Ind. Eng. Chem. Res. 2003, 42, 6007-6011
6007
KINETICS, CATALYSIS, AND REACTION ENGINEERING Kinetics of the Catalytic Oxidation of Lean Trichloroethylene in Air over Pd/Alumina A. Aranzabal, J. L. Ayastuy-Arizti, J. A. Gonza´ lez-Marcos, and J. R. Gonza´ lez-Velasco* Department of Chemical Engineering, Faculty of Sciences, Universidad del Paı´s Vasco/EHU, P.O. Box 644, E-48080 Bilbao, Spain
Kinetic modeling of the catalytic oxidation of trichloroethylene (TCE) over a palladium supported on alumina was evaluated. On the basis of previous studies on the effect of temperature, inlet TCE and oxygen concentrations, and the space velocity on the reaction rate, a five-step reaction network scheme, based on an Eley-Rideal-type mechanism, provided an accurate correlation of the experimental data, determined by a nonlinear least-squares regression. Introduction Chlorine-containing volatile organic compounds (CVOCs) are widely used in the industry as solvents, extractive agents, additive paints, and adhesives and in the synthesis of pesticides and polymers, leading to an increasing amount of their release to the environment and therefore contributing to the photochemical smog formation and depletion of the ozone layer in the stratosphere. Catalytic oxidation of halogenated volatile organic compounds (HVOCs) has become an advantageous strategy for reducing their emissions to the atmosphere, in comparison with the conventional thermal incineration, which requires higher operating temperatures (800-1000 °C), leading to the formation of highly toxic byproducts such as NOx, dibenzofurans, and dioxins.1 Broad catalyst compositions are being developed in order to improve these requirements and to meet the increasingly stringent environmental regulations.2-9 However, very little has been reported about the reaction mechanism of the CVOC complete catalytic oxidation. Literature involving kinetic rate expressions for the oxidation of chlorinated volatile organic compounds over platinum and palladium catalysts is rather limited. The reaction mechanism is believed to involve a reaction between adsorbed oxygen and an adsorbed reactant molecule (the Langmuir-Hinshelwood mechanism)10-12 or a reaction between adsorbed oxygen and a gas-phase reactant molecule (the Eley-Rideal mechanism).13-15 Both of these assumptions have been used in the development of kinetic rate expressions to describe the oxidative conversion of CVOC over supported noble metal catalysts. However, selectivity data have not been considered, limiting the applicability of the kinetic models reported in the literature. Rossin and coworkers10-12 took into account the inhibition effect of the product HCl (considering that all chlorine atoms were converted to HCl) by adding a product inhibition term to the reaction rate expression. * To whom correspondence should be addressed. E-mail:
[email protected].
The reaction mechanism of trichloroethylene (TCE) catalytic oxidation has already been studied, and was reported previously,16 by analyzing the effect of kinetic variables (temperature, feed composition, and space velocity), resulting in a five-step reaction network scheme. Continuing this research, the objective of this work is to determine the kinetic expression that best predicts the final effluent composition (CO, CO2, C2Cl4, HCl, and Cl2) when TCE is oxidized over a 0,42 wt % Pd/Al2O3 catalyst, prepared in our laboratory.17,18 TCE was chosen as a suitable model compound present in many off-gases, such us groundwater stripping emissions and dry-cleaning and degreasing processes.19 Experimental Section Descriptions of the catalyst, experimental setup, and conditions have been reported elsewhere.16-20 Results and Discussion The reaction pathway of lean TCE oxidation on a granulated Pd/Al2O3 catalyst had been studied previously16 by recording the effect of the temperature (200500 °C), space velocity (1158, 2290, and 3450 kg s molTCE-1), and inlet TCE (660, 1040, and 1320 ppm) and oxygen (0.8-21%) concentrations on the reaction rate and selectivity. Among the reaction products, CO, CO2, HCl, Cl2, and C2Cl4 were detected, as shown in the product distribution in Figure 1.16-18 The velocity of the gas stream was kept constant at 0.12 m s-1 to ensure conditions free of diffusional resistances.17 Prior the experiments, catalysts were exposed to the chlorinated feed at 550 °C for 8 h to ensure that no deactivation occurred in the kinetic experiments.20 Figure 2 shows schematically the conclusions from previous studies that have already been discussed elsewere.16,18 Oxygen is dissociatively adsorbed onto active centers. The dissociation step occurs very quickly in comparison with the adsorption, resulting in the full coverage of the active surface with atomic oxygen. Consequently, gaseous TCE reacts directly with adsorbed oxygen, leading to CO and CO2 according to a
10.1021/ie030286r CCC: $25.00 © 2003 American Chemical Society Published on Web 11/01/2003
6008 Ind. Eng. Chem. Res., Vol. 42, No. 24, 2003
Figure 2. Schematic reaction pathway of TCE catalytic oxidation.
be determined respectively by CO and C2Cl4 first-order and oxygen zero-order rate equations (eqs 10 and 11). Figure 1. Product distribution profiles in the oxidation of TCE over 0.42% Pd/Al2O3 as a function of temperature.
Eley-Rideal mechanism. The oxidative decomposition of TCE involves C-Cl bond dissociation, by a chemical interaction of the halogen with the precious metal and the support, resulting in precious metal (oxide)-chloride species, [M(Ox)Cly], on alumina and aluminum chloride. The metal (oxide)-chloride species can directly decompose to molecular chlorine (Cl2) and also react with the feed TCE by transferring chlorine (Cl2) to the double bond. The intermediate pentachloroethane is spontaneously dehydrochlorinated by HCl elimination, resulting in the more stable tetrachloroethylene (PER). CO and PER are also supposed to react with atomic oxygen from the gaseous phase, as an Eley-Rideal mechanism. This reaction is promoted at higher temperatures, reducing their concentration at the reactor exit. The above-mentioned results allowed us to model the reaction by a minimum of a five-step reaction to adequately account for all byproducts detected:
C2HCl3 + 2O2 f 2CO2 + HCl + Cl2
(1)
C2HCl3 + O2 f 2CO + HCl + Cl2
(2)
C2HCl3 + Cl2 f (CCl3-CHCl2) f C2Cl4 + HCl (3) 1 CO + O2 f CO2 2 C2Cl4 + 2O2 f 2CO2 + 2Cl2
(4) (5)
(7)
r2 ) Ao2 exp(-Ea2/RT)PTCE
(8)
r3 ) Ao3 exp(-Ea3/RT)PTCE
(9)
r4 ) Ao4 exp(-Ea4/RT)PCO
(10)
r5 ) Ao5 exp(-Ea5/RT)PPER
(11)
The kinetic rate equation for the Deacon reaction can be found in the literature, over CuCl2 and CuCl2-KClLaCl2. The mechanistic rate equations given by Shakhovtseva et al.21 and Kenney,22 based on the reoxidation of CuCl to CuCl2 as the rate-controlling stage, can be simplified to
rD ) AoD exp(-EaD/RT)kPHCl2
-rTCE ) rCO )
(6)
The individual reaction rate expressions of reactions (1) and (2) can be written as a first-order power rate equation with respect to TCE and zero order with respect to oxygen, as well as for TCE chlorination (3), as shown by eqs 7-9, assuming an Eley-Rideal mechanism as a reaction between the gas-phase TCE and adsorbed chlorine. The oxidation of CO and C2Cl4, eqs 4 and 5, can also be postulated to occur as an Eley-Rideal mechanism as the catalyst surface is saturated with chemisorbed oxygen. Thus, the rate of oxidation of CO and C2Cl4 can
(12)
because of the large excess of oxygen. Prasad et al.23 used a similar model for propylene combustion. Propylene is oxidized to CO and CO2 in two parallel reactions, and then CO is oxidized to CO2. The rate expressions are first order in propylene and CO concentrations, respectively. From these rate equations, the decomposition and formation rate of each component in the reaction scheme were calculated as follows:
The Deacon reaction should also be considered because under conditions of reaction it is very probable that HCl reacts with oxygen.
1 2HCl + O2 T H2O + Cl2 2
r1 ) Ao1 exp(-Ea1/RT)PTCE
rCO2 )
1
dPCO
1
dPCO2
1
PTOT d(W/FTOT) 1
PTOT d(W/FTOT)
rHCl )
(14)
) 2r1 + r4 + 2r5
(15)
dPPER
PTOT d(W/FTOT)
dPCl2
(13)
) 2r2 - r4
PTOT d(W/FTOT)
rPER ) rCl2 )
dPTCE 1 ) r 1 + r2 + r3 PTOT d(W/FTOT)
) r3 - r5
(16)
) r1 + r2 - r3 + 2r5 + rD (17)
dPHCl 1 ) r1 + r2 + r3 - 2rD (18) PTOT d(W/FTOT)
Ind. Eng. Chem. Res., Vol. 42, No. 24, 2003 6009
Figure 3. Comparison of experimental product distribution (symbols) with predicted product distribution (lines) at different space times and TCE feed concentrations: (a) 105 Pa, 2290 kg s molTCE-1; (b) 105 Pa, 1158 kg s molTCE-1; (c)105 Pa, 3450 kg s molTCE-1; (d) 67 Pa, 2290 kg s molTCE-1; (e)134 Pa, 2290 kg s molTCE-1.
A computer program based on the Nelder-Mead nonlinear least-squares algorithm and on the fourthorder Runge-Kutta numerical integration method was used to find the best combination of the preexponents and the activation energies for each kinetic constant for the set of differential rate equations. The nonlinear least-squares algorithm minimized the error between the predicted and experimental values of the partial pressures of the reactant and products. m 68
RSS )
∑i ∑j [Pexpt(i,j) - Pcalcd(i,j)]2
(19)
The kinetic fit parameters were determined over the entire range of conditions (temperatures, inlet concentrations, and residence times) simultaneously rather than by the standard technique of determining the fit parameters at each reaction temperature and then correlating the fit parameters using the Arrhenius equation. The stability of the nonlinear least-squares algorithm was greatly improved by referring the fit parameters to a midpoint temperature over the range of data as follows:
[ (
kr ) k600 exp
)]
-Ear 1 1 R T 600
(20)
Table 1. Calculated Kinetic Parameters for the Complete Oxidation of TCE
reaction 1. C2HCl3 + O2 f 2CO + HCl + Cl2 2. C2HCl3 + 2O2 f 2CO2 + HCl + Cl2 3. C2HCl3 + Cl2 f C2Cl4 + HCl 4. CO + (1/2)O2 f CO2 5. C2Cl4 + 2O2 f 2CO2 + 2Cl2
Ao (mol s-1 kg-1 Ea atm-1) (kJ mol-1) 1.2 15.7 167.34 2.8 × 105 1.1 × 107
53.4 67.1 80.4 119.0 157.3
The values of the kinetic parameters developed from the experimental data for each reaction stage in the oxidation of TCE on 0.42 wt % Pd/Al2O3 catalysts are given in Table 1. Fitting of the partial pressures of HCl and Cl2 is derived in very low regression because of the poor chlorine balance (within 70-90%) in catalyzed reactions and because of the interaction between chlorine from TCE and the catalyst (Figure 2).17,18,20 Nevertheless, this obstacle was easily overcome by leaving out the experimental data of the HCl and Cl2 concentrations and fitting only carbon-containing compounds (the carbon balance was complete). This decision was adopted because HCl and Cl2 concentrations are not included as variables in any kinetic rate equations of the TCE decomposition scheme, eqs 7-11, and therefore did not influence the formation and destruc-
6010 Ind. Eng. Chem. Res., Vol. 42, No. 24, 2003
covering the catalyst surface. TCE is oxidized simultaneously to CO2 and CO, leading to precious metal (oxide)-chloride species, which react with the gas-phase feed TCE to produce C2Cl4. Oxidation of both CO and PER, effected as the temperature was increased, occurs also by interaction with chemisorbed oxygen from the gas phase. The model becomes a valuable tool in end-of-pipe environmental control systems for optimizing operating variables according to the effluent composition. Acknowledgment The authors thank Universidad del Paı´s Vasco/EHU (9/UPV-13517/2001) and Ministerio de Educacio´n y Cultura (PPQ2001-0543) for financial support. Literature Cited Figure 4. Product distribution profiles in the oxidation of TCE under thermal combustion conditions (without catalyst) as a function of temperature.
tion rate of carbon-containing products. Of course, the kinetic rate equation for the Deacon reaction (12) was also omitted. Figure 3 compares the composition in the reactor exit at various temperatures, space velocities, and inlet concentrations. The solid lines are the partial pressures of the compounds involved in the TCE oxidation calculated by the kinetic model, and the points are the experimental values. The developed model provides an accurate fitting of the experimental data over the whole range of conditions, except under the following condition: above 400 °C the concentration of CO decreases, as a consequence of its oxidation (3). Above 450 °C its concentration increases again, and therefore the partial pressure of CO2 is lower than expected. A homogeneous reaction23 is thought to be the reason, according to the runs carried out without a catalyst (Figure 4). That is why the kinetic model, based on the heterogeneous reaction, cannot predict such behavior. Such differences are lower when the reaction space time is increased because conversion of CO, even in homogeneous oxidations, occurs faster, leading to a lower CO concentration and a higher CO2 concentration in the product composition (Figure 3c). However, this does not affect to the rest of the conditions because homogeneous oxidation of TCE is negligible below 450 °C (Figure 4). Likewise, homogeneous chlorination3 is also negligible in the entire range of temperatures, so formation of PER is only due to heterogeneous chlorination, as shown by the good fitting. Additionally, the activation energies calculated from the model are consistent with those reported in the literature for the oxidation of volatile organic compounds over supported noble metal catalysts.24 Conclusions The kinetic model based on a five-step reaction and based on a single-site Eley-Rideal-type mechanism provides an accurate correlation of the product distribution at the reactor exit for the investigated range of conditions. According to the model, the TCE molecule reacts from the gas phase with chemisorbed oxygen
(1) Olie, K.; Vermeulen, P. L.; Hutzinger, O. Chlorodibenzo-pdioxins and Chlorodibenzofurans are Trace Components of Fly Ash and Flue Gas of Some Municipal Incineratiors in The Netherlands. Chemosphere 1997, 6 (8), 455. (2) Chintawar, P. S.; Greene, H. L. Interaction of Chlorinated Ethylenes with Chromium Exchanged Zeolite Y: An in Situ FTIR Study. J. Catal. 1997, 165, 12. (3) Imamura, S.; Tarumoto, H.; Ishida, S. Decomposition of 1,2Dichloroethane on TiO2/SiO2. Ind. Eng. Chem. Res. 1989, 28, 1449. (4) Kieβling, D.; Schneider, R.; Kraak, P.; Haftendorn, M.; Wendt, G. Perovskite-type OxidessCatalysts for the Total Oxidation of Chlorinated Hydrocarbons. Appl. Catal. B 1998, 19, 143. (5) Sinquin, G.; Petit, C.; Hindermann, J. P.; Kiennemann, A. Study of the formation of LaMO3 (M ) Co, Mn) Perovskites by Propinates Precursors: Application to the Catalytic Destruction of Chlorinated VOCs. Catal. Today 2001, 70, 183. (6) Petrosius, S. C.; Drago, R. S.; Young, V.; Grunewald, G. C. Low-Temperature Decomposition of Some Halogenated Hydrocarbons Using Metal Oxide/Porous Carbon Catalysts. J. Am. Chem. Soc. 1993, 115, 6131. (7) Hutchings, G. J.; Heneghan, C. S.; Hudson, I. D.; Taylor, S. H. Uranium-Oxide-Based Catalysts for the Destruction of Volatile Chloro-Organic Compounds. Nature 1996, 386, 341. (8) Luo, M. F.; Zheng, X. M. Ce0.5Zr0.5O2 Solid Solutions as Novel Support for Preparing Highly Active Palladium Catalysts for Catalytic Combustion of Volatile Organic Compounds. World Congress on Environmental Catalysis, 2nd ed.; AIChE: Miami Beach, FL, 1998; p 28. (9) Jiang, X. Z.; Zhang, L. Q.; Wu, X. H.; Zheng, L. Catalytic Decomposition of Methylene Chloride by Sulfated Oxide Catalysts. Appl. Catal. B 1996, 9, 229. (10) Klinghoffer, A. A.; Rossin, J. A. Catalytic Oxidation of Chloroacetonitrile over a 1% Platinum Alumina Catalyst. Ind. Eng. Chem. Res. 1992, 31, 481. (11) Rossin, J. A.; Farris, M. M. Catalytic Oxidation of Chloroform over a 2% Platinum Alumina Catalyst. Ind. Eng. Chem. Res. 1993, 32, 1024. (12) Papenmeier, D. M.; Rossin, J. A. Catalytic Oxidation of Dichloromethane, Chloroform and Their Binary Mixtures over a Platinum Alumina Catalyst. Ind. Eng. Chem. Res. 1994, 33, 3094. (13) Wang, Y.; Shaw, H.; Farrauto, R. J. Catalytic Oxidation of Trace Concentrations of Trichloroethylene over 1.5% Platinum on γ-Alumina. Catalytic Control of Air Pollution, Mobile and Stationary Sources; ACS Symposium Series 495; American Chemical Society: Washington, DC, 1992; p 125. (14) Yu, T.-Ch.; Shaw, H.; Farrauto, R. J. Catalytic Oxidation of Trichloroethylene over PdO Catalyst on g-Al2O3. Catalytic Control of Air Pollution, Mobile and Stationary Sources; ACS Symposium Series 495; American Chemical Society: Washington, DC, 1992; p 141. (15) Lou, J. C.; Lee, S. S. Destruction of Trichloromethane with Catalytic Oxidation. Appl. Catal. B 1997, 12, 111. (16) Aranzabal, A.; Ayastuy-Arizti, J. L.; Gonza´lez-Marcos, J. A.; Gonza´lez-Velasco, J. R. The Reaction Pathway and Kinetic
Ind. Eng. Chem. Res., Vol. 42, No. 24, 2003 6011 Mechanism of the Catalytic Oxidation of Gaseous Lean TCE on Pd/Alumina Catalysts. J. Catal. 2003, in press. (17) Gonza´lez-Velasco, J. R.; Aranzabal, A.; Gutie´rrez Ortiz, J. I.; Lo´pez-Fonseca, R.; Gutie´rrez-Ortiz, M. A. Activity and Product Distribution of Alumina Supported Platinum and Palladium Catalyts in the Gas-Phase Oxidative Decomposition of Chlorinated Hydrocarbons. Appl. Catal. B 1998, 19, 189. (18) Gonza´lez-Velasco, J. R.; Aranzabal, A.; Lo´pez-Fonseca, R.; Ferret, R.; Gonza´lez Marcos, J. A. Enhancement of the Catalytic Oxidation of Hydrogen-lean Chlorinated VOCs in the Presence of Hydrogen-Supplying Compounds. Appl. Catal. B 2000, 24, 33. (19) Hylton, T. D. Evaluation of the TCE Catalytic Oxidation Unit at Wurtsmith Air Force Base. Environ. Prog. 1992, 11, 54. (20) Aranzabal, A. Actividad y Selectividad de Metales Nobles Soportados en la Destruccio´n Oxidativa Catalı´tica de Compuestos Orga´nicos Vola´tiles en Mezclas Mono- y Multicomponentes. Ph.D. Thesis, Universidad del Paı´s Vasco, Bilbao, Spain, 2000.
(21) Shakhovtseva, G. A.; Vasilyeva, I. B.; Avetisov, A. K.; Gelbstein, A. I. Kinetics and Mechanism of the Catalytic Oxidation of Hydrogen Chloride. I. Kinet. Katal. 1970, 11, 1469. (22) Kenney, C. N. Kinetics of Gas Reactions Using Molten Salt Catalysts. Proc. 4th Int. Congr. Catal. (Moscow) 1968, 501. (23) Prasad, R.; Kennedy, L. A.; Ruckenstein, E. Catalytic Combustion. Catal. Rev.-Sci. Eng. 1984, 26, 1. (24) Shaw, H.; Wang, Y.; Yu, T.-Ch.; Cerkanowicz, A. E. Catalytic Oxidation of Trichloroethylene and Methylene Chloride. Symposium on Emerging Technologies in Hazardous Waste Management III; ACS Symposium Series 518; American Chemical Society: Washington, DC, 1992; p 358.
Resubmitted for review June 30, 2003 Revised manuscript received June 30, 2003 Accepted August 28, 2003 IE030286R
