lab evaluation of act carbon for liquid phase applications - chemviron carbon
DESCRIPTION
This booklet enables customers to run testsunder suitable conditions and to evaluatethe test results scientifically.TRANSCRIPT
Laboratory evaluation of activated carbon for liquid phase applications
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www.chemvironcarbon.com
Laboratory evaluation of activated carbon for liquid phase applications
3029 Blue lab book cover:Layout 3 6/6/07 10:00 Page 1
1 2
Preface
We have developed this book based on our experience in activated carbon, acquiredsince the early years of the 20th century. This booklet enables customers to run testsunder suitable conditions and to evaluatethe test results scientifically.
This work is intended as a guideline and not as a substitute for personal contact with our experienced technical staff.
3 4
Table Of Contents1 Introduction 6
1.1 History 6
1.2 Activated Carbon 8
1.3 Forms of Activated Carbon 9
1.4 Adsorption 10
1.5 Recycling by Thermal Reactivation 12
2 Isotherms 13
2.1 Theory 13
2.2 Experimental 14
2.2.1 General Description 14
2.2.2 Selection of Experimental Conditions 15
2.2.3 Test Procedure 17
2.3 Interpretation of the Isotherm 19
2.3.1 Linear Isotherm 19
2.3.2 Comparison of Different Isotherms 20
2.3.3 Non-Linear Isotherms 23
2.4 Summary 25
3 Adsorption Studies in Columns 26
3.1 Introduction 26
3.2 Experimental 28
3.2.1 Test Location 28
3.2.2 Selection of Equipment 28
3.2.3 Columns 32
3.2.4 Pump 35
3.2.5 Flow Meter and Totaliser 35
3.2.6 Connections 35
3.3 Liquid Pre-treatment 35
3.4 Carbon Pre-treatment 36
3.5 Column Operation 38
3.5.1 Temperature 38
3.5.2 Linear Velocity of the Liquid in the Bed 38
3.5.3 Duration of Column Testing 39
3.5.4 Displacement, Sampling and Analysis 39
3.6 Interpretation of Pilot Data 39
3.6.1 Mass Transfer Zone Determination 40
3.6.2 Minimum Carbon Consumption 42
3.7 Fixed Bed: Single, Series or Parallel 43
3.8 Pulse Bed 46
3.9 Accelerated Column Test (ACT) 47
3.10 Sizing of the Adsorption System 48
4 Conclusion 49
Appendix 1 : Format For Reporting Isotherms Data 50
Appendix 2: Format For Reporting Pilot Data 51
Glossary of terms commonly used in Activated Carbon Technology 52
5 Headquarters and Technical Department 55
5
1 Introduction1.1 History
6
Adsorption on porous carbons was described as early as 1550 B.C. in an ancient
Egyptian papyrus, and later by Hippocrates and Pliny the Elder, mainly for
medicinal purposes. In the 18th century, carbons made from blood, wood and
animals were used for the purification of liquids. All of these materials, which
can be considered as precursors of activated carbons, were only available as
powders. The typical application was batch contact treatment where a
measured quantity of carbon and the liquid to be treated were mixed and,
after a certain contact time, separated by filtration or sedimentation.
At the beginning of the 19th century, the decolourisation power of bone char
was detected and used in the sugar industry in England. Bone char was
available as a granular material which allowed the use of percolation
technology, where the liquid to be treated was continuously passed through a
column. Bone char, however, consists mainly of calcium phosphate and a small
percentage of carbon; this material was only used for sugar purification.
Egyptian Hieroglyphics
In addition, Calgon Carbon Corporation and Chemviron Carbon pioneered a
great deal of the work on the optimisation of granular carbon reactivation.
Today many users are switching from the traditional use of powdered activated
carbon as a disposable chemical to continuous adsorption processes, and using
granular activated carbon plus reactivation. In doing this they are moving towards
a more sustainable process that recycles and minimises waste.
1.2 Activated CarbonActivated carbon is a porous material made up of a random structure of graphite
platelets. It is produced from various carbonaceous materials such as different
grades of coal, coconut shells, wood and peat. The majority of activated carbons
are produced from coal.
When these materials are chemically or thermally treated, changes occur in the
precursor’s original structure. When sufficient thermal treatment is applied, the
carbon atoms from the precursor begin to realign and form plate-like structures
similar to graphite, however, they do not form the extended, uniformly packed
structure that graphite has. Figure 1 and Figure 2 provide schematic diagrams
of graphite and activated carbon.
A review of these diagrams reveals the ordered structure of graphite and the
limited ordering in activated carbon. Note that the “pores’’ in activated carbon
are cracks and crevices.
In the early years of the 20th century, the first processes were developed
to produce activated carbons with defined properties on an industrial scale.
However, with both manufacturing processes, i.e. steam activation (V. Ostreijko,
1900 and 1901) and chemical activation (Bayer, 1915), only powdered activated
carbon could be produced.
During the First World War, steam activation of coconut char was developed
in the United States for use in gas masks. This activated carbon type contains
mainly fine adsorption pore structures suited for gas phase applications.
Carbons with more transport pore structures, necessary for decolourisation
of liquids, were only available as powders until the middle of the 20th century.
After World War II, Calgon Carbon Corporation (the parent company of
Chemviron Carbon) succeeded in developing coal-based, granular activated
carbons with a substantial content of transport pore structure and good
mechanical hardness. This combination allowed the use of activated carbon in
continuous decolourisation processes, which, for many end users, such as sugar
decolourisation, is an ideal combination.
7 8
Figure 1: Graphite structure Figure 2: Activated carbon structure
Aliphatic dislocation of platelet
Inter bondingof plates100 Ångströms
Graphiteplatelet
3.354 Ångströms interlayer distance
Graphic plates
Activated Carbon Cloth - Activated Carbon
is also available in special forms such as cloth
and fibres.
Briquettes - Activated carbons can be
agglomerated and formed into a variety of
briquettes.
“The customer can have any colour he wants solong as it’s black” – Henry Ford - For some
applications it is now possible to produce activated
carbon in a range of different colours.
Granular and pellet activated carbons are used in fixed beds with the liquid or
gas passing through the bed. For powder activated carbon, the powder is added
to the liquid stream, mixed and then removed by filtration or decantation.
Activated carbon is also available in special forms such as cloth and fibres.
1.4 AdsorptionAdsorption is a naturally occurring phenomenon in which molecules of a
liquid or gas are attached to, and trapped by, either an external or internal
surface of a solid.
Within a solid like activated carbon, each molecule is aligned with another so
that equal forces act upon it, resulting in a balanced state. The molecules on the
surface are not completely surrounded by other molecules and, consequently, are
not in a balanced state. As those molecules seek to reach a balanced state, surface
tension or energy is created. This surface energy is an attractive force and, if it is
strong enough to overcome the energy of other passing molecules, they will
adhere to, or be adsorbed onto, that surface. These forces are known as "London
Dispersion Forces" (see Figure 3).
10
The carbon precursor usually has an inherent pore structure associated with it.
The activation process enlarges these pores, which ultimately provide channels
for the adsorbates to penetrate into the activated carbon.
When an adsorbate finds an area where the graphite platelets intersect, or are
close to each other, adsorption occurs. Where the platelets intersect are the
micropores (adsorption pores) and the enlarged pores are the macropores
(transport pores). Adsorption only occurs in the micropores (high energy sites).
Due to its graphite platelet structure, activated carbon has a very large surface
area which gives it excellent properties of adsorption of organic compounds.
These are removed from solution and fixed on the activated carbon surface by
adsorption forces.
1.3 Forms of Activated CarbonThere are a number of different forms of activated carbon as described below.
The form will affect the tests used in its characterisation.
Granular Activated Carbon (GAC) - Irregular
shaped particles with sizes ranging from 0.2 to
5 mm. This type is used in both liquid and gas
phase applications.
Powder Activated Carbon (PAC) - Pulverised
carbon with a size predominantly less than
0.18 mm (US Mesh 80). These are mainly used in
liquid phase applications and for flue gas treatment.
Extruded Activated Carbon - Extruded and
cylindrical shaped pellets with diameters from 0.8
to 5 mm. These are mainly used for gas phase
applications because of their low pressure drop,
high mechanical strength and low dust content.
9
As a result of this high adsorption capacity, activated carbon can be used to
purify liquids in any part of plant operations. It is used for raw material
purification to improve final product quality, or to treat effluent to remove
undesirable compounds.
The efficiency of powdered activated carbon used in batch processes can be easily
predicted from pre-trials which require only a short time. In most cases, it can be
immediately scaled up from the laboratory to the industrial equipment.
The adsorption kinetics on granular activated carbon are primarily regulated by
diffusion processes. Percolation tests with the granular material should, therefore,
be conducted and evaluated using certain scientific procedures in order to obtain
suitable information for the design of an industrial plant.
1.5 Recycling by Thermal ReactivationOnce granular activated carbon is saturated or the treatment objective is reached, it
can be recycled by thermal reactivation for reuse. Reactivation involves treating the
spent carbon in a high temperature reactivation furnace to over 800°C. During this
treatment process, the undesirable organics on the carbon are thermally destroyed.
Recycling by thermal reactivation is a highly skilled process to ensure that spent
carbon is returned to a reusable quality. Chemviron Carbon operates Europe’s
largest reactivation facilities and recycles daily large quantities of spent carbon for
a diverse range of customers. Recycling activated carbon by thermal reactivation
meets the environmental need to minimise waste, reducing CO2 emissions and
limiting the use of the world’s resources.
1211
In simpler terms, for example, if you have a contaminant in water and the energy
exerted by the activated carbon for adsorption is stronger than the affinity of the
contaminant and the water, then the contaminant will stick to the surface of the
carbon in preference to remaining in the water.
Aliphatic dislocation of plateletFine transparent pore
Inter bondingof plates
Adsorbate in a liquid-like state
100 ÅngströmsGraphiteplatelet
Figure 4: Molecular structure of a loaded activated carbon
Feluy reactivation facility
Additivity of London Dispersion Forces
(10,000,000 x Magnification)
The adsorption force present at
the adsorption site is the sum of
all the individual interactions
between carbon atoms and the
adsorbate molecules.
Figure 3: Relation between structure and adsorption force
AdsorbateMolecule
CarbonAtom
Taking logarithms, we obtain:
log X/M = log k + 1/n log C
This is the equation of a straight line whose slope is 1/n and whose intercept is k
at C = 1. Therefore, if X/M is plotted against C on log-log paper, a straight line
should be obtained. There are, however, occasions when deviations will occur;
for example, isotherms of liquids containing multi-components may have a line
with multiple slopes.
2.2 Experimental2.2.1 General Description
Liquid phase isotherms are normally based on a Freundlich isotherm which is an
adsorption test showing the relationship between the residual concentration of a
compound against the loading of the compound on the carbon. This is sometimes
referred to as the liquid phase concentration versus the solid phase concentration.
First, the carbon is ground up to reduce kinetic effects allowing the carbon to reach
equilibrium as quickly as possible. Thereafter, increasing amounts of carbon are
added to different flasks containing fixed volumes of the liquid to be treated. One of
the flasks does not contain carbon - this is the blank. The principle of the isotherm
is illustrated in Figure 6.
14
An adsorption isotherm is a relatively simple method for determining the
feasibility of using granular activated carbon for a particular application.
A liquid phase isotherm shows the distribution of impurities between the
adsorbed phase and the bulk solution at equilibrium. It is a plot of the amount
of impurity adsorbed per unit weight of carbon versus that remaining in the
solution (Figure 5).
Generally, straight line plots can be obtained by making use of the empirical
Freundlich equation, which relates the amount of impurity in the liquid phase to
that amount adsorbed onto the activated carbon by using the expression:
X/M = kC1/n
where
X/M = loading, is the amount of impurity adsorbed (X) per unit weight
of carbon (M);
C = equilibrium concentration after adsorption;
k, n = constants.
13
2 Isotherms2.1 Theory
1
10
co
100
0.1 1 10 100C0
Concentration C (mg/l)
(X/M)
Load
ing
X/M
(mg
/g)
Figure 5: Adsorption isotherm
Blank
Increasing dosage of carbon
Figure 6: The Freundlich Isotherm procedure
It is known that the kinetics of adsorption onto activated carbon is controlled by
the process of diffusion. The transfer of the impurities from the bulk solution to
the internal surface of the carbon proceeds through three stages as indicated in
Figure 7:
1) Bulk diffusion of the compound from the liquid to the film around the carbon
particle
2) Diffusion through this surface film and
3) Diffusion through the internal structure to the adsorption sites in the carbon.
It is known from experiments that step 2 is normally the rate determining step.
In order to increase the kinetics of the rate determining step, it is necessary to
pulverise the granular carbon for testing by means of a ball mill or other suitable
mechanical device so that 95 % will pass through a 45 µm (325 US Mesh) screen.
Carbon dosages are dependent upon the quantity of impurities present in the
liquid and sufficient dosages should be used to establish an accurate curve.
Experience has shown that dosages ranging from 0.1 g/l to 10 g/l are sufficient
for most applications.
16
The flasks are then stirred, normally overnight, at a constant temperature to
reach equilibrium. The carbon is then filtered from the sample and the
remaining liquid analysed for the concentration of the compound being
adsorbed. The concentration in each sample and the blank is then determined.
From these measurements, all the values necessary to plot an isotherm may be
calculated (Appendix 1) and plotted onto a log-log graph. The Freundlich
Equation assumes a straight line, but in practice a curved or unusually shaped
line may result.
An isotherm can be carried out either using a sample of liquid to be treated
(containing the contaminant), or a synthetic isotherm is carried out using
distilled water, spiked with the contaminant.
Though an isotherm is useful for indicating whether or not a substance is
adsorbable and can be used to estimate the consumption of activated carbon, it
does not give required design information such as the contact time.
2.2.2 Selection of Experimental Conditions
There are a number of variables that can affect the efficiency of adsorptionwhen performing isotherms.
Adsorbate solubility and liquid viscosity are key parameters in carbon adsorption,and both are affected by temperature. In order to obtain data that is relevant tothe process, it is essential to run the isotherm under the same conditions as that ofthe process.
As described previously, a liquid phase isotherm shows the distribution ofimpurity at equilibrium. In order to obtain equilibrium, the carbon and liquidmust be in contact for sufficient time, preferably overnight for water and longerfor liquids with higher viscosities.
The solution should be tested at a pH which is consistent with the process.However, it may be worthwhile to evaluate the effect of pH and determine if thecarbon loading is increased sufficiently to warrant changing the pH in the process.
The solution pH may change by coming into contact with the carbon, either as aresult of the dissolving of a portion of its ash, or by preferential adsorption of acidicor alkaline components. Since some impurities in solutions are sensitive to pHchanges, it is imperative that the final pH should be adjusted to the same value asthe untreated liquid before any concentration measurements are made.
15
mm
Bulk diffusion
10 nm
Internal diffusion
Film diffusion Adsorption
1 nmmicron
Figure 7: Diffusion in an activated carbon particle
18
Stoppered Erlenmeyer flasks serve as satisfactory containers for the carbon
solution mixture.
Manual agitation, even at frequent intervals, is usually inadequate and a
mechanical shaker or magnetic stirrers are preferred. After the required contact
time has elapsed, the carbon must be removed from the liquid prior to analysis.
This can be performed using either vacuum or pressure filtration.
2.2.3 Test Procedure
1. Pulverise a representative sample of the appropriate granular activated carbon
(a 10-20 g sample is usually adequate) so that 95 % will pass through a
45 µm screen (325 US Mesh), or use material pre-prepared and supplied by
Chemviron Carbon. Oven dry the pulverised sample for three hours at 150°C.
2. Obtain a representative sample of the liquid to be tested. Any suspended
matter should be removed by filtration using a glass microfiber filter (grade
GF/C). pH adjustment, if required, should be performed on the sample before
filtration.
3. Transfer a minimum of six different weights of the oven-dried pulverised
sample to separate Erlenmeyer flasks. Sample weights should be chosen in
accordance with the carbon dosage suggestions given above.
4. To each container add 100 ml of test solution (V) from a graduated cylinder,
and clamp the flasks on the shaker. If a constant temperature bath is used,
the shaker should be so designed as to permit immersion of the lower part of
the flask containing the carbon solution mixture in the bath.
5. Agitate the mixture for a minimum of four hours, preferably overnight, in
some instances the equilibrium agitation time may exceed 24 hours. If the
sample contains a volatile constituent, special care should be taken during
manipulation to avoid loss of the constituent. The same volume of solution
should be added to a container without carbon and subjected to the same
procedure in order to obtain a blank reading.
6. After the chosen contact time has elapsed, filter the contents of the flask.
One should discard the first and last portions of the filtrate and save only the
middle portion for analysis. The blank should be filtered in the same manner
as the other samples.
17
M C X = (Co - C)V X/MCarbon dosage Concentration Constituent Constituent remaining in adsorbed adsorbed
solution per unit weight(g/l) (mg/l) (mg) (mg/g)
0 18.150.1 15.90 2.25 22.500.2 14.12 4.03 20.150.5 10.25 7.90 15.801 7.15 11.00 11.002 3.93 14.22 7.115 1.80 16.35 3.2710 0.95 17.20 1.72
7. Determine the residual concentration of the contaminant in the filtrate by the
relevant analytical technique.
8. Tabulate the data as shown in Table 1. The residual solution concentration,
C, is obtained directly from the filtrate analysis. The amount adsorbed on the
carbon X is obtained by subtracting the value of CV from the value of CoV,
the influent concentration. The amount of constituent adsorbed per unit
weight of carbon, X/M is calculated by dividing X by the weight of carbon M
used as follows:
X/M = (CoV - CV)/M
9. On log-log paper plot C, on the horizontal axis, against X/M, on the vertical
axis, and draw the best straight line through the points, as illustrated in
Figure 5.
Table1: Isotherm data table
2.3.2 Comparison of Different Isotherms
The performance of two carbons for the same application can easily be
compared by the examination of isotherms obtained under identical conditions.
Isotherms obtained for two carbons (A and B) using the same test solution are
plotted in Figure 8. The numbers adjacent to the experimental points in Figure 8
indicate the grammes of carbon used in the test and the relative performances
of the two carbons.
The two carbons can be compared at any desired degree of purification by
drawing a vertical line for that concentration from the horizontal axis and
reading the X/M values corresponding to the points of intersection of that line
with the isotherms. Such a line has been drawn at Co influent concentration.
On Figure 8, the following X/M values are obtained:
(X/M)A = 26.5 mg/g
(X/M)B = 41 mg/g
The relative efficiency (RE) of carbon A with respect to carbon B can be
calculated from the equation:
% RE = x100 = 64.6 %
20
2.3 Interpretation of the Isotherm2.3.1 Linear Isotherm
If a vertical line is drawn (Figure 5) from the point on the horizontal axis
corresponding to the influent concentration (Co), and the best line through the
data is extrapolated to intersect this (Co) line, the loading X/M of the activated
carbon can be read from the vertical scale. This value, termed (X/M)Co
represents the amount of impurity adsorbed per unit weight of carbon when
the carbon is in equilibrium with the influent concentration. Since this
equilibrium can be reached in a properly designed system, it represents the
ultimate adsorption capacity of the carbon.
From the value of (X/M)Co, the carbon dosage can be calculated for complete
removal of impurities:
Theoretical carbon dosage = Co/(X/M)Co
where
theoretical carbon dosage = theoretical amount of carbon used per volume of liquid;
Co = influent concentration;(X/M)Co = extrapolated loading per unit
weight of carbon at the influentconcentration Co.
The following is an example for a capacity calculation:
If we plot the isotherm as shown in Figure 5 and extrapolate the data, we obtain
a loading (X/M)Co = 26.5 mg/g at an influent concentration Co = 18.15 mg/l.
From these figures, the theoretical granular carbon dosage or usage rate to treat the
liquid may be obtained. In this example, the grammes of carbon required to treat
one litre of solution are:
However, there are some applications where the carbon dosage will be higher
than this value because the carbon which is removed from the system will not
be fully saturated.
Note that if we use a powdered activated carbon for the same application, we can see
from Table 1 that we need a carbon dosage of 5 g/l to obtain only 90 % purification.
19
1
10
co
100
0.1 1 10 100C0
Concentration C (mg/l)
(X/M)
Load
ing
X/M
(mg
/g)
Carbon A
5 g GAC/I
5 g GAC/I
Carbon B
26.5 mg/g41 mg/g
Figure 8: Parallel isotherms
Theoretical carbon dosage = 18.15 mg/l26.5 mg/l
= 0.685 g carbon/l solution
At concentrations higher than C1, carbon A has a greater capacity than carbon B;
at lower concentrations, the reverse is true. Carbon A would be preferable for
column operation because of its higher capacity at influent concentration. It should
be noted, however, that for batch treatment, carbon B would be preferable.
In general, the steeper the slope of the isotherm exhibited by a particular carbon,
the greater will be its efficiency in column operation. This example also illustrates
the necessity of obtaining isotherms that cover the entire concentration range
before comparing two carbons for a particular application.
22
Parallel isotherms were obtained in the case illustrated in figure 8. Consequently,
the relative efficiency will be independent of the purification achieved. However, if
the lines are not parallel, the relative efficiency of each carbon will vary depending
on the treatment objective required.
It should be emphasised that misleading results can be obtained by making the
erroneous assumption that two carbons can be compared for column
application by evaluating only the purification achieved by equivalent dosages.
In the example (Figure 8), 5 grammes of carbon B achieve 93 % removal.
The same amount of carbon A achieves 90 % removal. On this basis, the two
carbons appear almost equally efficient. However, the relative efficiency shows
that in a well designed column system carbon B will have a capacity X/M about
one and a half times greater than that of carbon A at any degree of purification.
In other cases, the isotherms for two carbons will cross as illustrated in Figure 9.
21
C0C111
10
co
100
0.1 1 10 100
Concentration C (mg/l)
(X/M)
Load
ing
X/M
(mg
/g)
Carbon A Carbon B
Figure 9: Crossed isotherms
2423
1
10
100
0.1 1 10 100
Concentration C (mg/l)
Load
ing
X/M
(mg
/g)
1
10
100
0.1 1 10 100
Concentration C (mg/l) Loading Z
Load
ing
X/M
(mg
/g)
11
10
100
0.1 1 10 100
Concentration C (mg/l)
Load
ing
X/M
(mg
/g)
A sudden change in slope, as illustrated in Figure 10b, could indicate that at leasttwo different types of adsorbates are present, showing markedly different capacities.The plot in Figure 10c, similar to Figure 10b, illustrates a situation in which at leastthree types of adsorbates are present, e.g. undefined "colour", but when all areadsorbed similarly, straight line isotherms should be obtained.The curve shown in Figure 10d indicates that the adsorbate has reached itsmaximum surface coverage at loading Z.
2.3.3 Non-Linear Isotherms
It was previously stated that, ideally, straight line isotherm plots are obtained.
However, there may be departures from linearity.
A curve similar to that shown in Figure 10a may be obtained if a non adsorbable
impurity is present in the liquid being treated. Subtracting C, concentration of
non adsorbable compounds, from C0 and replotting, the isotherm will usually
yield a straight line.
(b)
(d)
(c)
Figure 10: Non-linear isotherms
1
10
100
0.1 1 10 100
Concentration C (mg/l)
Load
ing
X/M
(mg
/g)
(a)
3.1 IntroductionIf an isotherm study indicates that the liquid can be treated to the desired purity
level using granular activated carbon at a reasonable dosage, then the next step
is to evaluate the carbon treatment in a dynamic test.
The purpose of the column test is to obtain the "breakthrough curves” under
dynamic conditions, which show how the concentration of the solutes in the
effluent will vary with the volume of liquid treated.
This data will allow Chemviron Carbon to establish the design criteria for a full
scale plant:
• the effective capacity of the granular activated carbon in an operating system
• the shape of the mass transfer zone and the bed depth to be taken into
account for optimum adsorption
• the contact time necessary to achieve the treatment objective in the most
economical manner
• influence of suspended solids or gas formation on the operating conditions
for the adsorption columns
• colloidal suspended solids will have to be removed prior to the adsorbers
or the system will have to use the granular activated carbon column as a
mechanical filter as well and include a backwash sequence in the operation cycle.
If gas evolution occurs, the adsorption system must be designed to prevent
formation of gas pockets within the carbon bed.
Depending on the application and flow, different types of adsorbers are used.
Therefore, certain additional information must be obtained before the column
test can be started. The technical department of Chemviron Carbon has
extensive experience in purification of liquids and is able to provide advice on
the type of adsorber to be used. In order to do this evaluation, the operating
capacity as well as the desired flow rate and bed depth must be established to
determine the optimum dimensions and the number of columns necessary for
continuous treatment.
3 Adsorption Studies in Columns
26
2.4 SummaryFrom the isotherm adsorption capacity, an estimate of the granular carbon
usage rate necessary to meet the treatment objective can be obtained. Isotherm
tests also afford a convenient method for comparing different carbons and for
investigating the effects of pH and temperature.
It is essential to realise that the lowest possible carbon usage rate is predicted
by a properly run isotherm test. If a lower usage is observed in column tests or
in the plant, then one or more of the following is true:
• the isotherm test was not at equilibrium
• volatile components were lost during the isotherm trial
• the interpretation of the isotherm was done incorrectly
• there was an error in the analytical techniques
• carbon fines may not have been completely removed from the solution
• the influent liquids tested in the isotherm and column tests were not
identical
• there are micro-organisms present on the carbon in the column which are
causing biological degradation of the adsorbed organic substances
compounds, thereby reducing the carbon usage
• the pH of the isotherm solution may have been altered
• the isotherm was run under different process conditions compared to the
actual process.
25
3.2 Experimental3.2.1 Test Location
The tests should be conducted at the plant if any of the following
conditions exist:
• the liquid deteriorates or changes in character during shipment
and/or storage
• the carbon dosage is so low that large quantities of liquid would be
required to conduct the test
• the conditions of the liquid stream being treated vary widely.
3.2.2 Selection of Equipment
The pilot test can be carried out in either fixed beds or in a single pulse bed
column. Depending on the application, it should be discussed which type of
adsorber should be considered.
A simple way to conduct a pilot test is by using fixed bed columns. From this
test we can obtain design data for either a fixed bed or a pulse bed system.
3.2.2.1 Fixed Bed Systems
In the fixed bed vessel (Figure 11), the solution enters the top of the vessels
and passes through the carbon until the carbon is spent or the outlet specification
is exceeded. Then, the entire volume of carbon is removed from the vessel and
replaced with either virgin or
reactivated carbon.
The fixed bed system will have
fluctuation in the outlet quality.
Initially, the carbon depth is sufficient
to contain the mass transfer zone.
However, as the carbon becomes
spent the remaining carbon is not
sufficient to remove all the impurities
and some of them begin exiting the
vessel; breakthrough occurs.
28
The rate at which the solutes are adsorbed by the carbon can only be
determined by dynamic column tests. This is the key to the system design.
Ideally, it would be desirable to be able to use mathematical techniques to
predict the carbon performance from the equilibrium data. There have been
many studies conducted and models developed, however, they all have their
limitations. The studies used to develop these models were based on the
presence, or assumed presence, of one or more of the following elements:
• uniform carbon particle size
• ideal isotherm
• dilute solutions
• single component
• adsorbate molecule was too large to enter the fine pore structure, so onlyfilm diffusion was controlling the kinetics
• flow rate was high so pore diffusion was controlled
• comparisons of predicted versus actual data were made on early part of breakthrough curve before carbon was saturated
• systems with relatively short mass transfer zones (MTZ) were studied.
Until such time as a universal model is developed, it is still necessary to test in
a dynamic system and use empirical methods to design the plant scale units.
Prior to conducting a column test many aspects must be considered. Some of
these are listed below and then discussed separately:
• location of the test
• size and type of pilot system column, diameter, number of columns, fixed orpulse bed
• carbon type and particle size
• linear velocity of liquid in the carbon bed
• temperature
• pH
• treatment objective
• presence of suspended solids
• heat of adsorption for organics during the wetting.
27
Figure 11: Fixed bed adsorber
Time on stream or volume treated
Fixed bed outlet concentration Treatment objective
Co
nce
ntr
atio
n
Figure 12: The Pulse Bed System
30
3.2.2.2 Pulse Bed Systems
The pulse bed system (Figure 13) provides the best control over the outlet quality
compared to any available technology. It is operated in a true counter current
fashion. The liquid to be treated enters the bottom of the column, passing
through the most spent carbon first. As the liquid rises through the column it is
purified; exiting the column at the top where the freshest carbon resides.
Because the adsorption process is equilibrium based, this operation best utilises
the carbon adsorption capacity. Carbon which has been in equilibrium with a
lower impurity concentration (in the top portion of the column) can come to a
new equilibrium with a liquid (in the lower portion of the column) having a higher
concentration than the original. This enables carbon to better accommodate
fluctuations in the inlet contaminant
concentration while maintaining
a consistent concentration in the
outlet.
Periodically, the flow of liquid is
stopped. Spent carbon is withdrawn
(pulsed) from the bottom and virgin
or reactivated carbon is added into
the top of the adsorber.
29
Time on stream or volume treated
Treatment objectivePulse bed outlet concentration
Co
nce
ntr
atio
n
Figure 14: Pulse bed adsorber
Figure 13: Pulse bed adsorber
32
3.2.3 Columns
The pilot unit consists of either fixed beds in series or a single pulse bed column.
Usually, full scale granular activated carbon beds are from 1.0 to 10 metres in depth
and from 0.3 to 4.0 metres in diameter. In a laboratory, the diameter of the
columns can be scaled down to 50 mm; a smaller diameter should not be selected
in order to avoid excessive wall effects. By an appropriate reduction in flow rate,
plant conditions can be simulated. The carbon bed depth or superficial contact
time, should be the same as, or greater than the full scale system.
If a pulse bed pilot column is used, full size plant conditions are rigorously
simulated. Strictly speaking, fixed beds in series do not reproduce pulse bed
conditions, however data obtained from such pilot systems can provide a reliable
basis for design.
Another way to conduct a column test is to use one of Chemviron Carbon's range
of mobile adsorbers (see Figure 15), from the Cyclesorb® series of vessels. They offer
several advantages compared to traditional pilot columns.
31
Figure 15: Cyclesorb® MA20 adsorbers in operation
3.2.2.3 Fixed Bed vs. Pulse Bed System
The choice of system depends on many factors. Some of the advantages
and disadvantages of each system are given below.
FixedBeds
•Requires less plot area•Can be controlled to have the
effluent quality close to thespecification value
•Has the lowest carbon dosage inapplications having a long masstransfer zone
•No peak consumption of waterduring the recovery of product
•If gas is present, or can begenerated in the carbon bed, anup-flow design is the betteralternative.
Advantages
•Lowest building profile•Little operator attention required•Self-filtering system•Easy to inspect vessel interiors•Requires lower inlet pressure to
pass through the system•Can handle large flow•Can be designed for backwashing.
Disadvantages
•Requires greater plot area•Higher capital investment than a
pulse bed system for the equivalentamount of carbon on stream
•If a single adsorber is used effluentquality will vary as a function ofthe on-stream time of the column
•The treatment objective is reachedat the same time as the column isdisconnected from the system.
PulseBeds
•Cannot treat liquids containingsignificant quantities ofsuspended solids
•Inspection and/or repairs can onlybe done by taking an adsorber offline and emptying the entirecarbon content
•Effluent contains carbon finesafter each pulsing operation andshould be filtered.
For a classic laboratory study, columns are mounted vertically and connected in
series for down-flow operation as shown in Figure 16. Up-flow treatment can also
be considered, but there is no inherent advantage.
Provision must be made for taking samples between each column so that
purification can be studied as a function of bed depth, as well as of time and
volume of solution treated. The carbon bed is supported by a stainless steel or
plastic screen, glass wool, or porous plate at the base of each column. Manometers
or pressure gauges should be used to monitor the pressure drop characteristics of
the operation. The liquid is fed to the adsorbers using a positive displacement pump
to allow accurate flow control. If necessary, the adsorbers can be jacketed or heat
traced to avoid a temperature gradient across the beds.
Figure 17 shows a set-up for a pulse bed pilot plant. The liquid is fed into the
bottom of the adsorber and flows upwards through the carbon bed. The liquid is
sampled before the adsorber inlet (sample 0), at different levels in the carbon bed
(samples 1, 2, 3) and at the adsorber outlet (sample 4). The adsorber can be
jacketed, if necessary. At regular intervals a fixed amount of carbon is withdrawn
from the bottom of the adsorber, and an equal volume of fresh carbon is added to
the top of the adsorber.
3433
Figure 16: Fixed bed pilot plant arrangement
INFLUENT
VentsSiphon-break
PI
Pump
Sample Ports(0)
(1) (2) (3)
(4)
The Cyclesorb® service is a mobile adsorption service combining adsorber and
transport vessel for the treatment of food grade, non-corrosive and corrosive liquid
streams. A pre-loaded Cyclesorb® is delivered to a site and larger vessels can be
moved by forklift where required.
The unit is operated until saturation of the activated carbon. A second unit is then
delivered after saturation, and the exhausted unit is picked up at the same time, in
order to minimise transport costs.
Once the unit is returned to a Chemviron Carbon facility it is discharged, emptied of
carbon, cleaned, inspected and refilled with either virgin or reactivated carbon.
The Cyclesorb® units range in size from bench scale to full industrial scale.
These carbon vessels provide great flexibility of operation, and can eliminate the wall
effects and other problems found using small scale pilot equipment.
The liquid volume treated during these tests is much greater than with a classic pilot
test so that plant variations can be evaluated more thoroughly. Due to their design,
the Cyclesorb® range is also very easy to handle and to start up.
Since the carbon is pre-wetted and contained within the Cyclesorb® there is no
need for the operator to come in contact with the carbon.
Figure 17: Pulse bed pilot plant arrangement
Prefilter
Spent carbon
ChargeTank Siphon-break
Outlet
(3)
(2)
(1)
(0)
(4)
INFLUENT
PumpPI
3.4 Carbon Pre-treatmentOne of the most common errors in adsorption column test work which leads to
poor results is that the carbon is not degassed prior to the adsorption test. When
this is not done, air pockets form in the column and result in: (1) channelling, (2)
high pressure drop, and (3) premature breakthrough of adsorbate.
The time required to degas carbon is a function of liquid temperature, as shown
in Figure 18.
The carbon should be wetted prior to being placed in the test columns. Indeed
it is extremely difficult to remove air pockets from the packed carbon bed and
contrary to popular belief, backwashing the carbon at low rates does not
remove the air pockets.
The amount of granular activated carbon required to fill each column may
be determined as follows:
Weight carbon/column = V x AD
where V = volume of carbon in column;AD = apparent density of carbon
36
3.2.4 Pump
A pump capable of uniform delivery rates with positive displacement is preferable.
The most important criterion is the continuous delivery of a constant flow with no
introduction of air.
3.2.5 Flow Meter and Totaliser
A flow meter and totaliser should be installed so that the flow can be controlled
and the treated volume recorded.
3.2.6 Connections
Rubber, glass, plastic or metal tubing can be used, depending on the corrosive
characteristics of the material to be processed. Usually, rubber and glass tubing
connections with glass tees and pinch clamps are the most convenient.
3.3 Liquid Pre-treatmentIf the isotherm investigation indicates that solids removal or pH adjustment is
necessary before carbon treatment, then the same treatment should be made to
the liquid before the carbon column studies. If the test liquid has been stored at
a low temperature for preservation purposes, subsequent heating to room
temperature or higher, may result in degassing the liquid in the carbon bed.
When these conditions exist, the liquid must be degassed prior to pumping
it through the carbon columns.
35
% W
ette
d (%
of a
vaila
ble
inte
rnal
vo
lum
e fil
led
)
Time (hours)25oC 80oC
0
0
100
12 24 36 48
Figure 18: Wetting time with water
3.5 Column Operation3.5.1 Temperature
As adsorption is a function of diffusion rate and as diffusion is affected by the
liquid viscosity, the columns should be operated at the plant process temperature
to eliminate this variable from the data analysis.
3.5.2 Linear Velocity of the Liquid in the Bed
Many studies have been conducted to examine if and how linear velocity affects
diffusion rate. These tests confirm that an optimum linear velocity exists and must
be considered for each application individually. Velocities that are too high can
extend the mass transfer zone and therefore the breakthrough curve (Figure 19).
Consequently, it is recommended that the linear velocity in the pilot column is
the same as that expected to be used in the full scale plant.
If time and money permit, it is advisable to investigate one or two other linear
velocities to ensure that the optimum is being used.
38
The required amount of granular activated carbon for each column should be
weighed into a suitable container and degassed or wetted using one of the
following procedures:
• soak at ambient temperature for a minimum of 24 hours
• boil for 2 hours.
The degassed carbon is charged to the columns as a slurry in small increments,
keeping a thin layer of supernatant liquid above the carbon during charging.
This is best accomplished by filling the column one third full with the wetting liquid
before starting to charge the carbon slurry. This charging procedure should be
repeated for each column in series, all connecting tubing and other void space must
be filled with liquid in order to avoid pushing an air pocket through the carbon bed
when starting the test.
Any gas build-up can be released through the vents at the top of the columns.
37
Figure 19: Influence of linear velocity on the mass transfer zone
0
100
Time on stream or volume treated
Co
nce
ntr
atio
n %
High linear velocity Low linear velocity Inlet concentration Treatment objective
From Figure 20 one can deduce:
• the length of the mass transfer zone
• the adsorption capacity of the carbon for the impurities to be adsorbed at
breakthrough and saturation conditions.
3.6.1 Mass Transfer Zone Determination
During the adsorption cycle in a column, the upper section of the bed will be
saturated with impurity while the lower section is still free of impurity. Between
these two extremes lies a zone in which the adsorption is actually occurring.
This is referred to as the mass transfer zone or MTZ. As the column becomes
saturated, this adsorption zone moves downwards through the bed and can be
regarded as an adsorption wave front moving through the column (see Figure 21).
40
3.5.3 Duration of Column Testing
Ideally, the test should be run until the last column is saturated, or until the
treatment objective of the liquid in the last column has been reached. If time and/or
the quantity of test liquids available do not permit the test to be run to ideal
completion, it should be run to achieve at least saturation in the first column and
breakthrough to 50 % of influent concentration in the second column.
3.5.4 Displacement, Sampling and Analysis
So that accurate breakthrough curves can be defined, the wetting liquid must be
displaced from the columns in order to establish a treatment starting point for each
column used in the study.
After the wetting liquid is displaced from the first column, the collection of data to
determine the breakthrough curve can be initiated.
The breakthrough curve is determined by sampling at regular intervals; analysing
the samples for impurities, and by measuring and recording the volume throughput
(Appendix 2).
The displacement procedure is repeated for each column until the wetting liquid
from every column has been displaced. The breakthrough curve for each column
must then be defined as previously described.
3.6 Interpretation of Pilot DataSamples are taken at the influent of the first column and at the effluent of each
carbon bed. Analytical data is plotted versus cumulative throughput or time. The
plot of this data gives the so-called breakthrough curve for each column.
Figure 20 shows typical breakthrough curves for complex multi-components
where the influent impurity concentration is constant. The sample (1) curve
corresponds to a superficial contact time “t”. Assuming all columns contain equal
amounts of carbon, the curves (2), (3) and (4) correspond to superficial contact
times of 2t, 3t and 4t respectively.
39
Figure 20: Typical breakthrough curves
0
100
Co
nce
ntr
atio
n %
Time on stream or volume treated
Sample 1 Sample 2 Sample 3
Sample 4 Inlet concentration Treatment objective
V1
4241
Figure 21: Mass transfer zone concepts
The length of time required for the appearance of the impurity and the shape of the
breakthrough curve provides an indication of the relative length of the adsorption
zone. If considerable time elapses before the impurity appears, and the breakthrough
curve is sharply defined and steep, then the adsorption zone is short in relation to the
overall bed depth. If, on the other hand, breakthrough occurs almost immediately, a
relatively long adsorption zone is indicated.
After treating a volume of liquid V1 (Figure 20), column no.1 no longer participates
in the adsorption process because it is saturated. The impurities adsorbed on the
carbon are in equilibrium with the impurities in the liquid at influent concentration.
The mass transfer then takes place mainly in columns 2 and 3, and to a minor
extent in column 4.
The variation in concentration of impurity through the adsorbers after treating
a volume (V1) can be represented as shown in Figure 22. This graph shows that
the minimum superficial contact time to purify the liquid from its value C0 down
to Ce is tm.
Figure 22: Concentration versus contact time
0
100
Co
nce
ntr
atio
n %
Inlet concentrationTreatment objective
Superficial Contact Time
(4)(3)
(2)
(1)(0)
tm
t 2t 3t 4t
3.6.2 Minimum Carbon Consumption
The carbon saturation rate is primarily a function of the working conditions, and
the amount and nature of impurity present in the liquid to be treated.
Since carbon consumption depends on the superficial contact time, a good method
of presenting data (Figure 23) is achieved by plotting the carbon consumption
for a specific treatment objective against the individual superficial contact time
of each column. The carbon consumption requirement is calculated by dividing the
weight of carbon in each column by the volume of liquid treated at the point at
which the treatment objective is reached. "The operating line" is the name given to
the curve of such a configuration i.e. a single fixed bed. The operating line allows
us to determine the optimum combination of superficial contact time and carbon
saturation rate.
Headspace
Saturated
Virgin
MTZ
MTZ
MTZ
MTZ
Inlet Inlet Inlet Inlet
OutletOutlet
Short Mass Transfer Zone
OutletOutlet
Long Mass Transfer Zone
Start test Middle of test Breakthrough
3.7 Fixed Bed: Single, Series or ParallelIn a single fixed bed system, the carbon contained in the column is only partially
saturated when the effluent impurity concentration reaches the treatment
objective (Figure 24A). The bed is taken off-stream to be reactivated or replaced
and the amount of carbon per unit of liquid treated exceeds the minimum value
obtained from an isotherm test.
As the depth of the carbon bed is extended beyond the length of the mass
transfer zone, the relative amount of fully saturated carbon increases, and the
carbon usage rate decreases (Figure 24B).
4443
Figure 23 : Operating line of a carbon system
Figure 24: Single and multiple column Mass Transfer Zones
0
100
Car
bo
n C
on
sum
pti
on
%
Superficial Contact Time
Column
t 2t 3t 4t
IsothermShort single column Long single column
Outlet
Inlet
Parallel mode
MTZ
MTZ
MTZ
MTZ
MTZ
MTZ
Outlet
Outlet
A B
C
D
Outlet
Series mode
Inlet
Inlet
Outlet
Inlet
3.8 Pulse BedA pulse bed operates as a large number of beds in series and therefore, is suitable
for applications having high carbon consumption and long mass transfer zone.
Some carbon is removed at a predetermined rate from the bottom of the column
and replaced at the top by fresh carbon to ensure that the mass transfer zone
remains within the column length, thereby ensuring full saturation of the carbon
before discharge. This pulse rate is selected based on the experience of Chemviron
Carbon in similar applications.
The column is sampled at points (0), (1), (2), (3), (4) and analytical data is plotted
versus cumulative volume or versus time. Figure 25 shows a representative set of
results obtained from a pilot pulse bed.
If the chosen pulsing frequency is too low, the mass transfer zone gradually moves
through the bed with the risk of exceeding the treatment objective. On the other
hand, if the selected pulsing rate is too high, the withdrawn carbon will not be
fully saturated and the resulting operating costs will be higher than necessary.
46
A system of two fixed beds in series permits a more efficient usage of the carbon.
Indeed when the effluent impurity reaches the imposed limit, the lead adsorber is
taken off stream and the carbon is reactivated. An adsorber containing fresh or
reactivated carbon is then put on stream in the polishing position (Figure 24C).
In this system, more fully saturated carbon is discharged. As a result, the operating
line for this system will indicate lower carbon exhaustion rates.
Similar operating lines can be established for a system of fixed beds in parallel
(Figure 24D). This figure represents a system of parallel adsorbers that have been
installed at different points in time. This set-up can be used when a part or the
full stream has to be treated at all points in time. The choice of that system
configuration may be investigated due to one or more of the following factors:
• if the flow rate is too high, then the size of the adsorbers in a single pass would
be too large to be economical
• space limitation could prevent the use of an extremely high column
• a minimum pressure drop would be required
• the adsorption process must be continuous with limited process fluctuation
• blending of effluent product is desirable.
Furthermore, a combination of adsorbers in series-parallel is desirable when
a lower carbon dosage has to be combined with the benefit of parallel operation.
45
Figure 25: Pulse bed test results
0
100
Co
nce
ntr
atio
n %
Sample (1) Sample (2) Sample (3) Sample (4)
Time or volume
Inlet concentration
Superficial Contact Time
Co
st
0.5 2 3 4
Investment costTotal cost Operation cost
3.10 Sizing of the Adsorption SystemThe sizing of a granular activated carbon system depends primarily on the length
of the mass transfer zone, the carbon saturation rate and the linear velocity.
These parameters are determined by the column tests.
Once the operating line is established, it is possible to select the combination of
carbon consumption and superficial contact time which will give optimum results
at the lowest cost. Since both capital and operating costs are involved, an annual
operating cost must be established to compare the systems.
The capital investment cost of the adsorption system is primarily dependent on
the superficial contact time. The operating cost depends mainly on the carbon
exhaustion rate. If the annual capital depreciation and operating costs are plotted
as a function of superficial contact time, the summation of the two curves will
give the total cost. The optimum design is indicated by the minimum of the total
cost curve as shown on Figure 27.
48
3.9 Accelerated Column Test (ACT)Pilot testing as described above predicts accurate carbon usage and produces data
appropriate for design, it can however be very time consuming.
Therefore, Chemviron Carbon has developed the Accelerated Column Test, an
improved technique for testing the removal of organic impurities in water
solutions. This test combines speed with the accuracy of a pilot column.
Acceleration of the carbon adsorption cycle is achieved through a scaling down
of the conventional column testing hardware. Except for their reduced scale, the
other components of the test system and the overall system design are essentially
identical to larger scale laboratory or field evaluation systems (Figure 26).
47
Figure 26: Accelerated Column Test equipment
Figure 27: Annual costs
The technology behind the ACT was developed by Calgon Carbon Corporation,
the parent company of Chemviron Carbon. This development enabled Calgon
Carbon scientists to further develop a computer model for the column adsorption
process. The Accelerated Column Test is based on this computer model. With this
computer programme the breakthrough curves for full scale adsorption systems
can be readily calculated from the data generated by the scaled down Accelerated
Column Test.
Appendix 1 : Format For Reporting Isotherms Data
50
In conclusion, it should be emphasised that in many cases there are alternativeways in which a laboratory test can be carried out for a given application. Therelative advantages of each method can be ascertained only by carefully workingthrough the problem, keeping in mind the particular requirements of the processto be tested.
Please contact Chemviron Carbon for further information prior to scheduling anytests. As a Producer and Reactivator of activated carbon since the middle of thelast century, Chemviron Carbon has extensive experience in the application ofactivated carbon in almost every field, and can assist in establishing process andoperational guidelines to answer your needs.
49
4 Conclusion
Cust
omer
nam
eA
pplic
atio
n
Type
of
carb
onD
ate
Carb
on s
ampl
e re
fere
nce
num
ber
Tem
pera
ture
(°C)
pH Conc
entr
atio
nTr
eatm
ent
Obj
ectiv
e
Cont
amin
ant
Agi
tatio
n tim
e
Volu
me
solu
tion
M
CX
= (C
0-
C)V
X/M
Carb
on d
osag
e (g
/l)Co
ncen
trat
ion
Cons
titue
nt a
dsor
bed
Cons
titue
nt a
dsor
bed
rem
aini
ng in
sol
utio
n (m
g)pe
r un
it w
eigh
t (m
g/g)
(mg/
l)
Glossary of terms commonly used inActivated Carbon Technology
5251
Appendix 2: Format for Reporting Pilot DataCu
stom
er n
ame
App
licat
ion
Type
of
carb
onD
ate
Carb
on s
ampl
e re
fere
nce
num
ber
Pilo
t de
scrip
tion
Tem
pera
ture
(°C)
Flow
(l/h
)
pHSu
perf
icia
l con
tact
tim
e (m
in)
Cont
amin
ant
Line
ar v
eloc
ity (m
/h)
Conc
entr
atio
n
Trea
tmen
t ob
ject
ive
Wei
ght
of c
arbo
n/co
lum
n
Volu
me
of c
arbo
n/co
lum
n
Sam
ple
N°
Dat
eTi
me
Vo
lum
e th
rou
gh
pu
tSa
mp
le V
olu
me
An
alyt
ical
dat
aR
emar
ks
Abrasion number The abrasion number of granular carbon defines theresistance of the particles to degrade on handling. It iscalculated by contacting a carbon sample with steel balls ona Ro-Tap machine and determining the percent ratio of thefinal mean particle diameter.
Acid washed activated carbon Carbon which has been contacted with an acid solution withthe purpose of removing some of the ash in the activated carbon.
Activated carbon Activated carbon is a crude form of graphite, with a randomor amorphous structure, which is highly porous, over abroad range of pore sizes, from visible cracks and crevices tocracks and crevices of molecular dimensions.
Adsorbate Any substance that is or can be adsorbed on the adsorbent.
Adsorbent Any solid having the ability to concentrate significantquantities of other substances on its surface. Activatedcarbon is an adsorbent.
Adsorber A vessel designed to hold granular activated carbon.
Adsorption The phenomenon whereby molecules adhere to a surfacewith which they come in contact.
Adsorption isotherms A measurement of adsorption determined at a constanttemperature by varying the amount of carbon used or theconcentration of the impurity in contact with the carbon.
Adsorption pores The finest pores in the carbon structure. Pores which haveadsorption capabilities.
Apparent density The weight per unit volume of activated carbon.
Ash The mineral oxide constituents of activated carbon. It isnormally defined as a weight percent basis after a givenamount of sample is oxidized.
Backwash An operating method used to remove suspended solids fromthe carbon bed. Water is pumped into the bottom of theadsorber, flows upward through the carbon bed, and exitsthrough the backwash outlet. The upward flow expands thebed and removes the suspended solids, carbons fines andentrained air. The percentage bed expansion time requiredfor backwashing is a function of the backwash rate andwater temperature.
Bed depth The amount of carbon expressed in length units, which isparallel to the flow of the stream and through which thestream must pass.
5453
Bed expansion The effect produced during backwashing: the carbonparticles become separated and rise in the column. Theexpansion of the bed due to the increase of the spacebetween carbon particles may be controlled by regulatingbackwash flow.
Breakthrough The first appearance of adsorbate material in the solutionflowing from an activated carbon unit. Breakthrough is afirst indication that reactivation or regeneration of thecarbon is necessary.
Carbon column A column filled with granular activated carbon whoseprimary function is the preferential adsorption of a particulartype or types of molecules.
Channelling The flow of water or regenerant taking the line of leastresistance through an activated carbon bed, as opposed tothe usual distributed flow through all passages of the bed.Channelling may be due to fouling of the bed, poordistribution design or low flow.
Chemisorption Adsorption where the forces holding the adsorbate to theadsorbent are chemical (valence) instead of physical(London's).
Colour bodies Complex molecules which impart colour (usuallyundesirable) to a solution. Carbon adsorption is often usedin such colour removal applications.
Critical bed depth In a carbon column the critical bed depth is the depth ofgranular carbon which is partially spent. It lies between thefresh carbon and the spent carbon and is the zone whereadsorption takes place. In a single column system, this is theamount of carbon that is not completely utilised.
Cross sectional bed area The area of activated carbon through which the stream flowis perpendicular.
Deaeration (wetting) The process of removing air (gases) from a bed of carbon. Ina volume of 1m3 of activated carbon, there areapproximately 400 l of void space, 400 l of pore volume and200 l of skeleton carbon.
Desorption The opposite of adsorption. A phenomenon where anadsorbed molecule leaves the surface of the adsorbent.
Heat of adsorption The heat given off when molecules are adsorbed.
Iodine number The iodine number is the milligrammes of iodine adsorbedfrom a 0.1N solution by one gram of carbon at a filtrateconcentration of 0.02N iodine.
Mass Transfer Zone (MTZ) The adsorption gradient that exists in the carbon bed. Itcorresponds to the gradual transition of the carbon from"spent" to "fresh".
Mesh size The particle size of granular activated carbon as determined bya U.S. Sieve Series. Particle size distribution within a Meshseries is given in the specifications of the particularCHEMVIRON CARBON carbon.
Moisture The percent by weight of water adsorbed on activated carbon.
Pressure drop A decrease in fluid pressure during its flow due to internalfriction between fluid molecules, and external friction due toirregularities or roughness in surfaces past which the fluidflows. In addition, accumulation of solids on the activatedcarbon can cause pressure drop.
Pulse bed A unique application which a single carbon column offers theefficiency of several columns in series. This is accomplished bythe removal of exhausted carbon from one end of the carbonbed and the addition of fresh carbon at the other end withlittle or no interruption in the process.
Reactivation The removal of adsorbates from spent granular activatedcarbon which will allow the carbon to be reused. This is alsocalled revivification.
Regeneration It refers to any desorption process other than reactivation(including hot gases, solvents, supercritical fluids anddisplacing compounds) commonly used to remove theadsorbate from the adsorption space within the carbon.
Surface area The surface area of granular activated carbon is determined bythe Brunauer, Emmet and Teller Method (BET Method), whichutilises the adsorption of nitrogen at liquid nitrogentemperatures in the calculation. Surface area is usuallyexpressed in square metres.
Transport pores Pores larger than the largest adsorption pores. They function asa diffusion path to transport adsorbates. Adsorption does notoccur in these locations even at near saturated conditions.
Voids The percent by volume of the interstices to total bed volume.
Wave front The wave front is the carbon loading gradient that exists in thecritical bed depth. It outlines the gradual transition of thecarbon from "virgin" to "exhausted".
55
5 Headquarters and TechnicalDepartment
European Headquarters Chemviron Carbon, Zoning Industriel C de Feluy, B-7181 Feluy
Tel +32 (0) 64 511 811 Fax +32 (0) 64 541 591
Corporate Office Calgon Carbon Corporation, 400 Calgon Carbon Drive
Pittsburg, PA 15205
Tel +1 (0) 412 787 6700 Fax +1 (0) 412 787 6713
Website www.calgoncarbon.com www.chemvironcarbon.com