Chrome Scrubber Design

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SCRUBBER DESIGNS FOR HARD CHROME PLATINGFrank B. PowerWilliam M. SchottKimre, Inc.Perrine, Florida8BSTRACT

Emission control in the chrome plating industry has seen anevolution in regulation. This has resulted in an evolution in thegas-cleaning or scrubbing technologies employed. Packed bedscrubbers and chevron eliminators offer reasonable efficienciesdown to 8-15 micron droplets. Knitted-mesh mist eliminators wereable to improve efficiencies even further.With the adoption of more stringent emission limits by Californiaand other regional air resource boards, a uniquely interlacedmonofilament structure has been proven to meet those strict limitsin a cost-effective manner. Systems designed using this interlacedmonofilament structure achieve efficiencies of 99+% on droplets ofone micron and larger. This paper discusses the changes inscrubbing technology in the plating industry and describes variousdesign ideas and technologies behind systems utilizing thisinterlaced monofilament structure.11. J NTRODUCTI ONThere are many types of scrubber or gas-cleaning equipment.Generic types, along with their mechanisms of collection, areconsidered. Fiber-type scrubbers and an advanced design usinguniquely interlaced monofilaments with a specific geometricorientation are considered in more detail. This will illustratethe design methods for successful scrubber operations.The emissions standards might be set, the operating limits might beset as far as water balance or space; however, the DESI GNparameters are really not set. The actual size distribution ofdroplets to be collected is not known, nor is a reasonable methodto generate the size distribution known. This makes the comparisonof alternatives extremely difficult.Scrubber selection is evaluated in terms of capital cost, operating -cost, and Risk. Risk arises from variability in operatingconditions, uncertainty in particle size distribution and in theuncertain progress in regulation. We prefer robust design foreasily adjustable performance and reduced maintenance. Upfrontdesign can minimize capital and operating cost. It also makes itmore likely that the unknowns mentioned above can be successfullydealt with. The technology described here has an enormous range offeatures (Figure 1).'

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One researcher has remarked that separation costs go up by a factora factor l o o 2 All the more reason for good, flexible design.of 10 when the size of the droplets to be eliminated decreases by111.

In hard and decorative plating, the object to be plated is placedin a bath with a chromic acid solution. The object is thenconnected at the negative electrode of an electrolytic cell. Whena voltage is placed across the cell, chromium is deposited on theobject.However, as a side reaction, water in the plating solutiondecomposes to oxygen and hydrogen. A s those gases rise to thesurface, the gas bubbles burst creating chromic acidmist droplets.These droplets are collected in hoods and exhausted to controldevices like scrubbers. The exhaust airflow is then emitted intothe atmosphere.

Traditionally scrubbers are one or more pieces of equipment thatprovide :1. Contact of a gas and an aqueous solution to absorb gases(vapor), and/or assist the collection of particulates (mistor dust).2 . Separation of the aqueous and collected particle from thetreated gas.Examples of such traditional scrubbers are:

Venturi scrubbersCross-flow scrubbersVertical Countercurrent packed towersCyclonic separatorsSpray towersas shown in Figures 2and 3.The obvious and distinguishing features are ttCONTACTtt ndttSEPARATIONtif liquid. The term t tSCRUBBERtt is applied more widelytoday and ft actually seems quite difficult to generatedefinition for scrubbers.

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V. BOW DO SCR BBERS WORK?The many different methodologies of equipment and the methods ofscrubbing in the classical sense have o nl yt wo significant effects:

1. Absorption, and2. Collection of liquids and solids

For some scrubbers enthalpy control is of paramount importance butit is not usually significant as applied to plating. It is notconsidered further here. In addition to this, of course, otherfunctions, not @'scrubbing1' functions, impact on the scrubbingrequirements, and may be incorporated in the same vessel.A . ABSORPTIONAbsorption involves the contact of a gas with an aqueous solutionin order to promote mass transfer of a soluble gas component intothe liquid stream. In the case of chrome scrubbing, the chromicacid is present in mist droplets, not as a gas. Absorption,therefore is not significant and is not considered further. It is,however, extremely important in other acid scrubbing applicationslike hydrochloric acid (HC1).B. REMOVAL OF SOLID PARTICULATES AND LIQUID MISTThe term s particulate^^ is used for solid particulate, liquid mistof all sizes, suspensions of solids in liquid mist, and all elsewhich is not absorption. This is a broader definition than EPAMethod 5. For a scrubber to function, the particulate must becollected, and combined into an aqueous phase, and the aqueousphase must be separated from the gas. Collection mechanisms arediscussed at length in the literature.4 Calvert is especiallythorough on the factors involved in collection. In design, one hasto consider separation and re-entrainment.General mechanisms f o r collection are:

1. Inertial impaction which applies to larger droplets orparticulates. Impaction occurs when the droplet'smomentum carries it into the fiber instead of followingthe gas stream around the fiber.2. Brownian movement or diffusion applies to submicronparticulates. The random movement of the particulates as

they collide with the gas molecules forces impingement onthe fibers. However, the fibers used are extremely fine(approx. 1.3 micron). Removal by Brownian diffusion istypically done by fine fiber elements known as llcandlestt.

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3. Interception applies to smaller droplets which arecarried around fibers with the gas stream. However theinertia created by this path carries the droplet intodownstream fibers. Its effect is most noticeable in thedroplet size range below where inertial impaction occursand above where diffusional impaction occurs.The uniquely interlaced monofilament structure relies oninterception and impaction mechanisms (See Figure 4). Moreover forsystems to achieve the new strict standards, interception isbelieved to be significant. When interception and impaction areused, collection is a function of:

GEOMETRYFiber diameterFiber orientationParticle diameterBed thicknessFLOWGas Velocity

mT ER IA L PROPERTIESLiquid densityLiquid viscosityGas densityGas viscosity

Smaller fiber diameter, larger particles and higher velocitiesfavor better collection. Unfortunately, the same factors favorhigh pluggage, high pressure drop, poor separation and increasedchemical attack. These are important factors in chrome scrubberdesigns.VI. "NEW TECHNOLOGY"The technology described here has been called "newly emerging" inan EPA study for NESHAP.5 With over 10,000 installations of thestructure worldwide ran ing from less than 1 CFH (0.5 l/h) to aboutmore than 10,000 units per month, neither the structure nor thetechnology can be considered new.What is this structure?

35,000,000 cfm (17000m 9/sec) and an additional disposable use of

The basic media structure is made in very long pieces six feet wideand is composed of round monofilaments interlaced in a patentedstructure (See Figure 5). The ladder-like arrangement specificallyorients the filaments to lie essentially perpendicular to the flowof the gas. Virtually any removal efficiency can be achieved bycontrolling:- Fiber diameter (from .002 to.0625 inches or 50 micronto 1.6" diameter), as shown in Figure 6.- Solid fraction (from - 0 3 to .OB )Material of construction in polypropylene, PVDF(KynarN), ETFE (Tefzel"), FEP (TeflonN), PFA (TeflonN),- Liquid rates, andGas velocity

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VII. CHROME SCRUBBER DESIGNS using the uniquely interlacedmonofilament structure.Each style of coarseness of this structure has its own performancecharacteristics - removal efficiency, liquid handling capacity,pluggage resistance, pressure drop, etc. Therefore, pad designscan run from simple elements comprised of one style, to morecomplex pads comprised of several styles-composite pads (Figure7 )The NESHAP report mentioned above looked at technologies used inchrome scrubbing. It summarized that ttcontrol techniquesincorporating the use of a composite mesh pad appear to be the mostpromising of all the control options.tg.5A composite pad utilizes the best qualities of each style toprovide a more versatile arrangement than the individual stylesalone could provide. Composite pads are important in chromescrubbing because high removal efficiency, high liquid handling andreduced maintenance are critical. Troubleshooting performanceproblems is also made easier. Styles may simply by added orremoved from composite pads to improve efficiency, pluggageresistance, pressure drop, or liquid handling capacity. This canbe done very cost-effectively.Irrigation of the media with recirculated water is recommended foroptimum performance of the scrubber. Indirectly, irrigationaffects the efficiency of collection. While the mechanisms ofcollection are unchanged with or without water, the viscous chromicacid droplets tend to cling to the media. Over time, the chromebecomes more concentrated and more viscous. This in turn increasesthe pressure drop, the potential for chemical attack on the fibersof the media, and the possibility of re-entrainment of the dropletsdue to pluggage or blinding of the fibers. Therefore, while thechrome scrubber can run ttdrytt,rrigation of the media provideslonger operating cycles before maintenance is required.In the earliest stages of chrome abatement (pre 1987) while chromewas under consideration for regulation under NESHAP, plating shopswere generally interested in controlling chrome to the point wheredeposition on the stack and nearby buildings and automobiles waseliminated. They wanted this type of control without the need formaintenance of the scrubber unit. In these installations onlycoarse fiber stiles were used. The coarse styles have theefficiency to eliminate the drops that cause spotting of cars(approximately 10 microns and up), while offering the liquidhandling capacity and pluggage resistance required for serviceswhere the minimum of maintenance is employed. We have receivedreports that units of this design have operated satisfactorily forperiods up to a year before maintenance/cleaning was required.

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Unfortunately, when units with this design were tested by the EPAfor chromium emissions the performance (98% removal, .078mg/ampere-hr) was not satisfactory for meeting the limits adoptedby CARB in 1988 for medium and large hard chrome plating shops. Itwas clear that the size distribution of the chromic acid mistcontained a significant percentage below the 8-10 micron sizerange. Therefore finer fiber styles would be required to meet thestandards.Composite pad mist eliminator systems then began to be employedutilizing medium to coarse fiber diameters. These designs are ableto achieve removal efficiencies of 99+% for droplets 3-5 micronsand up while maintaining a very reasonable amount of liquidhandling capacity and pluggage resistance. Whereas, such compositestructures may be able to meet emission limits for medium-sizedplating shops (this design has not been tested), it is our opinionthat such a design will not meet the standards for large hardchrome plating shops (99.8% or .006 mglampere-hr).Without a reasonable size distribution available and with a need tomeet the standards set forth by CARB, scrubber manufactures andend-users began to specify removal efficiencies of 99% at 1 micronand 99+% at 2 micron droplets in order to be certain to meet thelimits. On top of this level of performance, the mist eliminatorsystems must be designed for lowest maintenance cost.Removal efficiencies in the 1-3 micron size range necessitate usingthe finest fiber sizes (.002 to . 0 0 8 inches, 50 to 200 microndiameters), while the lower maintenance aspect requires coarsefiber sizes.The basic methodology to designing a system to meet the abovecriteria is to collect the mist and particulates in a stage-wiseapproach. The design would begin with coarse fiber styles on theupstream side and gradually work down to finer styles toward thedownstream. The coarse styles remove the majority of the dropletswhile protecting the finer styles from pluggage. The finer stylesthen only handle the smaller droplets ( < 3 microns) at a muchlighter liquid loading.It is possible to design a single stage mist eliminator using theuniquely interlaced monofilament structure that will achieve theCARB emission limits. However, the pad would be very deep and itwould be difficult to keep the inner layers of material clean orunblinded. Therefore, due to the potential maintenancedifficulties a multiple stage mist elimination system is preferred.(See Figure 8)

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Bypassing is probablythe single-most cause for a mist eliminator'sinefficiency problems. Bypassing refers to any portion of anairflow that is able to circumvent or short-circuit a misteliminator. The airflow which bypasses a mist eliminator carrieswith it a certain percentage of mist or particulates. Obviously,these particulates go uncontrolled by the pad. Bypassing occursbecause of fit problems between the mist eliminator and the housingor because of a design flaw in the scrubber, In either case, asingle-stage mist eliminator system is more susceptible to bypassproblems and has a greater potential to fail than a multiple stagesystem. With the exception of a serious design flaw in the vessel,the probability for gas bypassing all stages of a multiple padarrangement is small.Each stage of a multiple stage system has a specific designpurpose.The first stage of a multiple stage system would consist almostentirely of coarse fiber styles. This pad would eliminateessentially all of the droplets above 3-5 microns. A s in thedescription of a composite pad above, this stage would also offerprotection of finer stages downstream.The second stage is primarily comprised of finer fiber styles. Itis designed to operate as a coalescing element. In this case, thesmaller chromic acid droplets (1-3 microns) are collected andagglomerated into larger, easily collectible droplets. Thecoalescer pad operates in a ltfloodedtltate. This means themajority of the drops collected in this pad will be re-entrained.But the droplets will be quite large ( > 4 0 microns) and, therefore,easily eliminated.The third and final stage of the system is a simple re-entrainmentseparator pad comprised of medium to coarse fiber diameters. Thispad merely collects any of the droplets that may re-entrain fromthe coalescer pad upstream.For chrome scrubber designs, a horizontal flow with the misteliminator pads installed vertically is preferred. Horizontal flowallows better drainage of the collected chrome. Also the pads canbe designed for easy access on line for maintenance--in many cases ..without shutting the unit down.Vertical flow scrubbers are also acceptable. The basic designprinciples and collection mechanisms are the same as in horizontalflow units. However, the vessel design is a little morecomplicated with respect to drainage and maintenance, (See Figure91- 4

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The tough emission standards set down by the CARB can be met. Thishas been demonstrated by various technologies including unitsutilizing the uniquely interlaced monofilament structure. Thesuccessful application of this technology is due to the systematicinteraction and cooperation between the manufacturer of thisstructure, a few very skilled scrubber manufacturers, and the end-users. Because of this progression of the technology, scrubbersystems can vary from site to site based upon the resourcesavailable from the end-user and still be able to meet the strictstandards.Because of this versatility, chrome scrubber designs using theuniquely interlaced monofilament structure provide the most costeffective solutions to the industry's needs.

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REFERENCES1. Peder sen, Geor ge C. , I t El ect r opl at i ng: Evol ut i on of Scr ubberTechnol ogy" ; pr esent ed at El event h AESF/ EPA Envi r onment alCont r ol Conf er ence, M am , Fl or i da, Febr uar y 1990.

2 . Bur khol z, Ar m n, I I Dropl et Separ at i ont 1, CH Publ i sher s, NewYor k, NY, 1989.

3 . Mur cheson, Gar y et at , I I Proposed Ai r bor ne Toxi c Cont r olMeasur e f or Em ssi ons of Hexaval ent Chr om um f r om Chr omePl at i ng and Chr om c Aci d Anodi zi ng Oper at i onst 1, r epar ed bySt at e of Cal i f or ni a Ai r Resour ces Boar ds, J anuar y, 1988,

4 . Cal ver t , Seymour et al , "Wet Scr ubber Syst em St udy, Vol ume 1,Scr ubber Handbookt 1, APT, I nc. , Pr epar ed f or Envi r onment alPr ot ect i on Agency, J ul y 1972.

5 . I 1Chr om umEl ect r opl at i ng Chr om c Aci d Anodi zi ng - NESHAPI I ,br i ef i ng f or Nat i onal Ai r Pol l ut i on Cont r ol Techni quesAdvi sor y Comm t t ee, J anuar y 1991.


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FIGURE 4:MECHAMSMS OF COU C T I O N

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FIGURE 6:FIBER SIZECOMPARISON


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FIGURE 9: VERTICAL FLOWSCRUBBER

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Chrome Scrubber Design

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