3094
Ind. Eng. Chem. Res. 1994,33, 3094-3103
Catalytic Oxidation of Dichloromethane, Chloroform, and Their Binary Mixtures over a Platinum Alumina Catalyst Douglas M. Papenmeier and Joseph A. Rossin. Geo-Centers, Inc., Gunpowder Branch, P.O. Box 68,Aberdeen Proving Ground, Maryland 21010-0068 The complete catalytic oxidation of dichloromethane, chloroform, and their binary mixtures was examined over a 3% Pt/~-dA 1 2 0 3 catalyst at temperatures between 300 and 400 "C using a fixed bed catalytic reactor. The oxidation of chloroform and dichloromethane as pure compounds was nonlinear in the concentration of chloromethane and zeroth order in the concentration of oxygen. HC1, formed during the oxidation of each chloromethane, decreased the reaction rate. Kinetic rate expressions were developed to describe the oxidation of dichloromethane and chloroform as pure compounds. These expressions were derived by assuming that the reaction occurred via adsorption and decomposition of the chloromethane onto an oxygen covered platinum surface, with the reaction being inhibited by the presence of HCl. From the results of the pure compound studies, reaction rate expressions were developed to dexribe the oxidation of dichloromethane/chloroform mixtures. The resulting reaction rate expressions accurately predicted the catalyst's performance during the oxidation of dichloromethandchloroformmixtures over a wide range of conditions.
Introduction There is a growing interest in the use of catalytic oxidation to control vapor phase emissions (Spivey, 1987; Schmidt, 1992). Often times, contaminated streams will contain several compounds, and the interactions of these compounds with respect to the catalytic activity are not well understood. In general, the presence of additional components in the feed stream tends to inhibit the oxidation activity of the catalyst (Voltz et al., 1973; Yao and Kummer, 1973; Pope, et al. 1978; Gangwal et al., 1988; Barresi and Baldi, 1992). Ideally, one would like to be able to predict the reactivity of individual compounds in a mixed feed stream using only pure component reaction rate data. In this way, the data required to develop the mathematical model a n d or optimize the process would be greatly minimized. Attempts t o develop reaction rate expressions to describe the catalytic oxidation of mixtures of organic compounds have been limited. Voltz et al. (1973) formulated a complex kinetic rate expression to describe the oxidation of mixtures of carbon monoxide, propene, and nitrous oxide in air over a platinum catalyst at temperatures between 205 and 371 "C. The authors found the rate of oxidation for the target compound to be inhibited by the presence of additional compounds within the feed stream. The kinetic rate expression developed by the authors, whose fit parameters appear to have been derived from the mixture reaction rate data, provided a very accurate description of the experimental data. Gangwal et al. (1988) performed a fundamental investigation of mixture effects involving the oxidation of n-hexane and benzene over a supported platinum-nickel catalyst. The authors correlated the individual component reaction rate data using a Marsvan Krevelen rate expression and then extended the assumptions associated with the development of the rate expression to include the presence of the second compound. The model provided an accurate description of the single component reaction rate data; however, the reaction rate expressions were unable to accurately
* Author to whom correspondences should be addressed. Current address: Guild Associates, 5022 Campbell Blvd., Baltimore, MD 21236.
describe the oxidation of n-hexanelbenzene mixtures. Barresi and Baldi (1992)used dual component reaction rate data as a means of model discrimination in developing rate expressions to describe the oxidation of benzene-ethenylbenzene mixtures over a monolithic supported platinum catalyst. The authors were unable to develop the rate expression using only the pure component data, since both compounds exhibited zeroth order (in organic) kinetics. However, the results of the mixture studies coupled with the pure component data lead to the development of a kinetic rate expression capable of describing the oxidation of benzene-ethenylbenzene mixtures. The present study focuses on the oxidation of binary mixtures of dichloromethane (CHzClz) and chloroform (CHCl3) in humid air and is part of an ongoing effort aimed at determining reaction mechanisms for the oxidation of halogenated organic compounds. The objective of the present study was to develop an experimentally based reaction rate expression which would predict the reactivity of individual chloromethanes in mixed feed streams. Our approach to meeting the stated objective was to (1)record reaction rate data for the individual chloromethanes, (2) develop mechanistic reaction rate expressions to describe the oxidation of the individual chloromethanes, (3)extend the reaction rate expressions to include mixtures, and (4)evaluate the ability of the resulting reaction rate expressions to predict the catalyst's performance during the oxidation of dichloromethane/chloroform mixtures.
Experimental Section Materials. HPLC-grade dichloromethane and chloroform (99.9%purity) were obtained from Aldrich. Two compressed gas vessels containing either dichloromethane or chloroform in air were prepared by adding a known amount of liquid to a 45-L stainless steel pressure vessel and pressurizing to 20 atm with ultrahigh-purity air. The concentrations of chloromethane in each vessel were determined via gas chromatographic analyses t o be 6145 ppm for dichloromethane and 6091 ppm for chloroform. The catalyst employed in this study was a 3% P t / ~ - dA1203 purchased from Engelhard as 60/
0888-5885/94/2633-3094$04.50/00 1994 American Chemical Society
Ind. Eng. Chem. Res., Vol. 33, No. 12, 1994 3096
80 mesh granules. The BET surface area of the catalyst support was reported as 120 m2/g. Calibration gases, namely CO and COz in air, were obtained from Matheson. Catalyst Pretreatment. Prior to reaction exposure, several 5-g portions of catalyst were pretreated by calcining within a quartz tube furnace. The calcination was performed by exposing the catalyst to humid air (dew point temperature = 20 "C) flowing at 1 NL/min (NL refers to 1 L of gas at 0 "C, l-atm pressure) and raising the catalyst temperature from 25 to 425 "C in 2 h. The final temperature was maintained approximately 16 h. Following calcination, the individual portions of catalyst were combined in order to provide a uniform batch of catalyst from which to conduct the experiments. X-ray photoelectron spectroscopy (XPS) analyses of the catalyst performed following calcination revealed platinum t o be in the metallic oxidation state. Equipment. The fixed bed reactor system has been described previously (Klinghoffer and Rossin, 1992). Briefly, dry, oil-free air delivered from a PSA air drier was metered to the system using a mass flow controller of the appropriate range. Each chloromethane in compressed air was metered to the system using a mass flow controller, with the feed stream(s) being blended with the dry air stream just in front of a static mixer. Liquid water was injected into the feed stream at this point using a syringe pump. A temperature controller was used to maintain the temperature at the point of liquid injection at 110 f 1"C. The static mixer followed and served to blend the feed stream as well as dampen out oscillations associated with the syringe pump. Following the static mixer, the feed stream was delivered to the reactor. The reactor was housed in a 7-cmdiameter by 20-cm-long electrically heated aluminum block, which provided uniform heating of the catalyst bed. The temperature of the catalyst bed was controlled by controlling the temperature of the aluminum block. This control scheme allowed for maintaining a stable catalyst bed temperature to within f l "C of the desired value. Following the reactor, the entire effluent stream was delivered to a gas chromatograph (GC) located within a fume hood for on-line analysis. The reactor consisted of a 0.95-cm-0.d. glass tube approximately 25-cm long. The catalyst bed was supported on a plug of glass wool located approximately 8 cm from the bottom of the reactor. To minimize axial temperature gradients, the catalyst bed was prepared by diluting between 0.1 and 1.0 g of 60/80 mesh catalyst with crushed glass of a similar mesh size in order to achieve a bed volume of approximately 2 cm3. 12/20 mesh crushed glass was placed above the catalyst bed to serve as a preheat zone for the incoming feed stream. A single type-K thermocouple extended from the top of the reactor to approximately one-third of the way into the catalyst bed. Although the temperature profile within the catalyst bed could not be monitored, deviation from isothermal operation could be assessed by comparing the catalyst bed temperature to that of the aluminum block surrounding the catalyst. Reaction Rates of Chloromethanes. Kinetic rate data were recorded for each chloromethane as a pure component at reaction temperatures of 300, 345, 375 and 400 "C at a pressure of 1.5 & 1 psig. Four feed concentrations, namely 300,1000,3000, and 6100 ppm in humid air (2.6% water) were employed a t each reaction temperature for each chloromethane. Reaction rate data for dichloromethane/chloroform mixtures were
recorded at reaction temperatures of 345 and 400 "C a t a pressure of 1.5 f 1 psig. Feed streams consisting of between 300 and 3000 ppm of each chloromethane in humid air were employed at both reaction temperature. The residence time (based on the packing volume of the catalyst and calculated at 0 "C and 1atm pressure) was adjusted in an effort to keep the chloromethane conversion between 10 and 90%. The feed stream flow rates were typically varied between 0.01 and 0.25 NUmin. Flow rates greater than 0.25 NL/min resulted in an excessive pressure drop across the catalyst bed and thus were not employed. Experiments were conducted only during day-time hours. Overnight, the catalyst was exposed t o flowing, humid air at reaction temperature. In the morning, a standard process condition was repeated so that catalyst deactivation could be assessed. Rarely was the catalyst bed on stream for more than 72 h. All process conditions were maintained for a minimum of 2 h to ensure the achievement of steadystate operation. Online GC analyses were performed every 20 min during a given run. The GC was calibrated daily for chloromethane response. A flame ionization detector (FID) was used to analyze the effluent stream for dichloromethane and chloroform, while a thermal conductivity detector (TCD) was used to analyze the effluent stream for C02. Effluent Analysis. Reactor effluent streams were analyzed online using a Hewlett-Packard 5890 GC equipped with an FID and TCD. Dichloromethane and chloroform were separated using a 2-m-long x 0.32-cm0.d. Teflon column packed with Chromosil330 (Supelco) and analyzed using the FID. The concentration of COa in the reactor effluent was determined using a 2-m-long by 0.32-cm-0.d. stainless steel column backed with Porapak Q in conjunction with a TCD. In addition, remote effluent analysis of CO was performed using a Hewlett-Packard 5890 GC equipped with a 2-m-longby 0.32-cm-0.d. Carbosieve I1 stainless steel column and a TCD. CO analyses were performed only for selected experiments. Reactor effluent was quantitatively analyzed for acid gases, namely Clz and HC1, using Drager tubes. According to the manufacturer, concentrations determined using Drager tubes are f10-15% accurate. Analyses were performed for selected runs by attaching the Drager tube directly t o the reactor outlet using a short (less than 1cm) piece of Tygon tubing. In this way, no metallic parts were in contact with the effluent stream, and the entire feed stream was passed through the Drager tube. It should be noted that the reactor extended far enough below the aluminum block so that the Drager tube operated near room temperature. The Drager tube was removed after a known volume of gas had passed through the tube.
Data Analysis and Fit Parameter Estimation Reaction Rates of Pure Chloromethanes. All reaction rate data reported in this text were recorded in an integral, fixed bed catalytic reactor operating under isothermal conditions and in the absence of mass transfer resistances. The design equation for said reactor is as follows (Froment and Bischoff, 1979):
In a previous study involving the oxidation of chloroform over a supported platinum catalyst (Rossin and Farris,
3096 Ind. Eng. Chem. Res., Vol. 33, No. 12, 1994
1993)' a number of reaction rate expressions were evaluated for their ability to correlate the reaction rate data. The reaction rate expression which provided the best correlation of the experimental data was rate, =
z
80 60
kFiCi 1 + KPCP KiC,
-
+
ICHC131 1,250 pprn
where the terms in the denominator of the rate expression, KiCi and KpCp,represent inhibition resulting from adsorption of reactant and reaction product (HCl), respectively, onto catalytic sites. Assumptions associated with the derivation of eq 2 will be discussed later in the text. The reaction rate expression (eq 2) was substituted into the fured bed reactor design equation (eq 1) and integrated. The resulting equation was then incorporated into a nonlinear least-squares algorithm which was used to determine the kinetic fit parameters. In correlating the experimental data, the concentration of reaction product was determined from the reaction stoichiometry (C, = aC;x, where a = 2 for dichloromethane, 3 for chloroform). The nonlinear leastsquares algorithm minimized the error between the logarithm of the predicted and experimental values of WIF;: error = minimum =
0
1
20 v
0
40 80 120 160 Time-on-Stream, H
Figure 1. Conversion as a function of time-on-stream for the oxidation of chloroform in humid air: T = 400 "C, [CHC131= 1250 ppm, residence time = 0.33s.
and chloroform, respectively. The extra term in the denominator of each rate expression takes into account the adsorption of the additional chloromethane onto reactive sites. Equations 5 and 6 were substituted into the fixed bed reactor design equation (eq 1)and numerically integrated using a multistep method. The fit parameters determined from the correlation of the pure chloromethane reaction rate data were used without correction in predicting the mixture reaction rate data. For modeling purposes, the concentration of reaction product, C,, was determined from the reaction stoichiometry (C, = 2Cix1 3C&J. In predicting the mixture reaction rate data, an average value of the product adsorption equilibrium constant, K p ,was employed.
+
Error was minimized on the logarithm of WIF; rather than WIF; since WIF; was varied in excess of 2 orders of magnitude over the range of data. Taking the logarithm of WIF; resulted in a more uniform weighting of the data. The kinetic fit parameters were determined over the entire range of conditions (temperatures, concentrations and residence times) simultaneously. In this manner, both the pre-exponential factors and activation energiesheats of adsorption were determined simultaneously, rather than by the standard technique of determining the fit parameters at each reaction temperature, then correlating the fit parameters using Arrhenius and van't Hoff equations. The stability of the nonlinear least-squares algorithm was greatly improved by representing the fit parameters according to (Holland and Anthony, 1979)
(
A=A,exp-E
4
a1
---
R
(4)
where Tois the midpoint temperature over the range of the data and A, is a generic fit parameter determined at temperature To.It should be noted that only steadystate reaction rate data were used in determining the kinetic fit parameters. Reaction Rates of Chloromethane Mixtures. The reaction rate expressions used to describe the catalytic oxidation of mixtures of dichloromethane and chloroform were
k&ZG - 1 + KPCP+ K,C,
rate -
100 8
+ K2C2
(6)
where the subscripts 1 and 2 refer t o dichloromethane
Results Catalyst Deactivation. Prior to initiating the kinetic studies, catalyst deactivation was assessed by exposing the catalyst to 1250 ppm of chloroform in humid air at temperatures of 350 and 400 "C for extended periods of time. The purpose of these tests was not to determine a catalyst deactivation mechanism. Rather, the tests were conducted to ascertain how long the catalyst could be exposed to reactant without deactivation influencing the kinetic results. Figure 1 reports the conversion as a function of time-on-stream for the oxidation of chloroform at 400 "C. Over the duration of the run (160 h), the conversion of chloroform decreased from 92 to 86%. Similar results were observed at 350 "C. These results indicated that deactivation due to chloroform exposure over the temperature range investigated in this study would not influence the kinetic results, since rarely was the catalyst online for more than 72 h during the kinetic studies. An extended duration deactivation test was not conducted for dichloromethane, since it was felt that deactivation resuIting from chloroform exposure would be more severe due to the greater chlorine content of the chloroform. Tests for catalytic deactivation were also performed during the reaction rate studies. At selected times during a run, catalytic deactivation was assessed by returning to an initial set of process conditions. Conversions recorded at these times were typically consistent with those recorded earlier (rt2-3%), indicating that catalytic deactivation was not masking the experimental results. The absence of any significant catalytic deactivation is consistent with results reported by others involving the oxidation of chlorinated organic compounds. Bond and Sadeghi (1975) reported no significant deactivation during the oxidation of perchloroethylene over a supported platinum catalyst at 450 "C. Klinghoffer and
Ind. Eng. Chem. Res., Vol. 33, No. 12, 1994 3097 Table 1. Chlorine-Containing Reaction Products Observed during Oxidation of Dichloromethane and Chloromethane at 400 "C in Dry and Humid Air
100 CHCl3
2IC2HJ
t
2.6% H 2 0
:**
CH2C12
CHzClz CHzClz CHCl3 CHCl3
2.6% H 2 0
a
0
250
300
350 400 450 Temperature, C
500
Figure 2. Effects of water on the conversion of dichloromethane and chloroform in humid air as pure compounds: [CH2C121= 1000 ppm; [CHCILII= 1000 ppm; residence time = 0.33 s. The solid lines in the figure represent the data as predicted using the reaction rate expression.
Rossin (1992) reported minimal catalytic deactivation during the oxidation of chloroacetonitrile at temperatures to 400 "C over a supported platinum catalyst. Mendyka et al. (1992) reported virtually no loss of catalytic activity towards the oxidation of xylene in the presence of HCl during a 50-hour run. Farris and Rossin (1993) reported only a minimal amount of catalytic deactivation during the oxidation of chloroform at temperatures between 300 and 400 "C over a supported platinum catalyst. Effects of Water. Figure 2 illustrates the effects of water on the conversion of dichloromethane and chloroform (as pure components) as a function of temperature. Data reported in this figure were recorded for a 1000 ppm feed stream at a residence time of 0.33 s by decreasing the reaction temperature from 400 "C a t a rate of 40 "CAI and sampling the reactor effluent every 15 min (10 "C temperature intervals). The conversion of each chloromethane in the presence of water was similar to that obtained when water was absent from the feed stream, suggesting that water does not significantly affect the oxidation rate of either compound. Data presented in Figure 2 also reveal that chloroform is more readily oxidized than dichloromethane, which is consistent with a previous report by Lester (1989), who compared the reactivity of chloromethanes over a monolithic supported catalyst. Lester's results demonstrated a greater reactivity upon decreasing the number of chlorine atoms associated with the chloromethanes. Lester attributed this behavior to a decrease in the bond dissociation energy as a result of increasing the number of chlorine atoms. The carbon-containing reaction product distribution was not effected by the presence of water. The only carbon-containing reaction product observed in the reactor effluent was C02. No CO or COCl2 was detected under any conditions. Carbon balances, obtained during the kinetic studies, were typically between 95 and 110%. Chlorine-containing reaction products detected in the reactor effluent consisted of Cl2 and HCl, with the product distribution being dependent on whether water was or was not present in the effluent stream. Table 1 reports the chlorine-containing reaction products obtained during the oxidation of dichloromethane and chloroform in dry and humid air streams. For experiments conducted in dry air, the water content was estimated to be less than 500 ppm. When oxidizing either compound in dry air, significant quantities of Cl2 were observed. This is not surprising in the case of chloroform, since there is insufficient hydrogen associ-
867 867 860 860
c0.05
a
1.00
1086
86 86 ~ 0 . 0 5 92 1.00 92
a 1482
345 50 454 17
N/A 81
NIA 67
Cross sensitivity from Cl2 prohibits analysis.
ated with the molecule to form HC1 exclusively. However, the presence of significant quantities of Clz observed during the oxidation of dichloromethane in dry air was surprising, since there is sufficient hydrogen available to form HC1. The addition of water t o a feed stream containing either dichloromethane or chloroform significantly decreases the Cl2 concentration, with HC1 being the predominant chlorine containing reaction product. The decrease in the Cl2 concentration upon addition of water to the feed stream is consistent with results previously reported for the oxidation of chloroform (Rossin and Farris, 1993). Transport Effects. The effects of external mass transfer resistances were evaluated by repeating a set of process conditions while employing a different linear velocity. The process conditions evaluated were residence time 0.10 s, temperature 400 "C, and feed concentration 300 ppm chloroform. Chloroform was evaluated in this manner rather than dichloromethane due to its greater reactivity. Results of this experiment indicated that changes in the linear velocity did not affect the conversion of chloroform, to within experimental error. Therefore, it was concluded that external mass transfer resistances were not contributing to the observed reaction rate. Intraparticle mass transfer resistances were evaluated by computing effectiveness factors. At 400 "C, effectivenessfactors were calculated to be greater than 0.95 over the range of the data, indicating that intraparticle mass transfer resistances were not significant over that range of conditions evaluated in this study. External mass and heat transfer resistances were evaluated using a criteria outlined by Mears (1971).The j factor was calculated using equations presented by Froment and Bischoff (1979), and as an approximation, j d was assumed to equal jh. The remaining physical properties were obtained from Reid et al. (1977). The criteria of Mears were satisfied for elimination of both external mass and heat transfer resistances. On the basis of these calculations and the above stated experimental results, it is felt that our results were not influenced by interphase transport. Temperature excursions within the catalyst bed could be evaluated by comparing the temperature measured within the reactor to that of the aluminum block surrounding the reactor prior t o and following reactant exposure. In all cases, the increase in the catalyst bed temperature was less than 1-2 "C, indicating that axial temperature gradients were not influencing the experimental results. Reaction Rates of Pure Chloromethanes. Reaction rate data for the oxidation of dichloromethane and chloroform as pure components were recorded at reaction temperatures of 300,345,375, and 400 "C in humid air. Samples of these data are reported in Figures 3 and 4, which illustrate the conversion of dichloromethane (A) and chloroform (B) as a function of residence time (referenced t o 0 "C, 1 atm pressure) at
3098 Ind. Eng. Chem. Res., Vol. 33, No. 12, 1994
0 ' .I
.3
100
1 3 10 30 Residence Time, sec
I
100
I
300 ppm
8 c'
.-0
6091 ppm
.I
,3
1 3 10 30 Residence Time, Sec
I 100
Figure 3. Conversion as a function of residence time for the oxidation of dichloromethane (A) and chloroform (B) as pure compounds a t 300 "C in humid air. The solid lines in the figure represent the data as correlated using the reaction rate expression.
.
100 8 c' 80
3000 pprn e 6145 pprn
0 .-
E
>
60 40
0
20 01 .01
I
10
.I .3 1 3 Residence Time, sec
.03
7 300 pprn 1000 ppm 3000 pprn 6091 ppm
B .01
,I .3 1 3 Residence Time, sec
.03
10
Figure 4. Conversion as a function of residence time for the oxidation of dichloromethane (A) and chloroform (B)as pure compounds at 400 "C in humid air. The solid lines in the figure represent the data as correlated using the reaction rate expression.
reaction temperatures of 300 and 400 "C, respectively. The solid lines in the figures represent the data correlated using the reaction rate expression (eq 2)incorporated into the fixed bed reactor design equation. Results presented in these figures show that the oxidation of both chloromethanes is nonlinear in their respective concentrations. That is, the conversion of each chloromethane decreases upon increasing its concentration. In excess of 60 data points for each chloromethane were employed in determining the kinetic fit parameters. For dichloromethane, the standard deviation
between the predicted and experimental conversion was 0.57%,while that for chloroform was 0.36%. The kinetic fit parameters correspondingto each chloromethane are reported in Table 2 and follow the trends one would expect. All parameters are positive in sign. The reaction rate constant increases with increasing temperature, while the adsorption equilibrium constants decrease with increasing temperature. The heats of adsorption are on the order one would expect for chemisorbed molecules (Froment and Bischoff, 1979). Since chloromethanes produce HC1 as a reaction product, one would expect the product adsorption equilibrium constant, Kp,determined for each chloromethane t o be similar at each reaction temperature. Numerical values of Kpwere calculated from the van't Hoff expression at each reaction temperature and are reported in Table 3. As expected, the values of Kp determined for each chloromethane agree well over the range of reaction temperatures evaluated in this study. The reaction rate expression contains a term in the denominator which accounts for inhibition resulting from adsorption of product HC1 onto the catalytic sites. To ascertain the physical significance of this term, an experiment was conducted by which HC1 was added to the dichloromethane feed stream. Results of this test are compared to a similar test conducted in the absence of HCl and are reported in Figure 5. Data were recorded for a feed concentration of 1000 ppm dichloromethane and 5000 ppm HC1 in humid air at a residence time of 0.33 seconds by decreasing the reaction temperature from 450 to 350 "C at a rate of 40 'Ch and sampling the reactor effluent every 15 min. The solid lines in the figure represent the data as predicted using the reaction rate expression. If physical significance is to be assigned to the product inhibition term, one would expect the presence of HC1 in the feed stream to decrease the conversion of dichloromethane in a manner consistent with that predicted by the reaction rate expression. Otherwise, no physical significance may be assigned to this term, and any improvements in the data correlation may be attributed to an additional fit parameter. As the results indicate, the presence of HCl in the feed stream significantly inhibits the oxidation of dichloromethane, and the affects of the inhibition can be predicted reasonably well using the reaction rate expression. These results indicate that physical significance, namely, the reversible adsorption of HC1 onto catalytic sites, can be assigned to the product inhibition term in the reaction rate expression. Figure 6 reports the conversion of dichloromethane (A) and chloroform (B) as a function of the oxygen concentration in humid air. Decreasing the oxygen concentration from 42 to 1%did not significantly affect the conversion of either dichloromethane or chloroform, suggesting that the reaction is zeroth order in oxygen. The zeroth order dependency on the oxygen concentration has been observed by others during the oxidation of organic compounds over platinum catalysts (Patterson and Kemball, 1963; Cullis and Willatt, 1983; Otto, 1989; Hicks et al., 1990; Oh et al., 1991). Reaction Rates of Chloromethane Mixtures. The reactivity of dichloromethane and chloroform in a mixed feed stream were investigated at reaction temperatures of 345 and 400 "C for individual chloromethane concentrations between 300 and 3000 ppm. Data were recorded for constant feed concentrations as a function of residence time in an effort t o vary the conversion of the target compound between 10 and 90%.
Ind. Eng. Chem. Res., Vol. 33,No. 12, 1994 3099 Table 2. Kinetic Fit Parameters Determined for the Oxidation of Dichloromethane and Chloroform as Pure Components fit param
dichloromethane
chloroform
k R Ki
3.78(7) f 0.14(7) exp(-19 900 f 48/RT) 2.50(5) f 0.24(5) exp(+6900 f 15WRT) 1.27(4) f 0.16(4) exp(+lO 600 f 151/RT)
3.19(8) f 0.08(8) exp(-2l 100 f 32/RT) 9.28(6) f 1.17(6) exp(+2300 k 141/RT) 3.81(3) f 0.32(3) exp(+l2 100 f 127/RT)
KP
Table 3. Comparison of Reaction Product Adsorption Equilibrium Constant, Kp, Determined for Dichloromethane and Chloroform temp, "C
Kp,dichloromethane 1.4370(8) 0.7285(8) 0.4881(8) 0.3593(8)
300 345 375 400
100
o'e
$
60
0
40
0
Pule CHCIS
ap c' 80
Kp,chloroform
CH2C12
0 .-
1.5847(8) 0.7307i8j 0.4630(8) 0.3264(8)
20 0
CH2C12
,03
250
300
400 450 Temperature, C
350
8 loo 80 c-
500
0 .-
5
ICWC121. 300 pprn [CHZCIZI. 1,000 pprn
.
60
a,
$ 40
m
[CHc131 = 3000 pprn
20
. 100
.
/
1 3 10 30 Oxygen Concentration, %
.3
ap / Z H c' 80
I
-
[CHZCIZI 0 pprn CA [[CHZCIZI = 300 pprn
100
E 60 a, >
$ 40 0
20 "
-
[CHCI31 = 300 PPrn
80
.-0
E 60 a,
>
0' .I
0 ppm
0 .-
loo
c
-/A
ICHCl3l = 1000 pprn
0
20 0 ' .I
8
60
[CHC131 [CHCISI
$ 40
0
m
> 0
30
>
Figure 5. Effects of HC1 on the conversion of dichloromethane in humid air: [CHzClzI = 1000 ppm; [HCl] = 5000 ppm; residence time = 0.33 s.
.-0
.I 3 1 3 10 Residence Time, sec
1
0
8 c' 80
CHc13
$ 40 0
20
100
t
E 60 a, >
CHZCIZ + HCI
80
.-0
~
100
B I '
'
3
1 3 10 30 Oxygen Concentration, %
1 100
Figure 6. Conversion of dichloromethane (A) and chloroform (B) as a function of the oxygen concentration at 345 "C.
In some cases, however, certain conversion levels could not be attained due to limitations associated with the reactor system. The conversion of each chloromethane in the mixed feeds stream was predicted using the mixture reaction rate expressions (eqs 5 and 6). Figure 7 illustrates the conversion of dichloromethane and chloroform in a mixed feed stream consisting of equal concentrations (3000ppm) of each compound as a function of residence time at 345 "C. Data cor-
B .01
.I 3 1 3 Residence Time, sec
-03
10
Figure 8. Conversion of 300 ppm of dichloromethane in the presence of chloroform (A) and conversion of 300 ppm of chloroform in the presence of dichloromethane (B)as a function of residence time at 345 "C in humid air. Solid lines represent the data as predicted using the mixture reaction rate expression.
responding to the conversion of the pure compounds is shown for comparison. The solid lines in this figure represent the data as predicted by the mixture reaction rate expressions. Figures 8 and 9 illustrate the inhibition effects resulting from increasing the concentration of one chloromethane on the conversion of the other. Figures 8A and 9A report conversion as a function of residence time for the oxidation of 300 and 3000 ppm, respectively, of dichloromethane in the presence of increasing concentrations of chloroform. Figures 8B and 9B report similar data for the conversion of 300 and 3,000ppm, respectively, of chloroform in the presence of increasing concentrations of dichloromethane. Data presented in Figure 8 were recorded at 345 "C, while data presented in Figure 9 were recorded at 400 "C. The
3100 Ind. Eng. Chem. Res., Vol. 33, No. 12, 1994
80
predict the conversions of dichloromethane and chloroform in the mixed feed stream over the range of conditions investigated.
60
Discussion
40
Reaction Rates of Pure Chloromethanes. For the single component reaction rate expressions to be extended to include mixtures, the rate expressions must provide an accurate representation of the reactions occurring on the catalyst surface. Otherwise, gross errors may result upon extension of the reaction rate expression to include mixtures. The results presented in Figures 3 and 4 show that the reactions are nonlinear in the chloromethane concentration. This behavior suggests that adsorption phenomena occurring on the catalyst surface are influencing the reaction and must be taken into account. Results presented in Figure 5 demonstrate that HC1 inhibits the oxidation of dichloromethane; therefore, the reaction rate expression must take into account the adsorption of HC1 onto catalytic sites. Previous studies have shown that failure to account for adsorption of HC1 may result in a very poor data correlation (Klinghoffer and Rossin, 1992; Rossin and Farris, 19931, especially as the conversion exceeds about 50%. Often times, the effects of oxygen concentration are not evaluated in studies involving the oxidation of volatile organic compounds, since the reaction will typically be carried out in excesses of oxygen. Without evaluating the effects of oxygen, the reaction rate expression used to correlate the pure component reaction rate data (eq 2) can be derived following a number of dissimilar sets of mechanistic assumptions. Should the wrong set of assumptions be chosen, extension of the rate expression to include mixtures may prove disastrous. Evaluating the effects of oxygen provides information which may be used to discriminate the various sets of mechanistic assumptions. The pure component reaction rate expression (eq 2) may be derived by assuming that the reaction occurs between adsorbed reactant and gas phase oxygen, with the reaction being inhibited by the adsorption of HC1. Mechanisms similar to this have been proposed by Barresi and Baldi (1992) to described the oxidation of ethenylbenzene over a platinum monolith. Should the oxidation of dichloromethane and chloroform proceed according to the above mechanism, the reaction would be first order with respect to the concentration of oxygen. This clearly is not the case, as the results presented in Figure 6 indicate that the reaction is independent of the oxygen concentration. Because of this, the above mechanism may be discounted. Mechanisms which assume the reaction occurs between an adsorbed reactant molecule and adsorbed oxygen have also been discounted. A mechanism such as this would exhibit a dependency on the concentration of oxygen, which is inconsistent with the data presented in Figure 6 . In addition, the denominator of the resulting reaction rate expression would be squared. Correlation of the experimental data with rate expressions such as these did not yield a quality representation of the experimental data. Oh et al. (1991)studied the oxidation of methane over supported noble metal (Pd, Pt, and Rh) catalysts. The authors found the reaction to be zeroth order in the concentration of oxygen (for oxygen concentrations greater than stoichiometric) and first order in the concentration of methane. The zeroth order behavior in the oxygen led Oh et al. to conclude that the catalyst
100 8
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0 .-
E
a,
>
g
0
A
20 A
-
n.01
100 8
r'
ilCHCl31. lCHCl3l C H C l 3*l ~3000 3 0 0Pppm W/ l
80
A
.03 .I ,3 1 3 Residence Time, sec
10
ICH2C121. 0 . A
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.-0
E 60 a,
>
5
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40 20
B
0
d 0,100 I V
5
80
t
f
60
40 V
p
20
c
I! $ 0 0' 0
20 40 60 80 100 Exoerimental Conversion of CH2C12, %
Experimental Conversion of CHCIJ, %
Figure 10. Predicted versus experimental conversion for the oxidation of binary mixtures of dichloromethane and chloroform in humid air.
solid lines presented in these figures represent the data as predicted using the mixture reaction rate expression. Figure 10 reports a plot of predicted versus experimental conversion for the oxidation of mixtures of chloroform and dichloromethane a t 345 and 400 "C. In all cases, predicted conversions were calculated using the fit parameters determined from the pure component data. The results presented in this figure, along with results presented in Figures 7-9, indicate that the mixture reaction rate expressions are able to accurately
Ind. Eng. Chem. Res., Vol. 33, No. 12, 1994 3101 surface was saturated with adsorbed oxygen, and propose a mechanism whereby the reaction occurred by decomposition of methane onto the oxygen covered surface. Observations and mechanistic assumptions similar to that of Oh et al. have been reported by others to describe the oxidation of methane over supported platinum catalysts (Cullis and Willatt, 1983; Otto, 1989; Hicks et al., 1990) and the oxidation of olefins over platinum and palladium films (Patterson and Kemball, 1963). Gland et al. (1980) have shown that the surface of platinum is saturated with chemisorbed oxygen at temperatures between -123 and 427 "C. On the basis of this work and consistent with previously referenced studies, the zeroth order oxygen dependency (Figure 6) has been attributed to the catalyst surface being saturated with adsorbed oxygen. Assuming the catalyst surface to be saturated with chemisorbed oxygen, the reaction is postulated to occur via adsorption and subsequent decomposition of the chloromethane onto the oxygen covered surface, with the reaction being inhibited by the adsorption of HC1. The following rate equations are proposed to describe the oxidation of dichloromethane and chloroform over the supported platinum catalyst: CH,,ClY
Ki + 0, 0;
(11)
HCL
KP + 0,0;
(12)
-
+0
(13)
4
0; products fast
0+0,-0,
(14)
In the above equations, 0 represents a reduced site on the catalyst, 00represents an oxygen-coveredcatalytic site, 0;represents HC1 adsorbed onto an oxygen covered site, and 0;represents a reactant species adsorbed onto an oxygen covered site. Equations 11and 12 depict the competitive and reversible adsorption of chloromethane and product HC1 onto oxygen covered sites. In eq 13, the adsorbed reactant decomposes to yield reaction products, namely C02 and HC1, plus a reduced site on the catalyst, which is rapidly oxidized (eq 14). In the above reaction sequence, the decomposition of chloromethane onto the oxygen covered site is postulated to be the rate limiting step (eq 13). The rate of oxidation of dichloromethane and chloroform as pure compounds may be determined from
ratei = K,O;
(15)
where solving for 0:and substituting into eq 15 yields the reaction rate expression for dichloromethane and chloroform (eq 2). In developing the above rate equations, we assume that oxygen is strongly adsorbed onto the surface of the catalyst. That is, neither reactant nor product HC1 can appreciably compete with oxygen for reduced sites on the catalyst. Should either dichloromethane, chloroform, or HC1 be able to compete with oxygen for reduced sites, decreasing the oxygen concentration from 42 to 1%would have resulted in decreasing the conversion of the chloromethane. As shown in Figure 6, this was not the case, as decreasing the oxygen concentration from 42 to 1%had virtually no effect on the conversion of either chloromethane. From the
results presented in Figures 3 and 4, the rate equation may be used to accurately describe the oxidation of dichloromethane and chloroform over a wide range of process conditions. In developingthe reaction rate expression, the effects of water were not taken into account. For the kinetic rate study, the concentration of water in the feed stream was always constant and in excess. As a constant, the term would only be incorporated into the kinetic fit parameters. Further, results presented in Figure 2 show that the presence of water in the feed stream does not have a significant effect on the conversion of either chloromethane. The fact that water did not influence the catalytic activity (Figure 2) is also interesting. The addition of water to the feed stream (Table 1)shifts the reaction product distribution from Clz to HCl. Because HC1 was demonstrated t o be a strong catalytic inhibitor (Figure 51, one might expect the catalytic activity to decrease when oxidizing the chloromethanes in humid air, due to the formation of HC1. One possible reason that water did not affect the catalytic activity, to within experimental error, is that the inhibition effects of Clz are consistent with that of HC1. The reaction product distribution observed during the oiidation of dichloromethane and chloroform in dry and humid air provides W h e r insight as to the decomposition steps occurring on the catalyst surface (eq 13). During the oxidation of chloroform in dry air, reaction products consisted of significant quantities of Cl2 and presumably HC1, which would be expected assuming the overall reaction proceeds according to
CHC1,
+ 0, - CO, + HC1+ C1,
(16)
However, upon adding water to the feed stream, virtually no Clz was detected, with HC1 being the predominant chlorine-containing reaction product (Table 1). Since there is not enough hydrogen present in chloroform to account for the observed quantities of HC1, water must be contributing to the overall reaction scheme. Rossin and Farris (1993) observed HC1 as the predominant chlorine-containing reaction product during the oxidation of chloroform in humid air. The authors could not attribute the formation of HC1 to the Deacon reaction, since the Deacon reaction would favor the formation of Clz under the conditions of the test. The authors proposed a reaction mechanism by which the first step in the decomposition of chloroform was the abstraction of HC1 to yield an adsorbed phosgene (COC12)intermediate. The phosgene intermediate would then rapidly hydrolyze to yield HC1 and CO2. A similar mechanism may be assumed here to account for the observed reaction products. The decomposition of the adsorbed phosgene intermediate does not appear t o be the rate-limiting step in the reaction sequence, since the addition of water to the feed stream had no affect (to within experimental error) on the catalytic activity (Figure 2). In the case of dichloromethane, there is sufficient hydrogen associated with the parent molecule to yield HC1 exclusively. Unexpectedly, a significant quantity of Clz (although less than that observed during the oxidation of chloroform)was observed when the run was conducted in dry air. The addition of water significantly decreased the concentration of Clz in the reactor effluent, suggesting that water also plays an active role in the oxidation of dichloromethane. It is possible that dichloromethane may be decomposing on the surface of
3102 Ind. Eng. Chem. Res., Vol. 33, No. 12, 1994 the catalyst in a manner similar t o chloroform, with the abstraction of Hz or Clz being the first step in the reaction sequence. Since the addition of water to the dichloromethane feed stream altered the selectivity towards the formation of HC1, water must be interacting with an adsorbed intermediate. However, the interaction between the adsorbed intermediate and water does not appear to be the rate limiting step, since the addition of water to the feed stream did not affect (to within experimental error) the catalytic activity (Figure 2). Oxidation of Chloromethane Mixtures. In extending the pure component reaction rate expression to include mixtures, the adsorption of the second chloromethane, plus the HC1 generated during its oxidation, must be included into the rate expression. The mixture reaction rate expressions (eqs 5 and 6) are derived by assuming dichloromethane, chloroform, and HC1 compete for the same adsorption sites. Since the objective of this paper was to predict the mixture reaction rate data, the mixture reaction rate data were not used to improve the fit parameter estimation. The fit parameters used in conjunction with the mixture reaction rate expression were obtained solely from the pure component reaction rate studies. Results presented in Figure 7 indicates that the inhibition effects of the chloromethanes in the mixed feed stream are mutual. Each chloromethane decreases the reactivity of the other relative to that of the pure compound. This result was to be expected, since the adsorption equilibrium constants for each species were similar in magnitude a t each reaction temperature. As a result, no one species was able to dominate the adsorption sites and thereby prevent the adsorption and reaction of other species. Results presented in Figures 8 and 9 illustrate the effects of increasing the concentration of one chloromethane on the conversion of the other. These results follow the trends one would expect. Increasing the concentration of one chloromethane in the feed stream decreases the conversion of the other. This is because increasing the concentration of one chloromethane results in that compound, plus the HC1 generated during its oxidation, occupying a greater fraction of the adsorption sites, thereby rendering fewer sites available for the adsorption and oxidation of the other chloromethane. In all cases, the reaction rate expressions were able to accurately describe the oxidation of mixtures of dichloromethane and chloroform. The reaction mechanism which has been proposed to describe the oxidation of dichloromethane and chloroform, and mixtures of the two, may or may not represent the actual reactions occurring on the surface of the catalyst. However, the proposed mechanism is consistent with the zeroth order dependency on the oxygen concentration and accurately describes the reaction rate data for pure compounds and mixtures of dichloromethane and chloroform over a wide range of concentrations, temperatures and flow rates.
Conclusions The oxidation of dichloromethane and chloroform occur via similar mechanisms, that being adsorption and decomposition of the chloromethanes onto an oxygencovered platinum surface, with the reactions being inhibited by the formation of HC1. The presence of water in the feed stream appears to play a role in the overall reaction sequence, minimizing the formation of Cl2 in favor of HC1 without significantly affecting the
catalytic activity. The reaction rate expressions developed to describe the oxidation of dichloromethane and chloroform as pure compounds can be extended t o accurately predict the oxidation of mixtures of dichloromethane and chloroform. Competitive adsorption effects occurring in mixed feed streams may result in reversing the order in which individual species are converted relative to the order observed as pure compounds.
Acknowledgment The authors wish to thank the US.Army for supporting this work under contract DAAA15-91-C-0075.
Nomenclature C; = reactant feed concentration, mol/mL Ci = reactant concentration, molkm3 C, = HC1 concentration, mol/mL F: = flow rate of reactant, mol/s K, = reaction rate constant, mol/s-g-cat K, = reactant adsorption equilibrium constant, cm3/mol K, = HC1 adsorption equilibrium constant, cm3/mol R = gas constant, 1.9872 cal/mol-K rate = reaction rate, mol/s-g-cat W = mass of catalyst, g x = fractional conversion, dimensionless 0 = reduced site on the catalyst 0: = reactant species adsorbed onto and oxygen covered site 00= oxygen covered catalytic site 0; = HC1 adsorbed onto and oxygen covered site
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Received for review March 11, 1994 Revised manuscript received August 15, 1994 Accepted August 27,1994*
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Abstract published in Advance ACS Abstracts, November
1, 1994.
