scrubber technologies and their maintenance · 2018-06-13 · scrubber technologies and their...

3

SCRUBBER

TECHNOLOGIES AND

THEIR MAINTENANCE

''. .... I BY ART BROOKS SALES MANAGER

KCH SERVICES, INC.

Scrubber Technologies And Their Maintenance By Art Brooks, KCH Services

With the passing of the clean air legislation by the U.S . Congress and with the increased scrutiny on air emissions from various plating processes by state and local regulatory agencies, a well designed, well maintained air pollution control device, is no longer optional, but is mandatory for most platers and metal finishers across the U . S . Although the U . S . EPA has not yet promulgated a fed- eral atandard for chromium mists, most atate regulatory agencies have taken the lead, and have implemented their own stringent emiaaion standards for chromium; as well as for numerous other toxic air compounds.

In many states, compli- yzance must be demonstrated by stack tests. Also , in many areas, regulators require that proper .maintenance be demon- strsted through means including; but not limited to; pump flow indicators, air flow gauges, pressure'gauges, and or accessible maintenance re- cords, &c., to ensure that the scrubber continues to operate at its deaigned efficiency.

Scrubber technologies controlling emissions from metal finishing and plating operations, chemical manufac- turing, chemical processing, laboratories, municipal waste treatment facilities, etc., have greatly progressed in recent years. New scrubber packing designs are available for greatly increaaed *'ma88 transf erg* , ( i .e. transfer from

the gas to the liquid phase) and also for a more uniform distribution of the scrubbing solution throughout the packing section.

High efficiency mesh pad mediums are now regularly designed into "wet" and "dry" scrubber systems. These medi- . .urns are capable of removing acid or base mists with great efficiency. Some mesh pad configurations remove particles as small as one micron <um) in diameter.

Custom designed, high efficiency scrubbers are now required more than ever, and are

-1 . . more efficient than ever. Most are designed fo

resistance ast. However, if

:regular maintenance is neglected, theae units can become fowled or plugged, or the scrubbing solution can become too contaminated' for absorption to take place, resulting in excessive emissions, de- creased air flow or both. As any piece of equipment in your plant requires proper atten- tion to maintenance, so does the scrubber and it will not be ignored.

KCH SERVICES, I N 0 P.O. Box 1287

100 Westerly Hill Dr Forest Citv. NC 28043

.-. . . .

1

. ..-.

Emission Requirements The reaeon for the development of high efficiency acrubber technologies, and the reason it is so important to maintain them, is the ever tightening of emission standards for Tox- ic Air compounds (T.O.C.) across the U.S. Some states enforce the emission rates at the top of the stackhead, ae does California for chromium compounds. But most states enforce these emissions at the company's property boundary, fence line or nearest critical receptor. The maximum ground level concentration (G.L.C.) allowed at the company's property boundary is known by several different acronyms that vary from state to state. Some states refer to this as --either an Hazard Limiting Val- ue (H.L.V.), an Acceptable Ambient Level (A.A.L.), or an

._;;Acceptable Source Impact Level -(A.S.I.L.). For many compounds, the G.L.C. is estab- lished as one percent (1x1 or 1/100 of the Threshold Limit VaTue (.T.L.V.>, (i.e. the maximum concentration of a com- pound OSHA allows in the work place). However, for known carcinogens such as -Hexavalent Chromium, this property line G.L.C. is based on risk data and is much lower. Some examples are:

'-1

A. In North Carolina the A.A.L. for chromium compounds at the company's property boundary is 8.3 X 10.8. milli- grams per cubic meter '(.00000- 0083 mg/m3).

B. In Texaa the G.L.C. is .l microgram (1/10,000,800 of a gram) per cubic meter C.1 -:J ug/m3)

2

C. In New York the H.L.V. is .004 micrograms per cubic meter ( .804 ug/m3)

D. In New Jersey an individual risk assessment must be done for each chrome etack, demonstrating that the maximum incremental cancer risk is at or below one in one million.

To insure that these property line emission rates are met, most states require . proof of the maximum stack concentration (M.S.C.) through either, a calculation using reliable data, or a stack test. The resulting stack emission is then simulated to the company's property boundary through the use of computer modeling. Two methods of -modeling are used, (i.e. a !!screen model", or the more accurate and expensive "re-

. --fined model"). These meteor- --.-.rological simulations of emis- , ---sions crossing the company fence line use variables that include M.S.C., atack height, etack velocity, distance to the property boundary, ter- rain, wind factors, down wash, etc.

The Packed Bed Scrubber The best available control technology for reducing vapor- ous emissions of water soluble acids is still the wet packed bed scrubber. Other technologies exist that are considered more suitable for emissions that evolve in mist form, but for compounds that evolve as a gas or vapor, time. no measur- able particles in the air stream), and are aoluble in water, the w e t acrubber ia the accepted choice. The basic wet acrubber consists of a

KCri SERVICES, INC P.O. Box 1287

100 Westerly Hill Or Forest City. NC 28043

. . ' ,

vessel with adequate cross sectional area to provide the correct air velocity, a depth of packing media for distribution of the scrubbing solution, a recirculation system to provide the correct volume of solution throughout the packing, and a mist eliminator to prevent excess scrubbing solution from re-entraining into the exhaust stack. Most effective designs are either cross flow (horizontal flow) or counter current (vertical flow). ThB principal behind1 removal of gases in a wet scrubber is "gaa absorption" <i.e. the absorption of specific constituents of the gas stream by dissolving them into the scrubbing solution).

A scrubber operates by first reducing the velocity to, or below 400-500 fpm. This is done by designing the

zacrubber vessel with auffi- : d e n t cross sectional area relative to the volume of air being exhausted. A vessel with a crosa section too small wi3l result in a velocity too fast for gas 'absorption to occur. Secondly, the gas stream passes through a bed of structured, o r a random packed media. Many types, sizes, and configurations of random dump packing are marketed today, (i.e. spherical, tubular, sad- dle, tellerette, rings as well as polyhedron designs). They range in size from 1" to 5" diameter, iP; pressure d ~ c j p , and in square feet of contact area per cubic foot. They should be designed so that when randomly dumped into the acrubber vessel, sufficient void spaces are available to permit uniform irrigation thr- 'J oughout the packed bed. The packing is not to be a filter

'&>

. . __.

in itself, but to be the vehi- cle for uniform distribution of the scrubbing solution, enabling the solution to be more efficient for gas absorption. For example, if ineffi- cient packing channels solution to one area, resulting in another area of the pack remaining dry, the gas can not be absorbed in the dry area and will not by collected.

Scrubber spray chambers and spray nozzles should be designed to thoroughly. satu- rate the media. Most cross flow scrubber designs incorporate sprays in front of the pack to provide complete cov- erage of the face of the packing crossection and depending on the depth of the pack, top .mounted sprays to irrigate the packing from overhead. Coun- ter current scrubbers incorpo-

.-:rate sprays above the packed

.--.bed, enabling the distributed

down through the packing ef- fectively contacting the contaminated gas stream moving upward. Solid cone spray nozzles are effective distribu- tors, but have more potential for pluggage if solid particles are present in the recir- culated solution. Hollow cone nozzles are therefore a less maintenance intensive alterna- tive when the potential for pluggage is-present. However, it is critical to use a sufficient number of nozzles for packing ir i - igation to insure that no portions 05 the packed bed remain d r y .

and irrigation rates are utilized in scrubber design depending on the degree of dif- ficulty of the contaminant to be scrubbed. A one foot packed bed with an irrigation rate of

--.scrubbing solution to trickle '

Various packing depths .

KCII SERVICES, INC

100 Westerly Hill Or 3 P.O. Box 1287

K ~ ~ ~ F ~ CiL\ia b f c 2"343 ~- v

. . 0 .

1/2 to one gallon per minute 3 per square foot (GPM/ft2) is typically used to control low concentrations of high soluble contaminants. A three foot packed bed (standard for many scrubber manufacturers) with an irrigation rate of 2-4 GPM- /ft2 is often used for most contaminants typical to the plating industry.

However, for high efficiency control of high concentrations of . acid or base vapor evolving from pickling operations; (HCL) and ( H N 0 3 1 , bright dip operations; (NO or N02), chemical manufacturer; . ( C L 2 , N H 3 ) or municipal waste treatment systems; ( 5 0 2 , H2S), scrubbers should be custom designed based on the inlet concentration and solubility

.,characteristics of the con- --taminant. Packing depths of 5 to 8 ft with irrigation rates

.::from 4-8 GPM/ft2, are common .,for control of high concentra- -:tions of HCL and H N 0 3 . These designs also typically utilize a pH control eyetem to inject a baee pH solution (i.e. sodium hydroxide) into the recirculation system to help neutralize the acid vapor and increase the 'acrubbera cap- capability for abaorption. Multiple--stage systems using 2 or 3 scrubber units in series with 6-12 feet of pack each and en irrigation rate of 6-12 GPM/ft2 each, are often used in control of NO and N O 2 from bright dip operations, alumi- num or titanium etching processes, and also to control SO2 and H2S gases from municipal waste treatment systems. Each stage could incorporate a specific chemical additive into the recirculation aolu-

iJ tions to neutralize, oxidize, or deoxidize the contaminant,

-... -1

depending on the nature of that contaminant. (More information of specific custom scrubber designs is available upon request.)

The final stage of the scrubber must be an effective mist eliminator to prevent re-entrained scrubbing solution from going to atmosphere. The two most common mist eliminator designs are chevron baffles angles, and mesh pads. The chevron baffles are designed to create abrupt direc- tional changes in the air stream, thereby impinging wet particles onto the baffles surface or catchment. Chevron baffles are typically effective on the larger range of particle sizes (i.e. 12-100 microns) and due to the fact

._ -'.that their pluggage potential is minimal they are considered low maintenance. Mesh pad -eliminators, used in wet scrubbers, usually cohsist of wo- ,

--ven or knitted polypropylene monofilament, either in a random or specific configuration. They work by mechanical impingement and are velocity dependent. The particle to be collected must be traveling with sufficient velocity to impact and adhere to the fi- ber. These are designed for specific particle size removal depending on the diameter of the monofilament, the designed void spaces, and the number of mesh layers utilized. Smaller monofilaments and tighter void spaces are obviously more efficient in removing smaller particle sizes, (1-3 microns) but the down side is they have a greater potential for pluggage and require more maintenance and care. Therefore, the most logical scrubber mesh pad eliminator design is a

KCH SERVICES, INC

100 Westerly Hill Or Forest City. NC 28043

4 P.O. Box 1287

. ___-. - _

, ’. .

filament diameter and void space that is large enough to minimize the pluggage potential and have a functional liquid handling drainage capacity. This type of mesh pad configuration is usually effective on particle sizes 5-18 microns in diameter.

..“+I ’.. .

Maintenance Cozisiderations for the Wet Scrubber A new wet acrubber, with freah recirculation aolution and all components functioning properly, will obviously work at its designed efficiency. The main objective is to set up an effective maintenance schedule to keep the system functioning properly. The following maintenance objectives are relatively simple and should be faithfully executed:

1. Scrubber spray chambers cand nozzles should be regular-

. ly inspected (1-2 times a -week) to insure they are not plugged. A s was previously mentioned, if a’portion of the packing remains dry, that portion is ineffective. Poor irrigation can also cause the packing section or mist eliminator to become plugged or fowled. -Therefore, it is im- perative- thet each nozzle is spraying at its deaigned flow rate.

2. The packing sections should be inapected often to insure solids are n o t building up, plugging portions of’the pack. This could reault in increased static preaaure, reducing the ventilation rate, and could also prevent irrigation of other portions of the packed section, resulting in excess emissions. If build-up of contaminant on the

packed section occurs, the scrubber recirculation tank should be drained and replen- ished with fresh water, and perhaps an applicable solvent added. The packing aection should then be flushed to break down trapped solids. This process may have to be repeated, depending on the degree of the fowling present. If maintenance has been neglected and the severity of the pluggage ia too great, the tedious job of removing and cleaning the packing muat be done as a last resort. Most scrubber designs incorporate an access door or manway for this purpose. However, if maintenance inspections are done on a regular basis, this should not occur.

. --. ..3. The scrubber mist eliminator should also be regularly .inspected. The catchment on a :chevron baffle can become filled with solids rendering it ineffective. The mesh pad eliminator, as was previously stated, is even more prone to pluggage. Although most acrubber designs do not employ a washdown spray chamber for the mist eliminator, moat designs do provide access areas for spraying the eliminator manu- ally with a hose, or for removal of the mesh pad elfmin- ator for a more thorough cleaning.

4. The ecrubber recirculation system should be kept reasonable clean for aeveral of the following reasons:

A . To insure the solution is capable of gas absorption. C1.e. not scrubbing acid with acid, contaminant with contaminant, etc.)

KCH SERVICES, I N C

100 Westerly Hill Dr Forest City. NC 28043

5 P.O. Box 1287

- _---_

. . ,

._.

''3 --

B. To minimize build up of eolids in packed and mist eliminator sections.

C. To prevent pluggage of epray chambers and nozzles.

Most scrubber recirculation designs recommend a con- tinuous addition of freshwa- ter. Up to 5% of total recirculation rate should be added to the recirculation tank and simultaneously over- flowed to waste treatment. This serves to make up for evaporation of the recirculat- ed solution occurring in the scrubber and to help keep the solution from becoming too contaminated. The recirculation tank should also be kept clean of sediment settling to the bottom of the tank. These . ~ o l i d s are- easily stirred up and will inevitably contribute to pluggage of spray nozzles, packing section, and mist eliminator section.

Components for Preventative Malntenance Inspection and maintenance of the wet scrubber system can be made easier with the addition of accessories to monitor scrubber--operation and to ease the maintenance burden.

Pump Flow Meter Any scrubber design has a specification for the required recirculation rate based on scrubber capacity, inlet concentration, packing depth, etc. Monitoring this recirculation rate gives the assur- ance that the designed flow rate is maintained for proper irrigation of the packed section at all times. The pump flow meter reads out the number of gallons being sent to

the acrubber from the pump discharge. It is calibrated to read the designed recirculation rate on either an analog or digital display. If the reading is lower .than the designed recirculation rate, it is evidence that either the spray chamber or some spray nozzles are plugged.

Wye Strainer The wye strainer is a simple, inexpensive, filter installed in the piping between the pump discharge and the scrubber spray chamber. It simply col- lects particles from the recirculation flow that could potentially plug tlie spray system and perhaps the packing section. The advantage of the wye strainer is that it is .much easier to clean than the

. spray chamber, nozzles,- or packing.

.Magnahelic Gauge The magnahelic gauge is an analog gauge that gives a constant reading of the pressure differential, or preasure drop across a given medium. It is typically used to monitor the pressure drop acroas the scrubber packing media or mist eliminator. It. can be mounted on the 'scrubber vessel or in a control panel in a remote location. It is usually connected to the scrubber vessel via 114" plastic tubes, from the gauge, that penetrate the vessel on 'eithsr s i d e cf the media being monitored. Certain types of random or structured packing, and mesh pad configurations, have specific designed pressure drops, when clean, based on their density, void fractions, irrigation rates, gas velocity, etc. If the magnahelic gauge

KCH SERVICES, INC

100 Westerly Hill Or Forest Citv. NC 28043

6 P.O. Box 1287

. . . . .

. ..

''7

i'l ...

readings begin to increase above designed pressure drop, it is a sure indicator that the contaminant is building up on the media and corrective action should be taken.

Photohelic Gauges The photohelic gauge is aimi- lar to the magnahelic gauge except that it contains a set of relaya that are available to .perform a function (i.e. alarms, system shutdown, etc.- 1 . The set points are adjust- able for a varied.set of conditions (i.e. low air flow, pressure drop, etc...).

The High Efficiency Mist Eliminator Mist eliminator technology, for controlling water soluble

--chemistries that evolve in ::;.miat form, has. also greatly - --progressed in recent years. :-There was a time that the :=school of thought was that the best available control technology for all emissions, vapor, and mista from typical plating and metal finiahing procesees waa the wet scrubber . However, as emission atandards for. many of these chemiatries, especially chromium compounds, became much more stringent, the high efficiency miat eliminator became en increasingly viable option, with regard to removal efficiency, water savings, and economics. Many chemistries e v o l v e in mist form including but not limited to; sulfuric acid (H2S041, nickel compounds <NICL2)(NIS04), sodium hydroxide (NAOH), nitric acid (HN03) chromium compounda (CR03,CR06) etc.. .

The compounds that have received the most scrutiny,

have the most stringent emission standards, and are best controlled by the high efficiency mist eliminator are chromium compounda. High ef- f iciency multi-stage mist eliminators controlling chromium mists have been tested by the U . S . EPA, and private tes- ting companies, yielding su- perb results. Documented tests reports show that emission rates from some units exceeded the California standard by 60% and surpassed the expected U.S. EPA MACT standard by a factor of as much as 100.

These units have evolved into complex pieces of equipment to build, even though the principal of removing particles is relatively simple; (1.e. mechanical impingement or initial impactio .multi-stage designs .rate a principal of "variable -:stage removal". The rationale .._is to collect most of the particles in the gas stream in the first stage, thereby pro- tecting the subsequent atages which are designed to handle the smaller remaining particles in the gas stream. The best approach to accomplish thia has been 8 three or four stage unit, utilizing larger monofilament and void space configurations in the first stages, and gradually decreaa- ing. monofilament size, and void space downstream from stage to stage. Each stage is periodically flushed out with fresh water. Individual drain chambers should be utilized to prevent the wash water with the highest concentration of contaminant in stage #1 from flowing into stage #2 etc...

7 KCH SERVICES, INC

P.O. Box 1287 100 Westerly Hill Or

Forest City. NC 28043

"'7 Maintenance Considerations For the High Efficiency Mist Eliminator Due to the pluggage potential of high efficiency mesh pads and the thick consistency of many of the chemistries that it controls (i.e. chromium), these mist eliminators must be thoroughly maintained and washed down frequently,on a consistent schedule. If wash down is neglected for too long, the contaminants can solidify deep inside the pada containing smaller monofilaments, and flushing of the pad then becomes ineffective. A t that point, removal of the pad from the veeael for manual cleaning is the only option. This can be difficult, especially on larger units and can .be avoided if a proper washdo-

.:.wn schedule is faithfully exe- ::cuted. A magnahelic or photoh- -r:elic gauge should be used to :monitor the pressure drop of each individual stage of the eliminator. If the gauge readings begin to increase, that atage should be washed down immediately for the duration specified by the manu- f acturer .

A control panel for automatiS; timed washdown of the meah- pads is alwaya a good idea. I t remembers to wash t h e pad6 when your maintenance people do not; It consists of a 3 or 4 channel 365 day timer clock to initiate the wash down, timer relays on each stage for wash duration,-and solenoid valves plumbed into the eliminators spray chamber piping. This is a simple way of "fool-proofing" your system insuring that flushing of these mesh pad mediums can not be neglected.

Most high efficiency

eliminator designs incorporate access doors to remove the media, for maintenance pur- poses. Although it can be a difficult job, removal and thorough cleaning of the media should be done at least annu- ally, when production permits, as a preventative measure. Special care should b e taken when reinstalling the media into the vessel, to ensure proper fit, and that no gaps are left between the media and the vessels wall. Due to the high pressure drop across the mesh pad, even the smallest gap could provide a path for bypass, and with the extremely low emisaion rates required in many of todays atandards, a gap could cauae failure of compliance.

Technology for control- - ,ling acid or base _. emissions -::has never been better, but -::maintenance -responsibilities :zhave never been greater. Tec- . ~ hnology enables the latest emission standards to be met, but knowledgeable maintenance personnel and a well designed maintenance schedule ensures continued compliance.

KCn SERVICES, INC P.O. Box 1287

. 100 Westerly Hill Or Forest Citv. NC 28043: a

i

....... I"" ..L .... * - - - ..... -*..e ....

L . .(

''-1

.

Mr. Paul Ma&, P.E. BE&C Bn#nee.ts

SeorttIe, Wadhbgton 98124-220 P.O. BOY 07, MS 5F-31

. . . . . . . . .__ . . . . . . . . .-. ...... , 1

Dear Mr. hh8.Y:

BERtC Hn#aeets contracted JVU Test-& Quality, Inc, to qumtq c h r o d ~ emissions ht the {dat d butlet o€ &a KCH Sefvicea, Tnc. 6cmbber systea imtafled at Tslak 1316 in Buildhg 17-68 at Boelng'r Aubum facility. ThM (3) 180-minute

* lmfi chromium testa were performed on May 9, 1?91, Computtt phtouts of the

. . . . . ..

'. :. . . . ._ . . . . . . .."-.] :, . i _ . . . . . . . I

. , . , . . . . . . .

-. .

).- (J ' .

\

- results f tho 3 inlet samples and 3 outlet samples are enclosed, along Mth. copius of thi bath gmperoge reading6 and an exttmplb calculation of results for run 1 at the Met.

he removal effickncy for each individual run Is presented in Tablos 1 add 2 below:

Table 1. Hokkvaltnt chromium (Cr removal efficiency of tbe KCH Sedces, Inc. scrubbet By8tOm Lnftalled at Tank Ll '2 tn Buildhe 17-68 at BOehB's Auburn plnnt,

.. - 3 I

.:-.- . . . . . . ._

.

I . . . ij!

- n .o c

c .. -

6 *I*nbte 2.0. 1 texavaleiit ctiroiiiiuiri (Cr ' ' 1 eitiissioii tesl resulls froiii saiii Ies

Lhiiltliiig i 7 - O G ai 13oeiiig's Auburn, Wastiiiigtoii frtcitity oii May 29 a i d June 1-2, 1 Y92.

collected d llie Kc11 scrubber extiailst ftotti clitutiiic acid atiodizirig h i k # 1 s irt

i 0.00OG O.OO( 1.3 27.9 0.u 1 66 2 < 0.000 I < 0.0001 < 6.18 <. 0.005s 3 0.00 IO 0.0005 45.G 0.0302

Avcrnge - 0.0005 - o.ooo.? - 24.5 ' - 0.01 56 - -----

I t sliould be tinted (lint wlieii (lie results for t hee (3) ruiis are averaged together, if a

value is less ttiati ( e ) lIie tteteclioii Iiniit (IN,), i t is counleri as zero (0) iii tIie

average if at least I of ttie 5 tiitis is above the dctectioii Iitiiit. I f I o r 2 values are

11 -1 -

-1 -1 *

3 o.uuu5 0.0002 5.8 0.0025

0.000 I 4.0 0.00 18 1 Avcrnge 0.0004

7 1 -

1

I

!

_ .

1 #

1

I ' . . '

' : c

I

I

#

' . * . . l

I

.. .

FOCUS: Chromium Platin4

ong term environmental comp~i- mce, w+stereduction and riskmini- L mization require companies to be .

on the leading edge of technology. This has always been top priority for Court Industries Co. Limited, in St. Catharines, Ontario, and is integral to our providing a high-value service to our customers worldwide. while re- p as s i g maining competitive and viable. So in order to meet dcmand and improve s - upon the quality and servicc our cus- tomcrs have come to expect. it became necessary to increase our chromium

0 ‘c h r.0 1

Because our company is committed to preserving the environment, we look beyond current local legislatiorl and attempt to project what the future holds. Therefore, environmental compliance and risk minimization are integrated into all investment decisiom . . . By ROD BAR General Manager Court Industries Co. Limited St. Catharines, Ontario, Canada

plating capacity. Design for Zero Discharge. Review

of production, quality and servicc requirements indicated that a new facility was necessary. The goal of risk minimization necessitated incorporat- ing in the design and layout the use of state-of-the-art building and equipment considerations and the best available technology.

Wc recognized the need for and sought out a design that would set the standard for virtual “zero discharge” of chromium under any conditions, and insurc that ground contamination was impossible. The chromium tank pits werc scaled. the floors sloped to the pit and sealed, and the building perimeter dikcd to insure no spillage could seep into the cnvironmcnt if cvcr a tank was ruptured. And NO floor drains werc permitted.

Current Emission Requirements. To insure that the new facility would be truly state-of-the-art and the company

could continue its high-productivity operation-well into the future, it was decided that the exhaust ventilation and air emission control technology

could bc no less than the best avai The cumnt emission regulatior, forth by the Ontario Ministry (

Environment (MOE), requirechro emissions not to exceed five n grams per cubic meter at the PO Impingement (P.0.1.); deter1 through the use of computer mou Currently no “at the stack” limit: been set.

Emission Level 0bjective.Rc: was done on the chromium em regulations throughout North An to determine what to expcct i n MOErequirements. Wc found that mium emission standards vary jurisdiction to jurisdiction. Most enforced chromium emission IC\ the property boundaries or at po impingement. not unlike Ontar many areas the regulations wcrc siderably more stringent than i t tario. In addition, it was learnc the U.S. Environmental Protc Agency (US. EPA) would soor pose a tough fcderal standard on mium emissions to be measure! enforced at the top of the exhaust

During the building dcsign anc struction we felt that the toughest mium emission regulations to mee rhosc of California. The rcgul; requirc stack emissions io be rec to less than 0.006 milligram pc pcrc hour (mg/amp-hr), measur the stack. With the future in min decided that our specifications fo emission control equipment must or exceed Califomia standards.

HEXAVALENT CHROMIUM mist eliminator cuts emissions to 0.002 milligram per ampere hour.

===.-1r- E...... ..... - 1111 PI IRR3

Equipment Selection. The control equipment selected had to meet not only the lowest current emission standards in the U.S. and Canada. but had 10 surpass them due to the expectation of much tighter emission controls being introduced. Equipment vendors were asked for documented test results demonstrating equipment efficiency.

Other selectioncriteria includedcost: reliability; niaintenance requirements; corrosion rcsistance: and thc system should be a closed loop, requiring no discharge of scrubbing solution to waste treatment. The equipment supplier would have to guarantee the chromium mission ratc to bc at or lower tlian the California standard. The Spectra U ChronicEliniinator from KCH Serviccs, Inc., Forest City. North Carolina, met all these requirements.

We found that this four-stage. high- efficiency mist eliminator had been used in many applications where high- efficiency control of hexavalent chromium was required forregulatory compliance. Documented test results showed that it had exceeded the 0.006 mglamp- hrrequired in California and had beaten the expected U.S.EPA Maximum Achievable Control Technology (MACT) standard for new source:s by a factor of as much as 100.

The feature that clear1 y distinguished this system from other control tech- noIogies was that the unit was designed tQ operate "dry" with only periodic wash-down required for media cleans- ing. The volume of wash water used for these wash downs was minimal com- pared to that of conventional wet :5crub-

62 PRODUCI3 FINISHING

bers. Plus, the wash water is returned directly to the plating tanks. recovering the chromium and making up for tank evaporation loss, making the system essentially a closed loop and eliminat- ing water problems.

This dry unit is comprised of four mcsh pads in series and removes chromic acid in stages by particle size. The mesh pad media are specifically designed configurations. not random-type woven configurations. Thc first stage uses the iargcst monofilanient diani- eters and the largest relative void areas, designed to rcniove the larger particles of the gas stream. In stage two, smaller filamcnts and void sp;iccs rcniovc cvcii smaller mist particles. Thc final stages are the finest filamcnts and smallest voids to removc all chromic acid particles down to one micron. (The supplier feels that one micron particle removal is necessary to achieve the lowest emission rates required by Califor- nia.) The fourth stage acts as a re- entrainment chamber.

The first three stages are equipped with theirown wash-downspray changer for periodic rinsing with fresh water. The system is a horizontal flow device and that, along with specifically designed drain changers, allowseach stage to be separated and quickly drained. This prevents stage-one wash water from contaminating stage two, etc.

(Exhaust Wentilation .Eficiency- '.Worker Safety. Also extremely important was the efficiency of the exhaust/ventilation system. Our policy has always been to protect the health and safety of our employees. The envi-

JULY, 1993

AMBIENT AIR QUALITY TESTS indoors showed that the ventilation system maintained I chromium levels below the threshold limit volume of 0.05 mg per cu meter.

ronment of our employees must be sec- ond to none in air quality, therefore, the

: exhaust rates selected were the maxi- : mum recommended by the American 1 Conference of Industrial Government Hygienists (250 cfm/sq ft of tank surface area). This would insure that the air quality would remain excellent even though poly balls or fume suppressants ,are not used presently to reduce emis-, sions liberated from the tank.

Obtain the Permit. Through in- house engineering and help from the supplier, we obtained a Certificate of Approval from the Ministry of Envi-

ronment (MOE). During this process we made particular efforts to provide comprehensive engineering information, emission calculations, previous emission test data, detailed drawings of the control technology and the exhaust ventiiation system, and detailed systems descriptions. By providing this detailed engineering package and demonstrating the high degree of control efficiency of the system, we were able to obtain the Certificate from the Min- istry of the Environment in six weeks. (At the time, approval normally took three to four months.)

JULY. 1993 PRODUCTS FINISIfING 63

. .

P

in Canada and around the world in chromium plating, setting higher stan-

Fabrication andlnstallationon the Fast Track. Once the permit was ap- proved we asked for construction to begin on the fume control system. But, by this time the new building was al- most complete and customer demand made it essential IO begin plating production within six weeks. The supplier hit the fast track, complying with the project's revised construction schedule. The fume control equipment in- stallation andsystem startup took place according to plan. Stack Analysis. In compliance with

the MOE Certificate, a stack test analysis was done to demonstrate actual

. emission concentrations from the exhaust stack. Three separate tests were performed, and each test was done at high amperage to maximize emissions before control. Samples were collected using U.S. EPA method 425 and ana- lyzed for hexavalent chromium.

,The average results showedan emission rate of 0.002 milligram per ampere hour (mg/amp-hr) and 0.00 1 milligram per dry cubic meter o€ air exhausted (mddscm). The emission rate of 0.002 muamp-hr is 60 pct lower than required by California and the 0.00 1 mg/dscm surpasses the expected U.S. EPA MACT standard by a factor of 10.

Ambient air quality tests. were also performed inside the building to ensure the ventilation system was efficient enough to maintain contaminant levels below the threshold limit value (TLV) of 0.05 mg/cubic meter. The results showed that ambient chromium concentrations were below the test's mini-

To obtain more information on products or processes mentioned in this article, circle corresponding numbers on the Reader Service Card. Chromium scrubber systems .............................. Circle 290 To request an additional copy of this article, write on company letterhead to "Reprints," c/o PRODUCTS FINISHING, 6600 Clough Pike, Cincinnati. OH 45244.

64 PRODUCTS FlNlSHlNG JULY. 1993

scrubber technologies and their maintenance · 2018-06-13 · scrubber technologies and their...

Documents