Honeycomb Monolith

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

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 was a silicon-resistantcatalyst,which was able to withstand poisoning by silicon-organiccompoundspresent in the exhaust. Also, prior to the catalystloading, thefacility implemented a series of modifications inexhaust enclosureand distribution to remove the silicon-containingcompoundsintegral to processing semiconductors from the treatedstream. Thecatalytic oxidizer had been operated for more than fouryears at900°F to 950°F in a combustion chamber, compared to theoriginaloperating temperature 1,500°F. Temperature reductionresulted insubstantial fuel savings. The exhaust enclosuremodification,combined with the RTO conversion to an RCO, alsoavoided bedplugging by silicon that occurred in the RTO before theconversion. Silicon-resistant catalyst

While the addition of catalysts to RTOs has been anacceptedpractice for several years, it has not been a viable optionfor thesemiconductor industry. Exhaust from semiconductormanufacturingoperations contains silicon-organic compounds such asthehexamethyldisilazane (HMDS) commonly used in fabrication asanadhesion promoter on the wafer surface. In a typical RTO, theHMDSwould oxidize in the combustion chamber and formSiO₂compounds. These so-called "sand" particles wouldbuild up overtime in the unit, and result in plugging the ceramicmedia,channeling of the air flow, and increasing pressure dropacross thebeds (see Figure 2).

Figure 2b: Single monolith plugged fromitstop.

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 and testedinsimulated reactions of VOC oxidation under the influenceofsilicon-containing organics. Figure 1 showstimedependencies for catalyst activity during propane oxidation inthepresence of 50 ppm of tetramethylsilane. The tests wereperformedin a laboratory reactor with intense internal gas mixingthatyielded the reaction rate data. Relative activity in thechartordinate was calculated as a ratio between running andinitialrates of oxidation. Two noble metal catalyst samples weretestedalong with the base metal catalysts.

Sample1 in Figure 1 represents a common wash-coated noble metalcatalystwith active metals distributed over a thin film of porousaluminadeposited on a non-porous ceramic carrier. Another noblemetalcatalyst, Sample 2 in Figure 1, was obtained by impregnationamassive highly porous alumina carrier with noble metalsolutions.The base metal catalysts tested in Figure 1 includedmanganeseoxide and copper-chromite catalysts, both obtainedthroughextrusion of mixes of aluminum hydroxide and base metaloxides,followed by subsequent drying and thermal treatment.

Although the impregnated noble metal catalyst (Sample2)demonstrated higher stability than wash-coated (Sample 1),bothnoble metal catalysts deactivate very quickly compared to thebasemetal catalysts. The copper chromium catalyst showed thelowestrate of deactivation among all tested samples.

Aside from the VOC oxidation rate measurements, the testsincludedcontinuous measurements of inlet and outlet concentrationsoftetramethylsilane, thus it was possible to calculatetheaccumulation of silicon over the catalyst. Table 1presentsthe silicon accumulations over different catalyst samples,at whichthe reaction rate of VOC oxidation was decreased by 30percentcompared to the initial rate. This decrease was notconsidered highbecause the rate of reaction could be increasedagain to theinitial level through a moderate temperatureincrease.

The base-metal catalysts can trap a considerably higher amountofsilicon than the noble metals (see the comparison in Table 1).Themost resilient copper-chromium catalyst can absorb 0.4lbs/ft3without substantial decrease in activity. The experimentaldatasimilar to that shown in Figure 1 was used for predictingcatalystperformance based on information about concentrationofsilicon-containing organics in actual exhaust stream.

RTO retrofit design and installation

Inthe initial stage of the project it was understood thatthecatalytic operation would prevent bed plugging due toloweroperating temperature. Also, the facility made concertedefforts toremove HMDS from the exhaust stream in order to minimizesilicaformation in the oxidizer. This was an additional incentivefor theRTO conversion. The catalyst lifetime was estimated at fourto fiveyears, based on process gas properties and catalysttesting.

The copper-chromium catalyst recommended for RTO loading –suppliedby Matros Technologies Inc., Chesterfield, Mo., wasproduced byextrusion and shaped as Raschig rings (see Figure3) havingboth diameter and length of 15 mm. This shape wascompatible withthe monolith packing at linear velocities applied inthe RTO. Itwas determined that adding the catalyst would notincrease the bedpressure drop, but rather decrease it because ofreduced actualvolume of air through the bed at lower operatingtemperature.Pressure drop reduction contributed to operating costsavings asidefrom the reduction in fuel consumption.

Prior to the catalyst installation, the plugged upper layerofceramic monoliths was removed and the remaining bed wascleanedfrom above in every RTO canister. A bulk ceramic media wasplacedover the remaining 3-foot bed depth of the monolith. Thecatalystbed, at a depth of 8 inches, was placed above theadditional bulkmedia. In addition, a thin layer (3 to 4 inches) ofthe ceramicmedia was loaded above the catalyst to protect it fromradiant heatemitted by the burner.

It took two days to load the ceramic media, the catalystandprotective ceramics, and reseal the oxidizer chambers.Anadditional thermocouple was installed in one of the catalystbeds.The control system modification included reducing theset-pointtemperature in the combustion chamber from 1,500°F to950°F, andsetting the maximum allowable operating temperature to1,200°F; attemperatures above 1,200°F the copper chromium catalystwould beginto break down and the catalytic action would cease. Theoxidizerwas heated up and put into operation two days after thecatalystloading.

The original burners were designed for high temperatures andneededadjustments to operate at lower temperatures.

honeycomb monolith

Retrofitted unit performance

Figure 3: Catalyst applied forRTOretrofit

Ageneral operating strategy for oxidizer control can involvegradualor stepwise temperature increase with increase insiliconaccumulation over the catalyst. The higher temperatureimproves thecatalyst activity, thus reducing the effect of siliconpoisoning.Another strategy is to maintain a fixed operatingtemperatureduring most of the catalyst's lifetime. This temperatureis suchthat the system will have a sufficient reserve in theactivity forachieving high destruction efficiency while thecatalyst graduallydeactivates. The catalyst activity isperiodically monitored (atleast annually) using catalyst sampletesting and field emissiontests. The test results determine if thetemperature should beincreased to compensate for continuous silicondeactivation.Regular annual testing allows the operations team toproject thetime that the catalyst should be replaced. Once thecatalyst beginsto lose its effectiveness, the operatingtemperatures will need tobe increased to improve the reactionrates, and fuel costs willincrease. To avoid too-high fuelexpenditures, or prevent possibleloss of bed mechanical strengthdue to high operating temperature,the catalyst should bereplaced.

Initial performance testing of the retrofitted unit demonstratedthedestruction removal efficiency of VOCs at more than 99 percent,at apressure drop slightly lower than in the original RTO.Theconcentration of methane was subtracted from the concentrationoftotal hydrocarbons during the testing.

The retrofitted RTO has operated for about four years with nochangein the temperature set point and pressure drop.Performancemonitoring included the oxidizer emission and catalystactivitytesting. Most recent field test confirmed the systemperformance atmore than 97-percent destruction efficiency. Thecatalyst activitytests showed a moderate activity decrease in linewith the expectedsilicon accumulation and poisoning.

The actual fuel consumption in the original and retrofittedunitswas estimated based on the measured temperatures, VOC loadingsandflow rates. The method of estimations was based on a heatbalanceaccounting for energy spent for heating the process gasandcombustion air, and useful heat from VOC oxidation. The amountofthe combustion air and fuel were assumed to be equal tothedifference between outlet and inlet gas flow rates. Theestimationshowed that the retrofitted RTO decreased total fuelconsumption bytwo thirds, or as much as 15,000 MCF a year. Thesystem alsoprovided a noticeable reduction in material and laborcost forfrequent ceramic bed replacement and disposal.

Installation of the catalyst made it possible to combine thelowtemperature of catalytic oxidation with the high thermalefficiencyof regenerative heat exchange. This change had threeprimaryenvironmental benefits:

1. Due to the much lower oxidation temperature, 700°F to 900°F,theRCO operates using 50- to 60-percent less fuel, andgenerates40-percent less NO_X.

2. Due to the nature of the catalyst, a packing more resilienttoHMDS resulted, thus maintaining high destruction efficiencymuchlonger, improving energy recuperation, and reducingCO₂emissions.

3. A reduced volume of packing material for disposal.

Best results and longer operating periods were obtained withthereduction of silicon-containing compounds in the VOC stream.Thelesson learned: every effort should be made to minimizeoreliminate HMDS, and to maintain the highest operatingefficienciesin any thermal oxidation system.PE