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Honeycomb Monolith, Reducingthecost of VOC control in the semiconductor industry.
The regenerative the rmaloxidizer(RTO) is one of the standard pieces of equipment used to control the emission of volatile organic compounds(VOCs) in the semi conductor industry.In normaloperation, an RTO removes VOC susinggas-phasefree-radical reaction sofhomogeneous oxidationto CO2 andwaterat 1450ºF to1600ºF. Honeycomb Monolith
An RTO uses a regenerative heat exchangeintwoormorepackedbedsoperated with periodic flowreversals.Thebeds,filledwithaninert ceramic media, areconnectedbyacombustionchamberwhereone or more fuel burnersareinstalledforsystemstartup, andtomaintain necessarytemperatureatlowconcentrationsof VOCs.TheVOC-laden air enters theoxidizeratlowtemperature andisheatedthrough the heat exchangerwiththeinletceramic beds.Thisairstream then reacts inthecombustionchamberand returnsheattothe outlet beds, where itisabsorbed forthe nextcycle.Uponflowreversal, the bedfunctionschange suchthatasubstantialfractionof energy from VOCcombustionandburnerfiringisregenerated inthe upper fraction of thebeds.Amplesurfaceareaofceramicmaterial results in highthermalefficiencyachievingupto95percent in well-designedsystems.
Despite the high degree ofenergyregeneration,RTOscanstillrequirehigh fuel consumption –especiallyathighairflowrates.This isparticularly trueinthesemiconductorindustry,wherelarge volumesof airatlow-VOCconcentrations arethe norm.Analternative tothermaloxidationis acatalytic processthatoccursat lowertemperatures –600°F to900°F.As a resultoftransitionto aregenerativecatalyticoxidizer(RCO),fuelconsumption canbedramatically reduced,and inmanysituationstheinvestment inacatalyst is returned in averyshortperiod oftimedue to thefuelsavings.
Figure 1: Catalyst testing results. Test conditions: catalyst temperature 750 ºF, 2500 ppm of propane and 50 ppm of Si(CH3)4 mixed with air in inlet gas.
Semiconductor case study
Figures 2a and 2b show the top layers of ceramic media before catalyst loading in 2005. Figure 2a: Top bed in one of the canisters
Honeycomb Monolith
The conversion of an RTO at a large semiconductor facility inTexas demonstrates that some VOC control challenges for thatindustry can be overcome.
A key element of the technology wasasilicon-resistantcatalyst,which was able to withstand poisoningbysilicon-organiccompoundspresent in the exhaust. Also, prior tothecatalystloading, thefacility implemented a series ofmodificationsinexhaust enclosureand distribution to removethesilicon-containingcompoundsintegral to processingsemiconductorsfrom the treatedstream. Thecatalytic oxidizer hadbeen operated formore than fouryears at900°F to 950°F in acombustion chamber,compared to theoriginaloperating temperature1,500°F. Temperaturereductionresulted insubstantial fuel savings.The exhaustenclosuremodification,combined with the RTO conversionto an RCO,alsoavoided bedplugging by silicon that occurred in theRTO beforetheconversion. Silicon-resistant catalyst
While the addition of catalysts to RTOs has beenanacceptedpracticefor several years, it has not been a viableoptionforthesemiconductor industry. Exhaustfromsemiconductormanufacturingoperations containssilicon-organiccompounds such asthehexamethyldisilazane (HMDS)commonly used infabrication asanadhesion promoter on the wafersurface. In atypical RTO, theHMDSwould oxidize in the combustionchamber andformSiO2compounds. These so-called "sand"particleswouldbuild up overtime in the unit, and result in pluggingtheceramicmedia,channeling of the air flow, and increasingpressuredropacross thebeds (see Figure 2).
Figure 2b: Single monolith plugged fromitstop.
Honeycomb Monolith
Inan RCO, when a volatile molecule containing silicon atom(s)reactswith the catalyst surface, a practically unbreakable bond iscreatedbetween the active surface site and the silicon atom,inhibiting anycatalytic activity of that site. Deactivation bysilicon is referredto as masking. It is especially harmful forcommon platinum-metalVOC oxidation catalysts, containingrelatively few, albeit veryactive catalytic sites. Another type,the so-called "transition" or"base-metal" catalyst, contains a feworders of magnitude greaternumber of active sites, and thuspresents a good opportunity fortreatment of gases laden withsilicon-containing VOCs.
Several base-metal catalysts were synthesized andtestedinsimulatedreactions of VOC oxidation under theinfluenceofsilicon-containingorganics. Figure 1showstimedependencies for catalystactivity during propane oxidationinthepresence of 50 ppm oftetramethylsilane. The testswereperformedin a laboratory reactorwith intense internal gasmixingthatyielded the reaction rate data.Relative activity inthechartordinate was calculated as a ratiobetween runningandinitialrates of oxidation. Two noble metalcatalyst samplesweretestedalong with the base metalcatalysts.
Sample1inFigure 1 represents a common wash-coated noblemetalcatalystwithactive metals distributed over a thin filmofporousaluminadeposited on a non-porous ceramic carrier.Anothernoblemetalcatalyst, Sample 2 in Figure 1, was obtainedbyimpregnationamassive highly porous alumina carrier withnoblemetalsolutions.The base metal catalysts tested in Figure1includedmanganeseoxide and copper-chromite catalysts,bothobtainedthroughextrusion of mixes of aluminum hydroxide andbasemetaloxides,followed by subsequent drying andthermaltreatment.
Although the impregnated noble metal catalyst(Sample2)demonstratedhigher stability than wash-coated (Sample1),bothnoble metalcatalysts deactivate very quickly compared tothebasemetalcatalysts. The copper chromium catalyst showedthelowestrate ofdeactivation among all tested samples.
Aside from the VOC oxidation rate measurements,thetestsincludedcontinuous measurements of inlet andoutletconcentrationsoftetramethylsilane, thus it was possibletocalculatetheaccumulation of silicon over the catalyst.Table1presentsthe silicon accumulations over differentcatalystsamples,at whichthe reaction rate of VOC oxidation wasdecreased by30percentcompared to the initial rate. This decreasewasnotconsidered highbecause the rate of reaction couldbeincreasedagain to theinitial level through amoderatetemperatureincrease.
The base-metal catalysts can trap a considerablyhigheramountofsilicon than the noble metals (see the comparison inTable1).Themost resilient copper-chromium catalyst canabsorb0.4lbs/ft3without substantial decrease in activity.Theexperimentaldatasimilar to that shown in Figure 1 was usedforpredictingcatalystperformance based on informationaboutconcentrationofsilicon-containing organics in actualexhauststream.
RTOretrofit design and installation
Intheinitialstage of the project it was understood thatthecatalyticoperationwould prevent bed plugging due toloweroperatingtemperature. Also,the facility made concertedefforts toremove HMDSfrom the exhauststream in order to minimizesilicaformation in theoxidizer. This wasan additional incentivefor theRTO conversion.The catalyst lifetimewas estimated at fourto fiveyears, based onprocess gas propertiesand catalysttesting.
The copper-chromium catalyst recommended for RTO loading–suppliedbyMatros Technologies Inc., Chesterfield, Mo.,wasproduced byextrusionand shaped as Raschig rings (seeFigure3) havingboth diameterand length of 15 mm. This shapewascompatible withthe monolithpacking at linear velocities appliedinthe RTO. Itwas determinedthat adding the catalyst wouldnotincrease the bedpressure drop, butrather decrease it becauseofreduced actualvolume of air through thebed at loweroperatingtemperature.Pressure drop reductioncontributed tooperating costsavings asidefrom the reduction infuelconsumption.
Prior to the catalyst installation, the plugged upperlayerofceramicmonoliths was removed and the remaining bedwascleanedfrom above inevery RTO canister. A bulk ceramic mediawasplacedover the remaining3-foot bed depth of the monolith.Thecatalystbed,
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