4310
Ind. Eng. Chem. Res. 1995,34, 4310-4317
Catalytic Destruction of Volatile Organic Compounds Using Supported Platinum and Palladium Hydrophobic Catalysts Ramesh K. Sharma, Bing Zhou, Shimin Tong? and Karl T. Chuane Department of Chemical Engineering, University
of Alberta,
Edmonton, Alberta T6G 2G6,Canada
The catalytic destruction of volatile organic compounds (VOCs), namely, methanol, acetone, and methylene chloride, was studied over single Pt- or Pd-loaded hydrophobic catalyst and Pt-Pd bimetallic hydrophobic catalyst. A fixed bed reactor operated at atmospheric pressure and in the temperature range of 50-400 "Cwas used. The VOC destruction experiment was conducted for a broad range of concentrations for each single-component VOC feed and for two- and threecomponent mixtures. The best results were obtained on the Pt-Pd bimetallic hydrophobic catalyst, which exhibited higher destruction activity than either the Pt or the Pd single-metal catalyst in the range of temperature tested. The results suggest a possible alloy effect for the Pt-Pd bimetallic catalyst. The experiment also revealed strong VOC mixture interaction effects. Using the concept of reduction ability, the promoting and inhibiting effects exhibited by the VOC mixture tested can be explained. The data were correlated using a semiempirical Langmuir-Hinshelwood model which enabled the prediction of the destruction rate of each component among the mixtures studied.
Introduction The U.S.Clean Air Act of 1990 calls for a 90% reduction in emissions of toxic chemicals, 70% of them volatile organic compounds (VOCs), over the next nine years. For VOC destruction, catalytic oxidation is one of the most important air pollution control techniques in which the VOCs are oxidized over a catalyst at temperatures much lower than those required for thermal oxidation (Spivey, 1987). The subject of destruction of VOCs, which include halogenated and non-halogenated compounds, has been reviewed (Golodets, 1983;Prasad et al., 1984;Spivey, 1987;Schmidt, 1991). Both noble metal catalysts and metal oxides have been tested. In general, noble metal catalysts are mainly for non-halogenated VOC destruction, while the metal oxide catalysts are used for halogenated VOCs (Spivey, 1987). Although the reaction mechanism of VOC oxidation is not well understood, it is suggested that the reaction may involve both lattice and surface oxygen for metal oxides and reduced metal sites for noble metals. The noble metal catalysts make up about 75% of the catalysts used for VOC destruction. The catalyst support used has most frequently been alumina. Earlier literature also suggests that the reactions over noble metals follow either a Langmuir-Hinshelwood type mechanism (reaction between adsorbed oxygen and an adsorbed reactant) or an Eley-Rideal mechanism (reaction between adsorbed oxygen and a gas-phase reactant molecule). In the case of CO oxidation, it was reported that, below 150 "C, the rate-determining step was the adsorption of 0 2 on adjacent Pt sites (Barshad et al., 19851, whereas at higher temperatures, the reaction between adsorbed oxygen and gaseous CO was the rate-limiting step (Langmuir, 1921). The data suggest that with an increase in temperature the surface concentration of adsorbed oxygen increases while that of the adsorbed organic compounds decreases.
* To whom correspondence should be addressed. E-mail: KarlT.Chuang@ualberta,ca.FAX. (403)492-2881. + Present address: Western Research Centre, CANMET, Devon, Alberta TOC 1E0,Canada.
Among the noble metals, only Pt and Pd are generally used since other metals have been found to undergo sintering, volatility loss, and irreversible oxidation at high temperatures, besides being expensive (Prasad et al., 1984). It has been reported that Pt and Pd function in a reduced state especially at moderate temperatures (Germain, 1969).Volter et al. (1987)observed a pronounced oscillatory behavior of the oxidation in catalytic combustion of n-heptane. The observation was explained by a dual-site model: active metallic Pt competed in the oxidation with the less active oxidic surface complex [Pt4+lsites. The reduced sites were presumably generated by the organic compound n-heptane. Gates et al. (1979)reported that the Pt catalysts were more active compared to Pd catalysts for the oxidation of carbon monoxide. Shinjoh et al. (1989)found that the Pt catalysts were also more active for propane oxidation. However, the activities for propylene oxidation on Pt catalysts were found to be lower than on Pd catalysts. In some studies supported bimetallic Pt-Pd catalysts were tested. Cordonna et al. (1989)found that the Pt is susceptible to sintering at high temperatures. In contrast, Cullis and Willatt (1983)observed that, at temperatures above 450 "C, prolonged exposure of Pd catalyst t o oxygen caused structural changes, resulting in a loss of activity for methane dxidation, whereas the Pt catalyst could maintain its activity because no detrimental structural changes occurred. The degree of destruction for a compound may also depend on whether the compound is the only VOC in the feed, or is part of a mixture of VOCs. Earlier studies have found that a mixture effect exists in the oxidation of hydrocarbons, mostly in the form of inhibition of oxidation of one hydrocarbon in the presence of others (Gangwal et al., 1988;Tichenor and Palazzolo, 19871, indicating that the catalytic oxidation of a compound will be retarded by the other component. Therefore, when the inhibition effect is encountered, a higher temperature is usually needed to achieve a similar destruction level. Cullis et al. (1970)studied the oxidation of methane and observed the inhibiting effect of methylene chloride, but the catalyst activity and its selectivity t o COz and water for the oxidation of meth-
0888-5885/95/2634-4310$09.QQIQ 0 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34,No. 12, 1995 4311 ane could be completely restored when methylene chloride was eliminated from the feed (Spivey, 1987). One of the major problems for the noble metal catalysts supported on alumina is their low activity for the destruction of halogen-containing compounds (Bond and Sadeghi, 1975; Wang et al., 1992; Yu et al., 1992). It was conjected that alumina reacts with chlorinated hydrocarbon to form aluminum chloride which then blocks the surface sites or deactivates 'the noble metal. The alumina-supported catalysts may also deactivate due to excessive adsorption of process water on the support. Chuang et al. (1992) used a hydrophobic Pt supported catalyst for the destruction of benzene, toluene, and xylene. They obtained conversions of over 95% a t 150 "C. Attainment of high conversions was attributed in part to enhanced effectiveness arising from the hydrophobicity of the catalyst. As suggested, the organic compounds were more readily adsorbed on the surface of the hydrophobic catalyst than on a hydrophilic catalyst, thereby producing more reduced sites on the surface of the catalyst. This effect may become more profound in the destruction of VOCs where water, a rate inhibitor, is also present in the feed gas (ambient air contains 2-3% water vapor). The kinetics for the destruction of VOCs is a complex problem not only because the catalyst behaves differently for each individual VOC compound, but also because it functions differently when the VOC compound to be destroyed exists as a single compound in the feed or in a mixture. To objectively predict the activity of a catalyst for the destruction of VOCs, the mixture effect must be examined and development of comprehensive kinetics including the mixture effect is important for the description of a multicomponent VOC destruction process. Since the hydrophobic catalyst has been found t o have high activity for VOC destruction at relatively low temperatures, and is less susceptible to deactivation through surface concentration of water, the hydrophobic catalysts with either single-metal (Ptor Pd) loading or bimetallic (Pt-Pd) loading were tested in this study for the destruction of methanol, acetone, and methylene chloride, a typical mixture produced as waste from the coating industry. Tests were made using singlecomponent, binary, and ternary methanol-acetonemethylene chloride mixtures. The objectives of the study are to elucidate the reaction mechanism and t o establish an overall rate expression which can be used to estimate the conversion of the compound to be destroyed in the mixture under study.
Experimental Section Preparation of the Bimetallic and Single-Metal Catalysts. The catalysts were in the form of 1/4-in. (6mm) ceramic Raschig rings coated with Pt and Pd on fluorinated carbon. The method of preparation was similar to that described earlier (Cheng and Chuang, 1992). The stoichiometric amounts of Pt(NH&(NO& and PdClz were each dissolved to form methanol-water solutions, and a fluorine-treated carbon was added to each solution. The carbon was hydrophobic, and its contact angle with water was 110". The mixtures were then rotary dried at 95 "C and reduced in hydrogen a t 200 "C for 24 h. Each material was then mixed with water to form a slurry. Ceramic Raschig rings (obtained from Norton) were added to this slurry. The catalysts were dried and calcined in air a t 350 "C for 15 min. Since all metals were captured, the Pt and Pd loadings
were calculated on the basis of the initial weight of the chemical complexes. Experimental Setup. A fixed bed tubular reactor was used for the destruction of VOCs a t atmospheric pressure. The reactor was a 0.2-m-long U-shaped glass tube (25 mm in diameter), placed in an electric furnace. The temperature of the catalyst bed was controlled by an Omega temperature controller, using a thermocouple attached to the outside wall of the reactor tube. The reaction temperature was monitored by three thermocouples which were placed at the inlet, middle, and outlet of the catalytic bed. In all cases, the temperature variations among the three thermocouplereadings were less than 1 "C. The feed air was divided into three primary streams and one secondary stream. The flow rate of the four air streams was regulated by digital mass flow meters. The total feed rate was measured by a bubble flow meter at the vent. The three primary air streams were passed through humidifiers containing liquid methanol, acetone, and methylene chloride, respectively. The temperature of each humidifier was held constant using an ice bath. The three streams were then combined, preheated, and introduced into the reactor. This humidifier system generates a controlled level of VOCs in the feed. The concentration for each component in the feed was varied by adjusting the flow rates of the corresponding primary air stream, and analyzed by gas chromatography. After a steady state was obtained, the reactor eMuent was analyzed. Tests were carried out in both dry and moist conditions. In the case of moist conditions, the feed gas was saturated by passing through a water saturator. Between 5 and 20 g of fresh catalyst was used in each run. To minimize the temperature gradient in the reactor, the catalyst was diluted with inert support rings in a 1:3 ratio. Kinetic data were obtained using single-, two-, and three-component mixtures over hydrophobic catalysts loaded with Pt, Pd, or Pt-Pd. Single-component feeds were tested first, followed by binary and ternary mixtures t o enable examination of interaction effects. Reaction temperatures were in the range of 50400 "C, and the volumetric hourly space velocity (VHSV) was varied between 3000-15000 h-l. The inlet concentrations for various components were in the range of 950-6600 ppmv methanol, 550-2350 ppmv acetone, and 65-350 ppmv methylene chloride. On the basis of replicate runs, the reproducibility of the data was found to be within f 2 % . Analysis. Both the feed and reactor effluent were analyzed by an on-line gas chromatograph (HewlettPackard Model HP 5880A) equipped with a flame ionization detector (FID). The components were separated using a 4-m-long, 3-mm-diameter stainless steel column packed with 80-100-mesh Porapak Q, and were analyzed isothermally a t 170 "C. The detector was calibrated using mixtures of known composition. Carbon dioxide and water were the only products (i.e., no partial oxidation products). In the case of methylene chloride, it was assumed that HC1 and Clz also formed, because the results published by Aganvat and Spivey (1991) and Dowd et al. (1992) indicate that chlorine compounds found in the oxidation products were 95% HC1 and 5% Clz.
Results and Discussion Preliminary Test. The stability period of the catalyst was examined over a period of 160 h. It was found that for methanol and acetone the change of catalyst
4312 Ind. Eng. Chem. Res., Vol. 34,No. 12, 1995 100
80
-
-P0
Suppcltd Pd Catelytf
0
s"ppmaFicaluyst
80-
8
8
.-0 Ea,
r'
60-
8
40
-
20
-
c'
5 .-0 E9) 0
0
Supponad Fi-Pd CHeiyst
""
40-
Suppmed R.Pd Cauyst
20-
I
0
8
SuwmedFiCu.lyat
60-
100
200
I
300
400
1
0
500
400
Figure 1. Activity of the catalysts for methanol destruction.
activity was negligible for the range of concentrations used in the tests. For methylene chloride, the conversion decreased by less than 8%when its concentration in the feed was over 100 ppm. To test the effect of water on the destruction rates for the hydrophobic catalyst, runs were made in which water vapor was introduced with the VOC mixture containing methanol, acetone, and methylene chloride. The concentration of water in the feed was 3 mol %. The VOC destruction rates were not affected by the presence of water vapors over the range of conditions tested. Catalyst Screening. The activities of Pt, Pd, or PtPd supported hydrophobic catalysts were measured a t constant VHSV and VOC feed concentrations. The temperature was varied in the range between 50 and 400 "C. Preliminary runs showed that the activity of the catalyst for methanol destruction did not change with the amount of metal loading. However, the conversions of acetone and methylene chloride increased by only 5-8% when the metal loading was doubled from 0.1%to 0.2%. Cordonna et al. (1989)have also observed an increase in hydrocarbon conversion with increasing metal loading of P t P d catalysts, although the loading has no effect on the conversion of carbon monoxide. From the consideration of cost, the use of a durable, active catalyst with low Pt loading is always favored. In this study, a total metal loading of 0.1% was used. TPD measurements of the fluorinated carbon in air showed that the carbon is stable up t o 540 "C. The catalyst activity for the oxidation of three VOCs was stable during the steady state tests of several days. This suggests that there is no oxidation of the support up to a temperature of 400 "C. Effect of Pt and Pd Compositions on Destruction Activity. Six catalysts with different P W d ratios ranging from 0.02%/0.08% to 0.09%/0.01% have been tested. The catalyst with 0.075% W0.025% Pd showed the best activity. A comparison of the activities of three catalysts, 0.1% Pt, 0.1% Pd, and 0.075% W0.025% Pd, for the destruction of methanol, acetone and methylene chloride is presented in Figures 1-3. Figure 1 shows that the catalyst loaded with Pt exhibits a higher activity than the one loaded with Pd for the destruction of methanol, leading to conversions of over 95% at 150 "C. However, the supported bimetallic Pt-Pd catalyst exhibits the highest activity among the three catalysts. The lower activity of the Pd catalyst compared t o the Pt catalyst was attributed to the weak adsorption of oxygen on Pd (Spivey, 1987). Hicks et al. (1990) tested Pt and Pd catalysts for the oxidation of heptane and found that the Pt catalyst was more active than the Pd
6
0
Reactor Temperature, "C
Reactor Temperature, "C
Figure 2. Activity of the catalysts for acetone destruction. 100
80
-
8
$
'g
8
w40-
20
0
m
300
4w
500
Reactor Temperature, "c
Figure 3. Activity of the catalysts for methylene chloride destruction.
catalyst. Cordonna et al. (1989) also observed that, for the oxidation of CO, its conversion increased from 10% over Pd catalyst to 90% over Pt catalyst. The higher activity for the supported bimetallic Pt-Pd catalyst implies that alloy effects may play a role. The catalytic activities for the destruction of acetone are compared in Figure 2. It is interesting to note that below 300 "C the supported Pt catalyst shows higher activities compared to the supported Pd catalyst. In contrast, the supported Pd catalyst shows higher activity than the Pt catalyst at temperatures above 300 "C. However, of the three catalysts tested, the Pt-Pd catalyst exhibits the highest activity over the whole region of temperatures. The conversion of methylene chloride is shown in Figure 3. The supported Pt-Pd catalyst exhibits a much higher activity than either Pt or Pd catalyst. Similar observations were made by Cordonna et al. (1989) for the oxidation of hydrocarbons over various P W d catalysts. They found that the catalytic activity increased with increasing Pt content until the PdPt ratio was 4. These results suggest that for the Pt-Pd bimetallic catalyst the active surface is not a simple mechanical mixing of Pt and Pd. Some special structure which results in high activity is formed on the surface. Among the three VOCs tested, the methylene chloride is the most difficult compound to destroy. From the comparison of the results shown in Figures 1-3, the oxidation rate for the three chemicals decreased in the order methanol > acetone > methylene chloride
(1)
Ind. Eng. Chem. Res., Vol. 34, No. 12,1995 4313
A
M.(huloM(mylene Chlorid.
MoUumVMaf M e t h y h Chlorde
1W
50
0
150
200
250
300
Reactor Temperature,OC
0
Figure 4. Effect of acetone and methylene chloride on the destruction of methanol. 100 00 Morni-d
80
.
kahw/
w
AMaulM.(Nnov
reaction destruction of methanol destruction of acetone destruction of methylene chloride
60.
:
40-
XI-
,
0 0
~, 100
300
400
5w
Reactor Temperature,OC Figure 6. Effect of methanol and acetone on the destruction of methylene chloride.
M.thykMCnlUld.
Methyme Chlwib
0
200
Table 1. Promoting and Inhibiting Effect
A
8.
g ’L
100
2M)
300
400
500
600
reduction
pt4+ oxidation Pt According to this explanation, the reduction ability of the three hydrocarbon compounds in our study must follow the sequence of oxidation rates as above. The higher reduction ability exhibited by a chemical compound, the higher is the catalyst activity. A combination of Pt and Pd may somehow enhance the reduction of the Pt on the surface. Further study of the F’t-Pd interaction is required to clarify this phenomenon on the surface of the catalyst. Mixture Effect on Destruction Activity. Figures 4-6 show the effect of mixtures of VOCs on the destruction of each methanol, acetone, and methylene chloride, respectively. On the assumption that reduction ability affects the concentration of active sites, the rates of conversion of methanol, acetone, and methylene
promotinghnhibiting effect by methylene methanol acetone chloride inhibiting inhibiting promoting inhibiting promoting promoting
chloride mixtures shown in Figures 4-6 are readily understood. The compound with stronger reduction ability exhibits a promoting effect for the conversion of the compounds with weaker reduction ability because the enhanced reduction capability has increased the concentration of effective metallic Pt on the surface. On the other hand, compounds with weaker reduction ability show an inhibiting effect for the conversion of the compounds with stronger reduction ability. The inhibition effects have received wide acceptance as arising from competitive adsorption on the catalyst surface. Table 1 summarizes the experimental results for the promoting-inhibiting effect of the three compounds on each other. Similar observations on different reaction systems have also been reported by other researchers (Chashechnikova and Golodets, 1983; Tichenor and Palazzolo, 1987; Gangwal et al., 1988). In addition to the above explanation, an alternative explanation is that the presence of halogen causes a related inhibition effect. Klinghoffer and Rossin (1992) found that the inhibiting effect of methylene chloride in oxidation reactions was due to the formation of HC1 from the conversion of methylene chloride. The formation of HC1 from chlorine is dependent on the presence of water and is governed by the Deacon reaction: 2C1,
+ 2H,O
4HC1+ 0,
(3)
Whether the use of the hydrophobic catalyst is beneficial for preventing this reaction is presently u n k n o w n . Pope et al. (1978) studied hydrocarbon oxidation in the presence of halogen compounds, and found that the inhibition effect of the halogens was determined by the rate a t which the halogen compound was oxidized.
Kinetics of the VOC Mixture Destruction Effect of External Heat and Mass Transfer. Prior t o kinetic runs, experiments were made to determine
4314 Ind. Eng. Chem. Res., Vol. 34, No. 12, 1995
the effect of external heat and mass transfer on reaction rates. The stoichiometric expressions for the complete oxidation of methanol, acetone, and methylene chloride are CH30H C3H@ CH,Cl,
+ (3/2)0, - CO, + 2H,O
+ 40, - 3c0, + 3H2O
+ 0, - CO, + 2HC1
H = -676 kJ/mol (4) -rVOc
H = -1658 kJ/mol ( 5 ) H = -490 kJ/mol (6)
Due to the high exothermicity of the VOC oxidation, external heat transfer limitations may be significant, especially at low feed rates. This was demonstrated by measuring VOC conversions in our diluted catalyst bed at different feed rates. The VHSV was kept constant at 6600 h-l while the temperature was varied between 50 and 400 "C. For each of these runs, the supported Pt-Pd catalyst was used. The runs with high conversions were of special interest since more reaction heat in these runs was produced compared to that in runs at low conversions. At 350 "C and constant VHSV, the conversions of the three components studied were independent of the feed rate. The isothermal condition in the reactor was well maintained under the test conditions. Reaction Rate. The activity of the catalyst for the oxidation of VOCs depends on the rate of redox reaction on the surface: step 1
reduced catalyst
step 2
oxidized catalyst
Since the 0 2 concentration in the system is much higher than the VOC concentration, its effect on the reaction rate is assumed t o be constant. The reaction rate for each individual VOC component is assumed to be proportional to both its partial pressure and the sum of the reduction sites available on the surface.
-oxidized catalyst EO1
+ OM + + OMC)
= kVO$VOc(OO
(13)
where VOC represents methanol, acetone, or methylene chloride. In the Langmuir-Hinshelwood approach, the fraction of reduced empty sites is given by
+
Bo = 1/(1 KMPM
+ K A P A + KM$Mc)
(14)
Assuming that the reduction site generated by methanol, acetone, or methylene chloride is proportional to the adsorption rate between the VOC molecule in the gas phase and the empty site available on the catalyst surface, then
+
+ K A P A + KM$M,) 8, = kApA/(1 + KMPM + + KM$Mc)
8, = k ~ p ~ / (K,PM l
(15)
KAPA
(16)
+ KMPM + K A P A + KM$Mc)
(17)
and OM,
= kM$MC/(l
Substitution of eus 14-17 into eu 13and rearrangement
(7)
[hydrocarbon]
18)
reduced catalyst (8) By applying this principle to the VOC mixture destruction on the supported Pt-Pd catalyst, a schematic description of the catalytic cycle is:
t
19)
02
The above equations, although semiempirical, include both promoting and inhibiting terms in one equation for a mixture system. The terms bracketed in the numerator denote the promoting effect contributed from each component. The inhibiting effect contributed from each component is expressed in the denominator. In this study, it was observed that the destruction rates were not affected by the presence of water on the catalyst. The discussion about the water-repelling effect of this hydrophobic catalyst has been detailed by Chuang et al. (1992). Thus, no expression for an effect of water on reaction rates was included in the above kinetic equations. Parameter Search for the Rate Equations. In the search of parameters for fitting the experimental data, eqs 18-20 were integrated numerically t o calculate the conversions of various components. The parameters were estimated by nonlinear regression. The objective function which was minimized was the residual sum of squares (RSS) of experimental and calculated conversions of the three components:
Ind. Eng. Chem. Res., Vol. 34, No. 12, 1995 4315
where m, the total number of experiments, was 161. In eq 21, xexptl(ij) and x c a l c d ( i j ) are the experimental and calculated conversions of componentj in experiment i. To minimize the correlation between the activation energy and preexponential factor, the rate and equilibrium constants in the rate equations were reparameterized as I 5W
(Ow
1500
2000
2500
3WO
1ow
SMK)
Methanol Inlet Concentration, pprn Figure 7. Model check for predicting the conversion of methanol. 1w
where k ~ KT, , k ~ and ~ KT, , are the values of the constants at T and TR,respectively. E and H are the activation energy and adsorption energy, respectively. TRwas taken as 673 K. To simplify the search of the parameters, the rate eqs 18-20 are further simplified by considering the mixture effect as summarized in Table 1:
80
I
methanol
Acetone Inlet Concentration, ppm Figure 8. Model check for predicting the conversion of acetone.
acetone
Table 2. Estimated Values of Rate Parameters in VOC Oxidation
methylene chloride -TMC
+
+
= k ~ c " P ~ c ( 1~ M P M kAPA
+ kM$MC)/(l + KMCpMc) (26)
During the linear regression, various rate equations were tested by eliminating inhibiting and/or promoting terms in the equations. The simplest set of the rate equations fitting the experimental data is
K*M(TR) = 189.03 f 74.86 mL of methanol/(g of catalyst s atm) k*A(TR)= 4.19 f 1.5 mL of acetone4g of catalyst s atm) k*MC(TR)= 1.2 f 0.17 mL of methylene chloride/(g of catalyst s atm) EM* = 6.365 f 0.57 kcaYmol EA* = 5.585 f 0.78 kcal/mol EMC*= EM* k~ = 10245.25 f 4966.44 l/atm EM= 7.67 f 3.43 kcaVmol KAI(TR) = 38.70f 16.08 atm-l KM(TR)= 111.38 f 51.23 atm-l KMC(TR) = 363.64 f 145.32 atm-I HAI= -2.0 f 1.05 kcaVmol HM = HMC= HA^
methanol order of relative destructibility was found to be alcohols > aldehydes > aromatics > ketones > acetates > alkanes > chlorinated hydrocarbons
acetone
methylene chloride
-rMc = &*PMd(l+ KMCpMc)
(29)
The estimated values of the parameters along with their 95% confidence limits are given in Table 2. The highest value of the rate constant is for the destruction of methanol (KM*) which is followed by acetone ( k ~ *and ) methylene chloride (KMc*). Similar observations were made by Tichenor and Palazzolo (1987) in their studies on the destruction of different organic components using Pt and Pd honeycomb catalysts a t 260-425 "C. The
which is in agreement with the results of this study. The values of the activation energy (Table 2) for methanol, acetone, and methylene chloride are 6.4,5.6, and 7.7 kcallmol, respectively. These values are lower than those reported by Gangwal et al. (1988). The lower value obtained in our test results may be caused by the mass transfer effect. It is also possible that water adsorption in their hydrophilic support causes an increase in activation energy due to the fact that the activity of the catalysts was more significantly reduced by water adsorption a t low temperature than at high temperature. Model Check. The agreement between the regressed rate equations and the experimental data (Figures 7-9) shows that the rate eqs 27-29 provide a
4316 Ind. Eng. Chem. Res., Vol. 34, No. 12, 1995
kM = adsorption rate constant for methanol, l/atm = adsorption rate constant for methylene chloride, l/atm kA* = rate constant for acetone, mL of acetone/(gof catalyst s atm) kM* = rate constant for methanol, mL of methanol/(g of catalyst s atm) KMC* = rate constant for methylene chloride, mL of methylene chloride/(g of catalyst s atm) kM(TR)= adsorption rate constant for methanol at 673 K, l/atm kA*(TR)= rate constant for acetone at 673 K, mL of acetone/ (g of catalyst s atom) ~M*(TR =)rate constant for methanol at 673 K, mL of methanol/(g of catalyst s atom) kMC*(TR)= rate constant for methanol at 673 K, mL of methylene chloride/(g of catalyst s atm) k T = rate constant at temperature T , mL of VOC/(g of catalyst s atom) kTR = rate constant at temperature TR,mL of VOC/(g of catalyst s atom) KA = adsorption equilibrium constant for acetone, l/atm KM= adsorption equilibrium constant for methanol, l/atm KMC= adsorption equilibrium constant for methylene chloride, l/atm KT = adsorption equilibrium constant at temperature T , l/atm K T= ~ adsorption equilibrium constant at temperature TR, l/atm K A ~ ( T R=) adsorption equilibrium constant of acetone at 673 K in rate eq 26, l/atm K ~ ( T R=) adsorption equilibrium constant of acetone at 673 K in rate eq 27, l/atm KMC( T R )= adsorption equilibrium constant of methylene chloride at 673 K, l/atm m = number of experiments P A = partial pressure of acetone, atm PM = partial pressure of methanol, atm PMC= partial pressure of methylene chloride, atm -rA = reaction rate for methanol, mL of acetone/(g of catalyst s) -rM = reaction rate for methanol, mL of methanoll(g of catalyst s) -rMc = reaction rate for methylene chloride, mL of methylene chloride/(g of catalyst s) - ~ O C= reaction rate for VOC, mL of VOC/(g of catalyst kMC
I
o
100
2w
300
409
Methylene Chloride Inlet Concentration, pprn
Figure 9. Model check for predicting the conversion of methylene chloride.
good model for the experimental data over a wide range of concentrations and temperatures.
Conclusions The supported Pt-Pd bimetallic catalyst exhibited the highest activity for the destruction of methanol, acetone, and methylene chloride in the range of temperature up to 400 "C when compared with the supported single Pt or Pd catalyst. Over 90% conversions of methanol and acetone were achieved using Pt and/or Pd hydrophobic catalysts. The Pt-Pd bimetallic catalyst also showed a high activity to the destruction of methylene chloride, about 60% conversion at 400 "C at VHSV values in the range between 3000 and 15 000 h-l. A mixture effect was found to exist in the destruction of VOCs in the methanol-acetone-methylene chloride system. The concept of reduction ability and subsequent competitive adsorption on the surface explained well the promoting-inhibiting effect which was exhibited by the VOC mixture destruction kinetics. Semiempirical rate equations including promoting and inhibiting effects were developed for the threecomponent methanol-acetone-methylene chloride mixture system. The rate equations enable prediction of the conversion of methanol, acetone, and methylene chloride in a mixture under a range of conditions.
Acknowledgment Financial support from the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged.
Nomenclature EM= activation energy for methanol adsorption rate, kcall mol EA* = activation energy for acetone reaction rate, kcdmol EM*= activation energy for methanol reaction rate, kcall mol EMC*= activation energy for methylene chloride reaction rate, kcallmol HA^ = adsorption energy of acetone for adsorption equilib, rium constant K A ~kcal/mol Hm = adsorption energy of acetone for adsorption equilibrium constant Km, kcallmol HMC= adsorption energy of methylene chloride for adsorption equilibrium constant KMC,kcal/mol kvoc = rate constant for VOC, mL of VOC/(g of catalyst s atm) kA = adsorption rate constant for acetone, l/atm
S)
R = universal gas constant, 1.987 cal/(mol K) RSS = residual sum of square T = temperature, K T R = reference temperature at 673 K xexptl(ij)= experimental conversion of component j in experiment i (j = acetone, methanol, and methylene chloride, i = 1, 2, ...I &&d(ij) = calculated conversion of component j in experiment i (j = acetone, methanol and methylene chloride, i = 1, 2, ...) [ox*] = oxidized site on the catalyst surface [red*] = reduced site on the catalyst surface Subscripts A = acetone M = methanol
MC = methylene chloride Greek Symbols 80 = fraction of empty reduced site on the catalyst surface 8~ = fraction of site occupied by adsorbed acetone OM = fraction of site occupied by adsorbed methanol
Ind. Eng. Chem. Res., Vol. 34,No. 12, 1995 4317 fraction of site occupied by adsorbed methylene chloride
eMC =
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Received for review January 19, 1995 Revised manuscript received July 3, 1995 Accepted July 21, 1995@ IE9500578
Abstract published in Advance A C S Abstracts, October 15, 1995. @
