these regulations, it is expected that the recovery and recycle of CFCs and HCFCs will become extremely important in the 1990s.

One way IO achieve the reduction of organic solvent emissions is through the use of carbon adsorption. The recovery and recycle of CFCs, such as CFC-113 and CFC-11. is currently being achieved using activated carbon. Carbon adsorption has been used for many years to recover solvent vapors in areas such as dry cleaning. &greasing, electronics.

Keywords: chlorofluorocarbon. hydrochlorofwrocarbon. aclivaled carbon, adsorpfion. recycle, 1 J-dichloro-l- flwroelhane

MARCHIAPRIL 1993

and film coatings. Solvent vapors escaping from the processing unit are caphued and recovered from air by carbon adsorbs. Activated carbon is used in the solvent recovery process since it preferentially adsorbs organic compounds. Depending upon the species adsorbed. activated carbon is capable of holding up to 50 percent of its weight in adsorbed material.‘

The objective of this work is to determine if present-day carbon adsorption technology is suitable for the recovery of l.l-dichloro-l-fluoroethane @CFC 141b). TWO solvfflts, Genesole ux)o and Genesole 2004, are being used as transitional solvents to eliminate CFC-113 in many of its applications.

SOLVENTRECOVERY Adsorption is simply defined as a phenomenon in which

molecules are held at the surface of a solid by van der Waal‘s amactive forces. Because the molecule loses most of its molecular motion. heat is r e l d (exothermic). Factors that s e c t the rate of adsorptisn are the partial pressure of the adsorbate.itsdiffmionrate.intothepom of theadsorbent, and the degree of saturation of the carbcm bed2 The amount of s o l v e n t a d s o M i w i t h solventconcurmation (partial pressure) and decnases with inmasing temperature. Once the bed becomes saturated. the adsorbate. must be removed (stripped) from the a d s o h n r

Desorption is usuaUypaformed by passing steam through the carbon bai. thus displacing the adsorbate by a combm- tion of vaporization and physical displacement. Water vapor isadsorbeQdisp~ingtheadsorbatethatispresentatalower equilibrium partial pressure.. Steam desorption is generally regarded as the most efficient method of stripping the bed of adsorbedsolvent whenwinparedto hating. vacuum.orinert gas stripping? Following bed ” a t i o n with steam, the excess water left on the bed must be removed by hot-air drying.

EXPERIMENTAL A laboratory-scale carbon adsorber was designed to con-

duct dynamic admptioddesorption experiments. A schematic of this apparatus is provided in Figure 1. Nitrogen is passed through a 1-L thermal-jacketed vessel containing the

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Pressure relief valve

To water/solvent separator .._. I

Bypass , Bed

- GC - Absorption

t Desorption __..

\ Q - Gauge b# - Needle Valve w -Toggle Valve

- 3-way Valve

- Pressure Indicator

TW - Thermowell FM - Flowmeter

Figure 1. Schematic diagram of carbon adsorber sysrem.

HCFC-14lb-baSed blend. The temperature of this vessel is maintained at 5.0 f3"C using a chiller bath. The chiller bath also saves to condense the steadsolvent mixture during desorption. A portion of the solvent is vaporized and recnm- bind with the main nitrogen flow. The resulting concentration of solvent in nitrogen is propoltional to the flow rate of nimgen intothevesseL Thisconcentration is measuredbyby- passing the carbbn bed ami sampling the gas stream directly.

The flow rate of the nitrogenholvent mixture is measured prior to contact with the 3 in. x 5 in. carbon bed. The temperature of the solvent/nitrogen gas mixture was typically 25 So. The. bed was charged with 297 g of Calgon XInsorb 600 granular activated carbon, as previously determined from adsorption isMherm work. The system flow r a t e d for these experimentswas 19.5Umin whichresultedinacontacthe between the gas stream and carbon bed of 1.8 see, the same parameters used inthemvery ofCFC-113. Becauseadsop tion is exothermic, the change in temperature of the carbon bed as the solvent progresses through the bed was monitored by a thennocouple kept in a thennowell. The thennowell is a conductive jacket to protect the thermocouple.

Agaschromato~hwasusedtoperiodicallymonitorthe outlet stream of the carbon bed for breakthrough of the solvent Breakthmugh occurs when the carbon bed is saturated or nearly saturated with solvent Once breakthmugh occurred. low-pressure steam was used to strip the bed of solvenrThesteamispassedthroughthebedcountercurrentto adsorption to minimize the distance the solvent/steam mixture

34

must travel. The pressure of the steam was 6 to 7 psig, used at a ratio of 1 lb of steam/l Lb of carbon. The solvenr/steam mixture was then condensed and gravity separated. The f d stepinthedesorptionprocessistoremovewa~r~thebed using air heated to approximately 75'C.

Gassamples werecollectedduringthedesorptionperiodto monitor the formation of breakdown products that may form when steam comes into contact with HCFC-14lb. This was accomplished by evBulILting and freezing (in liquid nimgen) slainless steel sample cylin&rs. The cylinders we& then placed in d e s with the water/solvent separator and opened to collect gas samples. These samples were then analyzed by gas chromatografiy//mass spectrometry (Gc/Ms).

RESULTS AND DISCUSSION A calibration curve of nitrogen flow rate into the solvent

vessel v e r s u s H C F C - 1 4 l b c t r a t i o n was genmtedandis shown in Figure 2. llis curve allows for rapid determination of the feed concentration into the carbon bed. Linear rem- sion of chese data correlated well and the equation for this curve is:

y = 0.0236~ - 1.1705,R2 = 0.99

where: y = flow rate of nitrogen into vessel(cm3/min) x = feed concentration @pm by volume) R = conelation coefficient

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Calibration Curve for Inlet Concentration of HCFC14lb Into Carbon Bed System Flowrate = 19.5 L/min

-20 I . . . . ; I 0 200 4ca 6m m 1 m 0 1 m

WM CQxa7lralkmol HCFCl4lb (ppmbY-1

Ygure 2. Calibration curve for inlet concentration of HCFC- 41b vs.flow rate of nitrogen into solvent vessel.

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The breakthrough of HCFC-141b was measured as a unction of timeandinlet (feed) concentration. Figure 3 shows wo examples of HCFC-141b breakthrough. one using a feed oncentrationof 5ooppm by volumeandthesecondug825 lpm by volume. As expected, at a higher feed concenmtion. ~reakthrough occurs earlier and the magnitude of HCFC- 41b concentration increases morerapidly than at lower feed oncentrations. In addition, the amount of HCFC-14 lbrecov- red increased with an increase in feed concentration. TXese wultsarepvidedinTable l.Thesedatacorrespondtoa6.0 J 11.0 percent capacity on a gram-of-adsorbate-to-gram-of- dsorbent basis and is lower than that for CFC- 113, which is ypically 7.0 to 15.0 percent

Table 1. Recovery of HCFC-141b at Several Feed Concentrations

Feed Breakihrough HCFGlBb Capacity concentration time (hours) recovered (grams r-vered)

(ppm by volume) (grams) (grams of carbon)

500 7 17 0.06 825 5 27 0.09 1026 3.5 34 0.1 1

The feed concentration versus breakthrough time results :hen inTable 1 were also plotted. This plot is shown in Figure i. The results of a linear regression on these data are as Ollows:

Breaklhrough of HCFC-141b Versus T i m e System Flowrate = 19.5 L/min

0 2 4 6 a IO 12

Time (hours)

Figure 3. HCFC-14lb breakfhrough times as function of HCFC-14lb concentration (ppm by volume) in incoming airstream at system airflow rate of 195 liters per minute.

y = 41.5787 + 3532.2~8' = 0.94

IARCWAPRIL 1993

Figure. 4. Breakthrough times for HCFC-141 b (1s an inverse function of inlet concentration of HCFC441b in the oir- stream at a systemflow rate of 195 liters per minute. where: y = feed concentdon of HCFC-141b x = Ibreakthrough time (hours') R = correlation coefficient

The pointat 825 ppmconcentration didnot titwell with the other two points. This resulted in a lower correlation coefti- cient. The graph shows that the breakthrough time increases with a dearase in concenmtion. Research was also conducted on GenesolVe W. Results showed that this system behaved very similarly to HCFC-141b in that brealahrough occurred at approximately the same time for a given feed

35

concentration. The breakthrough sequence for this blend was methanol, nitromethane. and HCFC-14lb. During steam desorption, the methanol and nitromethane were lost in the aqueous phase.

During the initial stages of steam desorption, gas samples werecollectedandanalyzdforbreakdownprodu 141b. GC/MS results did not show the presence of any breakdown products. In addition, gas samples collected during the middle stage of desorption did not show any breakdown products. It may be possible to use higher pressure steam without any adverse effects. This would decrease the time and the amount of steam used for desorption.

Levels of HCFC-14lb Seen in the steam condensate were typically 1200 to 1500 ppm by weight. Water levels in the HCFC-14lbphase were found tobeapproximately200 to 500 ppmbyweight.Theseresultsindicatethatthewatwmayhave to be treated before sewage and that the recovered HCFC- 141b may need to be dried using molecular sieves prior to

Multiple adsorptioddesorption cycles were performed on the same charge of carbon in the bed. The carbon appeared to remain intact and did maintain the same adsorptive capacity from cycle to cycle. Visual observation of the 316 slainless steel adsorber showed no signs of corrosion after repeated adsorptioddesorption experiments. This is not the case for l.l,l-trichloroethane(TCA). wherea bakedphenolic coating is needed to separate the carbon from the metal. When TCA is steam stripped, the stabilizers are almost completely lost, creating a highly corrosive condition. Therefore. special metals are required to handle recovered TCA. and it must be neutralized and restabilized prior to reuse?

reuse.

CONCLUSION The work presented in this paper shows that HCFC-14lb

can be recovered using carbon adsorption. The aasOrptive capacity of Calgon Xtrusorb 600 for HCFC-14 l b is typically 6.0 to 1l.Opercent on agram-of-a~~ate-to-gram-of-adsorbent basis. This capacity is slighlly lower than that for CFC- 113, which is typically 7 to 15 percent It is expected that by optimizing the adsorptioddesorption parameters, the capc- ity for HCFC-14lb can be improved.

36

Future work planned for carbon adsorption includes in- vestigating adsorption as a function of temperature, system flow rate, humidityof theinlet gas s-, and particle sizeof granular-activated carbon. For the desorption mode. future. research includes determining the effect of steam pressure on the efficiency of removal of adsorbed HCFC-14lb. the time needed for desorption. and its effect on the breakdown of HCFC-14Ib.Oncetheseparameters havebeen optimized, the research will be scaled up to a commercial carbon adsorber.

# ACKNOWLEDGMENTS

We wouldlike to IhankL. M. Sfachura for hisassisfame in rhe laborafov. We wouldalso like IO thank Dr. L. Wmg and R. Selanick for helpful discussiOns.

REFERENCES 1. AdsorptionHandbock Calgon Carbon Corporation,Pins-

burgh, Pennsylvania (1990). 2. Avery. D.A.. and Boism, D.A., “The Recovery of Sol-

vents From Gaseous Effluents.“The Chemical Engineer, pp. 8-1 l(January/Febmq 1969)

3. CheremiSinoff. P. N., and Ellerbusch, R.. Carbon Adsorp- tion Handbook, Ann Arbor Science Publishers. Inc., pp. 371-387. Ann Arbor. Michigan (1978).

4. Larson. D.M., “Control of Organic Solvent Emissions By Activated Carbon,“ Metal Finishing. pp. 62-70 @ecem- ber. 1973).

Resented at the 38th Annual Technical Meting of the Instimte of Environmental Sciences. Nashvillc.Tc~ersec. May. 1992.

Reader Response Panel Pleasc help the Journal ofthe IEss editors serve you better by mIhg your intemt in this article on the Reader Service Card.

I High 313 Medium 314 Low 315 1

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Solvent Regeneration of Activated Carbon Contact Hugh McLaughlii. Ph.D., P.E. Waste Min, Inc. 151 W R o a d Groton, MA 01450 tel508-448-6066 fax 508-448-6414

Summary description of technology

Adsorption by activated carbon is a technology widely applied for the removal of chemical species from waters and wastewaters, as well as for the removal of organic chemicals from vapor streams. In normal applications, the activated carbon gradually accumulates the chemical species removed from the liquid or vapor stream, causing a progressive reduction in the carbon's ability to remove additional chemicals from the stream being treated. At some interval, Le., when the activated carbon has become "spent", it must be replaced or regenerated to restore its adsorptive capacity.

Solvent Regeneration of Activated Carbon uses organic solvents to dissolve adsorbed material out of the pores of the activated carbon, thereby regenerating the adsorptive capacity of the activated carbon. Subsequently, the solvent is removed by low pressure steam.

In term of capacity, solvent regeneration typically restores 70% to 90% of the adsorption capacity of virgin activated carbon. In terms of cost, solvent regeneration of activated carbon is only a few cents per pound more expensive than steam' regeneration and up to an order of magnitude less expensive than thermal regeneration options or replacement of the activated carbon.

Solvent regeneration can be applied to both vapor phase and liquid phase activated carbons. The technology is applicable to virtually all applications currently utilizing thermal regeneration or one time use of activated carbon.

Comparative advantage of technology

Solvent regenerated activated carbon is a proven technology for the purification of water & wastewaters and air pollution abatement. However, there are many others. Alternatives exist for treating the spent carbon, such as replacement after one time use and thermal regeneration, and other treatment technologies exist, such as membranes, biological digestion, and advanced oxidation methods for water and incineration methods for air pollution. In general, the choice of technologies comes down to two issues - effectiveness of the technology for a given application and cost of implementation.

In terms of treatment effectiveness, solvent regenerated activated carbon is appropriate whenever virgin or thermally regenerated activated carbon can be successhlly utilized. The solvent regeneration process provides 70 to 90 percent of the adsorption capacity of virgin activated carbon and the impact of trace residual adsorbed material and solvent can be accommodated in the design of the carbon regeneration facility.

In terms of energy consumption, activated carbon manufacture requires between 150,000-200,000 W (kilojoules) per kilogram of virgin product. (NOTE: to convert to Btu'dpound, divide the kJkg in half, since 1 kTks = 0 479 Rtii's/nniind) Thekal reeeneratinn remiires hetween 40 000-70 000 kTke nf

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02/17/97 18:52:32

0 -- --13 - -1 --__ . - , - - - . . , - - - _ _ 0 -" ~

I____ - - _ _ carbon and destroys the desorbed material (and part of the carbon backbone) as part of the thermal regeneration process. In comparison, solvent regeneration requires only 5,000-10,000 kJkg of carbon, recovers the desorbed material intact (for recycle or use as a fuel - which typically cuts the overall fuel requirement by 40 to 75 percent), and does not deteriorate the carbon backbone due to the mild regeneration conditions.

In addition, the solvent regeneration process can be integrated into an existing industrial operations. If steam and cooling water are available at an industrial site, the steaming of the carbon at the end of the regeneration cycle and the operation of.the solvent recovery operations simply draw from the available site utilities. In contrast, thermal regeneration and virgin carbon manufacture require dedicated facilities with multiple hearth furnaces or rotary kilns.

Technical performance and cost data

The solvent regeneration technology has been demonstrated on a commercial scale (over a million pounds of carbon regenerated each year), with continuous operating data over a ten year period, providing for the removal and recovery of phenol and mixed aromatics from industrial wastewater.

Multiple industrial applications have been demonstrated on a benchtop scale. The cost data is based on actual plant performance and calculated energy and equipment requirements for specific applications.

Developments

The core technology is in the public domain and has been implemented by private industry in isolated applications over the years. In general, these applications are not well publicized. However, there has been a steady stream of academic studies over the years, with gradual delineation of the underlying physical phenomena.

The technology was broadened by a NYSERDA sponsored Universityhdustry research project (98O-EEED-IC-87), performed in 1987 & 1988. These efforts are documented by Energy Authority Report 88-16, titled SOLVENT-REGENEFUTED ACTIVATED CARBON. The study demonstrated that the technology had many potential industrial applications and enjoys a significant cost advantage over alternative treatment technologies.

Since 1992, Waste Min incorporated has been further studying the core technology and extended the capability of the technology to the regeneration of vapor phase activated carbon. Current designs allow both liquid and vapor phase carbon to be regenerated in the same facility.

Regulatory Considerations

The technology is viewed favorably by the regulatory community because it allows the reuse of the activated carbon and recovers the desorbed material intact, allowing for recycling. The facilities are small chemical handling facilities, built to operate with minimal solvent losses, but do require appropriate siting and permitting.

Status of intellectual property rights

The patents of the core underlying technology are expired. Waste Min has proprietary design and

02/17/97 185232

operating expertise, based on operating a commercial scale facility and subsequent developmental efforts.

Type of situation sought

Waste Min inc. is looking for a corporate partner to team with or merge into in order to commercialize the technology. Three types of partners have been identified: Organizations currently utilizing large amounts of activated carbon, organizations who have industrial sites with excess low pressure steam (thereby supplying the primary operating energy source without additional investment and at attractive rates), and organizations that service the waste treatment/disposal market with a variety of technologies.

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NORIT ACTIVATED CARBON AND GAS/AIR PURIFlCATION

1. INTRODUCTION

Adsorption onto activated carbon has been recognized for a long time as a very cost effective technique for gaslair purification.

As a result of more than 75 years experience supported by dedicated research and product development, NORIT is able to supply specialized carbon types for gaslair applications.

This Bulletin describes the theory behind the application of these carbons and the relevant parameters determining its performance.

2. ACTIVATED CARBON

Activated carbon is a microsporous carbonaceous material which is produced from various raw-materials, such as peat, wood, lignite, anthracite, h i t pits or shells. The raw materials are converted into activated carbon using steam (temperature above 900 C) or acid (temperature above 450 C). The activation process creates pores in the carbon, whose surface area may exceed 1500 d / g . It is by far the most important adsortive media due to its wide range of pore sizes.

The adsorption affinity of the internal surface (V/d Waals adhesion forces) together with the pore size distribution (cohesion forces), constitute an activated carbon's purification capability. Physical adsorption forces are not always sufficient to adsorb a component, and in these circumstances, the intemal surface may be used as a carrier for an active component, or chemical compound.

In such a case the activated carbon (or adsorbent) together with the active component either form a catalyst, or the impurity (or adsorbate) is removed by chemisorption.

Depending upon the application different physical forms are utilised:

Powdered Carbon 0 GranularCarbon 0 Extruded Carbon

In the gadair applications, predominantly extruded carbon types are to be preferred, because they offer the lowest pressure drop by way of their regular shape.

The NORIT extrusion process produces a hard carbon, which gives high abrasion resistance (ASTM hardness of more than 99%) and minimum dust formation during usage, even at a high degree of activation.

3 . ADSORPTION THEORY

3 . 1 . General

Several models do exist to define the adsorption process. The BET (Brunauer, Emmet and Teller) theory was most commonly favoured and is still used as a measure of the activation degree of activated carbons (Table 1)

02/17/97 175601

... .- .............

Table 1

BET-surface of NORIT activated carbon . . . . . . . . . . , , , . . . ;, * . . . . . . . . . . .

r-'+ . . . . . . . . . . $Quality Density (gA) F a 1 surface area (m2/g) I . - ___-___I . . . . . . . . . . . . . . . . . . . . . -_.

kORBONORIT . .. '1400 ..... 1 j l ........................

SORBONORIT

, " U T ........ RB .....

......... ..

.....

* Typical value for a 3 mm extrudate

The BET-theory however is less suitable for process calculations. For these calculations the Dubinin-theory is applied. This theory defines the equilibrium adsorption capacity as function of the adsorption temperature and the adsorbate concentration in the gasphase (Figure 1).

Figure 1 :Adsorption isotherm

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0.0001 0.001 0.01 0.1

PiPo (relative vapan pressure) I

0 p = actual adsorbate partial pressure 0 PO= saturation pressure 0 T = adsorption temperature

3.2. Dubinin theory

According to Dubinin the equilibrium adsorption capacity is influenced by:

0 The adsorbent type 0 The adsorbate type n TL.. "-&...,I ....... ,......a:&:.,-..

e

02/17/97 1 7 5 6 : 0 9

u l l l d aCI1ua.I p1uc;c;ss c;ulIuIlIuIIs

3.2.1. Influence of the adsorbent type

The type of adsorbent can influence the equilibrium adsorption capacity in two ways

The total pore volume of the adsorbent.

Adsorption occurs by condensation of the adsorbate in the adsorbents pores. A larger total pore volume will therefore result'in a higher maximum amount of adsorbate being adsorbed. This maximum adsorption capacity, however, can only be obtained if the gadair flow is totally saturated with the adsorbate.

The pore size distribution of the adsorbent.

We distinguish (according to IUPAC) .

Micropores : 0 - 2 nm diameter 0 Mesopores : 2 - 50 nm diameter 0 Macropores : diameter > 50 nm.

The actual adsorption occurs almost only in the micropores. The macropores will determine the accessibility of the adsorbent, while the mesopores influence the transport of the adsorbate from the gas phase to the micropores. An adsorbent with a high activation degree, and therefore a high total pore volume, will possess a high maximum adsorption capacity.

At low adsorbate concentrations adsorption occurs almost only in the smallest micropores, possessing the highest adsorption energy. The higher the activation degree of the adsorbent, the larger the average pore diameter and the lower the content of very small micropores. This means that the adsorbent with the highest activation degree will possess the lowest equilibrium adsorption capacity at low adsorbate concentrations. This concentration influence on the equilibrium adsorption capacity implies that carbons with a high activation degree are less suitable for air purification purposes (Figure 2A). This effect will be enhanced by adsorbent density. This applies particularly in the air purification field, where the adsorbent is used on a volume basis, the volume adsorption capacity (grams adsorbed per 100 ml activated carbon) instead of the weight adsorption capacity is the relevant parameter (Figure 2B).

Figure 2 :Benzene adsorption capacity at 20 C

A

Adaorption Isotherms "Il N.V. Amemso&. The Neiherlenda

02/17/97 175616

B

As shown in figure 2B, the adsorbent with the lowest activation level still possesses the highest volume adsorption capacity in a significant part of the concentration working range.

3.2.2. Influence of the adsorbate type

Three parameters related to adsorbate type influence the equilibrium adsorption capacity namely:

0 The molecular size of the adsorbate 0 The density of the liquid adsorbate 0 The boiling point and the structural shape of the adsorbate

It is clear that the adsorbate must be small enough to fit into the adsorbent’s pores. If the adsorbate fits

3).

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into the micropores it is adsorbed by condensation. The maximum volume which can be adsorbed is given by the total pore volume but the weight adsorption is given by the density of the liquid adsorbate (Figure

The first two parameters influence the adsorbed amount. The strength of adsorption is influenced by the boiling point and the structural shape of the adsorbate. In fundamental terms an adsorbate with a higher boiling point will be adsorbed more strongly than an adsorbate with a lower boiling point. In multi-component gadair streams, the higher boiling adsorbate will therefore displace the lower boiling adsnrha te from t h e adsnrhent Tn addition t h e shane n f t h e adsnrha te nlavs a rnle as well

02/17/97 175625

Figure 3.

Adsorption capacity on NOFUT RE3 3 at 20 C

e

A component like benzene will fit better into the adsorbate pores than a component like, for instance, hexane. The strength of adsorption is commonly expressed as the adsorption affinity factor, R.

3.2.3. Influence of the process conditions

There are two process conditions which innuence the equilibrium adsorption capacity directly :

0 The adsorbate concentration 0 The adsorption temperature

Other process parameters will not influence the equilibrium adsorption capacity itself, but they will

equilibrium adsorption capacity. At low adsorbate concentrations, adsorption will only occur in the

larger pores, with lower adsorption energy, will be used as well, resulting in a higher equilibrium adsorption capacity, When the gas is completely saturated with adsorbate (p/pO=l) even the macropores can be totally filled with liquified adsorbate. The adsorption temperature influences the energy content of the adsorbate which again influences the equilibrium adsorption capacity. In other words the higher the

influence the time required to reach this equilibrium. The adsorbate concentration is directly related to the

smallest micropores, the pores with the highest adsorption energy. At high adsorbate concentrations the

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adsorption temperature, the lower the equilibrium adsorption capacity, - 3.3. Kinetics of adsorption

The Dubinin-theory defines~equilibrium adsorption capacities. In actual applications however, the time which is needed to obtain this equilibrium will play an important role. After a certain adsorption time the adsorbent bed can be divided into three different zones Figure 4):

I - That part of the bed where the adsorbent is used to its full -equilibrium- capacity,

02/17/97 175629

I1 - That part of the bed in which this adsorption capacity is being built up -the Mass Transfer Zone or MTZ-.

III - A part of the bed which is not -yet- used. The gas which passes this zone is virtually free of adsorbate.

Figure 4

Adsorption capacity in relation to bed depth

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During the adsorption time the MTZ will move through the adsorbent bed, until this zone reaches the end of the bed. At this point in time the residual adsorbate concentration in the outlet will slowly increase. When the adsorbate concentration in the purified gas reaches the emission limit, adsorption should be stopped and the adsorbent should be replaced or regenerated. This means that at the end of the adsorption a part of the adsorbate is not M l y used. The average adsorption capacity in the MTZ will be approximately 50% of the equilibrium capacity. The length of the MTZ therefore plays an important role in the total amount of adsorbate which can be adsorbed until too high a breakthrough occurs. This total amount is known as the Effective Loading Capacity. It is clear that for optimal adsorbent use the MTZ should be as short as possible. The length of the MTZ is determined by the physical shape and the activation level of the adsorbent, the adsorbate type and the prevailing process conditions. L

Within the NORIT extruded activated carbon programme all carbons are in principle available in 0.8 ; 1 ; 1.5 ; 2 ; 3 and 4 mm diameter. This variation in extmdate diameter together with the possible variation in activation degree enables NORIT to select a carbontype which combines a minimum MTZ length with the lowest possible pressure drop over the carbon bed (Figure 5). -

Figure 5 Typical pressure drop curves in relation to the carbon diameter

02/17/97 175634

NORIT’s experienced staffwill be pleased to assist you in selecting the most cost effective design. It is clear that this design will strongly depend on the actual process conditions. Therefore, we need to know all relevant process data for your specific gadair problem. (Please ask for our Questionnaire GadAir purification and solvent recovery”).

Based on the completed Questionnaire NORIT will provide the relevant basic design details

Our services range from design, through the construction of the installation to start-up and operational assistance, as required.

This technical bulletin (issue 09-96) replaces previous issues.

Return

02/17/97 17:56:40

SIMPLIFIED APPROACH FOR DESIGNING CARBON ADSORPTION COLUMN By S. L. Ong,’ A. M. ASCE

lNlRODUCTlON

Activated carbon adsorption processes play an important role in cleaning up industrial and municipal wastewaters. Typically, activated carbon adsorption processes are applied to: (1) The removal of specific contaminants resistant to biodegration which are toxic and readily adsorbable (e.& phenol); (2) advanced treatment of wastewaters for organic removal superior to what is now biologically attainable; and (3) the recovery of process stream for reuse (1).

Activated carbon adsorption processes are commonly designed with the aid of breakthrough-curve concepts (3,4). A major part of the breakthrough-curve analysis concerns d e t e d i g the fractional capaaty of the adsorbent at the break-point, and the depth of the adsorption zone (3). The traditional method (3,4) for determining these two parameters requires that engineers construct three graphs and perform a series of graphical integrations, but this design approach is very tedious and time consuming. Thus, it is desirable to develop a simplified procedure that can be easily implemented on either an advanced programmable calculator or a computer. Such an approach would allow the engineer to concentrate on activities worthier than routine numerical calculations.

This paper presents a simplified method for determining the fractional capacity of the adsorbent at the break-point, and the depth of the adsorption zone.

.. GRAPHICAL APPROACH

The major steps used in the traditional graphical method are briefly summarized as follows (detailed information can be found in Refs. 3 and 4):

1. Construct the equilibrium isotherm curve by plotting C* versus q, in which C* represents the concentration of the solute in solution in equilibrium with adsorbed concentration, and q represents the mass of solute adsorbed per unit mass of adsorbent. These data are usually obtained in laboratory.

2. Superimpose the operating line, a straight line which passes through the origin and the equilibrium curve at C, (the concentration of solute contained in the influent to the column), on the graph constructed in

’Lect., Dept. of Civ. Engrg., National Univ. of Singapore, Kent Ridge, Sin- gapore 0511.

Note--Discussion open until May 1,1985. To extend the closing date one month, a written request must be filed with the ASCE Manager of Technical and Profes- sional Publications. The manuscript for this paper was submitted for review and possible publication on February 22, 1984. This paper is part of the Journal of Enoironmentul Engineering, Vol. 110, No. 6, December, 1984. BASCE, ISSN 0733- 9372/84/OLW&1184/$01.00. Paper No. 19317.

1184

Step 1. The ordinate of a point on the operating line is designated as C, which represents the effluent solute concentration under the operation condition.

3. Evaluate the expression V - V,/VE - V B corresponding to various values of C by graphical integration, using the plot of l /(C - C*) versus C. Where V = total volume of water treated, V , = total volume of water passed through the column until the break-point is reached, and VE = total volume of water passed through the column to the exhaustion point.

4. Determine the fractional capacity of the adsorbent at the break-point, f, by solving Eq. 1, defined as follows:

c v-v, 0 c. VE- VB

1

f = (1 - -) d(-j .. .. . . . . ..... . . . ... . . ... . . . . . . . . . . . . . (1)

This is done by graphical integration, using the plot of C/C. versus v - V,/V, - v,.

5. Determine the depth of the adsorption zone using Eq. 2:

in which Z,, = depth of the adsorption zone, L; Q = volumetric flow rate into the adsorption column, L/T; Kh = overall mass transfer coefficient, T’; C, = concentration of solute in the effluent at the break- point, MIL3; and CE = concentration of solute in the effluent at the exhaustion point, MIL3.

6. Determine the degree of saturation, D, , at the break-point. 7. Compute the total amount of solute adsorbed, M,, at the break-

8. Estimate the break-point time, TB . 9. Estimate the total volume of water treated, V,, to the break-point.

point.

SIMPLIFIED APPROACH

Equation Development.-Many theoretical and empirical methods have been developed to model adsorption isotherms. Although no single equation can describe all mechanisms and shapes of adsorption isotherms, the Freundlich model (2) is very useful for dilute solutions over small concentration ranges (4). It is particularly well-suited for application to the adsorption of impurities from a liquid solution onto activated carbon. The Freundlich model can be expressed as

. . . . . . . . .

Upon some manipulations, the fractional capacity defined in Eq. 1 can be re-expressed as . ,

I II

. rct , , . \

f = A J c B [ l - k ) ( & ) d C ................................. a (5)

...................................... (6)

Substituting Eq. 6 into Eq. 2 yields

(7) ZA = - ....................................................... aQ KlA

Solution Method.-In the last section, it was shown that the first five steps of the graphical approach can be summarized into two equations, namely, Eqs. 5 and 7. Since the empirical constant n in both equations need not be an integer value, it is clear from the calculus that no closed- form solutions are in existence under the general conditions for both equations. However, they can be easily solved with the aid of numerical methods. For example, one can apply the Simpson's rule to solve Eq. 6 as follows:

AC 3

... .......... (go + 481 + 282 + 4g3 + + 28h-2 + 2g2,-, + g?,") (8) a=-

in which AC = (C, - CB)/2m; gj = l / ( C j - bC;); Cj = CB + jAC (0 5 j c 2m); and 2m is the number of subintervals in which the interval of integration is divided.

Similarly, Eq. 5 can be approximated as

in which AC' = (C, - CB)/2m' ; k, = (1 - C,/C,)/(C, - E;); and C, = CB + jAC' (0 < j < 2").

In practice, it would be convenient to choose m = m ' ; since the numerical integration of both equations can be performed simultaneously. Although the accuracy of Eqs. 8 and 9 will improve in principle as the values of m and m' increase, a value from 10-20 for both m and m' should be adequate for most practical purposes. These numerical calculations can be easily performed on any programmable calculator or microcomputer.

In order to solve Eqs. 8 and 9, it is first necessary to estimate the values of K and n in Eq. 3. Since Eq. 3 is only a simple function, its parameters can be easily estimated using the least-square technique. This type of operation can be performed on most scientific calculators. It can also be programmed on any programmable calculator or microcomputer.

Once the values of a and f are determined from Eqs. 8 and 9, the values of ZA , Ds, M,, TB and V , can be obtained at once, using the appropriate equations given in the literature. All design procedures can be easily programmed on any programmable calculator or microcomputer.

IuusTRnnvE EXAMPLE To illustrate the simplified method developed and to compare its re-

1186

sult with that obtained by the graphical approach, it would be useful to choose a problem solved by the latter method and performed by an independent person. With this in mind, an example problem presented by Sundstrom and Klei (see Ref. 4, example 9-3) was chosen. Briefly, this problem mainly concems the determination of the degree of saturation (of carbon) at the break-point, the break-point time, and the volume of water treated to break-point by a fixed bed of activated carbon with a height of carbon in the column of 4 m. The column is fed with wastewater containing 20 mg TOC/L at a superficial flow rate of 0.2 m'/ m2-min. The break-point and exhaustion concentration are to be taken as 2 mg TOC/L and 18 mg TOC/L, respectively. The bulk density of carbon and the overall mass transfer coefficient, K I . , are assumed to be 600 kg/m3 and 15 min-', respectively. Equilibrium data were given in an earlier example in the same chapter of the book. Accordingly, the values of K and n were estimated by Sundstrom and Klei (4) as 0.008 and 1.176, respectively. With n = 20, the values of a and f, as evaluated from Eqs. 8 and 9, are 18.66 and 0.45, respectively. The corresponding values obtained by Sundstrom and Klei (4) using the graphical method are 16.3 and 0.46, respectively. The values of a found by both methods differ by about 12.670, whereas the values off obtained from both methods are reasonably close to each other. The difference in the value of a is not surprising, since the graphical method is generally more suscep- tible to numerical errors. IIhese errors can be particularly significant when the results obtained graphically are, in turn, used as input for further graphical analysis.

Based on the values of a and f obtained from Eqs. 8 and 9, the values of Z,, D,, M E , TB and V, are found to be 0.249 m, 0.972, 238 kg, 41 days, and 11,900 m3/m2, respectively. These are compared to the corresponding values of 0.217 m, 0.975, 229 kg, 40 days and 11,400 m3/m2 obtained by Sundstrom and Klei (4). Except for 2, , the value found by this study is 12.9% higher than that obtained by the graphical method, and the values for the various parameters found by both methods agree fairly well with each other despite the fact that the values of a found by the two methods differ by 12.6%. This finding indicates that the final results-in terms of break-point time, degree of saturation at the break- point, amount of solute adsorbed and volume of water treated to break- point-are not sensitive to a slight error in the estimated value of a.

EFFECTS OF NUM0ER OF sU0lNTERVALS USED IN NUMERICAL lNTEGRATlON

In order to assess the effects the number of subintervals on the values of a and f, Eqs. 8 and 9 were solved using different values of m and m'. In this analysis, both m and m' were assigned the same value. The results of this analysis are summarized in Table 1, which shows that the values of a and f converge very rapidly with respect to m or m'. This finding indicates that a value for m (or m') ranging from 10-20 should be adequate for most practical purposes. Although a further increase in the value of m (or m') may slightly improve the accuracies of a and f, the inaemental computational effort does not seem justifiable for the negligible gain in accuracy obtained. This low value of m (or m ' ) required confirms that the relevant model is suitable enough to implement on an

1187

TABLE 1.-Effects of Number of Subintervals Used In Numerical integration

4 6 8 10 12 14 16 18 20 30

5,wo

m or m' I u I f

18.759 0.446 18.690 0.447 18.673 0.447 18.667 0.447 18.665 0.447 18.664 0.447 18.663 0.447 18.663 0.447 18.663 0.447 18.662 0.447 18.662 0.447

1188

1

' FORMATION AND FATE OF BROMOFORM DURING DESALINATION

By Ning-Wu Chang' and Philip C . Singe?

INTRODUCTION

One of the more promising desalting processes for both brackish and seawater is the reverse osmosis (RO) process. This process requires a feed water of relatively high quality in order to prevent fouling of the membranes by particulate materials. Until 1982, the U S . Office of Water Research and Technology (OWRT) operated a pilot-scale desalination facility at WrightsviUe Beach, N.C. to evaluate various RO systems for desalting seawater. A key feature of the Wrightsville Beach Test Facility (WBTF) was a Central Pretreatment System (CFE), the objective of which was to provide a relatively uniform, particle-free water for the various RO units being tested.

The ClTS (see Fig. 1) consisted of a coagulation, sedimentation and pressure-filtration system. Chlorine was added at the seawater intake by an electrolytic hypochlorite generator for the purpose of controlling biological growths through the transmission line and the pretreatment system. The main purpose of the activated carbon was to remove any residual chlorine in order to protect those RO membranes which were not resistant to chlorine. An alternative pretreatment system consisting of ultrafiltration was also evaluated, as shown in Fig. 1.

- R 0 Feed

Filters Corbon Column

ICsntral Pr.treOtmF"l system. CPTS)

FIG. 1.-Flow Diagram of Desallnation Pretreatment 'Research Asst., Dept. of Environ. Sci. and Engrg., School of Public Health,

Univ. of North Carolina, Chapel Hill, N.C. 27514. 'Prof., Dept. of Environ. Sd. and Engrg., School of Public Health, Univ. of

North Carolina, Chapel Hill, N.C. 27514. Note.-Dimssion open until May 1,1985. To extend the closing date one month,

a written request must be filed with the ASCE Manager of Technical and Profes- sional Publications. The manuscript for this paper was submitted for review and possible publication on March 7, 1984. This paper is part of the Joumd of En- oimnmnctal Engineering, Vol. 110, No. 6, December, 1984. OASCE, ISSN 0733- 9372/84/0006-1189/sO1..00. Paper No. 19317.

1189 I

December 12, 1996 Purchasing

Index will decline Watch for falling chemical prices through the first half of 1997. PURCHASING'S index of industrial chemical prices is forecast to decline slowly from an average of 107 in the final quarter of 1996 to 106 in the first quarter of next year. The index will dip to 104 in the second quarter of 1997. Slowing demand, excess capacity, and reduced raw material costs will drive the decline. In the second half of next year, the index for domestic commodity chemical prices will flatten out at 105.

The Purchasing Forum is a great place meet your

colleagues

During November, the index bounced back slightly after dropping 2.49 points to 107.58 in October. The index rose 1.16 points to 108.74 in November. Chemicals with higher price tags included caustic soda, up $2/ton; butadiene, up l$/lb; toluene, up 2$/gal; phenol, up l$/lb; and titanium dioxide, up 0.5$/lb.

The exclusive data published here from a monthly survey of

500 chemical buyers conducted by PURCHASING Magazine. Data

may be reproduced only with written vermission from the editor

of PURCHASING, Editorial contact: Cheryl Lewis, Senior

Editor,Chemicals, 617-837-0148. 0 PURCHASING 1996. * CPI

Slowing demand and weakening raw material prices finally are

then quickly dropped back to 89.66 in November. Expect further depressing plastic resin prices. The index peaked at 9 1.61 in October

declines in the coming months. Purchasing estimates + Does not include applicable taxes Average U.S. prices, November 4 through

November 18, tankload or greater quantities, FOB producer.

Leadtimes represent time % of buyers Averages (weeks) Change from between order placement Off the 1-3 4-6 Over 6 This Month Year month ago and delivery shelf wks wks wks month ago ago (weeks)

*PURCHASING estimates.

Average U.S. prices, November 4 through May 18, tankload or greater quantities, FOB producer.

Data published here may be reproduced only with written permission from the editor of

Purchasing. 0 Purchasing 1996.

~

L

Purchasing transact

0.24-0.265 0.50-0.65 0.35-0.31

!on price survey results

0.24 0.22 0.53 0.55 0.35 0.34

Contract average

~

Contract range

spot average

I 0.23 Acetic acid (synth) 0.22-0.24 I 0.25 I I I

Acetone $Ab I 0.25 I 0.225-0.26 I 0.25 Acrvlic acid $Ah 0.55 I 0.45-0.65 I 0.57

Acrylonitrile $Ab I 0.37 I 0.35-0.39 I 0.36 Ammonia $/ton 200.00 I 155-265 I 210.00

0.21 IlButanol $Ab I 0.32 IlCalcium carbonate I . . _ _ _

I 215.00 Calcium chloride (anhvd) $/ton , .

!Carbon black $Ab I 0.325

0.120 Caustic potash (commercial) $Ab Caustic soda (liq, , 50%. diaphragm 249.00

I

//grade) $hon 1 Caustic soda (liq, 151, rayon grade) 1 340.00 $/ton

0.315 Caustic soda beads

0.25-0.35

68-130

198-245

0.32-0.335

0.33

115.00

210.00

0.37

0.105-0.135 0.120

300-380 340.00 t 0.31-0.32

175-270 240.00

lkhlorine $Ab I 188.001 125-245 I 195.00 Citric acid (USP, anhyd) $Ab Cyclohexane $/gal Diethylene glycol (standard) $Ab 0.235

Six-month forecast spot contract

0.24-0.26 I 0.22 I 0.23 I

200-230 I 205.00 I 215.00 0.35-0.40 I 0.31 I 0.38 1.70-1.90

0.20-0.22 0.21 0.23-0.39 0.3 1 0.32

95-145 110.00 115.00

200-220 215.00 210.00

0.36-0.375 I 0.325 I 0.37 11

I I

165-230 I 195.00 I 188.00 11 0.71-0.75 I 0.715 I 0.73 (1

I I

1.10-1.20 I 1.20 I 1.15 I I

0.18-0.25 I 0.20 I 0.23 1

The exclusive data published here come from a monthly survey of over 500 chemical buyers conducted by Purchasing Magazine. Data may be reproduced only with written permission from the editor of Purchasing. Editorial Contact: Cheryl Lewis, Senior Editor/ Chemicals, 617-837-0148. 0 Purchasing 1996.

I 0.46 I 0.435-0.50 I 0.44 I 0.38-0.55 I noldine) $Ab

0.41

0.465

0.455

HDPE (injection molding) $Ab LDPE (extrusion) $Ab LDPE (film liner) $Ab

Six-month I 1 forecast spot

0.38-0.45 0.39 0.34-0.43 0.34 0.37

0.46-0.47 0.47 0.44-0.50 0.38 0.38

0.44-0.47 0.415 0.41-0.42 0.44 0.32

0.46 I 0.43 11

0.46

0.41

LLDPE (film grade) $Ab LLDPE (gen purp) $Ab

0.43-0.505 0.41 0.38-0.43 0.38 0.35

0.38-0.43 0.385 0.36-0.44 0.34 0.325

Iresins) pvc ( d ~ s i 0 n 7 0 , 5 8 $Ab 1 0.56-0.605 1 0.56 0.53-0.59 0.53 0.55

0.40

0.31

Polyvinyl chloride (film) $Ab Polyvinyl chloride ken PUT) $Ab

Copyright0 1996 Reed Elsevier. Inc.

0.40 0.41 0.39-0.43 0.41 0.40

0.305-0.32 0.33 0.31-0.35 0.30 0.31

@ Return to subject index , or go to this issue’s table of contents

BARNEBEY & SUTCLIFFE TECHNICAL BULLETIN e CORPORATION

NO. TB-642 11/95 Page 1 of 3

Removal Capacity of Standard Activated Carbon for Various Contaminant Vapors

This table shows the relative capacity of s tandard activated carbon (Type AC) to remove selected chemical compounds from air under typical ambient conditions.

Average index values are shown, b u t a d s o r p t i o n capac i ty will va ry with changes in the following conditions:

Temperature: adsorption ca- p a c i t y d e c r e a s e s a t h i g h e r temperatures. Pressure : capacity becomes h i g h e r a s s y s t e m ope ra t ing pressure increases. Relative Humidity: capacity depends on type of contaminant. At higher humidity (RH > 50%), capacity will be higher f o r w a t e r miscible so lvents (Acetone, Methanol), but lower for immiscible or partially

Substance Index

* Acetaldehyde Acetic acid Acetic anhydrite Acetone

* Acetylene Acrolein Acrylic acid Acrylonitrile Adhesives. Alcoholic beverages

* Amines * Ammonia

Amyl acetate Amyl alcohol Amyl ether Animal odors Anesthetics Aniline Asphalt fumes Automobile exhaust

2 4 4 3 1 3 4 4 4 4 2 2 4 4 4 3 3 4 4 3

immiscible solvents (Toluene, Benzene , C h l o r i n a t e d solvents). Concentration: adsorption capacity improves with increases in contaminant concentration. Type of Contaminant: capacity generally increases for materials with a higher boiling point and molecular weight.

Standard activated carbon has limited effectiveness with certain chemically reactive gases like ammonia and formaldehyde. Special impregnated carbons, however, have been formulated for many reactive gases, and can be recommended for the substances noted with an asterisk y).

The four capacity index values are defined as follows:

Substance Index

Benzene

Bromine Butadiene Butane Butanone Butyl acetate Butyl alcohol Butyl cellosolve Butyl chloride Butyl ether

' Bleaching solutions

+ Butylene Butyne

* Butyraldehyde Butyric acid

4 3 4 3 2 4 4 4 4 4 4 2 2 3 4

Caprylic acid 4 Carbolic acid 4 Carbon disulfide 2 Carbon dioxide 1

*Special impregnated carbon available for this material.

High capacity. One pound of carbon adsorbs about 20% to 40% of its own weight. More than 70% of the substances listed fall into this category. Satisfactory capacity. These constitute good applications b u t the capacity is not as high as for category 4. Carbon adsorbs about 10 to 25% of its weight. Borderline. Capacity is no t high but might give good service under the par t icular condi t ions of operation. These require individual checking. Not recommended. Adsorp t ion capacity is too low for these materials to satisfactorily remove them under ordinary circumstances.

~

Substance Index

Carbon monoxide 1 Carbon tetrachloride 4 Cellosolve 4 Cellosolve acetate 4 Cheese

* Chlorine Chlorobenzene Chlorobutadiene Chloroform Chloronitropropane Chloropicrin Cigarette smoke odor Citrus and other fruits Cleaning compounds Coal smoke odor Combustion odors Cooking odors

* Corrosive gases Creosote Cresol

4 3 4 4

~

3 3 4 3 4 4

Barnebey & Sutcliffe Corporation P.O. Box 2526 * Columbus, OH 43216 . 1-800-886-2272 614-258-9501 - Fax: 614-258-364

This data and information is presented to assisl a technically knowledgeable customer in the evaluation of carbons produced by Earnebey & Sutcliffe Corporation. However, due to variations in the content 01 specific gas or liquid streams. and the fact that the use of the carbon is beyond the Control of Earnebey & Sutcliffe, no guarantee or warranty. expressed or implied, is made as to such use, any effects incidental to such use. or the results to be obtained. Earnebey 8 Sutcliffe expressly disclaims responsibility therefore and the user accepts lull responsibility lor performance of systems using carbon based on this data. Please contact Earnebey & Sutcliffe lor a more detailed review of your application. before proceeding.

IY NO. TB-642 11/95 Page 2 of 3

BARNEBE 6 SUTCLIFFE TECHNICAL BULLETIN Q CORPORATION

Substance Index Substance

Crotonaldehyde 4 Ethylene dichloride Cyclohexane 4 Ethylene oxide Cyclohexanol 4 Essential oils Cyclohexanone 4 Eucalyptole Cyclohexene 4 Exhaust fumes

Decane Decaying substances Deodorants Detergents Dibromethane Dichlorobenzene Dichlorodifluoromethane Dichloroethane Dichloroethy lene Dichloroethyl ether Dichloromonofluormethane Dichloronitroethane Dichloropropane Dichlorotetrafluorethane Diesel fumes

* Diethylamine Diethyl ketone Dimethylaniline Dimethylsulfide Dioxane Dipropyl ketone Disinfectants

Embalming odors

Ethane Ether Ethyl acetate Ethyl acrylate Ethyl alcohol

Ethyl benzene Ethyl bromide Ethyl chloride Ethyl ether Ethyl formate Ethyl mercaptan Ethyl silicate

Ethylene chlorohydrin

EPOXY

* Ethylamine

Ethylene

4 Fertilizer 4 Film processing odors 4 Floral scents 4 Fluorotrichloromethane 4 Food aromas 4 * Formaldehyde 4 * Formicacid 4 Fuel gases 4 Fumes 4 3 Gasoline 4 4 Heptane 4 Heptylene 4 Hexane 3 ' Hexylene 4 * Hexyne 4 Hospital odors 4 Household smells 4 Hydrogen 4 Hydrogen bromide 4 * Hydrogen chloride

* Hydrogen cyanide 4 * Hydrogen fluoride 4 * Hydrogen iodide 1 * Hydrogen selenide 3 Hydrogen sulfide 4 4 Industrial wastes 4 Ink odors 3 Iodme 4 Irritants 4 Isophorone 3 * Isoprene 3 Isopropyl acetate 3 Isopropyl alcohol 3 Isopropyl ether 4 1 Kerosene 4 Kitchen odors

*Spec ia l i m p r e g n a t e d carbon avai lab le for th is mater ia l .

Index Substance Index

4 Lactic acid 4 3 Lingering odors 4 4 Liquid fuels 4 4 Liquor odors 4

Lysol -4

3 Masking agents 4 - 4 Medicinal odors 4 3 Menthol 4 4 Mercaptans 2 2 Mesityl oxide 4 3 Methane 1 2 Methyl acetate 3 3 Methyl acrylate 4 -

Methyl alcohol 3 4 Methyl bromide 3

Methyl butyl ketone 4 4 Methyl cellosolve 4 4 Methyl ceUosolve acetate 4 3 Methyl chloride 3 3 Methyl chloroform 4 3 Methyl ether 3 4 Methyl ethyl ketone 4 4 Methyl formate 3 1 Methyl iodine 2

1 Methylcyclohexane 4 1 Methylcyclohexanol 4 1 Methylcyclohexanone 4 1 Methylene chloride 4 1 Mildew 3 1 Mixed odors 4

Monochlorobenzene 4 3

3 Lubricating oils L?d greases 4

4

1 Methyl isobutyl ketone ' 4

. ~ ~ ~~~

3 Naphtha (coal tar) 4 4 Naphtha (petroleum) 4 4 Naphthalene 4

1

4 Nicotene 4 3 * Nitric acid 3 4 Nitro benzenes 4 4 Nicroethane 4 4 * Nitrogen dioxide 1

Nitroglycerine 4

4 Nitropropane 4 4 Nitromethane 4

~

B a m e b e y & Sutc l i f fe Corporat ion - PO. Box 2526 * Columbus, OH 43216 * 1-800-666-2272 * 614-258-9501 * Fax: 614-236-3464

This data and information is presented to assist a technically knowledgeable customer in the evaluation of carbons produced by Barnebey 8 SutcliHe Corporation. However, due to variations in the content of specific gas or liquid streams, and the fact that the use of the carbon is beyond the control of Barnebey & Sutcliffe. no guarantee or warranty, expressed or implied, is made as lo such use. any effects incidental to such use, or the results to be obtained. Barnebey 8 Sutcliffe expressly disclaims responsibility therefore and the user accepts full responsibility for performance of systems using carbon based on this dala. Please contact Barnebey & Sutcliffe for a more detailed review of your application, before proceeding.

NO. TB-642 11/95

BARNEBEY 6 SUTCLIFFE Q CORPORATION TECHNICAL BULLETIN Page 3 of 3

Substance Index

Nitrotoluene Nonane Noxious gases

4 4 3

Octalene 4 Octane 4 Odorants 4

Ozone 4

Paint and redecorating odors 4 Palmitic acid 4

Paradichlorbenzene 4

Pentane 3 Pentanone 4

Organic chemicals 4

Paper deteriorations 4

Paste and glue 4

* Pentylene 3 * Pentyne 3

Perchloroethylene 4 Perfumes, cosmetics 4 Persistent odors 4 Pet odors 4 Phenol 4

Pitch 4 Plastics 4

Pollen 3

Phosgene 3

Poison gases 2

Propane 2 * Propionaldehyde 3

Propionic acid 4 Propyl acetate 4 Propyl alcohol 4 Propyl chloride 4 Propyl ether 4 Propyl mercaptan 4 Propylene 2

* Propyne 2 Putrescine 4 Pyridine 4

Radiation products 2 Rancid oils 4 Resins 4 Reodorants 4

Substance

Ripening fruits Rubber

Sewer odors Smog Smoke Solvents Stoddard solvent Stuffiness Styrene monomer

* Sulfur dioxide ' Sulfur trioxide

Sulfuric acid

Tar

Tetrachloroethane Tetrachloroethylene Toluene Toluidine Trichlorethylene Triochloroethane Turpentine

Urea Uric acid

Valeric acid Valericaldehyde Varnish fumes Vinegar Vinyl chloride

Xylene

* Tamishinggases

Index

4 4

4 4 4 3 4 4 4 2 2 4

4 3 4 4 4 4 4 4 4

4 4

4 4 4 4 2

4

c

Special impregna ted ca rbon avai lab le for this material.

~ ~~~ ~

B a m e b e y & Sutc l i f fe Corporat ion - P.O. Box 2526 * Columbus, OH 43216 * 1-800-886-2272 614-258-9501 Fax: 614-258-3464

This data and information is presented to assist a technically knowledgeable customer in the evaluation of carbons produced by Barnebey 8 Sutcliffe Corporation. However, due to variations in the content of specific gas or liquid streams, and the fact that the use of the carbon is beyond the control of Barnebey 8 Sulcliffe, no guaranlee or warranty. expressed or implied, is made as lo such use, any effects incidental to such use. or the results 10 be obtained. Barnebey & Sutcliffe expressly disclaims responsibility therefore and the user accepts full responsibility for performance of systems using carbon based on this data. Please contact Barnebey 8 Sutcliffe for a more detailed review of your application, before proceeding.

Distilling and reusing solvent to clean roll waters and spray guns that apply adhesive to office furniture panels is paying for itself at Haworth. Inc., Hol- land, MI. The adhesive-laden solvent formerly was disposed of by partial treatment, hauling to a landfill or selling to a reclaim company.

From 10 to 12 gal of me ylene chlo-

from a laminating line where PVC is bonded onto panels. About 20 gal of a blend of foluene. MEK. hexane and naphtha are used daily in cleaning adhesive used in assembling sheet metallhoneywmb panels.

The methylene chloride and solvent blends are distilled in separate autc- matic systems that deliver recycled solvent that is 99.5% pure. Each system

ride is used daily in cleani x g adhesive

leaves an adhesive residue in a plastic bag for wnvenient disposal.

Fred GaNer. manager of environmental and energy conservation. said the solvent is being distilled and reclaimed at a w s t of about 20clgal. compared to a purchase cost of $3.541 gal for methylene chloride and $3.641 gal for the solvent blend. Haworth’s formula in determining payback time shows 0.99 year for the unit that re- claims the solvent blend and 1.51 years for the system reclaiming methylene chloride.

Reclaiming solvent has enabled Haworth to reduce solvent purchasing by one-half. The small amount of adhesive residue waste products wI- lected in reclaiming has slashed the need to haul waste products to a land-

. ..

-.

Paul Russcher. sment recovery operator, checks data lor the unit that can recover 70 gal a day of soivent blend used in cleaning adhesive from application equipment.

28-INDUSTRIAL FINISHING-11183

RECOVERY /-

was being spent annually to have the hauled away; about $30.000

was spent a year on solvent purchase. Tie distiller that recovers methylene

chloride has a 14-gal boiler and a 13- gal reclaimed solvent reservoir and can recover up to 20 gallday. The other unit has a 35gal boiler and a 67-gal reclaimed solvent reservoir and can re- mver 70 gaVday. The boilers and reser- voirs are stainless steel.

Each unit works essentially in the same way. Contaminated solvent methylene chloride is recovered; about lid is opened. enters the boiler, where electric heaters immersed in a heat-transfer liquid warm the mlvent to its boiling temperature. me generated solvent vapors pass through a water-cooled condensing coil where they are wnverted to a solvent liquid, which WllActS in the dean solvent reservoir. Each unit has a foot pedal to activate an eXplOsioO-p~f motor to pump the clean solvent to another container.

The smaller unit has a roll-out boiler for access convenience. The larger unit's boiler is reached by pneumati-< are: acetone 132 to 134F, heptane 19,)

cally opening the boiler lid. A nonpo- rous nylon bag liner that withstands boiler heat and is resistant to solvent and adhesive attack is positioned into the boilers before adding the dirty solvent. Thus, the solvent boils away. leaving the adhesive residue in the bag. The bag and contents are deposited in a Waste drum. POlymerizatiOn prevents the adhesive from being reused.

Recovery efficiency is high. About three-fourths of the adhesive-laden '

nine-tenths of the adhesivecontami- nated solvent blend is recovered.

Operation of the units Is simple. After filling the boilers with solvent and push- ing the r o l l a t boiler securely into place and closing the lid on the larger unit, a pushbutton starts the process. The adhesives. boiling temperature control is left in the same position. but is set to the boiling point of the solvent being distilled. The distillers can process solvents at up to a 40gF-boiling.=

'Boiling points of the solvents us&

to 214F. hexane 151 to 16OF, MEK 174 to 177F, methylene chloride 102 to 106F. naphtha 240 to 32OF and toluene 228 to 232F. Acetone and heptane are included in the adhesive and are re. claimed with the solvent blend but not with the methylene chloride due to its much lower boiling point.

Elaborate safety precautions are built into the recovery units to prevent explo- Sion or fire. An interlock on the lid of the larger unit turns off electricity when the

Suppliers Zerpa Industries supplied the RS14

and A70 recovery units. H.B. Fuller supplies the SciOi2 solvent blend and the SC1931 and SC1948 contact bond

For mare information on the solvent recovery systems described. Circle No. 10 on the reader information card.

A For an e m wpy of mi. a w e , uss m8d~infOrm.U.n~.ClrclsNaII

1s 1lY

a: T

1IC al 3u as iaii n v . lair

E 5an enti imn the The thrc whf liqui VWl

Ped mol othi

T for unit

... ., ~ ... :

trenq?iinsisdeposltedintoa wastedrum

1 lI83-INOUSTRIAL FINISHING-29

here is nothing old about acti vated carbon, even though itr earliest known use as a water pu T rification agent dates back 3,00(

years to a Hindu recipe for making potable water. In a simple process, the water was boiled in a copper pot anc filtered through charcoal to make il safe to drink.

In much the same way as it wac then, filtration with activated carbon is a cost-effective method for process water treatment. Its primar, use is fox

i the removal of volatile organic compounds (VOCs), soluble organic com. pounds (SOCs) and other adsorbable materials.

In addition to being inexpensive, activated carbon has passed the test 01 time and has proven reliable. In fact, the 1986 Amendments to the Safe Drinking Water Act named adsorption with granular activated carbon (GAC) as the standard by which other treatment technologies are to be evaluated for control of SOCs.

While the cleaning capability of activated m b o n has been known for thou- sands of years, scientists still study how it works. It is known that the key to its abilities lies in its vast surface area. In‘fact, a teaspoon of activated carbn has as much surface area as a football field.

I Examination under a microscope re- veals that its porous surface is a net- work of holes that lead to smaller and

-smaller holes d e d micropores. The micropores are so small that molecules get trapped onto the immense surface area of a granule. The larger the surface area of an activated carbon granule, the more it can adsorb.

Organic contaminants have different adsorption affinities. Some organic solvents - including chlorinated organics, such as trichloroethylene - and aromatic solvents such as toluene are adsorbable due to their low solubility in water. Polynuclear aromaties and other higher-molecular-weight compounds such as dyes and surfactants are adsorbed effectively. However, water soluble compounds such as alcohols and aldehydes are poorly adsorbed.

A key advantage to the carbon adsorption process is that not only does the carbon remove the contaminants from the water, but the equilibrium es-

. . ~

..

.~

carbon sets the standard

America, Inc

94 CHEMICAL ENGINEERINGIJULY 1994

ablished also concentrates the conk” nants and holds them within the car- mn granule.

Materials for activated carbon 4n important consideration in select- ng activated carbon for p m s s water reatment is what it’s made from. The jrimary materials used to produce ac- ivated carbon - lignite, wood, coconut md bituminous coals - impart differ- *at qualities. Cfinerally speaking, the nore dense the starting material, the ettcr the activated c a r h product re- ipnds M reactivation and reuse.

Coconut-based carbon has the high. !st density. Ilowever, it also has a large lumber of extremely small pores, vhich limits its applicability. Lignite- lased carbon, on the other hand, has he Imst hardness, density and abra- ion resistancc. Carbon from bituminous coal has su-

ltrior hardncss and high-bed density LS well as broad porc-size distribution, rhich yields thc most efficient adsorp ion capacity per volume. It has the est ability to remove trace organic

contaminants such as VOCs. tt bi

organic compounds, activated can also remove free LG

water through a surfac at free chlorine reacts ’ w

In addition to physical adsorption

carbon to form surface oxides, as following chemical reaction illustr

C + HOC1 ---> CO + HC + C1-

where C and CO represent acti carbon and the surface oxide tion will happen quickly an on all carbon surfaces.The of raw material to activated volves three major steps: d bonization and activation.

ar ta vi, - ?.! fu mi th

w - Th ad tra in each of the steps. During the wa step, water is driven off as ste

the second step, organic om driven off in somewhat is t

re1 ner as the water. The c remain are “carbonized O V carbon. In the last step, ott, granules are enlarged with ste tiot give activated carbon its uniqu 1

ads structure and vast surface area. Activated carbon is used ‘ tre

powder forms. While limited application of

may be added to the I and filtered out later in

the process - more commonly used for water treatment is the granular form, which is frequently installed in a down- flow fixed-bed adsorber system.

Selecting the filtration method The choice of a filtration method depends on the contaminants in the water. Chemical analysis will identify the contaminants and help determine if activated carbon adsorption is an efficient treatment. Additional tests can be conducted to determine the feasibility of adsorption for the test water. For example, the isotherm test for liquid- phase adsorption is the basic preliminary evaluation tool.

The isotherm technique is a batch test in which a sample of water is tested with varying amounb of carbon. The carbon is pulverized to reduce the length of testing time required to reach equilibrium. If the results of the isotherm test are favorable, the next test is dynamic adsorption to determine when "contaminant breakthrough" curs. Breakthrough signals the capacity limit for the activated carbon has been reached.

The scaled column test is the most common and direct way to obtain operating design information. Columns with granular activated carbon are arranged in a series. Each column contains enough activated carbon to provide a specific contact time, usually 7.5-15 midcolumn. The scaled- and full-column studies provide good information on carbon use and breakthrough characteristics.

When to use carbon adsorption There are situations where the carbon adsorption system may be the only filtration treatment. Dechlorination of water from an incoming water source is one situation where carbon adsorption is the treatment of choice. Treatment of relatively clean groundwater for trace organic contaminant removal is another application where carbon adsorption is recommended.

There are other times when carbon adsorption is part of an overall water treatment scheme. If the water being

treated contains suspended solids or ii upstream process operations include precipitation or settling operations for removal of biological solids or metals, a separate filtration step is needed.

Another unit filtration step, such as sand, may be used prior to the carbon adsorption step. Subsequent, additional filtration steps such as ion exchange or reverse osmosis may be employed downstream from the carbon adsorption unit to produce highly pure or ultrapure water.

Systems operation Process water treatmeh with granular activated carbon (GAC) can be tailored to fit a vast range of parameters, such as system size, and influent and effluent water quality. For example, low volumes of high-quality rinse water can be generated by dechlorinating municipal potable water with GAC in fixed beds of a size similar to that installed for use as household water filters. In some cases the water may not require post treatment to achieve the desired quality.

For general purposes or for steam generation, larger fixed beds usually are preferred. Beds can range from 4-12 R dia. and contain 1,000-225,000 Ib of GAC. These beds operate under conditions that are consistent with typical recommendations for fixed beds.

Normally hydraulic loadin is held to

tact time exceeds 15 min. Flows range from 50-1,000 g a m i n and several adsorbers are placed in both series and parallel operation to optimize GAC consumption and hydraulics. Depending on the source of the water, be it potable or raw surface water, groundwater or recycled wastewater, the effectiveness and service life of GAC will vary.

An often-overlooked opportunity for reducing the cost of water and wastewater treatment is to use water generated from chemical reaction. Most often, this water is simply treated as wastewater when, in fact, it is of a quality higher and more easily treated than the potable water used for various plant processes. At Elf Atochem's Beaumont, Tex., plant, chemically generated water from one process is treated with GAC and then used in another process, displacing treated potable water.

between 4-8 gamin per R f and con-

Regeneration When activated carbon is spent, it is ex- changed for a fresh supply. Spent carbon that is disposed can be landfilled or incinerated as solid waste. But these disposal options are expensive and carry certain liabilities.

Regeneration is a viable and cost-effective altemative, particularly for coal-based GAC. In a process similar to initial carbon activation, reactivation removes and pyrolyzes the more volatile organic contaminants from the carbon's surface, and decomposes the less volatile compounds held within the pores of the carbon particles.

The handling and reactivation of granular activated carbon will cause product loss. It may be as much as 10-20% of the bituminous coal-based carbon, and higher with s o h r lignite- based carbons.

An onsite thermal reactivation facility is cost-effective when demand exceeds 10,000 Ib/d of carbon. but rarely does the process water treatment con- sume that much material. Another on- site regeneration process is in situ, em- ploying steam or hot gas treatment, but the results are not as good compared with those of thermal reactivation.

In situ regeneration will restore some capacity, but it cannot remove all of the compounds from the adsorption pores. That limits the ability of the reactivated carbon to effectively remove contaminants to low levels.

Rather than invest in a dedicated carbon regeneration system, some chemical processors contract for off-site services. The merchant facilities take advantage of economy of scale to regenerate carbon and resupply the proper grade to its customers. A few facilities can also regenerate carbon that is contaminated with hazardous waste.

Edited by Deborah Hairston

The author

CHEMICAL ENGlNEERlNGlJULY 1994 95

Adsorption of Methylene Chloride Vapor on Activated Carbons: \\/en-Tien Tsai & Ching-Yuan Chang* (;r:tduate Inst i tute of Environmental Engineering, Na t iona l Ta iwan University, 71 Chou-Shan Road. .l.ijpui 106. Ta iwan

,~:.~.~.ivc,J 17 January 1993; accepted I I April 1994)

Abstract: A laboratory investigation on the adsorption 01 hazardous methylene chloride (METH) vapor on the commercial activated carbons BPL and PCB. which were made lrom bituminous coal and coconut shell, respectively, was conducted at 283. 293. 303. and 313 K. The physical properties and surlace l u n c t i o i ~ ~ r o u p s n l the 1wn ;activ:itcd c:irhnns wcre :iko measured and compared , u t11 c i , , -%-y. i i i c ~ ~ . i . L . ~ a ~ t t c ~ ~ i , ~ , r o L t i 1 x i i i d iL , i~c ; h . g t ( ! I C ti of c:trbo{ PCB isjightly!iigher than that 01 cubon BPL. I t was Langmuir, F r e u i m ~ % ~ ~ ;ind I)uhiiiin~ll:idushkevich adsorption equ:itions were well li~ted by the measured ;idsorption data. Tlic villues 01 the parameters of the adsorption equations w c r ~ ' dctcrntincd for the two adsorbents. The physical properties (c.g. microporc v o l u m e ) 01 the adsorbents are consistent with the p:ir:u"ers nht:iincd lrom the ;idsorption results.

Key words: :Idsorption iioihcrm. ~ n c t h y l e n e chloride, granular activ:itcd cxbon. I'

N O I ' ; \ T I O N

\diorption potential (J mol- ' ) :\ilsorbate concentration (mol m-') Adsorbate liquid density (g cm-') 1':ir:tmeter ol Freundlich adsorption isotherm [mol kg- l (m' mol-')''"] Adsorption equilibrium constant of Langmuir isotherm (m' mol-') Molecular weight o f adsorbate (g mol- ' ) .\J.;orption intensity defined by Freundlich isotherm Idimensionless) Ikfractive index of adsorbate liquid (dimensionless) Equilibrium pressure o f adsorbate vapor (Pa) Elcc!ronic polarization of adsorbate (dimensionless) S:'turated vapor pressure of adsorbate (Pa) Adsorption capacity (mol k g - ' ) l\dsorption capacity at equilibrium (mol kg - ' ) Gas Constant (=8.314J mol- ' K-') Temperature (K) ,'Idsorption capacity (cm' g-1) ;\cti.ge pore volume (cmJ g-1)

'i!5!itY coefficient~(dimensionl~ss). . . . .

Structural constant (dimensionless) . _.

. , i.4- Methylene chloride (METH), also known as IL lor0 methane. is a colorless, volatile non-flammable liquid with a penetrating, ether-like odor.' Its vapor denjiry is three times that ol air. The compound i s an sxccllent solvent and i s especially attractive as a cleaning solvent because of its effectiveness and non-flammabilir!. I t has been widely used as a blowing agent o f polyurcthane (PU) flexible foam and as a solvent for many applicarions. including photoresist stripping, aerosol formulations. and to a large extent in paint stripping formulations. I t \vas also formerly used as an extraction solvent in food and pharmaceutical processing where its high volatility is desirable.2 The annual consumption of this solvsnr has exceeded 500000 metric Ions world-wide,' and a\sra$es 8000 metric tons in Taiwan.*

The current American Conference of Governmsnt:il Industrial Hygienists (ACGIH) threshold limir v31lte (TLV) for METH is 50 ppmv as a n 8 h time-aeiphted average (TWA).5 Repeated contact with METH may result in dermatitis. The liquid and vapor of METH are irritating to the eyes, skin, and upper respirator! tract. Symptoms ~ or exposure- include headache:-dizzifl<ss. nausea, giddiness, stupor, irritability, and tingling in the

absorbed to a considerable extent through the ce 1985 the emphasis of METH toxicity has around i ts carcinogenic effects.' The ACGIH

ified it ;is a suspected human carcinogen (Group iuse of the toxicity of METH, i t has been listed f the hazardous air pollutants or air toxics in ' leun Air Act Amendments (CAAA) of 1990, and e toxic and priority pollutants in many environ-: nd workplace regulations.' l i ng to the US Environmental Protection

(US EPA) Toxic Release Inventory (TRI), :missions are the largest source of carinogenic in the atmosphere. During 1990. about 46 142 ns of the compound were released to the ~ i i r . ~ t le 111 of the CAAA, the organic vapors are to be , requiring their emission sources to install i achievable control technologies (MACTs). MACTs for organic vapors are condensation, on. carbon adsorption, and liquid absorption.

technologies, carbon adsorption i s a very one because i t olfers some advantazes over the 'hr ;idv:iiit:i?e? i n c l i i d ~ t l ie pnccihilit?. -nf'.!lic Ui 11iw i i i : i tcria~s ur I I U ~ C products 1ur rcvvc14,2,

tL1

pore volume and surface functional groups. The particle density was measured by a mercury displacement method using a mercury porosimeter (Autopore I 1 9200; Micro- meritic3. Inc., GA, USA) . The true density was determined by a helium displacement method using a pycnometer (AccuPyc 1330; Micromeritics, Inc., GA, USA). There- fore. the particle porosity can be computed from the particle density and true density."

The BET surface area, total pore volume and micropore volume of GACs were determined by nitrogen adsorption/desorption apparatus ASAP 2000 (Micro- meritics, Inc., GA, USA), using a continuous Row method. The BET equation was used to calculate the surface area of GACs. The Kelvin equation and r-plot method were used to calculate the total pore volume and micropore volume. respectively."

ln 'order to obtain the appropriate information of surface functional groups from the Fourier transform infrared (FTIR). spectroscopy analysis using a Bomen DA3.02 system (Bomen Inc.. Quebec, Canada), the spectra were measured with K B r discs containing only : I ~ O [ I I W 1 q 11.1 \VI''' <,llh,. l i i i c c T r r i i i i i < l :iciv:if,,cl I-:irthovc

. . ... itions. and the low fucllenergy C O S ~ S . ' ~ . ~ ~

study, the adsorption isotherms of two dilljcrcnt cirbons and M E T H vapor were investigated. icobility 01 v:irious common ;idsorption iso- ;is tested. I n addition. this paper dcscribcs t l i c imt ion of activated carbons by means of various rs siicli :is BET surlucc area, pore volume. pore ih i t t iw. :iiid surface functional $ r o u p The i i p hctwccn t l ic u I ) x r v c d vii lucs O i t l ic :idso~q)- win parameters and t l i e physical/chemic;il i of activated carbons are also discussed.

2 EXPERIMENTAL

trials

'mt kinds of the commercial granular activated :jACs) made from bituminous coal (BPL) and i e l l (PCB) with a particle size of I 2 x 30 mesh ied by Calgon Carbon Co. (Pittsburgh. USA) in t h i s study. The activated carbons were sieved anges or 20 x 30 (average particle diameter of ), and dried at 393 K for at least I O h before d in the experiments. METH. supplied by odl Co. (Kentucky, USA) was over 99.9% pure ised without any further purification.

ICC characterization measurements

:c characterization-ofcarbons BPL and PCB .ired by using precise instrumentill techniques 1 rt iclc x i i d ' rr uc -dens i tics,- 8 ET-su rface a rcnT '-

The spectra were obtained with 20.18 sc:ins i i t 4 c m - ' rcsolutioii. The FTI R spectroscopy measurements wcre ;iIso uscd to coiilirm the function;il groups un tlie surface o i t l ic ( i , \Cs ohscrvcd by t h e X-ray pliotoclcc!ron spectroscopy ( X 1's) ~ i i c : i surc~ i i c~ i ts . The XI'S experiments

1 p i ' were performed with ii V G ESCALAB-110 ( U K ) system

using :i M g K , X-ray source in ;I v:icuum of IO- 'mbnr. S.implcs w r c prcp:ircC ((11- Y I'S ; i i i : t l y i s I)! yiiidiii: iiiilli_cr:iiii qwi i1 t1t1cs , t i 1111: c : u h m , ;ind t l i c ~ r c;irbuii i d

oxygen contents detcrmined. The carbon I s photoelectron pwk. which \viis uscd ;is ;I reference for the chemical shift. w;is assumed to have ii binding energy (BE) value of ZS1 .6 cV.

I B. : -: \ j '7'

2.3 Adsorption apparatus and methods

The apparatus used for the measurement of adsorption isotherms is shown in Fig. I. The dried (<I":, relative humidity) and purified air was metered by using two rotameters with needle valves. The constant llowrate of gas through a n xt ivated curbon column of 1.5 cm i.d. resulted in a linear velocity of about 0.21 m s - ' . For all experiments. the mass of the activated carbon was about 2-4 g. I t w:is packed in ii carbon bed to a height of 2,s-5.6 cm. The adsorption temperatures, performed at .?S3.293,303. and 3 I3 K, were maintained by a refrigerated circulating constant bath ( k0 . I K ~iccuracy). The experiments were conducted iit inlet concentrations of M E T H vaporsriingingfrorn4.16 x IO-'to8.32 x IO-'mol m-' (i.c 100--2000 ppmv).

Small nmoiints of the etlluent Bits of M E T H i a i r were withdrawn into LI gas-tight micro-syringe and measured b y gas 'L.lirom:ito~rapliy (GC, Hcwlett-Pnckard 5890

. ~~

L

W.-T, Tsai, C.- Y. Chang

WAVENUMBER (cm-') FTI R spectra 01 activated carbons BPL and PCB.

.I, :u1> 111 111c,1 . 4 1000 C l l l ' 1 c ~ l o l i . - 1111* ! c ~ l < , l l

:tcristic of t L c C - ' 0 btrctching vibr:ttions, ;~nd Ibably arises from the phenolic :tnd/or cxboxylic

! 3 comparcs the XPS spectra of carbons BPL I. The C I s spectra have asymmetric pmk shapes long tails at the higher binding energy sides

tin peak. Thrce peaks are obscrvcd. which occur ifO.9-1.7. 2.9-3.1. and 5-0-51 eV from the main lese may be attributed to the C-0 (phcnolicl ), C=O (carbonyl), and C O O H (carboxylic) ilities, respectively."-" I t c:m be seen t l i i i t the igncd to C-0 lor the carbon BPL occurs ;it :I

.e slightly higher than that Tor the carbon PCB.

orption isotherms

rrption of M E T H vapor on GACs ill wrious urcs may be littrd by t l ie IineLir form of the r. Frcundlich. and Dubinin-R:idusli~evicli >therms.'*.'' The isotlierm cqu:ttions ;ire givvn .

!S.

~

-283 K -283 K -303 K -313 K 3-

2.5 - -

0 0 20 40 EO 80 100 120 140 1 6 0

1IC (m3 mob')

Fig. 4. Langmuir plots 01 METH adsorption on carbon BPL at various temperatures.

\bIicrc I / 15 t l ic i u ~ i u u n t adsorbcd pcr unit ni i iss 01 tlie xlsorbcnt. i.c. adsorption capitcity. (/," is thc adsorption c:ipacity at monolayer saturation. C i s the :idsorbate equilibrium concentration. and K is the adsorption equilibrium constant. Figures 4 and 5 present the Langmuir isotherm fits ( I /y vs I iC) of M E T H to the measured adsorption data for carbons BPL and PCB. rcspcctively. The Langmuir isotherm appears to f i t the data reasonably well. A least square method lias been used to give the Langmuir isotherm parameters of qm and K ofcarbons BPL and PCB. The adsorption capacity of carbon PCB is seen to be slightly higher than that of carbon BPL (Table 2). The values of K decrease with increasing temperature, indicatinz that the adsorbents have ii. higher adsorption affinity at lower tempentures.

The Freundlich isotherm was used in t l ie form

In q = Ink + ( [ / t i ) In C (2)

whcre k and II are empirical constants. and q and C are i is prcviously defined. In general, 11 has ii viiluc greater tlian unity, and 1/11 represents the intensity factor or the edsorption; A~liigher--ualue or r! incrcases t l ic~adsorpr ion~~~~--- c:tpacity o l thc adsorbent.'" Figurcs 6 and 7 prcscnt the

tngmuir isothcrni'cin be rcprcscniiil I;?

I/</ = (I/</,,,) + ( I j q , , ,K) ( I j C ) ...-

' . ~ ~

-. -+I ) - . typic~l-Frei~ndIich-plotultt-vnriou~tzm~rtlt~trzs~-A~lmujt~-~

,,-

Adsorptioit of methylene chloride vapor on activated carbons I49

1.5

TABLE 2 Langmuir Par;imeters for Adsorption of M E T H on Activated

Carbons :it Various Temperatures

*283 K +293 K *303 K -313 K -

TABLE 1 Freundlich Paromerers for Adsorption of M E T H on Act ivated

Carbons at Various Temperatures

Adsorhrnr Adsorpr io ,~ L w p i u i r pnrnrnerers r 2

renrpcmrire -~ ( K ) 4 m K

(,,!,>I k, - ‘ j (d mol - ’ ) BPL 283 1.708 83.39 0966

291 2-875 56.67 0.976 303 2.7 I4 38-88 0976 311 2-504 21.63 0.992

PCB 281 4-406 81.08 0.954 291 3.826 58.85 0.940 301 3-920 33.09 0-994 313 1.566 19.44 0.955

1-4.5 -4 -3.5

In C Fig. 6. Frcundlich plots ol rMET1-I ;idsorption on carbon U P L

iit various tcmper:~turcr.

2 * 2 8 3 K + 2 9 3 K + 3 0 3 K * 3 1 3 K

1.5 -

U C -

BPL 283 293 IO1 I13

PCB 283 293 303 313

8-503 8-002 7.175 9-479

11,692 11,948 10.598 10.371

2.5 I5 0.998 2.250 0.991 2. I 34 0.993 1.504 0-999

2.547 0-995 2-137 0-982

1.668 0-970 2-01? 0-997

straight lines were obtained. The Freundlich parameters. k and 17. were computed and iirc given in Table 3. I t i s seen that the value of 11 dccrmses with incrsasing )4iipcr;iturc. The va lues of tlic esponent i t \vers in the

t(lso;&siIucs 10Ckwictrbo11 PCB :ire larscr t1i:in [t iow o l c x b o i I3PL;it tI1c siinic tcmpcraturc. This is conjtitciit with the rcsults hased on tlic I-iingmuir ;innl!sis. i.2. tliiit carbon I’CB liiis slightly higher c:ipiicity iiiid iiivorabilit! for MIITI-l v:ipor t l i i i ~ i c:irhoii DPL.

t\cccircIiiig to t i i t ~ u b i i l i n h h i i y i tlicvr!.’. the :idsorption isothcriii d vapor c;in be rcprcscntcd h! the lollowing D~ R cqu:ition:

.~ . . I ..~. -... ; i , . , I < ; . \ . < : , i , $ ,,,$i,ii;

. -.-

I l l II’ I l l I [ ; , /kI .I i i l 2 1 :I

,I = KTIti(l’J1’) (41

wlicrc 11‘ i s tlie Limount of adsorb;ite ;idsorbed ps: unit mass or carbon, IV, i s the iictive pore volum? c i tlic cubon. ti i s :I constant related to the pore strui!urs. /I

adsorption potential o i the test adsorbate with rh31 o l ii referencc adsorbate, R is the gas constimi. T :I t h e absolute temperature. Po i s the saturation vapor pisi jurc at T, P is the adsorbate equilibrium pressurc. .md .-I 15

detined as adsorption potential. The :itfinit!. cosriiaent /J lor the adsorption o i an adsorbate on ac t i~ i t ed i z rbon may be approximated by the ratio of the clsc:tonic polnrization (P,) of the adsorbate [P, (vapor)] l o ! k i t u i the. reference adsorbate (Le. benzene) [P, irci)] iiv ;?alar organic adsorbates better than by the moliir \oicnic or pariichor mcthods.l” The approxim;ilion gl ie j

is an iilfinity coelticienr which permits comparison d i the L

~~

wlicrc :LI is the molecular weight, (/,.,is the liquid J:tijity. ond 11. i s the relractivs index ol the ;tdsorb;rtc.

~ ~ Figurcs Y and. 9 illustratc tlie typical K ~ - R pii.lj 3s

111 CV vs , I 2 for carbons BPL ;ind I’CU. rcspccti$~‘i!. The -pirdmeiers.ti and LL; in thc D-I< cqii;ition w r c a~b:,iincd

.

W.-T. Tsai, C.- Y. Chanq

50 1W 150 2 W 250 300 350 400 450 !

A2x 10-6 (J mol.1)2

. D-R plot lor METH adsorption on carbon RPL.

~ ~~ ~

0453 I 5.49 x 10." 11,9797 0.5744 5.20 x l o - " t).Y6??

01 METH is 063.

:best fit with the slope -~/[l', and the intercept nploying leal square analysis. The valucs of 1.1: ~r the two activated carbons are summarized in

Several noteworthy feeaturcs m a y bc obtoined : results shown in Table 4. The values of Il,;l of PCB arc larger [hiin those o f~carbon BPL. ng [he fuct that ciirbon PCB cont:iiiis iiiorc re space. This is also consistent with tlie results 11 Tables 2 and 3. I t should be inotcd iIi;it the )I micropore volume given iii Tablc I were d for micropores in tlie range of 1.5-2.0 niii only' of tlie limitatioii of- the : i n ~ ~ I ~ t i ~ n l iiistriiiiieiit.

' e , the mcnsured microporc \wlui i ics listed in rrc-lcss.rhnn thc computcd.vnlucs of IC,, Fitrrhcr.

the structural constant, K, of carbon PCB is slightly less than that of carbon BPL. A relatively small value of K IS a reasonable indication ora high percentage of narrow micropores in the carbon." Thus, the carbon PCB has a larger micropore volume.

4 CONCLUSIONS

The adsorption of methylene chloride, listed as one of [he hazardous air pollutants and priority pollutants, on the two commercial granular activated carbons BPL and PCB were investigated. Also, the adsorption isotherms have been obtained at various temperatures. All these adsorption isotherms obey the Langmuir, Freundlich and Dubinin-Radushkevich isotherm equations. I t was found that the adsorption capacity of carbon PCB i s slightly higher than that of carbon BPL, which is consistent with the measured physical properties (e.g. micropore volume) of the activated carbons and the computed parameters 01 the isotherm equatiods. The results :iIso indicate that IIll.rc ; c G!c,l!!i,::!!,t , ~ F ~ , ~ : r ~ p ~ ~ ~ l ~ m . .. .. . . , I ~ , : ! ~ ~ . , . , . ~ ~ . . . ,I>,. . . . - ,!:irl.TcI1....

t l ic ;idsorption c:tppcitic:S and th:it (21 t ~ i e suriacc functioniil groups of t l i e two g iA f i k i r ;&iv;ttcd carbons examined by FT lR and XPS ;tnalyses in t h i s study.

. .

~ \ C K N O W L E I ) ( ~ E ~ I I N ' I ' S

The authors arc gr;ttclul to !he National Science Council, .Etiwnn for the liti:inci;il suppor t 01 this project (Contr;ict Yc,: N % ' S I - l l ~ ~ ~ - l ~ - l l t l ~ ~ i ~ ~ t l - ~ ~ . ' ~ l ~ ~ 1 ~ ~ 1 1 i i i c : i I ; ~ \ ~ ~ \ I : L I I C C

prov~dcd b! t h e 1';nrticul;itc Technology L;ibor;itory o i the Department of Chemical Engineering. and Professor C. E. L in of the Department of Chemistry of National Taiwan University. is very much appreciated.

F

Adsorption of methylene chloride vapor on activated

7. Agency lor ToxicSubstances and Disease Registry (ATSDR). Toxicological I'rojle (or M e l k y l m c Chloride. Atlanta. USA. 1989.

8. Kokaszk:i. L.C. & Flood. J. W., Ei~oironmeninl Manugemen1 Ifundhook-Toxic Chemical ~Materiols ond Wastes. Marcel Dekker, New York. NY, USA, 1989.

9. Dempsey. C. R.. A comparison of organic emissions from hazardous waste incinerators versus the 1990 toxic release inventory air releascs. J . Air Waste Mailage. :Assuc.. 13 (1993) 1374-9.

IO. Ruddy, E. N. & Carroll. L. A., Select the best VOC control stratcgy. Cliem. Eny. Progress, 89(7) (1993) 28-35.

I I . Spivey, J . J., Recovery of volatile organics lrom small industrial sources. Enuiro,um"iml Proyress. 7 (1988) 3140.

I?.. Lowell, S. & Shields, J . E.. Powder SurJace Area and P o m s i i y . 3rd edn. Chapman nnd Hall, London. UK. 1989.

13. Gregg. S. J . &Sing, K. S. W.. Adsorption. Sur/oce A w n and Purosily 2nd cdn. Academic Press. London. UK, 1982.

14. .Ishizaki. C. & Marti. I., Surface oxide structures on a commercial activated carbon. Cnrbon, 19 (1981) 409-12.

15. O'Reilly. J . M. & Mosher. R. A,, Functional groups in cirbon black by FTIR spectroscopy. Curbon. 21 (1983) 47-51.

16. Richiirdson. 1. T., Principles o/ Cnralgst Deuelop,neni. Plenum Press. New York. NY, USA. 1989.

17. I:ricdcl. R. A. & Hofer. L. J . E., Spectral characterization ,,r :Iciiv:trccI c:lrhon. .I IV,,,V~ ri,rr,l.. 74 (1970) x i 2.

I S I L ICI ICI . 1:. ,\. Lk ~~,,,i><,,~, ti. L.. UIIIcrclll cL,b",L,Lc<>,,> ~ii;itcri:~Is :~nd Ihcir infr:i-rcd :ind Raman spectr:i: rcarsign- imcn1s for c w l spcctr:~. Fm,l. 51 (1972) 194-8.

IO Mcldrum. I1.J. Sr Rochcstcr. C. 1-1.. In situ infrared study o f ihc surliice oxidation of :!ctivnlcd carbon in oxygen : i d

S(,I i

carbons 151

20. Colthup, N. B. & Daly. L. H., Inrroducrion io Infrored and R U I I I U I ~ Speclroscopy. 3rd cdn. Academic Press, San Diego. CA, USA. 1990.

21. Ishitani. A.. Aoolication of X-rav ohotoelectron ':oec!:aicoov I . .,

to surhce anhlysis o l carbon fiber. Curbor!: 19 11951) 269-75.

22. Takahagi. T. & Ishitani. A., XPS studies by use olihe di-tal diKerence spectrum technique of functional groups on the surFacc of carbon fiber. Curbon. 22 (1984) 43-6.

23. Bradley. P. H., Ling. X. & Sutherland, 1.. An invesii:srion of carbon fiber surface chemistry and reactivity hzsd on XPS and surface free energy. Curbon, 31 11993) I I IS-20.

24. Ruthven. D. M.. Principles OJ Adsorpiion nnd Adsorpiion Procuss. John Wiley. New York, NY. USA, 1984.

25. Suzuki. M.. Adsorpiion Enyineeriny. Elsevier. Amste rd~m. The Netherlands. 1990.

26. Aho-Elela. S. I. & El-Dib. M. A,, Color remom \ia edsorplion wood shaving. Sci. Toral Enuiron., 66 11937) 269-73.

27. Treybal. R. E.. Muss TronsJer Opcmlioris. 2nd cdn. McGraw-Hill. New York. NY. USA, 1980.

28. Dubinin. M. M.. Porous structure and xdsorpiion proplies of :ictivc carbons. In Cliemislry and Pbysics o/ Carhon. sd P. N. Walker Marcel De.k&, New York. NY. USA. 1966

20 R c w o f i . P I . Simosc -.W I T . ,p. .Inn:ir. I.. ,I.. Wr?':,m ~ i ~ q x ~ t u . ICII I . I IC~ I J . P ~ J ~ . ~ i ~ m . . 71. \ . , , - - I

30. Troiit. I).. Ilreyssc. 1'. N., 1-la11. -I.,'Grn. M. S Ri,b!. T.. I le~erniin:it ion of organic vapor respiratory carrrid:e v:iri:ihili!y in terms of degree of ;ictiv;ition of Ihc w b u n :ind c:irtridgc packing density. Am I r d 1 1 r q :IJSIIC.. J . . 41 1 I ~ ~ X O I 4'1.1 0.

-- 3 5 2 0 ~3 I . ?-'" . .. ,I,

Chapter I11

VOC Separation Systems for Gaseous Wastes

Similar to the recovery of aqueous VOC wastes, several technologies exist that can be used for recovering VOC's fiom gaseous wastes. The most widely used technologies for recovering VOC's from gaseous wastes are liquid absorption using heavy oils/hydrocarbons, activated carbon adsorption, and condensation using coolants/refrigerants. In addition, recently developed membrane-based technologies can be used in conjuction with one of the above technologies to improve system efficiency and/or overall operating. The following is a summary of these technologies, a description of the technology, and some advantages and disadvantages of these technologies, Additional information and modeling equations of these mass transfer operations can be found in literature such as McCabe et al. (1993), Perry and Green (1984), Geankoplis (1983), Henley and Seader (1981), King (1980), and Treybal(l980). Destructive technologies, such as biofltration, catalytic oxidation, and thermal oxidation, are not reviewed since this work focuses solely on recovery technologies.

VOC Recovery Technologies for Gaseous Wastes

0 VOC gaseous emissions flow into the bottom of a packed or tray column and is distributed throughout the absorption column.

0 A heavy oilhydrocarbon flows into the top of the column and the VOC is transferred (an amount based on solubility levels) from the ga: to the oil via direct contact, thus, the airstream is "scrubbed."

0 The VOC/oil mixture exits the bottom of the column and is subsequently distilled to allow separation of the VOC and the oil.

0 "VOC-fiee" gas exits the top of the column.

Advantages -1 :i

' 1 ' i ' 1

! :i P P ~ ) .

.I

0 Can achieve high recovery efficiencies (95-98%). 0 Can be used for a wide range of gas flow rates (2,000-100,000 cfm). 0 Can handle a wide range of inlet VOC concentrations (500-5,000

0 Good for high humidity (>50% r.h.) air streams.

:I Disadvantages

0 May result in the generation of a wastewater stream. 0 May result in column packing plugging or fouling ifparticulates are

:I 1 1~ ~' 0 Some of the liquid absorbent may be transferred to the exit gas present in the gaseous waste stream.

stream, thus creating a new pollution concern. . .....

F

02/17/97 18:24:55

Keturn to 'l'able o t Contents

_____- - .................... _ ............ - ....... _..._..__......-.-I .......... ................ .................. ..... .- ............ - . ..........

Figure 3.1

A Schematic Representation of an Absorption Process for VOC Gaseous Wastes

Clean A r to Atmosphere or Carbon Adsorber

Liquid Absorbent (Oil)

VOC Gaseous W

Liquid Absorption Column

Packed or Tray Tower

VOC and Absorbent to Distillation

0 VOC gaseous emissions flow into the top or bottom of an adsorption

the carbon bed.

advantages, and disadvantages listed in this section correspond to the temperature-swing adsorption process.

.. r . . . . . . . . . . . .

02/17/97 182S:OS

VUL ana loses its capacity ror aaaitioniu aasorption ims resuits in me concept of "breakthrough" where significant quantities of VOC become apparent in the gas stream exiting the adsorption process When this occurs the carbon must be regenerated for re-use or replaced with virgin carbon

0 Multiple fixed beds are generally employed so that as one or more beds are adsorbing at least one bed can be regenerating Regenerating a bed of activated carbon typically involves the direct injection of steam, hot nitrogei or hot air to the bed which causes the VOC to release &om the carbon and exit the bed via a vapor or condensate stream The regenerated stream, containing a higher concentration of the VOC than the original wastewater stream, is subsequently condensed Ifthe VOC is immiscible in water, the condensate will form an aqueous layer and a solvent layer that can be separated using a decanter If the VOC is miscible in water, additional distillation can be used to hrther separate the VOC and water

0 "VOC-&ee" gas exits the adsorber after contacting the activated carbon

Advantages

0 A widely used technology with well established performance levels 0 Can achieve high recovery efficiencies (90-98%). 0 Can be used for a wide range of gas flow rates (100-60,000 cfm) 0 Can handle a wide range of inlet VOC concentrations (20-5,000 ppm) 0 Can efficiently handle fluctuations in gas flow rates and VOC

concentrations.

Disadvantages

0 VOC's having high heats of adsorption (typically ketones) can cause carbon bed fires.

0 Carbon attrition properties (permanent bonding of small quantities of VOC through each adsorption cycle) requires the periodic replacement of carbon witb virgin or reactivated carbon. Spent carbon may need to be disposed of as a hazardous waste depending on the VOC(s) adsorbed

0 Carbon efficiency decreases for high humidity 050% r h 1 air streams

Return to Table of Contents

___ ~ ---- ~~~~.~~~~ ............ . . ~ . ~ ~ ~ ~ ~ ~~~~~~ ~ ~ . . ~ ~ ~ . . . ~ ........................... ~~~~ ~ . . ~ ~ ~ ~ ~ . ~ ~ ~ ~ ~ ~ ~ . ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ . ~ ~ ~ ~ ~ . ~ ~ ~ ~ ~ ~ ~ . . ~ . ~ ~ ~ ~ ~ ~ ..... ~ .................... ~ ~ . . . ..........,.... ~ . . ~ . ..... ~. ......... ~ ..... ..... .. . ..... . .. .....

- Figure 3.2

A Schematic Representation of a Carbon Adsorption Process for VOC Gaseous Wastes

02/17/97 18:25:05

VOC Gase Activated Carbon Adsorbers

Adsorption Mode

Condenser To Atmosphere or Carbon Adsorber

Recovered V O C RecycledorSent to Distillation Steam orHo Wata Efluent

Decanter DischaFgedor Sent to Air or Steam Skipix Regmeratian Mode

L

02/17/97 18:25:12

Volume 16. Issue 4 327

A REVIEW OF MODELS DEVELOPED TO PREDICT GASEOUS PHASE ACTIVATED CARBON ADSORPTION OF ORGANIC COMPOUNDS

Authors: Martin D. Werner School of Public Health The University of Texas Health Science

San Antonio, Texas

Nancy L. Winters Department of Wastewater Management Monitoring and Testing Division San Antonio, Texas

Center

Referee: Emesl S. Moysr Laboratory Invcrtigstions Section I n j q Rcvcnlion Rewareh Branch Division of Safely Racareh National lnriiiuic for Occupalional Safely and Heallh Center for D i ~ m Conuol public Health Service Depanmcni of Health & Human Services Morganlown. Wcsl Virginia

SYMBOLS

= Reversible isothermal work of compression = Cross-sectional area of adsorbent bed = Moles of adsorbate adsorbed = Partial molar surface area of adsorbate i = Primary adsorption centers for water = Adsorption value for water at a given relative pressure = Parameter whose value depends on the pore size distribution of the activated

= Henry's law constant of adsorbate i = Constant related to the heat of adsorption = Selected adsorbate breakthrough concentration = Influent adsorbate concentrations = Diameter of adsorbate particle = Effective coefficient of internal pore diffusion of adsorbate = Diffusion coefficient of adsorbate vapor = Characteristic adsorption energy = The change in the free energy of the system = Fugacity of the ith component in the gas phase = Fugacity that ith component would exert as 'a pure adsorbate at the total ad-

= Saturated fugacity of pure unadsorbed liquid at adsorption temperature = Weight of adsorbent = Mass velocity of air mixture through column = Fractional relative pressure = ' Critical bed depth

carbon

sorbate volume of a mixture

,' , ~ ,.

. . .

. : .'. i ; . , ; . ; . : . . . . . . .... . ... ,. . .

= Adsorption rate constant = Length of adsorbed bed = Molecular weight = Refractive index = Moles of component adsorbed = Number of moles of component i in the surface phase = Maximum number of moles of component i in the surface phase = Number of components = Partial pressure of the adsorbates in the test air stream = Equilibrium vapor pressure of adsorbate vapor at a given temperature = Vapor pressure of adsorbate in the compressed state (or adsorbed state) at a

= Saturated vapor pressure of the ith component at the temperature of the ad-

= Saturated vapor pressure of the liquid adsorbate = Total atmospheric pressure = Volumetric flow rate = Universal gas constant = Selectivity Coefficient (i.e., selective adsorption of component 1 relative to I

= Temperature (Kelvin) = Time to adsorbate breakthrough = 50% Breakthrough time = Molar volume of component i = Carrier gas linear velocity = Molar volume of the ith component as a saturated liquid at its boiling poin = Molar volume of water = Volume of organic adsorbate adsorbed = Volume of adsorbate necessary to cover the entire surface of an adsorbent

= Maximum adsorption space in micropores of activated carbon = Mole fraction of component i in the adsorbed phase = Mole fraction of component i in the vapor phase = Polarizibility of organic compound = Adsorption rate Coefficient for water vapor = Desorption rate coefficient for water vapor = Affinity coefficient of organic adsorbate = Equilibrium adsorption potential = Ratio of intemal to external porosity = Corrected adsorption potential of component i considering interference by

given temperature

sorption isotherm

component 2 in a binary mixture)

form a monomolecular layer

water vapor = Surface tension = Activity coefficient of component i in adsorbed-phase vacancy solution '~., = Wilson's interactive parameters

= Viscosity of air mixture = Chemical potential of component i = Spreading pressure = Fugacity coefficient of component i in bulk gas mixture = Argument of the normal probability distribution curve corresponding to a

lain breakthrough time (1,)

Volume 16, Issue 4 329

= Density of air mixture = Bulk density of carbon = Standard deviation of breakthrough curve = Relative standard deviation of breakthrough curve = Fractional coverage of adsorbent surface = Parachlors of organic compound

I. INTRODUCTION

Activated carbon adsorption is an important control process for treating air contaminated by organic compounds. Like other engineering processes, rational methods are essential for reliable and economically effective design and operation of a gaseous phase adsorption process. Current methods and recent development concerning successful design and operation

dispersion of the contaminant breakthrough curve, adsorption of organic mixtures, and

ature of the ad-

:nt 1 relative to

11. PREDICTION OF ADSORPTION CAPACITY

1 A. Isotherm Equations Several equations have been used to describe the adsorption of organic contaminmts on

microporous adsorbents, such as activated carbon. Among these are the Freundlich, Lang-

Although the Freundlich Isotherm has been shown to fit experimental data, it bas very limited predictive ability.'.' It is an empirical equation whose form is simply desigfled to fit the parabolic shape of a typical adsorption isotherm. Equation 1 is the simplest form of [he Freundlich Isotherm.

:

where W = volume of organic adsorbate adsorbed, P = partial pressure of the adsorbents in the test air stream, and K,,f = coefficients of the equation (0 < f < I ) . The coefficients K, and fare unique for each adsorbent-adsorbate pair and operating conditions of adsorption. Because this is an empirical equation, the values of K, and f cannot be predicted for untested conditions (e.g., other temperatures) or untested adsorbent-adsorbate pairs. Tests must be

Unlike the Freundlich Isotherm, the Langmuir Isotherm (Equation 2) was theoretically

W = Wm K,P/(I + K,P)

where Wm = the volume of adsorbate necessary to cover the entire surface of an adsorbent to form a monomolecular layer and K, = coefficient of the Langmuir Isotherm; the other parameters have been defined previously.

Assumptions of this equation include ( I ) adsorption occurs at definite localized sites, (2)

330 CRC Critical Reviews in Environmental Control

energetically homogeneous adsorbent surface, and (4) no interaction between individu adsorbed molecules. These assumptions are not met during the adsorption of organic mol; ecules on activated carbon or other microporous adsorbents.' Therefore, even if vapor adsorption data appear to fit the Langmuir Isotherm, one cannot reliably use it to p adsorption of other substances or adsorption under different conditions. \

The BET Equation has limitations similar to the Langmuir Isotherm for predicting va phase adsorption. Many of the assumptions of the Langmuir Isotherm are included in BET Equation (Equation 3); however, adsorption is assumed to be multimolecular on t surface of the adsorbent.'

W/Wm = C.x/(l - x)(l - x + COX)

where C, = a constant related to the heat of adsorption, x = relative pressure of adsorb (P/P,), and P, = the saturated vapor pressure of the liquid adsorbate.

Like the Langmuir Isotherm, the BET Equation does not accurately predict vapor ph adsorption due to the inappropriate assumptions.' However, the BET Equation is useful making surface area determinations of activated carbon.

B. Potential Theory of Adsorption I . Theoretical Background

The Potential Theory of A d s o r p t i ~ n ' ~ ~ ~ ~ or the Theory of Volume F i l l i ~ ~ g ~ - ~ is speci applicable to the adsorption of organic vapors on microporous adsorbents (e.g., ac carbon). The theoty is based on rational physical and chemical concepts rather than empirical fit of experimental data. Since the theory is rational, parameters of the adso and adsorbate which affect the adsorption process can be identified. By determining acc values for these parameters, the theory can be used to predict the adsorption of untes organic adsorbates on an activated carbon which has been previously characterized.'O process of characterizing an activated carbon is summarized later.)

Because the Potential Theory of Adsorption has become central to predictive effort vapor phase adsorption, its theoretical basis will be reviewed here in detail. Initially, the adsorption of a single organic adsorbate will be considered. More complicated a cations of the theory, including adsorption of vapor mixtures and vapor, will be considered in subsequent sections of this review.

The idea of an adsorption potential involving the adsorbent and which is central to this theory.',"J2 Adsorption potential is defined as forces which transfer an organic (adsorbate) molecule from the vapor phase to a co state within the pores of the carbon. The potential can be conceptualized as a field exten outward from the surface of the carbon and decreasing in force with increasing dist from the surface. Manes'= and Manes and Greenbank" described the analogy betwe adsorption potential and a gravitational force field. Mathematically, the adsorption PO

can be expressed in terms of thermodynamic q u a n t i t i e ~ ' ~ ~ ~ ~ ~ ' ~ ~ ' ~ as in Equation 4.

E = A -AF = RT In(P,/P,)

where E = equilibrium adsorption potential, A = reversible isothermal work of compre AF = the change in the free energy of the system, R = universal gas constant, temperature (Kelvin), P, = equilibrium vapor pressure of the adsorbate vapor at a temperature, and P, = vapor pressure of adsorbate in the compressed state (or adso state) at a given temperature. Thus, the adsorption potential is actually a change in the energy of a system, which results from the conversion of an organic from a vapor to a li state. The conversion is a result of compressional forces within the micropores of th

xbate is a con


FIGURE I. homogeneous surface.

Adsorption potential field above the surface of an adsorbent for the idtalized case of a plane

When the compressional forces within the micropores exceed the adsorption potential (E), adsorption of the involved organic vapor will occur."

The concept of the adsorption potential field is illustrated in Figure I. The E , represents the adsorption potential (with E* > E , > E,,). W, is the volume of space in the carbon pores in which the potential compressional forces are greater than or equal to the corresponding e , value. For example, E 2 within the adsorption space represented by W,. At E,,, the adsorption potential reaches zero. Therefore, W, is the limiting adsorption volume of the microporous carbon. At the surface of the carbon, W = 0, and the adsorption potential is at its maximum value (emax).

Several assumptions underlie the Potential Theory of including

I .

2 .

The magnitude of the adsorption potential at a given point above the carbon surface is independent of temperature, (i.e., [a~,laT], = 0). The adsorption potential at any point is independent of the compressed adsorbate between it and the carbon surface.

3. Interaetions between adsorbed molecules are the same as those between unadsorbed molecules.

4. 5.

6.

The vapor phase behaves in an ideal manner. The adsorbed phase has reached its maximum compression and cannot be compressed to a greater extent. Below the critical temperature of the adsorbate, the density of the adsorbed phase equals the density of the liquid adsorbate at the same temperature.

Considering assumptions 5 and 6 above, Equation 4 can be rewritten as follows: ,

E = RT ln(P,/P) ( 5 )

Assumptions I through 3 characterize the dispersional component of van der Waals' ad- sorptional forces. Therefore, during the adsorption process, when dispersional forces are decisive and electrostatic interactions are minimal, the Potential Theory of Adsorption is applicable, 1.6.7.9.IS-I9 These conditions are valid for a wide variety of activated carbon types and organic adsorbates.

Equation 6 results from assumptions 5 and 6 .

where W = volume of adsorbate adsorbed, a = moles of adsorbate adsorbed. and V = _. molar volume of adsorbate. Thus, the amount of organic adsorbed can be expressed as a volume, in moles, or as a mass.

332 CRC Critical Reviews in Environmental Control 3 .

!

..

.“~.

Dubinin and associates‘.6~q have advanced the Polanyi Potential Theory of Adsorptiodi. il number of ways which facilitate its use for practical vapor phase adsorption applications.

sorption space for one vapor (A) times a constant is equal to the adsorption potenti First, they noted that the adsorption potential (E) corresponding lo a given volume of ad-

corresponding to that same volume for a second vapor (B).

(7 (E,/E”)W = (%/~”)a“ = P

where = the affinity coefficient, or

(E*)W = P ( 4 W

The value of p relating these two adsorbates can be considered to be constant for filled (W) ranging from 10 to 94% of the total microporous volume of the carbon ( The value of p is apparently independent of the nature of the microporous bent.6~7.‘4~‘0~2’ Thus, given the relationship between W and E for an organic vapor and p value which relates that organic compound to another compound, the relationshi W and E can be calculated for the second compound over a wide range of volu Plots of W vs. E are termed characteristic curves of the ads~rbent . ’ .~~.” . ’~.“ The cons relationship of the characteristic curves between different organic vapors is due t “affine” nature of the organic adsorbates.‘ Knowing the characteristic curve for an and the P values for adsorbates permits one to calculate isotherms by using Equations 6. Independent methods for calculating P values for pairs of organic compounds w presented later.

A second major advancement made by Dubinin6.’ was to define a general mathemati form of the characteristic equation which is applicable to many microporous carbon ads bents. The general form (Equation 9) is based on both theoretical considerations and empiri results .6.7

W = W, exp( -K, e2)

where W, = maximum adsorption space in micropores of activated carbon and K, coefficient of the equation.

By combining Equations 5, 6, 8, and 9:

a = WdV[exp( - B/P’[RT In(Ps/P)j2)]

where B = a parameter whose value depends on the pore size distribution of the activat carbon.

Equation 10 is commonly referred to as the Dubinin-Radushkevich Equation. The lin form of Equation IO is

In(a) = ln(WdV) - B/P’[(RT In[P,IP1)21

or

In(W) = In(W,,) - B/P*[(RT In[P,/PJ)’I

By plotting In(W) against [RT In(Ps/P)]2 for several values of P of a reference adsorba W, (y intercept) and B/p* can be determined. W, is a function only of the adsorbent. value can be considered to be a constant for a given adsorbent once the value is relia

: of volume filled.

mrs is due to the

ig Equations 5 ana ompounds will be

ieral mathematical rous carbon adsor- lions and empirical

:arbon and K, =

In of the activate

the adsorbent.


determined by one adsorption trial. Additionally, the value of B can be determined from the slope if the value of P is known. The value of B should also depend only on the characteristics of the activated carbon. Once the values of W, and B are determined for an activated carbon, that carbon is characterized relative to the requirements of the Dubinin- Radushkevich Equation. These values of W, and B can be used to predict adsorption of other organic vapors on that activated carbon under varying ambient conditions.

In summary, an activated carbon can, theoretically, be characterized after a single adsorption trial using a representative adsorbate. Knowing values for W,, and B and the P which relates the compound of interest to a reference compound, one can calculate an adsorption isotherm for an untested adsorbate by using Equations 10, I I , or 12.

Limitations of the Dubinin-Radushkevich Equation will be discussed later.

2. Parameter Estimation

tigated reference compound is defined in Equation 13. The "affinity coefficient" relating an untested organic compound to a previously inves-

Knowing the isotherm for the reference compound and the value of P relating the reference compound to an untested organic compound, one can calculate points along the isotherm for the untested compound using Equation 12. Several methods have been suggested for estimating the value of p based on physicochemical propenies of the untested and reference adsorbates. Polarizability of the individual compounds should be important in determining the value of P (Equation 14) since dispersional forces play a dominant role in determining adsorption interactions.6

where a = polarizability of the respective compounds. According to Dubinin, the ratio of molar volumes can also be used as a first approximation

of p.

Errors between predicted and experimental values of varied from 3 to 26% (mean = 16.6%) when using Equation 15 for eight organic compounds.' The ratio of parachlors of the organic compounds resulted in closer estimates of the P values.

where fl = parachlors2' = yl"M/p, y = surface tension, M = molecular weight, and p = density of the liquid compound.

The error between predicted and actual values using Equation 16 was between 0 and 6% (mean = 2.6%) for 1 I organic compounds (many were the same compounds for which Equation 15 was tested).'.9

argued that by using electronic polarization (Pe) more accurate values of P could be predicted (Equation 17).

Reucroft et

where Pe = [(n' - I)/(+ + 2)] (M/p), n = refractive index of the organic compound measured at the sodium D wavelength. and M = molecular weight of organic compound.

334 CRC Critical Revieiss in Environvzental Corirrol

Funhermore, when compounds with polarities similar to test compounds were used t calculate @, more accurate estimates were made. Thus, grouping test and reference corn- pounds by polarity before calculating @ is advantageous. Reucroft et aL2' demonstrated better p estimates by grouping compounds by polarity for 15 compounds using electronic polarization, compared to predicted values obtained using the ratio of parachlors. Other investigator^'^^'^-'^ also implied that compounds having characteristics in common with the. test compound should be used to calculate values of p by any of the above techniques. Foi example, p values calculated for a homologous series of saturated hydrocarbons were cloS& to experimental values, but for a dissimilar series the calculated p value was in error f r o 2 10 to 20%.'4.'5.26 Although the use of electronic polarization to calculate p values has bee;' strongly recommended by Jonas and co-workers,"-". 2g-30 other investigators continue to use' the ratio of molar volumes or parachlors with apparently good results.

3. Characrerizarion of Activated Carbon Urano et al." have attempted to estimate values of the parameters which characte

activated carbon (e.g., W, and B) without experimental testing. These authors investiga e the adsorption of 13 organic compounds on 7 commercial carbons. For all the carbons tested,'' the value of B was (2.8 2 0.2) x mo12/J'. The suggestion was made that this value< of B could be used for many commercial microporous carbons manufactured for vapor ph

Additionally, an empirical relationship was established to estimate the value of W,."

W, = V,,, + 0.055

1

adsorption. I ... '3

!

where V,., = cumulative volume of all micropores with diameter S 3.2 nm. This relationship was accurate for the seven carbons tested. However, Equation 18 assn

lhat the microporous activated carbon being characterized was manufactured for vapor phg adsorption of organic compounds."

3 Thus, techniques exist of estimating values for the three parameters, B, W,, and p, which; characterize the adsorbent-adsorbate system of the Dubinin-Radushkevich. Equation. Using; these techniques, Urano et al." predicted the adsorption capacities of the I activated carbpi for 13 organic adsorbates within 6% of values obtained experimentally. The same technique were used to predict the adsorption capacity of trichloroethylene on a microporous activate carbon.' At six trichloroethylene influent concentrations (ranging from 0.S to 4 orders:( magnitude below concentrations used by Urano et al.'O), predicted adsorption values 8 to 25% higher than experimental v a l ~ e s . ~ Suggested techniques for estimating the p- eters of Equation 12 appear to be well founded. However, additional comparisons betwee predicted and actual values under various operating conditions are required to asses reliability of these techniques.

4 . Limitations of the Dubinin-Radushkevich Equation Dubinin' suggested that Equations I O through 12, which are based on the Polanyi Pot?@

Theory of Adsorption, are not appropriate if the volume of adsorption pore space fi1led.e is below 0. I to 0.2 W,,' or above 0.94 W,.',In general, this represents a range of adso@$ relative pressures (Le., P/P,) of For example, Equation 12 is apPare!$ relevant for benzene adsorption within a range of relative pressures from IO-' t0:10.$ This range includes most concentrations which would involve applications of activated C* for environmental air pollution control. At very high relative pressures, condensation Of$ organic vapor occurs within the transitional pores of the carbon, and W can exceed the of W,.' (Transitional pores connect micropores with macropores. Unlike micropore sorptional forces from opposite sides of a transitional pore do not overlap. Thus, adsorpt?&

to

.c

i: 8

[. . i" f;: .. . . '


test compounds were used Jping test and reference c o ~ ~ ; i i ~ ~ ~ :eucroft et al." demonstratpd.~i;.;. 5 compounds using electron :he ratio of parachlors. 0 cteristics in common with t of the above techniques.: rated hydrocarbons ted p value was in to calculate p values has r investigators coni d results.

arameters which charac .g. These authors investi ms. For all the carb tion was made that manufactured for vapor

:stimate the value o

ieter < 3.2 nm.

meters, B, W,, and p, whic

:ies of the I activated car entally. The same t e c h '

:ing from 0.5 to 4 orders

ies for estimating the p itional comparisons bey IS are required to assess

3, Equation 12 is app :ssures from~l0-S to Aications of activated essures, condensati and W can exceed

The thermodynamic models discussed in the previous section have the potential tO accurately predict the adsorption capacity of the carbon after the carbon has been characterized

forces are less in a transitional pore than in a micropore.) At these high adsorbate pressures. the Dubinin-Radushkevich Equation is not linear.'." However, this nonlinear portion of the isotherm does not invalidate the use of Equations IO, 1 I , and 12 for predictive purposes I.: lien lower relative pressures are involved.

Properties of the adsorbate molecule must also be considered when predicting p values and when using Equations 10 through 12 to predict adsorption capacities. Properties of the organic molecule which may be important include: solubility,"."." steric conf ig~ra t ion ,~ , '~ size,' polarity," and the nature of the functional groups. The value of p may not account for these properties unless the reference compound is similar in critical aspects to the test compound as previously stated.2'.25.26 For large molecules, the carbon pores may not be large enough to transmit or contain the molecule. Thus, the carbon pores may not adsorb some molecules regardless of.the apparent affinity of the carbon for the molecule. According !!) Dubinin,' molecular size should not be a limiting factor for adsorption of compounds ivith a molecular weight below 150,g/g/mol. Properties of the organic molecule are potentially important considerations when using Equation 14 through 17 to estimate values of p.

5 . Structure of Activated Carbon The structure of activated carbon is determined by the method of its manufacture and the

nature of its parent material.3s As previously stated, two parameters, W, and B, of the Dubinin-Radushkevich Equation define the activated carbon for use as a microporous adsorbent. "." W IS the volume of the microporous space, and B is a measure of the pore -ize distribution of the carbon. The value of B decreases as the frequency of small pores increases. In general, the smaller the value of B, the steeper the rise in the initial portion of the adsorption isotherm. A sharp initial rise in the isothenn is advantageous because it demonstrates the adsorption efficiency of the carbon even at the lower influent concentrations of the organic conta~ninant. ' .~~

As the activation process for carbon is intensified, in terms of heat applied and duration of time, a greater amount of carbon is "burned out", which results in increased values for W, and B.".'2.'4 Based on common processes by which activated carbon is manufactured and experimental evidence (such as molecular sieving properties of carbon), most carbons produced for vapor phase adsorption have a narrow range of microporous sizes.'"'b This characteristic permits Equation 12 to be used for describing and predicting the capacity of

general equation for predicting vapor phase adsorption on other types of microporous adsorbents. For example, when the adsorbent has bimodal rather than unimodal distribution of pore sizes, Equation 19 can be used.

~

.' -

-

the carbon for organic vapor during adsorption.'6''"o D~binin'~.'' also presented a more . ~~

L

W = W,, exp - (dph,)"' + W,, exp - (E/PEJ (1%

~ where E,; = characteristic adsorption energy in micropores, E,,! = K,B,,-In, K, = constant for similar types of activated carbon, m = variable exponent (m = 2 if range of microprous sizes within a size distribution is narrow), and E = RT In(P,/P).

Notice that this Equation reduces to Equation 12 for activated carbons with unimodal distribution of micropore sizes and narrow ranges of sizes within that d i~ t r ibu t ion . " .~~~ ' Suzuki and Sakodd2 also considered equations which would describe adsorption on adsorbent with a wide distribution of micropore sizes. Evaluation of the predictive accuracy of equations for the more unusual types of activated carbon have not been located in the literature.

~

-111. KINETICMODELS

and the adsorption pattem of a reference vapor is known. As described, the Dubini Radushkevich Isotherm Equation is the major predictive equation developed for that purpos For many applications of activated carbon vapor phase adsorption, knowledge of the shap of the contaminant breakthrough curve is also desirable. The time to breakthrough of a ;i critical concentration of adsorbate is often the criterion used to determine the time at which :2 the carbon unit should be taken out o f service. The “critical concentration” may be based on effluent standards, levels at which adverse health effects may result, or similar pollu ‘ control objectives.

Several groups of researcher^^.^^.^'.^'-^* have developed and tested kinetic models to pre the time to breakthrough and the shape of a contaminant breakthrough curve under variou$ operating conditions. These kinetic models require either some experimental data on $ adsorbent-adsorbate pair or extensive characterization of the adsorbent, adsorbate, and ani:’ bient conditions. Confusion remains as to the best predictive approach to be taken. Seve current approaches wil l be presented in this review. Data required for use of the vario models and reliability of the method wi l l be discussed as the models are presented.

I

i,.

and Burgess4* have modified the Theory of Statistical Moments to predict poing along an entire contaminant breakthrough curve for an activated carbon system. The theory accounts for the dependence of the breakthrough curve shape on critical factors of adsorption such as: concentration of the contaminant, velocity of the carrier gas, size of the carbon 3; panicle, and length of the adsorption bed. While adapting the Theory of Statistical Moments..? >+el to carbon adsomtion. several simnlifvine assumntions were fA comolete version :c,

qi;, * i$

$ : 2 I

4

:$ . ’ j

The Theory of Statistical Moments applied to carbon adsorption is based on the premise;g that the velocity of an adsorbate molecule through a carbon column is determined by randomly distributed phenomena. The velocity o f the molecule is influenced not only by adsorption3 and desorption steps but also by other factors. For example, heterogeneous gas flow velocities~ and diffusional pattems within the column influence the velocity of the molecule. The$ in tum, result from the distribution of carbon particle sizes and of various surface groups: on the carbon. (Note, carrier gas flow velocity does not equal adsorbate molecule flow, through the column. The adsorbate molecule is slowed by adsorption - desorption ’.’

readsorption, etc.) Since the distribution on molecular velocities results from random p nomena, one would expect that a Gaussian probability curve could be used to describe th... distribution (see Figure 2). Applied to the adsorption problem, Figure 2 demonstrates the frequency distribution of individual adsorbate velocities. The greatest frequency occurs. ai the mean velocity; the mean velocity corresponds to the time at which 50% of the influe adsorbate concentration exits the column. If diffusion and adsorption within the colum were instantaneous, the velocity o f al l molecules through the column would be the me

.

. . .. velocity, and complete contaminant breakthrough would occur at one time. .$ When the values of Figure 2 are converted to a cumulative probability plot, the relationship>

shown in Figure 3 results. This plot resembles a typical contaminant breakthrough curve fOt

a microporous adsorbent. The resemblance i s expected, assuming those factors causing the velocities of individual adsorbate molecules are normally distributed with regard to intensityy :i . ,

and influent adsorbate concentration (C,).‘* The relationship between these parameters is, given by: ly

i


Adsorbat8 Velocity

Frequency of adsorbate molecules velmiiies assuming a Gaussian distribution. F IGURE 2.

FIGURE 3. molecde~ are described by n Gaussian distribution.

Typical Conlaniinant breakthrough curve when the frequency of the wlmities o f the adsorbate

,

338 CRC Crilical Reviews in Environtnenml Conrrol

Therefore, for a given influent concentration and carrier gas velocity, if t,, or W is known, the other one can be calculated regardless of the dispersion of the curve. Additionally, with the same assumptions, i f only two points are known on the breakthrough curve, then t,, W, and the entire breakthrough curve can be calcu'lated..''The procedure for these calculations ~

can be demonstrated most clearly by an example similar to that presented by Grubner and" Burges~ . '~ Assume the times to 5% (1,) and 40% (I4,) breakthrough of contaminants a and 11.5 hr, respectively. From the tabulation of a noma1 probability distribution c the argument for values of 0.05 and 0.40 correspond to - 1.65 and -0.25, respective1 The standard deviation of the breakthrough curve can be calculated using Equation 21.

,::2 where (PlIb, = argument of the normal probability distribution curve corresponding to .i3 certain breakthrough time (tb) and u = standard deviation of the breakthrough curve. , ..:?

Substituting the example values into Equation 21 results in a value for u of I .07. Knowin;: u and the breakthrough time for any effluent concentration less than C,, one can calculate: the breakthrough time of any other concentration (including t,,, which in this case is 11.8" hr). Thus, the entire breakthrough curve can be calculated by knowing only two points on'. it. Equation 22 is derived from Equation 21 and can be used to calculate any breakthrough3

G.1 .,<,

time after t,, is calculated. ,.>.

!+ Note, the value of u can be calculated without the use of a normal probability table by thei equation u = t,, - t,, since +50 and +,6 are 0 and - I , respectively. Grubner and Burgess??: introduced another parameter called the relative standard deviation of the breakthrough cuNe?

1; I - ' .

(uJ given by Equations 23 or 24.48 ,,

&3 v

! u. =

or

uc = (2/15)"* R(v,/E, D;L)"'

where R = radius of adsorbent granule, ep = ratio of internal to external porosity, Di.? effective coefficient of internal pore diffusion of adsorbate, L = length of adsorbent and'v, = linear velocity of the carrier gas. Thus, the value of u, can be calculated from2 experimental data (Equation 23) or parameters of the adsorbent-adsorbate system (Equatii 24). The authors argued that ur is independent of adsorbate influent c~ncentration. '~~'~ OG u~ is calculated fnr one inf luent cnncentration of adsorbate (C). it can be used to constm

[>:. L .

&> <- ,

lii

Ye

b?> . .

. '.

b? an entire breakthrough for an untested influent concentration (if t,, or W is known for 9s untested concentration). Remember, W can also be calculated for an untested set of conditiG$ by using Equation 12. Equation 25 results when u, is incorporated into Equation 22. ~ $ ..

u.

i ' k: I::

.,.7 F'.:. . . >~.*

'Yr g>.

I '

, .

The data in Table I demonstrate the degree of variability among u, values for a ra influent concentrations of several organic compounds. Using Equation 24 to estimate benzene (Table I ) resulted in a value of 0.21. Grubner and Burgess of values for cr, is reasonably narrow and the use of Equation 24 results in valid es for that parameter.49 As reviewed, the use of u,, in the manner s ~ g g e s t e d ~ * . ~ ~ C

powerful tool for calculating breakthrough curves for untested organic compounds.

5 :; argued that the r

,


Table 1 u, VALUES FOR A RANGE OF INFLUENT

ADSORBATE CONCENTRATIONS OF SEVERAL ORGANIC COMPOUNDS

1"flW"t concentrillion u. u,

Compound range (ppm) Range Mean Source

Benzene 125-2000 0 .154.20 0.17 49' Methyl acetate 100-2000 0 . 2 W . 2 5 0.23 4Y Acetone 100-2Wo 0.15-0.22 0.18 49' Vinyl chloride 100-2000 0 .124.23 0.19 49' Trichloroethylene 50-300 O . I W . 1 7 0.13 3

' Dala from References 2 and 45.

Grubner and Burgessd9 combined the Theory of Statistical Moments with the general Lewis Adsorption Isotherm" to increase predictive power. The Lewis Adsorption Isotherm results in a general plot of adsorbed liquid volume vs. the natural logarithm of the relative adsorbate fugacity multiplied by absolute temperature and divided by molar volume. The resulting curve describes the adsorption of similar adsorbates (e.g., paraffins or olefins) on carbon for a wide temperature ~ a n g e . ~ ~ . ~ ~ In the example presented by Grubner and five adsorbates f i t the form of the general Lewis Isotherm with a correlation coefficient of 1i.98. The polynomial equation obtained is

y = 0.749 - 8.307X + 14.82X' (26)

where y = W/G, G = weight of adsorbent, and X = (p/M) In (PslP). Incorporating the Theory of Statistical Moments results in

t, = I(24.1 X IO9) G p B,y]/60 M C, Q (27)

where Q = volumetric flow rate, B, = 1 + 0.365+, (W2) (Q/Vb~.Di)'", V, = volume of adsorbent bed, and E, = extemal porosity of adsorbent.

A detailed example using Equations 26 and 27 is included in Reference 49. The authors wamed that calculations using this model will fail for superficial linear velocities (v,) outside the range of 10 to 60 cm/sec and for certain bed g e o m e t r i e ~ . ~ ~

B. Mecklenburg Equation The Mecklenburg Equation was one of the first kinetic equations developed to predict

breakthrough times from microporous adsorbent beds. The equation was developed by combining Mecklenburg's mass balance approach (Equation 28) with a definition of the critical bed depth (Equation 29).54

t, = WA,/Q C, (L - I) (28)

where A. = cross-sectional area of adsorbent bed and I = critical bed depth.

I = Ila[(DGJp)"" (@pa D,)0.67 In(C,/C,)] (2%

where a = superficial surface of adsorbent per unit volume of bed, D = diameter of adsorbent particle, G, = .mass velocity of air mixture through column, = viscosity of

r ?

340 CRC Critical Reviews in Environmental Control

air mixtures, p, = density of air mixture, D, = diffusion coefficient Of adsorbate ;q and C, = selected adsorbate breakthrough concentration.

The value of the critical bed depth is independent of the nature of the carbon dependent on properties of the air mixture and granular characteristics of the adsorbent Notice also that the value of I is dependent on familar hydraulic parameters. specifica the Reynolds and Schmidt numbers.”l A combination of Equations 28 and 29 results in Mecklenburg Equation.

. ~ . ..,

I, = WA/QC, [L - I/a ([DCJ&]”.“) (t~lp,D,)”-“’ In(C,lC,)I I.

In their extensive research on vapor phase adsorption. Nelson and other^'.^^^''^^^^^^ gc erated data which were used to demonstrate the agreement between predictions of ,I Mecklenburg Equation and experimental results. The data tested represent 12 I orgai compounds in 1 I chemical classes, plus a number of compounds not included in any of(..- classes. Nelson’s data2.45-4’ indicated that in some cases the Mecklenburg Equation coula be used to satisfactorily predict breakthrough time for some of the tested compounds’: However, in general the equation predicted longer times to breakthrough than those whidh: actually occurred. Additionally, the more volatile compounds tested were predicted with less accuracy than the less volatile compounds. Nelson et a1.1.46.4’ attributed pan of 163 problem to the presence of water vapor in the test air (50% relative humidity). Apparently” A water vapor more adversely affected adsorption of both the more volatile and the more water -3 soluble compounds than it did less volatile and relatively insoluble compounds. Deviations of predictions from experimental results ranged from 0 to 360%.’ The reader is referred t the original papers for detailed information on the error involved.’.‘’

Although under certain conditions the Mecklenburg Equation accurately predicts break: through times, its use is intractable. For example, it requires that the organic vapor, activated carbon, and operating conditions be characterized by as many as 25 different variable~.~’.~’.S$ Accurate values for many of these variables may be difficult or impossible to obtain. In, addition, the Mecklenburg Equation cannot be used to predict breakthrough times for c, 0.4 C,.

C. Wheeler Equation

to various levels of contaminant breakthrough.

ji

The Wheeler Equation is another kinetic equation which may be used to predict the ti

1, = W G Q IG - (P ,QK) WC,ICJI .(311

where p, = bulk density of carbon and K, = adsorption rate constant. , i Nelson et al.46 compared predictions of the Wheeler and Mecklenburg Equations to ex-

oerimental results for more than 30 oreanic comvounds. The com~arisons indicated that in general the Wheeler Equation is more accurate than the Mecklenburg Equation. However the accuracy of the Wheeler Equation was also unacceptable for volatile and water solu compounds.” Additionally, both equations failed to reasonably predict breakthrough ti for greater than 40% of the influent adsorbate concentrations. The Wheeler Equation h fewer variables than the Mecklenbxg Equation, therefore i t is less burdensome to use. Bo equations require accurate values for the amount of contaminant adsorbed (W), which can be obtained experimentally or by using Equation 12. Note, Nelson et al? did not consider water vapor as a second component of their adsorption system while testing the above equations. This likely contributed significantly to the inaccurate prediction of the equations since the test air had relative humidities of UD to 90%. Humiditv can have a substantial

1 !

effect on the adsorption of organics as discussed in a later section of this paper.


Assigning accurate values to the adsorption rate constant, K,. is important for using the Wheeler Equation. Jonas and c o - ~ o r k e r s ~ ’ ~ ~ ’ ~ “ - ’ ~ ~ ~ ~ ~ ~ ~ ~ ~ ’ have performed extensive research

determine reliable methods for predicting K,. They reponed that the value of K, varied ,, ith temperature,”.” adsorbent panicle size,6‘ superficial carrier gas vel~ci ty , ’~.~‘ and the ,,mlecular weight of the organic a d ~ o r b a t e . ~ ~ - ’ ~

The rate limiting step affecting K, during the adsorption process may be mass transfer of gas to the external surface of the carbon, internal diffusion within the carbon pores, or the actual adsorption of the adsorbate molecule onto the adsorbent surface. For practical purposes of physical adsorption, the last possibility can be ignored.54 The assumption that mass transfer to the external surface of the carbon is the rate limiting step results in Equation 32.2’

(32) K, = lo(v;Z M‘”2 0-1‘2 p-V2 T I

where P, = total atmospheric pressure and M = molecular weight of the mixture. Jonas and Rehrmann” made the assumption that the total atmospheric pressure (P,) is

one. Since the adsorbate is at a relatively low concentration, the molecular weight of the air mixture (M) is that of pure air (28.8 @mol). With those assumptions, Equation 33 results.

K, = 1.86 v l2 D-’I2 (33)

Hence, the adsorption rate constant can be estimated if the supeficial carrier gas velocity and diameter of the adsorbent panicle are known for the system. Other authors used similar equations to estimate K,.2.43-+7.56 1 nherent in Equation 33 are the additional assumptions of common diffusivity and viscosity of all adsorbates. This implies that mass transfer rates are independent of the nature of the adsorbate. Jonas and Rehrmann” presented experimentally determined and predicted values (using Equation 33) for five compounds within several classes of organic compounds; the average deviation between the values was 8.6%.

In a subsequent paper, Jonas and Rehnnanns9 demonstrated that Equation 33 has mathematical weaknesses as v, approaches either zero or infinity. Additionally, Equation 33 does not predict K, accurately over a wide range of gas velocities. Therefore, based on the shape of the resulting curve when K, is plotted against vir the foIlowing empirical equation was suggested?

.

K, = a’ + b’/l + (a’/b’) exp - (a’ + b‘)c‘v, (34)

where a’, b’, and c‘ = empirical coefficients of the equation. The empirical coefficients of Equation 34 are determined using experimental data within a range of conditions of interest. Notice that Equation 34 does not include a parameter for adsorbent panicle size. Apparently, the effect of particle size is included in one or more of the coefficients of the equation (a’, b’, or c’).’~ Although experimental data fit this empirical equation quite well, it has serious limitations. Primarily, experimental data are required to calculate values of the coefficients. Additionally, there are no rational bases for the equation. If data are available corresponding to the adsorption conditions of interest, perhaps Equation 34 could be used to provide accurate estimates of K, for use in the Wheeler Equation.

In later papers, Jonas and c o - ~ o r k e r s ~ ~ ” ~ returned to Equation 32 for predicting K,, but made the assumption that the rate controlling step of the adsorption process is internal rather than external diffusion. Now, the paramear M represents the molecular weight of the adsorbate rather than that of air as in Equation 32. Thus,

K, = K’ M-IIZ (35 )

L

342

where K' = 10v,''2/D''21PT''2. The parameter, K', is a constant for adsorption of diffe adsorbate compounds at a given flow rate and adsorbent particle size. Therefo 36 is valid under constant operating conditions.

CRC Critical Reviews in Environmental Control

(K, MIJ2), = (K, M"')..,

Thus, the value of K, can be determined for any adsorbate, i . i f its molecular weight is known and if K, has been determined for some reference adsorbate under the sam of adsomtion (i.e., carrier gas flow rate, total gas pressure, and adsorbent pa

r

Predictive values of K, using Equation 35 for five organic vapors ranged from 2 15% of I their actual values.30

of whethzr the rate limiting step is internal or extemal diffusion. Various aut arguments for the use of one or more of these equations to predict K, and presented data which supported the validity and accuracy of each equation.'~22~2'~28~'0~44~~'~~o~*9~6' F reasons, the equation used might be selected on the basis of available information. Fo example, if only the adsorbate changes from a previous adsorption trial, Equation 36 might be accurately used to predict K,. For situations in which extensive laboratory data are available under common conditions of adsolption, Equation 34 might be the best choice to accurately assign a value for K,. When little information is available for an adsorbent- adsorbate system, one might select Equation 33.

D. Yoon-Nelson Theory Yoon and Nelson50.51 have developed a kinetic model which describes the adsorption o

organic compounds on activated carbon. Their theory is based on ( I ) theoretical principles of gas adsorption kinetics and (2) changes in the probability of a breakthrough of a con inant molecule at vacious stages during the adsorption process. Equation 37 is the basic expression of the Yoon-Nelson Theory."

An important consideration in selecting from among Equations 33.34. and 36 is k I

1

t, = t,, + I/k'[ln(C& - CJl

where k' = a dimensionless constant of proportionality. Values fort,, and k' can be obtained from experimental data by plotting t, vs. the qu

In (CdC, - CJ. Application o f the Yoon-Nelson Theory as a predictive tool depe obtaining values for these parameters. After t,, and k' are determined, the entire breakthro curve can be calculated for a given contaminant concentration and constant ads conditions by using Equation 37. Predictions for several concentrations of methyl and vinyl chloride, a single concentration of toluene, and a single concentration o f five different trichlorinated hydrocarbon compounds closely matched experimental A comparison between experimental data' for trichloroethylene and calculated values 01 tained by using Equation 37 is presented in Figure 4. As with the comparisons presente by Yoon and Nelson,5o a close agreement is demonstrated between predictions a imental results.

One of the assumptions inherent in this theory is that the breakthrough curve is symmetr about the inflection point (Le., 50% breakthrough point or Cm). That assumption was alr made by Grubner and and leads to the following useful relationship^.^^

.:

. .. W = C, Q t, or t,, = WIC, Q

The parameter, k', of Equation 37 is further defined as:


i o 0

90

80

70

60

50

40

30

20

i o

0 0

FIGURE 4. Continuous trichloroethylene breakthrough CUIYCS far activated carbon as predicted by the Yoon- Nelson Method compared to experimental data (points on curves). Trichlorwthylene influent concenlrations were IC03 mum' for curve A and 6CQ mum' for curve B.

(39) k' = k" C, Q/W

where k" = a constant for a given adsorbate and adsorbent. The constant, k". is independent of adsorbate concentration and carrier gas flow rate through the carbon column.50 Therefore, i f k" is determined for an adsorbent-adsorbate pair, it can be used for other adsorbate concentrations and conditions of adsorption for the same pair. Only t, or W would need to be determined or estimated (e&, by using Equation 12) for the untested adsorbate concentration to utilize Equation 37 to predict the breakthrough curve.

Yoon and NelsonSa discuss differences and similarities between Equation 37 and the Mecklenburg and Wheeler Equations, The reader is referred to that discussion to ascertain the clear parallels between these three kinetic equations. The Yoon-Nelson Equation is more convenient to use than the other equations if laboratory data are available to estimate values of the equation parameters. Another advantage is that breakthrough times for the entire breakthrough cycle can be determined. The Mecklenburg and Wheeler Equations fail at breakthrough concentrations above 40% of the influent.

Yoon and Nelson5' further developed their theory in a second paper. They introduced a useful third term, a, of an adsorbent-adsorbate system. To obtain "a", the principle of chemical kinetics,. which states that the half-life of a reactant for an n-th order reaction is inversely proportional to the reactant raised to the (n - l)th power, is applied. For vapor phase adsorption, Equation 40 results.

t,, a l/cI"-" (40)

For two cases differing only with respect to contaminant concentration, Equation 41 results.

344 CRC Crificol Reviews in Environmenful Confrol

~,,Jt,,, = [C,,,JC,,,,I’

where a = (n - I ) , C,,,, and C,,,, = two different adsorbate influent concentrations for the same adsorbate compound, and tSWI, and tSn2, = 50% breakthrough times for C,,,, C,,,,, respectively.

parameter, a, are sound.

t,,Jt,*, = [C,,,JC,,,,]”

W,,JW,,, = ~ ~ , , l J ~ r , ~ , l ’ - a

Equation 44 was also derived by Yoon and Nelson.5’

log t = log k,, - log Q - a log C,

where VWQ = coefficient of the equation. The development of this equation and a complete definition of k‘ can be found in Yoon and Nelson.” Note, Equation 44 is parallel in stmc to a form of the F;eundlich Equation (Equation 45). The Freundlich Isotherm Equatio known to fit experimental data but up to this p i n t had no theoretical basis.

log t = log B + A log C,

where A and B = coefficients of the Freundlich Equation. The slope of Equation 44, “a”, should remain constant for different carrier gas flow ra

when log t is plotted against log C,; only the intercept (log k’, - log Q) will vary Additionally, k’,, is also independent of flow rate and can be determined from the plot of the experimental data if the values of Q are known. Equations to calculate k‘, also been developed by Yoon and Nelson.5‘ After the values of k‘,Q and a are determ Equation 44 can be used to calculate breakthrough times at various adsorbate in concentrations and carrier gas flow rates.

The Yoon-Nelson Theory appears to present substantial advantages over previous1 veloped kinetic models. For example, the equations are theoretically based and simple

and other operating conditions before they can be used with confidence in actual adso

IV. ADSORPTION OF MIXTURES

selectivity of microporous adsorbents for air m i ~ t u r e s . ~ ~ - ~ The present review will not r the material of the previous papers. Instead, important qualitative observations conce adsorption of mixtures will be summarized, and three successful predictive methods WI

briefly presented. These are the Grant-Manes Method, Ideal Adsorbed Solution ( Theory, and the Vacancy Solution Model (VSM) Theory.


because most environmental applications of activated carbon involve mixtures of contaminants. The ability to describe their concurrent adsorption is highly desirable since one or more of the components involved may be required to be removed from the air stream. Many early attempts to predict concurrent adsorption of organics from air by microporous adsorbents were based on the BET or Langmuir equati~ns.~’.~’ These methods are still often used, although their assumptions are not consistent with gaseous phase adsorption of multicomponent systems and fail to accurately agree with experimental data over wide ranges of pressure.= Although methods based on the BET and Langmuir equations are usually convenient to use, they are not recommended for multicomponent gaseous phase adsorption.

A. Qualitative Observations Lewis and co-workers6’-’0 presented experimental data for several mixtures of simple

hydrocarbons over a range of vapor pressures. They made several observations conceming the results, which may be relevant to predictive approaches. First, mutual interference between components results for all mixtures tested. The amount of a particular vapor adsorbed at a given partial pressure was less when that organic vapor was a component of a mixture than when it was present as a pure vapor. Second, the preferentially adsorbed vapor of a mixture was the one for which the adsorbent had the highest capacity as a pure vapor. Third, the quantity of total vapor mixture adsorbed per unit weight of carbon was within the range of the adsorption capacities for the individual pure vapors at the pressure of the mixture. Fourth, while different activated carbon adsorbents had different adsorption capacities for a mixture, they had essentially the same preference for a given compound within a given mixture of hydrocarbons.6’ These Observations were based only on adsorption of mixtures of simple hydrocarbons on microporous ads or bent^^'-'^ and may not be consistent with results obtained for more complex, less ideal mixtures.

B. Selectivity

or preferential adsorption of one compound in a mixture relative to the others.= Selectivity is an important parameter for adsorption of mixtures. It refers to the separation

SI, = (XJYI)/(X2/YJ (46)

where x, = mole fraction of component i in the adsorbed phase, y, = mole fraction of component i in the vapor phase, and S,, = selectivity coefficient (in this example, adsorbents affinity for component 1 relative to component 2 for a binary mixture).

When the value of the above selectivity coefficient is greater than one, preferential adsorption of component 1 has occurred. Both adsorptive capacity and selectivity are important considerations for choosing a microporous adsorbent for various environmental control applications. An adsorbent should be selected which would preferentially remove the compound($ of greatest concern. According to results published by Lewis et al.,67-” selectivity would be expected to be similar among activated carbons manufactured by common processes for common applications. At the macro level, activated carbon is a hydrophobic and or- ganophilic adsorbent. While many organics adsorb efficiently onto activated carbon, the carbon exerts greater selectivity for compounds with higher molecular weights when other chemical and physical characteristics of the adsorbate are similar.670.’* At the micro level, heterogeneity on the carbon surface may impart selectivity for organics which vary between sites on a given activated carbon particle.

C. Liquid Phase Adsorption of Mixtures Several resear~hen”~’~~”.’~.’’ argue that adsorption of mixtures of organic compounds on

activated carbon from the gaseous phase should be analogous in prinicpal to that from the

346

aqueous phase. Techniques to model adsorption of mixtures from the aqueous phase wh are based on sound thermodynamic p r in~p les” . ” . ”~~~ may also be relevant to gaseous phase adsorption. Therefore, progress made in liquid phase adsorption research should be cons id^ ered in future efforts in order to develop predictive equations for gaseous phase adsorption :

CRC Critical Reviews irr Environmental Conrrol

vapor mixtures.

D. Methods to Predict Adsorption of Gas Mixtures I . Grant-Manes Method

Grant and Manes“ extended the basic Polanyi adsorption theory to predict adsorption of multicomponent gas mixtures on microporous adsorbents. Their method assumes: ( I ) th ’

adsorbed mixture behaves in an ideal fashion and (2 ) the total adsorbed volume determine the adsorption potential of each of the pure components in the mixture. Adsorbed investigated by Myers and Prausnitz” did not deviate greatly from ideal behavior some mixtures that were nonideal as free liquids. Their results provided empirical e for the validity of the first assumption. Grant and Manes” suggested that the strong forces, which compress the vapor to a liquid (as envisioned by Polanyi), tend to interaction between the different adsorbate molecules. Intermolecular interactions co to the nonideal behavior of mixtures. At high adsorbate concentrations, the mixture proaches a free liquid on the surface of the microporous adsorbent, and nonideal beha may become a significant factor.20Thus, the Grant-Manes Method may become less accu at high adsorbate concentrations, thereby limiting its use for certain applications.’0 B on these assumptions, this method should be most reliable for adsorbate mixtures w approach ideal behavior as free liquids, such as a homologous series of hydrocarbons.20, .. Increased deviation from predictions apparently occurs with mixtures of dissimilar co ponents.”

The assumption of ideal behavior by the adsorbed mixture permits the use of Raoult’ law (Equation 47) to determine the relationship between the mole fraction ofeach compone and its potential pressure in the overlying gas phase.

f, = xi

where f j = fugacity of the i-th component in the gaseous phase, x, = mole fraction of th i -th component in the adsorbed mixture, and f,* = fugacity which the i -th compon would exert as a pure adsorbate at the total adsorbate volume of a mixture.

Equation 48. Substituting fugacity for pressure in the Polanyi adsorption potential equation yi

E! = RT In(f/f*),

where f5 = saturated fugacity of pure unadsorbed liquid at the adsorption temperature. Substituting Equation 47 into 48 results in Equation 49.

E; = RT In xj(f,/Oj

The “affine” nature of organic adsorbates on microporous adsorbents (Equation 50) an the concept of the characteristic curve for a given adsorbent permits the use of Equation 4 for various adsorbates of a mixture (Equation 51). . .

.. . .?

If r vali

\dl

P,, is0 in

atL of

.~

F< E( ~

fr: of m C i

7

at

P

I e

r

1:

C

c

F


If one assumes that gas fugacity can be replaced by pressure* and that Equation 15 is ~ a l i d . * ~ . ~ ~ Equation 52 can be derived.

P, = V,IV,,, (15)

I/V,[ln(P,, xJP, y,)l = I/Vdn(P,, xJP, yJ1 (52)

where V, = molar volume of the i-th component as a saturated liquid at its boiling point, P,, = saturated vapor pressure of the i-th component at the temperature of the adsorption isotherm, P, = total vapor pressure of mixture, and y, = mole fraction of the i-th component in the vapor phase.

Equation 52 is the form in which the Grant-Manes Method usually appears in the liter- ,mm.66.88.8q Equation 53 is the second condition which must be met during the adsorption uf a binary mixture.

x, + x2 = 1 (53)

For a given temperature, total vapor pressure, and gas phase mole fraction (y, and y2), Equations 52 and 53 are solved simultaneously to provide a unique solution for the mole fractions of the adsorbed components. Grant and Manes" gave examples of the accuracy of this method for binary mixtures of simple hydrocarbons. The authors also presented a method to estimate gas fugacity with pressure in Equation 52. The Grant-Manes Method can be extended to predict adsorption of mixtures with more than two components.20

2. Ideal Adsorbed Solution ( IAS) Theory The IAS theory,",w like the Grant-Manes Method, is based on the assumption that the

adsorbed phase forms an ideal solution. Thus, the partial pressure of the adsorbed component is assumed to be the product of the mole fraction in the adsorbed phase multiplied by the pressure it would exert if it were a pure adsorbate at the temperature and spreading pressure of the mi~ture ."~"~~' The concept of the spreading pressure of a mixture is central to the 14s Theory. Spreading pressure is calculated from the integrated Gibbs adsorption isotherm equation using a single gas isotherm. The integration requires an iterative solution proce- d ~ r e , ~ ~ unlike the Grant-Manes Method which has an exact solution. The basic thermodynamic equation which describes spreading pressure of a gas mixture follows:

N

A d n = n , d h i = l

where A = specific surface area of a solid, 71 = spreading pressure, ni = moles of component i adsorbed, N = number of components, and pi = chemical potential of component i.

Notice that Equation 54 is a form of the ideal gas law with volume being replaced by area A and pressure by spreading pressure, 71." For pure gas isotherms, Equation 54 can be integrated as shown:

m N R T = lm0 (n/P)dP

An adsorption isotherm is incorporated into this basic equation to calculate the number of inoles, n, of a component for each value of pressure, P." To permit ease and efficiency in

* "e, this assumption may lead 10 a significant emor far some adsorbales

348

use, the integration is performed with respect to the energy distribution rather than pressure. An excellent review of the method and its use, including a complete algorithm for its solution,

CRC Critical Reviews in Environmental Control

former is based on thermodynamic principles of a mixture, whereas the latter is an extensio

disadvantage of the thermodynamic approach is that it requires iterative methods or graphi techniques for its solution.

for the nonideality of the adsorbed solution by introducing activity coefficients for the adsorbates based on the Wilson E q u a t i ~ n . ~ ’ . ~ Like previous methods de~cribed,2’,’’.~

“bulk vacancy solution” and the “surface vacancy solution”. The “vacancies” are hy+ pothetical entities which represent a void volume in the vapor or adsorbed phase. The void volumes can be occupied by molecules of the gas contaminant. The basic isotherm for a pure component system is derived by equating the chemical potential of the contaminant in the two phases (vapor and liquid adsorbate) and by considering vacancies in both phases.%

P = ((nF-/b,)(e/l - e)l[A,,(l - [ I - Av,]e)l/[A,v + ( I - A,,)eI

exp [ -Av,( l - A v , ) W I - [ 1 - AvllQl

- [ ( I - A l v ) W k v + [ I - Atvle)l

where P = pressure, ns- = maximum number of moles of component I in the surface phase, b, = Henry’s law constant of adsorbate 1 , 8 = fractional coverage (nT/n?% nf - number of moles of component I in the surface phase, and A,,, A”, = Wilson’s interacti

.~

L

.-. +; yi P = 7; xi n:(ns.- A, Jn2- bJ exp(Av, - 1) exp(v H,/RT)

where +j = fugacity coefficient of component i in bulk gas mixture, ys = activity cceffici of i in adsorbed phase vacancy solution, n; = total number of moles of mixture ill

= partial molar surface area of adsorbate i. The following equations are also agplied to obtain the solution of the VSM.


(59) N

CY<= 1 i - 1

Simultaneous solution of the equations of the VSM using an iterative approach yields the solution for adsorption of a gas mixture. Valenzuela and MyersM can be consulted for a detailed solution algorithm of the VSM.

Danner and co-workers"-% suggested that the VSM is simpler to use than similar thermodynamic approaches (e&, IAS). 'It is also the most generally applicable method since it considers nonideal behavior of mixtures. Specifically, the VSM was shown to be superior to other methods when the mixtures exhibited azeotropic behavior." However, when the mixture approached ideal behavior, the IAS method performed slightly better than the VSM,M.x In the cases evaluated, the VSM may have been hampered because the pure component isotherm had not been determined over a sufficient range of pressures to give physical significance to the parameters used by the method.%

4 . Comparison of Predictions with Experimental Data Valenzuela and Myers" presented prediction errors based on selectivity for the three

methods reviewed. Comparisons were made between predictions and experimental data for 22 mixtures adsorbed at various temperatures and pressures on activated carbon, silica gels, or zeolites. Average calculated errors, considering all the variables, for the IAS, VSM, and Grant-Manes methods were 35, 41, and 80%. respectively. The authors" pointed out that calculating errors by senstivity comparison may suggest greater errors than actually exist. ror example, a 3S% error in selectivity corresponds to an average mole fraction error of only 0.04. Therefore, all three methods are actually quite accurate. Valenzuela and Myers& concluded that although none of the methods were acceptably accurate for all situations, the IAS and VSM methods were more accurate overall than the Grant-Manes approach. Other authors have also found the IAS method in pruticular to be in good agreement with their gaseous phase experimental data.*%

-

V. EFFECT OF RELATIVE HUMIDITY ON CARBON ADSORPTION

Water vapor in the carrier gas may substantially and adversely affect the adsorption of organic vapors on activated ca~bon.*."."-"' This effect has k e n demonstrated for different applications of activated carbon, various organic adsorbates, a range of relative adsorbate pressures. and various ambient conditions of adsorption. In short, experimental results and actual applications of activated carbon indicate that air humidity is a factor which should be considered during the design and operation of carbon adsorption processes.

Numerous factors influence the extent to which humidity affects the vapor phase adsorption process. First, the adsorption of various adsorbates is affected differently, depending largely on their chemical characteristics.'m,'M'08 In general, organic adsorbates which are soluble in water and volatile are affected by water vapor to a greater extent than others.6.'m.'w Second, the partial pressure of the adsorbate has a significant influence at a given level of relative humidity."' Decreasing the adsorbate pressure generally increases the deleterious impact of "' Th' ird, the level of humidity influences the magnitude of its effect. The statement is often made that below 40 or SO% relative humidity the effect is minimal, and the effect increases with increasing humidity beyond that humidity range.4"4'.'a'.'~N.''' This Statement is consistent with the observation that water adsorption isotherms on activated carbon often increase sharply at about 50% relative humidity.'"."4-"6 although the precise point at which the slope steepens depends on the type of activated carbon."4~"J~"7 H owever, analyses of the humidity effect at several low concen- [rations of trichloroethylene indicate that at a relative humidity as low as 25% significant deleterious effects on adsorption can occur."'

?.. 6- 350 CRC Critical Reviews in Environmental COntrd

Characteristics of the carbon adsorbent also influence the magnitude of i vapor.'" Water molecules are believed to adsorb onto oxidized sites of th

The greater the number of oxidized sites on the adsor the interference waer molecules can exert on the adsorption of an organic vapor. Stomg of carbon in a moist atmosphere may increase oxygen complex formatio surface, thereby reducing its efficiency for organic adsorption."9 Loss of car when stored in humid conditions has been dem~nstrated.~~.~' . ' 'O The loss in efficiency be due to both increasing the number of oxidized sites and preadsorption of wat which occupy the adsorption sites.

The presence of water vapor not only affects the adsorption capacity of the carb may also influence the kinetics of a d s o r p t i ~ n . ' " ~ ' ' ~ ~ ' ~ ' This effect, which is likely due inhibition of mass transfer of the organic molecule by interference from water molecules,! can be manifested as an increased width of the contaminant breakthrough curve."

carbon surface (bulk diffusion) or diffusion within the carbon pores (pore diffusion)."3

A. Dubinin Water Isotherm Understanding the manner in which water molecules adsorb onto activated carb

aid in developing methods to describe its interference with the adsorption of organic va Dubinin and co-workers16-18,120.'24 have extensively investigated the adsorption of wat on carbonaceous material and have developed a model to describe the process. The anism of adsorption for water molecules is different from that of organic Dispersional forces greatly influence the adsorption of organics, whereas hydrogen to oxidized adsorption sites on the carbon is the major influence during water mo adsorption. 16-'8.''3 Thus, volume filling of the micropores occurs with organic vapor, adsorption of water begins at specific sites on the carbon surfaces. The adsorb molecule acts as a secondary adsorption site and other water molecules can attach to hydrogen bonding. 16-'8.' ' ' At a sufficiently high relative water pressure, capill sation occurs to fill the micropores with water.'"~''s~'22 (This phenomenon apparently b to occur at approximately 50% relative humidity, depending on the type of c most extensive interference of organic adsorption occurs when the micropores with water."4."6

equilibrium between adsorption and desorption (Equation 60).'6-'8.'2'

transfer inhibition can result from inhibition either of diffusion from the air stream to . ~~

-

Development of the adsorption equation for water began with the assumption of a dyn

a,(a + 4 (1 - k,aJh = a2 a,

where a, = adsorption rate coefficient, a2 = desorption rate coefficient, k, = computing decrease in adsorption centers as pores fill up with water, a,, = primary centers, rt. = adsorption value for water at a given relative pressure, and h = relative pressure. The left side of this equation expresses the adsorption rate of wa molecules. This rate is proportional to the total number of adsorption centers (a,, + a,J to the relative pressure of water vapor (h). The term (1 - k,aJ accounts for the dec in the number of adsorption centers with increased water adsorption. The right side equation accounts for desorption of water molecules, which is proportional to the of water already adsorbed. Equation 60 can be transformed to the final form of the isotherm:

;l/h = 1'3% + 4 (I - k, a J l


one isotherm with water on an activated carbon and used for other conditions. The manner in which this determination is performed can he found in papers by Dubinin and co-work-

The water isotherm equation has proven to be accurate for predicting water vapor adsorption on activated carbon."."' However, to increase its practical value. i t needs to be extended to include the interaction between water and organic compound molecules during their concurrent adsorption. Simultaneous adsorption of these dissimilar substances influ- ,.ices the adsorption capacity and, possibly, the adsorption kinetics of each other. Future research efforts to predict the effect of water vapor on the adsorption of organic molecules should benefit from Dubinin's clarification of the adsorption process of water.

B. Manes Predictive Method for Organic Adsorption from Humid Air Interference of organic adsorption by water vapor is a case of multicomponent adsorption

which requires special attention since some level of water vapor will almost always be present. Manes12 presented a method to predict the adsorption of organic vapors in the presence of water vapor. The method is based on the Polanyi Adsorption Potential Theory ,xith the assumption that adsorbed water reduces the pore space available for adsorption of an organic compound on a one-to-one volume basis.I2 Additionally, Manes assumed that at 100% relative humidity adsorption of any organic vapor is equivalent to its adsorption from a water solution. At 100% relative humidity, the net adsorption potential for the contaminant is its calculated potential (without interference from water vapor, E;) diminished by the adsorption potential of an equal volume of water (E,) which the organic must displace from the carbon pores.

ers,14-18.1z1

€.;/vi = €;/VI - EJV, (62)

where E$' = corrected adsorption potential of component i considering interference by water and V, = molar volume of water.

According to Manes'' Equation 62 applies to any immisible organic under the condition that the organic adsorbate volume is less than the volume of adsorbed water. If the organic volume is greater than that of water, no interference results from the presence of water. Characteristic curves for the pure organic vapor and for water are necessary in order to use this method. The curves may be determined from either experimentation or calculation. If less than 100% relative humidity is present, an additional term is required to describe the effect of the water vapor.

E;/V; = 6JVi - EJV, - RT In h/V,

The added term is based on sound thermodynamic principles."~'* This treatment of organic adsorption is also applicable to adsorption of organic mixtures, i2

using the techniques developed by Grant and Manes" and Greenbank and Manes." The method was tested for four single compounds and two tertiary mixtures at relatively low concentrations (59 to 265 ppm).'26 Tests were conducted with dry air and air with 80% relative humidity. The method performed well for one group of single compounds ( I to 12% error) but poorly for another group (62 to 67% error). The authors'26 attributed the poor performance of the method for the second group to the use of an inaccurate affinity coefficient for those compounds. Predicted values for the individual components within the mixtures were in error from 0 to 33% (with one exception). Predicted values fo; methylene chloride (the exception) were different-by 56 to 60% than the actual values. The large discrepancy was due to the relatively high solubility of methylene chloride. The Manes method was developed for organic compounds which, unlike methylene chloride, are immiscible in water. l2.Iz6

(63)

Grant et a1.I2" concluded that the Manes method was adequate to predict the effect of moisture on adsorption of immiscible organic compounds and mixtures of compounds under the conditions of adsorption which were tested. However, the method has not been rigorously tested. Additional tesling i s needed for wider ranges of organic compounds, relative humidity, and influent contaminant concentrations in order to establish the reliability o f the method.

the carbon. Therefore, the model may not perform well under conditions during which the organic compound(s) and water reach their equilibrium capacities concurrently on the carbon and when competition for specific adsorption sites is occurring. Concurrent adsorption may involve different mechanisms than displacement of previously adsorbed water by the organic compound

ACKNOWLEDGMENTS

This project was supported by the U.S. Air Force Office of Scientific Research. Special thanks are due to Carolyn E. Marshall, who made valuable contributions to every aspect of the effort.

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13(1 I ) . 745. 1972. 58. Nelson. G. 0. and Harder. C. A,. Remirator canridse efficiency studies. IV. Effects of steady-state and . . -

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59. Jonas, L. A. and Rehrmann, J. A., The rare of gas adsorption by activated carbon. Carbon. 12, 95.

60. Jonas, L. A.. Boardway, J. C., and Mereke. E. L.. Prediction of adsorption behavior of activated

61. Rehrmann. J. A. and Jonas, L. A,, Dependence of gas adsorption rates on carbon granule size and linear

62. Brunauer, S., The Adsorption of Gnses and Vnponr.~. Oxford University Press. New York, 1944 63. Young, V. M. and Crowell, A. D., Physicul Adsorprion o/Cnrrr. Buttcnuonhs. London. 1962. 64. Sirear, S. and Myers, A. L., Surface potential theory of multilayer adsorption from gas mixtures. Chcm.

65. Jaroniee, M., Current state and perspectives of developments in the theory of mixed-gas adsorption on

66. Valenzuela, D. and Myers, A. L., Gas adsorption equilibria. Sep. Pur. Mrdiods. 13(2). 153. 1984. 67. Lewis, W. K., Gilliland, E. R., Chertow, B., and Cadogan, W. P., Adsorption equilibria hydmcarbon

gas mixtures. lnd. Eng. Chem.. 42(7). 1119. 1950. 68. Lewis, W. K., Gilliland, E. R., Chertow, B., and Hoffman, W. H., Vapor adsorbate equilibrium. 1.

Propane-propylene an activated carbon and on silica gel, I . A m Chon. Sac.. 72. 1153, 1950. 69. Lewis, W. K., Gilliland, E. R., Chertow, B., and Milliken. W., Vapor-adsorbate equilibrium. 11.

Acetylene-ethylene on activated carbon and on silica gel. I . A,,,. Chrm. Soc.. 72. 1157. 1950. 70. Lewis, W. K., Gilliland, E. R., Cherlow, B., and Bareis, D., Vapor-adsorbate equilibrium: the effect

of temperature on the binary systems ethylene-propane. ethylene-propylene o w silica gel. J . Am. Chem. Soc.. 72. 1160, 1950.

71. Myers, A. L. and Prausnilz, J. M., Thermodynamics of mired gas adsorption. AlChE I . , I l ( 1 ) . 121, 1965.

72. Myers. A. L. and Sirear, S., A thermodynamic consistency test for adsorption of liquids and vapors On

solids. J. Phyr. Clrem.. 76(23). 3412, 1972. 73. Creenbank, M. and Manes, M.. Application ofrhe Polanyi adsorption potential theory to adsorption

solution on scrivated carbon. XI. Adsorption oforganic liquid mistures from wotersolution, J . Phys. Ch 85(21). 3050. 1981.

74. Sircar. S. and Myers, A. L., Statistical lhermodynamicr of adsorption from liquid mixtures on solids.' Ideal adsorbed phase, I . Phys. Chem.. 74(14). 2828. 1970.

75. Sircar. S., Myers, A. C., and Molstad, M. L., Adsorption of dilute solutes from liquid mixtures. T m Fwoiloy Soc.. 66. 2354, 1970.

76. Larionov, 0. G. and ~ y e r s , A. L., Thermodynamic trolytes. Chem Eng. Sci.. 26. 1025. 1971.

77. Radke. C. J. and Prausnitz, J. M., Thermodynamic of multi-solute adsorption from dilute solutio AlChEJ. . 18(4). 761, 1972.

78. Minka, C. and Myers, A. L., Adsorption from lcmary liquid mixtures on solids. AIChEJ.. 19(31. 45'9 1973. ....

EXR. Sci.. 28, 489, 1973.

solid surfaces. Thin Solid Film. 50. 163, 1978.

adsorption fro,,, nonideal s o ~ u t i ~ n s of nanelec- . .

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:tical model for respirator ' ' 1

A theoretical model for ,,;;< .. 1.. 45(8). 517. 1984.. j&

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gkins, D. J., Respirator ,(2). 105. 1972. I. Preparation of test at.

rator canridge efficiency r n Ind. Hyg. Asroc. 1..

ffects of steady-state and

:arbon, Carbon. 12. 95.

m behavior of activated

,n granule size and linear

New York, 1944. ;, London. 1962. 'om gas mixtures. Chem.

mixed-gas adsorption on _. >, 13(2). 153. 1984. a equilibria hydracarbon

adsorbate equilibrium. 1. , 1153, 1950. Isarbate cquilibnum. 11. . 1157, 1950. le equilibrium: the effect silica gel. 1. Am. Chem.

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8 . AlChE I.. I l(1). 121.

heory to adsorption fm iolution. 1. Phys. Chc

lid mixtures on solids.

n liquid mixtures. Tra

79. Sircar, S. and Myers, A. L.. Prediction of adsorption at liquid-solid in1erf:tce fmm ;alrorplion isuthcrms ofpure unsilturated vaporc.AlChEJ.. lY(1) . 159. 1973.

XD. Schenz, 7'. W. and Manes, M.. Application of the Polanyi adsorption poleinin1 thcury to ;Idsorption fmm solution on activated carbon. VI. Adsorption of some binary organic liquid rmixlurcs. J . PIzw Chcm., 79(6). 604. IY75.

81. Rasene, M. R.. Ozcan, M., and Manes. M., Application of the Polanyi ;~drorption potential theory to adsorption from sol~l ion on activated carbon. VIII. Ideal. nonideal. and competitive adsorption of rome solids from water solution. I . Phys. Chem.. SO(23). 2586. 1976.

82. Rosene, M. R. and Manes, M., Applicalion of the Polanyi adsorption theory 10 adsorption from solution on activated carbon. IX. Competitive adsomtion of temary solid solutes from water solution. J . P h w Chon.. 81(17). 1646. 1977.

83. Jossens, L., Prausnitz., J. M.. Fritz, W., Sehlunder, E. U.. and Myers, A. L., Thermodynamics of multi-solute adsorption from dilute aqueous solutions. C h m . Eng. Sci.. 31. 1097. 1978.

84. Digiano, F. A., Baldauf, C.. Friek. B.. and Sontheimer, H.. A simplified compctitive cquilibrium adsorption model. Chem Eug. Sci.. 33. 1667. 1978.

85. Myers, A. L. and Sircar. S., Theory of conespondence for adsorption from dilute solulions on heterogeneous adsarbem. in T r " w o/ Wulrr by Gromdnr Aclivored Corboo lAd,n!!c~,r in Chrat irw~ Series 2021. McGuire. M. I . and Suffer. I. H.. Eds.. American Chemical Society. 1983. 63.

86. Tien, C., Recent advances in the calculation ofmullicomponent adsorption in fixed beds. in Treomenr o/ Woler by Granular Acrivoled Cnrbmt (Advancer in Chemisrry Series 2021. MeGuire. M. 1. and Suffer. I. H.. Eds., American Chemical Society. Washington. D.C.. 1983. 167.

87. Juhola, A. J., Package sorption device system study. NTIS. PB 221. 138. Springfield. Va. 88. Huang, J. C. and Madey, R., Application of polential theory 10 adsorption of binary mixtures on activated

carbon. Crnrbon. 20(2). 118. 1982. 89. Smoot, D. M. and Smith, D. L., Development of improved respirator canridge and canister test methods.

Pub. 77-209. NIOSH. Depanment of Health. Education and Welfare, Washington. D.C.. 1917. 90. Myers, A. L., Adsorption of pure gases and their mixtures on heterogeneous surfacer. in F,,,,~l',,,,~,,~,~,l~

o/Adsorprim Proc. E ~ s . Fobuttd. C o ! ~ . . Myen. A. L. and Bellon. G.. Eds.. Bavaria. W. Germany. 1984. 365.

91. Tsai, M. C., Chen, W. N., Cen, P. L., Yang, R. T., Kornorky. R. M . , and Holcombc. N. T.. Adsorption of gar mixture on activated carbon. NTIS DE84W6665. Springfield. Vs.

92. Myers, A. L., Minka. C., and Ou, D. Y., Thermodynamic properties of adsorbed iiiixtures of benzene and cyclohexane On graphitized carbon and activaled charcoal at 30'C. AIChE J . . 28( I). 97. 1982.

93. Suwanayen, S. and Danner. R. P., A gas adsorption isotherm equation based on vacany solution theory. AIChEJ. . 26(1). 68. 1980.

94. Suwanayuen, S . and Danner, R. P., Vacancy s ~ l ~ t i o n theory of adsorption from gas mixtu~es. AIChE J . . 26(1), 76, 1980.

Y5. Hyun, D. H. and Danner, R. P., Equilibrium adsorption of ethane. ethylene. isabutme, carbon dioxide and their binary mixlures on 13x molecular sieves. I . Chem. Eng. Data. 27(2). 196, 1982.

96. Wilson, R. J . and Danner, R. P., Adsorption of synthesis gas-mixture components on activated carbon. 1. Chem. Eng. Darn. 28(1). 14. 1983.

97. Danner, R. P. and Chai. E. C. F., Mixture adsorption equilibria of ethane and ethylene an 13X molecular sieves. I r d Eng. Chum Fundam.. 17(4). 248. 1978.

98. Frederich, R. 0. and Mullins, J. C., Adsorption equilibria of binary hydrocarbon mixtures an hamo- geneous carbon black at 25°C. Ind. Eng. Chenz. Fundam.. I l(4). 439. 1972.

Y9. Burrage, L. J. and Allmand, A. J., The effect of moi~lure on the sorption of a carbon tetrachloride from an air Slream by activated charcoal, 1. Soc. Chem. h d . , 57( 12). 424. 1938.

IW. Maggs, F. A. P. and Smith, M. E., Adsorpiion of anaesthetic vapourr on charcoal beds. Amerrhesio. 31, 30. 1976.

101. Baku, E., Characterizatian of respirator adsorbent filters by means o f penetration curve parameters. 11. Effect of relative humidity. Ann Ocmp. Hyg.. 20. 375. 1977.

102. Henry. N. W., 111 and Wilhelmc, R. S., An evaluation of respirator canirtr.rr 10 ncrylonltrilr vapors. A m Ind. Hyg. Arror.. I . . 40(12). 1017. 1979.

103. Henry, N. W., 111. Industrial canridge and canister evaluation. NIOSH lntl. Resp. Res. Workshop. Murguntown. W.Va.. September. 1980.

104. Ballcu, E., Effects of water vapor on the performance of respirator gas and vapour liltcrs. NIOSH Intl. Resp. Res. Workshop. Morgantown. W.Vil.. September. 1980.

105. Howard, A. C. and Pirric. R., Sampling uf chlorinated and bromated halogcnr from humid utmuspherer. A m . Orrrrp Hyg.. 2 4 0 ) . 167. 1981

106. Kalab, P., Effccl DI water vapor on the determination of lrnce substances trapped on activated carbon. Collrmr. C r d . Chcrn Commen.. 47(9). 24Y1. 1982.

..:.,.<;,

356 CRC Critical Reviews ;ti Erivironmenral Corirrol

107. Stnmpfeer. J. F.. Rcrpirntur t;mislrr rvaluatiun lor nine relrcted organic vapors. A r i l . Iml. H J , ~ . 43(5). 319. 1982.

108. Mayer. E. S., Review of inl1uroti;d I~cturraffeciing the pcriormnnce ol'or:mic v:ipor:tir [purifying respirator cdnridgcs. Ant. Irtd. H?:q. Airor. 1. . 44(1). 46. 1983.

IOY. Gregory. E. D. and Elia. V. J.. Siimplc retentivity of p;isive urgaiic $apor r;ihlplcrr ;md chxcoaI tuber undnr viirious cuiiditiiins of r a m p k lhdtng. rclativc humidity. zero e x p o ~ ~ t r c IevcI pcrwd\ :~nd ii competitive s o l v m t . At,!. / , id. H V , ~ . AWJC. 1.. 4 4 3 . 88. 1083.

I 10. Josils. L. A,. Sansone. E. 8. . ;md Fnrris. 'T. S., The dim u i nnu>i>turs L)II the :tllnrlmm ~ f c h l o r o f ~ ~ by actiwted carbon. Am. / r i d . Hys. Arsuc. 1.. 46(11. 20. 1985.

I I I. Werner, M. D.. The effect o i relative huinidity on thc vapor philrc adsorption 01' irichlurocthylene by artivated carbon. A,,,. /tu/. H j s . A . V . ~ ~ . J . . 46(10), 585. 1985.

I I?. Polorzhinskei, 1. U. and Serpionava, E. N., Adsorption of vapors of organic solvcnts frum a stream of moist air by activated carbon. T w M o r K h i m Teklmol. Insf . , 65. 79. 1970.

113. Dubinin, M . M . , Nikolaev. K. M., Polyakav,N.S.. Pirozhkov.G. L., and Filimonor. N. V.. Adsorption of water and microporous s~mucturcs of capban adsorbents. VI. Effect of the panmctcrs of the microporous stmctures of activated carbons an the kinetics of adsorption of vapors o i water. I:?. Akocl. +k., fS.fSRJ Ser. K h i n . . I I, 2429. 1982.

114. Okazaki, M.. Tsman, H.. and Toel, R., Prediction of binary adsorption equilibrra 0 1 solvent and water vapor on activated carbon. J. Clte,,i. Eng. J p m , I l(3). 209. 1978.

115. Barton, S. S. and Koresh. J. E., Adsorption interilction of water with m~croparous ;dwrbentr. I. Water- vapour adsorption on activated carbon cloth. J. Cltent. SOC. Fororlm. 7rmr.7.. 79. 11-17, 1983.

116. Dubinin, M . M.. Kutkov, V. S., Larin, A. V., Nikolaev. K. M . , and Polyakov. N. S.. Caadsorption of vapors of erg" substances and water by microporous carbon adsorbcnts. I. Cvadrorplion of vapors of water and cyclohexane on uniformly damp activated carbon. l i v . Aknd. i V o d . (SSSRI Srr. Kltinz.. 6. 1231, 1983.

117. Mafsumura, Y . , lgaku, S., and Kenkyiyo, S., Adsorption of the paseous components of air pollution with activated carbon, USAF. FTD-lD(RS)T-0438-84. 1984.

118. MeDermol, H. L. and Amell, J. C., Charcoal sorption of water by hydrogen-treawd charcoals. Defcence Research Chemical Laboratories. Canada. 1954.

119. Billinger, B. H. M. and Evans, M . G . . The growth of surface oxygen complexes on the surface of activated carbon exposed to moist air and their effect on methyl iodine-l3l cetentmn. I. Clzott. Plzw Clre,!!. B i d . . 81(11-121. 779. 1984.

120. Dubinin. M. M., Adsorption of water vapor and the microporous stm~ture a i carbon adsarbents. Proc. Acnd. Scl. USSR Chev,. Ser. . I. 9. 1981.

121. Dubinin, M. M. and Serpinsky, V. V., Isotherm equation for water vipor adsorption by microporous carbonaceous adrorbentr. Cnrbon. i9(5). 402. 1981.

122. Dubinin, M. M. and Nikolaev, K. M., Waler adsorption and microporous ~ t m c t ~ r e s oicarbon adrorbmts. ViII. Interrelation between the parameters of the microporous ~lmcture of activated carbons and the equilibrium constants of the isotherms of water adsorption, h v . Aka</. Nnek. (SSSRI Srr. Klzim. 4. 743. 1984.

123. Ackerman. F. J. and Grens, J. 2.. Mechanisms that affect the adsorption o i methyl iodide an charcoal under humid conditions. Repon #UCRL- 14990. Lawrence Livermore Laboratory. University of Califomia, Livemore. Calif.. 1966.

124. Dubinin, M . M. and Kulkav, V. S., Methods for the study of the dynamics of adsorption of organic vapors in the presence of water. ZIr. Fiz. Kltim.. 56(4). 985. 1982.

125. Dubinin, M. M. and Serpinsky, V. V., Isotherm equation for water vapor adsorption by microporous carbonaceous adsorbents. Ext. Abstr. Pmg. Bienn. Coni.. Corbori. 15. 214. 1981

126. Grant, R. J. , Joyce, R. S., and Urbanclo, J. E., The effect of relative humidity on the adsorption of water-immiscible organic vapors on activated carbon. in Fsrtrlnmenndr ofAdmrp,io,t Prvc. E!,$. Formd. Cotf.. Myers. A. L. and Bclford. G.. Eds.. Bavaria, W. Germany. 1984. 219.

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