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KINETICS, CATALYSIS, AND REACTION ENGINEERING Catalytic Hydrodechlorination of Chlorinated Olefins over a Pd/Al2O3 Catalyst: Kinetics and Inhibition Phenomena Salvador Ordo´ n ˜ ez, Fernando V. Dı´ez,* and Herminio Sastre Department of Chemical and Environmental Engineering, University of Oviedo, Julia´ n Claverı´a s/n, 33006 Oviedo, Spain

The gas-phase hydrodechlorination of tetrachloroethene (TTCE), trichloroethene (TCE), and 1,1dichloroethene (DCE) dissolved in toluene over a commercial Pd/Al2O3 (0.15% Pd) catalyst has been studied in a fixed-bed reactor working at space times in the range of 0-5 min‚g/mmol of organochlorinated compound. The study includes both the influence of the temperature (in the range of 175-300 °C) on the kinetics of the hydrodechlorination of the single compounds and the kinetics for mixtures of these compounds at 250 °C. The observed order in reactivity is DCE > TCE > TTCE. The influence of the temperature on the kinetics can be satisfactorily modeled by a pseudo-first-order model and by considering an Arrhenius dependence for the kinetic constant. Important inhibition phenomena have been found by working with mixtures of organochlorinated compounds. These phenomena can be modeled considering a LangmuirHinshelwood model in which hydrogen (dissociatively chemisorbed) and all of the organochlorinated compounds are adsorbed in the same active sites. 1. Introduction Substantial amounts of chlorinated olefins are found in industrial waste streams. Among them, trichloroethene (TCE; used in metal-degreasing and electronic industries), tetrachloroethene (TTCE; used in drycleaning of textile fibers), and in minor extension dichloroethene (DCE; a byproduct in the synthesis of a vinyl chloride monomer) are considered to be especially important.1 The catalytic hydrodechlorination of these wastes has been shown in the literature as an interesting alternative to the incineration, because no harmful products are formed in the reaction and the economy of the process can be favorable if the hydrogen is well managed, because its cost is the most important in the process.2,3 The critical point in the development of a hydrodechlorination industrial process is the selection of the catalyst. In the first works dealing with hydrodechlorination of chlorinated compounds in organic wastes, hydrotreatment catalysts (supported metal sulfides) were proposed because of the similarity between hydrodechlorination and hydrotreating operations in the oil industry.4-7 Although these catalysts are very active for the hydrodechlorination of most chlorinated olefins, they work at very high pressures (7-10 MPa) and can be poisoned by the HCl formed in the reaction.8,9 Noble metal catalysts have been studied for many years for the hydrodechlorination of chlorinated compounds and chlorofluorocarbons (CFCs) at low concentrations in gaseous streams10-12 (chlorinated plus hy* To whom correspondence should be addressed. Phone: 34985103507. Fax: 34985103434. E-mail: [email protected]. uniovi.es.

drogen and occasionally an inert gas) but have been not used for hydrodechlorination of organochlorinated compounds in organic matrix. In a previous paper,13 we showed that supported noble metals, especially Pd, Pt, and Rh, are more active than hydrotreatment catalysts for the hydrodechlorination of most aliphatic organochlorinated compounds. Further works14 indicated that the stability of these catalysts (especially Pd catalysts) is also greater than that of hydrotreatment catalysts, in addition to working at less severe conditions. A better performance of the alumina-supported Pd catalysts, when compared with carbon-supported catalysts, was also found.14,15 Despite their importance, because they are essential for the development of industrial processes, as far as we know, the only kinetic studies available for these reactions are some simple (pseudo-first-order approach) studies devoted to hydrotreatment catalysts8 and a work on the hydrodechlorination of TTCE over a Pd/Al2O3 catalyst.15 Another important lack in the growing volume of the literature on hydrodechlorination processes is the absence of studies dealing with the simultaneous hydrodechlorination of mixtures of organochlorinated compounds. To the best of our knowledge, there are no such studies, neither over noble metal catalysts nor over hydrotreatment catalysts. This is so that an industrial hydrodechlorination process should deal with mixtures of organochlorinated compounds and that, in the case of analogous reactions such as hydrodesulfurization (HDS), hydrodenitrification (HDN), and hydrodenitrogenation (HDO), important mixture effects were observed.16-18 This work attempts to fill this lack, studying the kinetics of the gas-phase hydrodechlorination of the chlorinated compounds most commonly found in indus-

10.1021/ie010679v CCC: $22.00 © 2002 American Chemical Society Published on Web 01/11/2002

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Table 1. Composition and Textural Characteristics of the Catalyst Used in This Work composition (wt %) BET specific surface (m2/g) BJH desorption pore volume (cm3/g) average pore diameter (nm)

0.15% Pd, balance γ-Al2O3 104.3 0.40 15.1

trial organic wastes (TTCE; TCE, and DCE), both alone and in mixtures. 2. Experimental Setup Materials. Experiments were carried out over a commercial Pd on an γ-alumina catalyst (0.15% Pd) supplied by BASF, whose main textural parameters (determined by nitrogen adsorption in a Micromeritics ASAP-2000 apparatus) are given in Table 1. The catalyst, available in extrusions, was milled and crushed, and the fraction between 100 and 200 µm was selected. The catalyst was introduced inside the reactor diluted with inert corundum (Janssen, Geel, Belgium, maximum particle size 100 µm). TTCE, TCE, and DCE were supplied by Panreac (99.9% purity) and were dissolved in toluene (99.5% purity, Panreac, Barcelona, Spain). Hydrogen (>99.9995% purity) was supplied by Air Liquide, Madrid, Spain. Reaction Studies. Reaction studies were carried out in a continuous packed-bed reactor. The reactor was a stainless steel tube of 9 mm i.d. and 450 mm length, placed inside an electrically heated furnace. Five thermocouples measured the reactor wall external temperature at different heights. The reaction zone was located in the center of the bed, with a height of approximately 30 mm, with the remainder of the reactor length being filled with inert corundum. The temperature in the reaction zone was measured by a thermocouple inserted in the reactor that provided the control signal for a proportional-integral-differential controller acting on the electric furnace. The flow rate of hydrogen fed to the reactor was controlled by a Brooks 5850TR/X-5879E mass controller. The liquid feed was impelled by a Kontron LC T-414 metering pump with a flow rate between 3 × 10-4 and 3 × 10-3 L/min. At reaction conditions the reaction mixture remained as a gas in all of the experiments. The cooled reactor effluent was collected into a 1 L stainless steel Teflon-coated cylinder, acting as a gas-liquid separator and a reservoir for liquid reaction products. The pressure inside the reactor was controlled by a Tescom back-pressure regulator that vented gas reaction products. The pressure was fixed at 5 bar because, as demonstrated in a previous work,19 higher pressures do not lead to substantially higher conversions and lower pressures lead to errors in the sampling. A two-valve system allowed periodical withdrawal of liquid samples. The amount of catalyst charged to the reactor was 1 g mixed with 2 g of corundum. The catalyst was reduced in situ before use by passing 0.8 NL/min hydrogen at 350 °C for 6 h. Reactions were started after catalyst activation, maintaining the catalyst at the reaction temperature in hydrogen flow for 3 h first and then starting the liquid feed. Before samples were taken, the catalyst was aged at the reaction conditions for 5 h. To avoid transient effects, samples taken after changes in the liquid or gas flow rates or temperature were discarded. In addition, some points were randomly repeated in order to check possible deactivation phenomena.

The reaction feed and liquid products were analyzed by gas chromatography in a Varian Star 3400 apparatus equipped with a 50 m DB1 column. Analysis and experimental points were replicated in order to get an error estimation. In all of the cases, the error was lower than 3%. Light alkanes formed in the reaction were analyzed by injecting samples of the gas reaction product into the same chromatograph using a Carbowax packed column. To check the chlorine mass balance, the hydrogen chloride concentration was measured by absorption of the gas in distilled water and titration of the resulting solution with NaOH. 3. Results All of the experiments were carried out by feeding the reactor with solutions of the organochlorinated compounds in toluene. In previous works, in which toluene and decane were tested as hydrodechlorination solvents, it was observed that there was no significant effect of the solvent on the conversions attained.15 Toluene was chosen as the solvent because it provides a higher stability to the catalyst and reacts in a very low extent, yielding only methylcyclohexane, whereas aliphatic solvents yield many cracking hydrocarbons which make the analysis of the reaction products difficult. In the experiments presented here, the solvent conversion was below 0.05% in all cases. In the range of space time and temperature tested, the main reaction product for all of the reactions was ethane, with selectivities being higher than 96% for TTCE and TCE and above 99% for DCE. Small amounts of TCE were detected in the hydrodechlorination of TTCE, with no intermediate products being detected in the hydrodechlorination of DCE and TCE. These results are very important because the main scope of this process is the transformation of organic chlorine to inorganic chlorine. The chlorine mass balance was periodically tested in order to check both the good operation of the reactor and the possible existence of unidentified reaction products. Hydrogen was in high excess over the stoichiometric amount, even in the less favorable case (hydrodechlorination of the ternary mixture). Preliminary experiments without catalyst were carried out up to 350 °C, with negligible conversions for the three compounds studied in this work being attained. In a separate work,19 the plug-flow reactor (PFR) like behavior of the reactor and the absence of internal and external mass diffusion limitations were theoretically demonstrated for the hydrodechlorination of DCE, which is the most reactive compound, and hence this behavior can be extended to all of the experiments presented here. The first set of experiments was devoted to the study of the effect of the temperature in the range of 175300 °C on the kinetics of the hydrodechlorination of single compounds. The concentration of the organochlorinated compound in the liquid feed was 0.548 mol/L, and the H2/organochlorinated compound molar ratio was 75:1. The liquid and gas flow rates were varied between 4 × 10-4 and 3 × 10-3 L/min and between 0.4 and 2.5 NL/min, respectively. Results are given in Figures 1-3. A second set of experiments was devoted to a more detailed study of the kinetics of the hydrodechlorination of these compounds, including their binary and ternary mixtures. These experiments were carried out using seven different solutions of TTCE, TCE, and DCE

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Figure 1. Evolution of TTCE conversion with space time at 200 °C (]), 225 °C (4), 250 °C (O), and 300 °C (0). Lines correspond to the pseudo-first-order predictions.

Figure 2. Evolution of TCE conversion with space time at 200 °C (]), 225 °C (4), 250 °C (O), and 300 °C (0). Lines correspond to the pseudo-first-order predictions.

Figure 3. Evolution of DCE conversion with space time at 175 °C (×), 200 °C (]), 225 °C (4), 250 °C (O), and 275 °C (0). Lines correspond to the pseudo-first-order predictions.

(whose compositions are given in Table 2), working with a constant flow of hydrogen (0.8 NL/min) and variable liquid flows (4 × 10-4-3 × 10-3 L/min). The temperature was fixed at 250 °C in order that the three studied compounds react to a certain extent but none would be fully converted. Results for pure compounds are given in Figure 4. Concerning the mixture effects, the evolutions of the conversions of TTCE, TCE, and DCE in the different mixtures are shown in Figures 5-7, respectively. 4. Discussion and Modeling 4.1. Reactivity and Inhibition. In the first set of experiments (Figures 1-3), it is observed that DCE is the most reactive compound, whereas the reactivities of TTCE and TCE are very similar, with TCE being slightly more reactive at higher temperatures (above 230 °C). The same trend is observed in the second set of experiments (Figure 4).

This order of reactivity, DCE . TCE > TTCE, can be explained considering that the reaction mechanism consists of the catalytic hydrogenation of the organochlorinated compound double bond, followed by the elimination of HCl and the regeneration of the double bond. At the reaction temperature, the elimination of HCl is fast (the presence of saturated chlorinated compounds has not been observed in any case), with the catalytic hydrogenation of the chloroolefin being the limiting step. This mechanism was proposed by some authors for several hydrodehalogenation reactions,20-22 and similar mechanisms have been reported in the literature for HDN, HDS, and HDO of nonheterocyclic heteroatoms bonded to olefinic or aromatic carbons (ref 16 and references therein). The hydrogenation of the double bonds is favored as the number of substituents of the double bond decreases, especially when these atoms are very electronegative (as chlorine atoms) and absorb electron density from the π bond. Ethylene, formed according to the proposed mechanism, would be quickly hydrogenated on the surface of a very active hydrogenation catalyst such as Pd (the conditions mentioned in the literature for ethylene hydrogenation are much less severe than those used in this reaction23). The similar behavior of TTCE and TCE, unexpected according to this mechanism, could be caused by a partial compensation of the above-mentioned phenomenon by steric effects. These effects have been shown to play a determinant role in the catalytic hydrogenation of alkenes.23 Other mechanisms proposed in the literature, such as the one proposed by Weiss and Krieger, do not explain properly the different reactivity of the studied compounds.10 These authors stated that the heterogeneous catalyst stabilizes the tautomeric forms of the chloroolefin in which a chlorine atom supports a positive charge, so that the canonic form is hydrogenated to the chlorinated alkane, whereas the tautomeric form is hydrodechlorinated. Because TCE presents a more stable tautomeric form (in which the hydrogen atom stabilizes the negative charge of the carbon), the hydrodechlorination of TCE to DCE isomers should be faster than the hydrodechlorination of DCE, and so important amounts of DCE should be formed in the TCE hydrochlorination. In addition, this model predicts the formation of quantifiable amounts of hydrogenated chlorinated compounds (such as 1,1,2,2-tetrachloroethane in the case of TTCE), which have not been observed in our work. It is important to remark that the works of the above-mentioned authors have been carried out below 100 °C, and at this temperature polychloroalkanes could be more stable. Concerning the inhibition effects (Figures 5-7), it is observed that DCE is the compound affected in lower extension by the presence of TTCE or TCE, whereas the conversions of both TTCE and TCE are severely inhibited by the presence of the other two compounds studied. It is also observed that TCE presents the lower capability to inhibit the hydrodechlorination of both DCE and TTCE, whereas the inhibition capability of DCE is slightly higher than that of TTCE. The different inhibition capability of the organochlorinated compounds is related to their adsorption on the metallic surface, which is a complex phenomenon. So, two different aspects must be considered: the interaction between the double bond (π orbital) and the d orbital of the metal and the interaction between the chlorine atoms and the metal

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Table 2. Composition and Specific Gravity of the Different Mixtures Used in the Detailed Kinetic Study experiment/compound TTCE TCE DCE specific gravity

mol/L % w/w mol/L % w/w mol/L % w/w

4

3

2

4/3

4/2

3/2

4/3/2

0.554 10.0 0.000 0 0.000 0 0.909

0.000 0 0.695 10.0 0.000 0 0.904

0.000 0 0.000 0 0.930 10.0 0.893

0.579 10.0 0.731 10.0 0.000 0 0.959

0.572 10.0 0.000 0 0.977 10.0 0.938

0.000 0 0.717 10.0 0.971 10.0 0.932

0.598 10.0 0.755 10.0 1.022 10.0 0.981

Figure 4. Evolution of the conversion of TTCE (0), TCE (4), and DCE (O) with the space time at 250 °C (experiments with single compounds). Lines correspond to the fit with the proposed LH rate equation (eq 5).

Figure 7. Evolution of DCE conversion alone (+) and in the presence of TCE (O), TTCE (b), and TCE and DCE (4). See concentrations in Table 2.

4.2. Activation Energies. To calculate the activation energies from the experimental data in the range of temperatures and space times tested in this work, pseudo-first-order kinetics with respect to the organochlorinated compound and zeroth-order kinetics with respect to hydrogen (present in great excess) were assumed. So, considering the reactor as integral plug flow, the following integrated expression is obtained:

x ) 1 - exp(-kWP0/Fi0)

Figure 5. Evolution of TTCE conversion alone (+) and in the presence of TCE (O), DCE (b), and TCE and DCE (4). See concentrations in Table 2.

(1)

where x is the conversion of the organochlorinated compound, W the weight of catalyst (g), P0 the initial partial pressure of the organochlorinated compound (in this series of experiments it is 0.057 bar), Fi0 (mol/min) the initial molar flow rate of the organochlorinated compound (this value was changed in the experiments in order to achieve different space times), and k the kinetic constant (mol/bar‚min‚g). The dependence of the kinetic constant on temperature is expressed by the Arrhenius equation:

k ) k0 exp(-Ea/RT)

Figure 6. Evolution of TCE conversion alone (+) and in the presence of TTCE (O), DCE (b), and TTCE and DCE (4). See concentrations in Table 2.

surface. The electron density, which is correlated to the strength of the interaction between the double bond and the metallic surfaces, increases in the order DCE > TCE > TTCE. However, the order in the number of chlorine atoms, related to the chlorine-surface interaction, is the opposite. The observed behavior could be explained by these two opposite effects.

(2)

where k0 is the preexponential factor, Ea is the activation energy (kJ/mol), T is the reaction temperature (K), and R is the ideal gas constant. Experimental data were fitted to eqs 1 and 2, using the least-squares fitting algorithm (Powell algorithm) implemented in the commercial program Scientist (MicroMath). To avoid the mathematical problems caused by the cross-correlated dependency of the preexponential factor and the activation energy, the reparametrization proposed by Mezaki and Kittrell24 was used. These authors propose the change of the above-mentioned constants by the following expressions:

k′0 ) k0 exp(-Ea/RTm)

[

(

k ) k′0 exp -(Ea/R)

(3)

)]

1 1 T Tm

(4)

Ind. Eng. Chem. Res., Vol. 41, No. 3, 2002 509 Table 4. Kinetic and Adsorption Constants for the Hydrodechlorination of TTCE, TCE, and DCE (Experiments with Single Compounds), According to the Proposed LH Model (Eq 5) ji [mmol/(g of catalyst)‚min] Ki (bar-1) KH (bar-1) r2

Figure 8. Evolution of the pseudo-first-order kinetics constant with the temperature for the hydrodechlorination of TTCE (0), TCE (4), and DCE (O). Table 3. Pseudo-First-Order Activation Energies and Preexponential Factors for the Hydrodechlorination of TTCE, TCE, and DCE (Disappearance of the Reactant) compound

Ea (kJ/mol)

k0 (mol/bar‚min‚g)

TTCE TCE DCE

29.32 38.64 38.05

8.398 80.8 221.4

where Tm is the average temperature at which the experiments were carried out. The evolutions of the kinetic rate constants with temperature (Arrhenius plot) for the three compounds are shown in Figure 8, and the values of the activation energies and preexponential factors are reported in Table 3. It is observed (Figures 1-3) that pseudo-firstorder kinetics provides good fitting for the experimental data and that the pseudo-first-order kinetics constant follows the Arrhenius equation (Figure 8). Kim and Allen reported Ea values of 62.5 kJ/mol for both TCE and DCE and of 50 kJ/mol for TTCE, while working with hydrotreatment catalysts.8 Compared with the present work, the tendency of the activation energies for the organochlorinated compounds considered is the same, although the values reported here are lower, as expected considering the higher activity of the Pd catalyst.13 No experimental estimations of Ea are reported in the literature for hydrodechlorination of polychloroolefins over noble metal catalysts, except the work of Weiss and Krieger, who reported an activation energy of 112 kJ/mol for hydrodechlorination of different DCE isomers over Pt catalysts, but supposing zerothorder kinetics.10 Values in the range of 56-80 kJ/mol are reported for other chlorinated compounds such as dichlorodifluoromethane or tetrachloromethane, assuming first-order kinetics.12,25,26 Kim and Allen also found that the activation energy for the hydrodechlorination of chlorobenzenes over sulfided Ni-Mo catalysts is approximately twice that for chloroolefins.8 Considering our results and those reported by Coq et al., who found activation energies in the range of 80-125 kJ/mol for the hydrodechlorination of chlorobenzene over Pt, Rh, and Pd catalysts,25,26 it can be concluded that this empirical rule holds also for Pd catalysts. 4.3. Detailed Kinetic Study. Hydrodechlorination of Single Compounds. Although pseudo-firstorder kinetics can be a fair empirical approach for the study of the hydrodechlorination of single compounds, it does not provide any insight on the reaction mechanism and it is not useful for the prediction of the inhibition effects that occur while working with mixtures.

DCE

TCE

TTCE

7.53 311.6 1566 0.999

3.70 299.1 1311 0.994

2.11 521.0 1240 0.994

The most common approach to the modeling of similar reactions such hydrogenations, hydrodesulfurization, or hydrodenitrogenation is the use of the LangmuirHinshelwood (LH) postulate of reaction occurring between chemisorbed species.16,27,28 In the case of the hydrodechlorination of organochlorinated compounds over a Pd catalyst, the most plausible supposition is the reaction between dissociatively chemisorbed hydrogen and the chemisorbed chlorinated compound. These chemisorptions can take place both on the same or in different active sites. This reaction scheme is widely used in the literature for hydroremoval of heteroatoms and hydrogenation of olefins (according to the previously proposed reaction mechanism, the starting point is the hydrogenation of the double bond). Analogous models have been proposed in the literature for the hydrodechlorination of CFCs and other organochlorinated compounds.29 Other kinetics models have been found not to be able to explain the inhibition phenomena (Eley-Rideal) or not to have a clear physical sense for hydrogenation reactions (Mars-Van Krevelen; although Coq et al. suggested that the hydrodechlorination of chlorobenzene follows this mechanism25). Another variation that has been suggested for LH models for hydrodechlorination reactions is the inclusion of the inhibition of HCl released in the reaction. In our case, despite the resulting increase in the number of parameters of the model, the inclusion of this term did not improve the fitting of the model to the experimental results. So, the following kinetic rate expression was used for the hydrodechlorination of single organochlorinated compounds:

-ri )

jixKHpHKipi (1 + xKHpH + Kipi)2

(5)

where pi and pH are the partial pressures of the organochlorinated compound and H2, respectively, ji is the intrinsic kinetic constant, and KH and Ki are the adsorption constants of hydrogen and the chlorinated compound, respectively. In this case, as for the experiments carried out with mixtures, analytical integration is not possible, and the differential equations resulting from the inclusion of the rate expression on the PFR design equation were numerically integrated using the Episode-Stiff integrator package implemented in the commercial program Scientist (MicroMath). The values obtained for the model parameters are given in Table 4. As can be observed in Figure 4, the model fits fairly well the experimental results for the three compounds. It can be observed also that the calculated values of KH are very similar for the three compounds, which is in a good agreement with the assumptions of the simple LH model (the hydrogen adsorption constant should not depend on the organochlorinated compound). Hydrodechlorination of Mixtures. When eq 5 is extended to the hydrodechlorination of mixtures, the

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Figure 9. Experimental conversion profile simulated with the results of hydrodechlorination of single compounds (dashed line) and calculated profiles (solid line) for the hydrodechlorination of TTCE (0) and TCE (4) as a function of the space time for mixture 4/3.

Figure 11. Experimental conversion profile simulated with the results of hydrodechlorination of single compounds (dashed line) and calculated profiles (solid line) for the hydrodechlorination of TCE (4) and DCE (O) as a function of the space time for mixture 3/2.

Figure 10. Experimental conversion profile simulated with the results of hydrodechlorination of single compounds (dashed line) and calculated profiles (solid line) for the hydrodechlorination of TTCE (0) and DCE (O) as a function of the space time for mixture 4/2.

Figure 12. Experimental conversion profile simulated with the results of hydrodechlorination of single compounds (dashed line) and calculated profiles (solid line) for the hydrodechlorination of TTCE (0), TCE (4), and DCE (O) as a function of the space time for mixture 4/3/2.

resulting equation is

Table 5. Kinetic and Adsorption Constants for the Hydrodechlorination of TTCE, TCE, and DCE in Mixtures, According to the Proposed LH Model (Eq 6)

-ri )

jixKHpHKipi (1 + xKHpH +

4

mixture

(6)

Kjpj)2 ∑ j)2

where pi and pH are the partial pressures of each organochlorinated compound and H2, respectively, ji is the intrinsic kinetic constant for compound i, and KH and Ki are the adsorption constants of hydrogen and the chlorinated compound, respectively. As a first approach, the behavior of the mixtures was simulated using the parameters calculated in the fitting for the single compounds (Table 4) and an average value of KH of 1373 bar-1. It is observed that the model predicts reasonably well the behavior of the different mixtures of polychloroolefins (Figures 9-12). This aspect also corroborates the validity of the kinetic model proposed. The fitting between predictions and experimental results is especially good for the binary mixtures, whereas in the case of the ternary mixture, simulation predicts slightly lower conversions. Finally, all of the experimental results obtained with mixtures were fitted to eq 6. The parameters obtained are given in Table 5, and experimental results and model predictions are shown in Figures 9-12. As expected, the fitting is better than that when the parameters for single compounds are used. The devia-

3/4 2/4 2/3 2/3/4

j4

K4

j3

K3

j2

K2

KH

r2

2.20 512.2 3.86 298.3 1310 0.993 2.58 490.3 10.97 417.1 1017 0.997 3.64 287.0 11.32 422.7 1297 0.998 2.41 488.2 4.21 318.8 12.01 486.5 982 0.997

tion between predicted and experimental values does not follow any clear trend. It is also important to remark that the values obtained for KH and the different ji and Ki are very similar in all of the experiments. This fact supports the theory that the proposed model has a physical sense, in addition to fitting fairly well the experimental results. When alternative models were applied (such as supposing that the adsorption of hydrogen and the organochlorinated compound takes place on different active sites or considering the inhibition caused by hydrogen chloride), the results were worse, with the values obtained for the kinetic and adsorption constants being not congruent in the different experiments. Acknowledgment This work was financed by a research grant of the Spanish Commission for Science and Technology (PPQ2000-0674). We acknowledge Mr. Rolla´n (BASFSpain) for supplying the catalyst. Professor Thatcher W. Roof (University of WisconsinsMadison, Madison, WI) is acknowledged for the revision of the manuscript.

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Nomenclature DCE ) dichloroethene Ea ) activation energy, kJ/mol Fi0 ) molar flow rate, mol/min ji ) intrinsic kinetic constant for the organochlorinated compound i (Langmuir-Hinshelwood models, mmol/(g of catalyst)‚min k ) kinetic constant (pseudo-first-order kinetics), mol/bar‚ min‚g k0 ) Arrhenius preexponential factor, mol/bar‚min‚g k′0 ) reparametrized Arrhenius preexponential factor, mol/ bar‚min‚g Ki ) adsorption constant for the compound i (organochlorinated or hydrogen), bar-1 P0 ) initial partial pressure of organochlorinated compound, bar pi ) partial pressure of compound i (organochlorinated or hydrogen), bar R ) universal gas constant 8.314 J/K‚mol -ri ) reaction rate for component i, mol/min‚g T ) catalyst temperature, K Tm ) average temperature of experimental serie, K TCE ) trichloroethene TTCE ) tetracloroethene W ) weight of catalyst, g x ) fractional conversion Subscripts 2 ) DCE 3 ) TCE 4 ) TTCE H ) hydrogen

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Received for review August 13, 2001 Revised manuscript received October 26, 2001 Accepted October 30, 2001 IE010679V