A Multistep Surface Mechanism for Ethane Oxidative Dehydrogenation

Ind. Eng. Chem. Res. 2005, 44, 3453-3470

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A Multistep Surface Mechanism for Ethane Oxidative Dehydrogenation on Pt- and Pt/Sn-Coated Monoliths Francesco Donsı`,† Kenneth A. Williams, and Lanny D. Schmidt* Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455-0132

A computational study of ethane oxidative dehydrogenation to ethylene on Pt- and Pt/Sn-coated monoliths is presented as an improvement to previous kinetic models in reproducing experimental findings over a wide range of feed conditions. The multistep surface mechanism containing 20 reversible reactions among 11 surface species is based on published reaction steps for hydrogen and methane oxidation combined with lumped steps for ethane surface chemistry and coupled with an established homogeneous mechanism to form the detailed chemistry model. Simulation results at 1 atm are in good agreement with experimental data obtained on Pt at variable C2H6/ O2 and C2H6/O2/H2 ratios and predict experimentally observed phenomena such as ignition temperatures and homogeneous ethylene formation. The model is also used to predict Pt monolith performance over an industrially relevant range of space velocities (0.7-3.4 × 105 h-1) and pressures (1-10 atm). Furthermore, the Pt mechanism is extended to a Pt/Sn catalyst by changing two parameters in the H and CO oxidation steps, and agreement with experiments is obtained with and without H2 addition. 1. Introduction Ethane oxidative dehydrogenation (ODH) has been investigated as an alternative to ethane dehydrogenation for ethylene production.1,2 However, none of the catalysts investigated for ethane ODH in conventional reactors are suitable and profitable for large-scale applications because of the low yields and coking that occur by decreasing feed dilution and the ethane-tooxygen ratio when shifting from laboratory scale to industrial scale.3 Ethane ODH can be carried out in short contact time reactors (∼5 ms residence time) while obtaining ethane conversion as high as 80% and an ethylene selectivity of ∼70%.4 Improved ethylene yield has been attained by tuning catalyst composition and feed conditions.5-7 A Pt/Sn catalyst with hydrogen addition gives ethane conversion and ethylene selectivity comparable to those of the present industrial process.7 Adding H2 decreases C2H6 conversion to ∼70% but increases C2H4 selectivity to ∼85%. Originally, the process was believed to occur through purely heterogeneous chemistry on the basis of the experimental observation of large differences in synthesis gas yield among various noble metals during methane partial oxidation.8 Accordingly, a heterogeneous model was proposed to describe ethane dehydrogenation on a Pt catalyst in 23 steps.9 All chemistry was assumed to occur on the catalyst surface, and gas-phase reactions were ignored. However, later experimental results showed that significant homogeneous reactions occur at the high temperatures reached in the reactor (∼1000 °C). Indeed, experimental data has suggested that only COx and H2O are formed on the surface of a Pt catalyst at low temper* To whom correspondence should be addressed. Tel.: 612625-9391. Fax: 612-626-7246. E-mail: [email protected]. † Current address: Dipartimento Ingegneria Chimica, Universita` di Napoli Federico II, Piazzale Tecchio 80, I-80125 Napoli, Italy.

atures;10-12 at higher temperatures ethylene yield observed at the exit of the reactor was attributed entirely to gas-phase reactions. Variable bed length experiments with effluent sampling downstream of the catalytic monolith showed that most of the ethylene is formed in the gas phase.13 The finding that performance similar to that of noble metals could be obtained on a completely different catalyst (e.g., LaMnO3) further excluded the direct involvement of the catalyst in ethylene formation.14,15 Nevertheless, even if ethylene formation occurs entirely in the gas phase, the catalyst is extremely important in igniting and sustaining gas-phase reactions16,17 and making the process feasible at short contact times. On the basis of experimental evidence of the role of gas-phase reactions, Huff and co-workers proposed a kinetic model for ethane ODH (Huff model), assuming that the catalyst’s role was to oxidize a fraction of the fuel to COx and H2O (heat released on the surface then drove the homogeneous formation of ethylene).18 Although in good agreement with the experimental results without H2 addition, the Huff model seems to fail in the case of H2 addition and was based on the assumption of dissociative adsorption of ethane to C and H followed by oxidation of C and H. The rate constants of hydrogen and oxygen adsorption/desorption and ethane dissociative adsorption were considered adjustable parameters. Additionally, it was assumed that oxygen could adsorb noncompetitively on the catalytic surface while all other species adsorbed competitively in order to reduce computing time. Experimental and theoretical studies have shown that Pt step sites bind oxygen atoms more strongly than terrace sites, which leads to a much higher level of O2 dissociative adsorption in steps than terraces.19-24 In addition, experimental evidence of noncompetitive O2 adsorption was reported in a study of CO oxidation on supported Pt particles.25 Recently, Zerkle and co-workers proposed a detailed model for ethane oxidation on a Pt catalyst.26 The ethane surface chemistry was based on the model of

10.1021/ie0493356 CCC: $30.25 © 2005 American Chemical Society Published on Web 04/05/2005

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Wolf et al.27 for nonoxidative methane conversion on Pt and was combined with oxidative steps. This coupling led to a heterogeneous mechanism consisting of 19 surface species and 82 elementary reactions. The heterogeneous mechanism was implemented together with a detailed homogeneous mechanism in a full 2D model, since the authors claimed that mass and heat transfer are extremely significant factors in the short contact time reactor. In particular, reactant mass transfer from the bulk of the gas phase to the catalyst surface and transfer of heat from the surface to the gas phase were considered crucial points. The model predicted the amount of ethylene formed on the catalyst to be dependent on feed composition; ethylene was formed mainly on the surface when H2 was added to the reacting mixture. These findings of purely heterogeneous ethylene formation are not in agreement with experimental findings.11-13 In this work, the surface chemistry of the detailed kinetic model is based on well-established reaction rates for hydrogen and methane oxidation to COx and H2O. These steps are combined with lumped steps for ethane surface adsorption and dissociation to obtain a simple and flexible mechanism that can be used to predict process performance over a wide range of experimental conditions. Creation of the mechanism was motivated by experimental evidence for gas-phase ethylene formation.11,13 Additional effort is devoted to predict the catalyst’s performance outside the temperature range of interest for ODH and in particular the catalyst ignition temperature and product distribution before the threshold temperature for gas-phase reactions is reached. 2. Computational Methods 2.1. Reactor Models. As a first approximation, the system was modeled as a plug-flow reactor. The plugflow assumption is an extremely useful tool for building a kinetic mechanism considering the tradeoff between the extremely low computing time (seconds) and the accuracy of the model. A computational study of CH4 surface oxidation in short contact time reactors showed that the predicted differences between a plug-flow and a complete 2D model are limited to entrance effects for honeycomb monoliths.28 It must be emphasized that during CH4 oxidation under atmospheric pressure homogeneous reactions account for less than 10% of fuel consumption. In contrast, during ethane ODH probably only 10% of the fuel is consumed on the surface, and the remaining ethane undergoes gas-phase reactions. Hence the validity of the assumptions of infinitely fast heat and mass transfer is critical only in a short fraction of the modeled reactor length. Furthermore, the ideal Reynolds number ( 1), the effect on ethylene selectivity is less significant with only a moderate increase, while ethane conversion starts to drop. These features are well predicted by the model (Figure 6a,b). Simulated methane and acetylene selectivities (Figure 6c) show similar trends to the experimental data, with CH4 increasing and C2H2 constant with increasing H2/ O2. Simulated H2 formation lies between experimental values (Figure 6d). 3.4. CH4 and H2 Chemistry on Pt. The present mechanism was developed in part from an assembly of different surface mechanisms developed for H2/O2 and CH4/O2 chemistry. Since the mechanism was tuned on the basis of ethane ODH experiments, the performance of CH4 and H2 chemistry subsets was also compared to methane partial oxidation experiments to ensure that the carbon and hydrogen chemistry was still working properly. Specifically, the model was compared to experimental data obtained in short contact time reactors with the same reactor morphology (45 ppi ceramic foam monoliths), same catalyst (Pt), and lower flow rate (4 slpm

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Figure 5. Simulation results (lines) and experimental data (black symbols,34 white symbols5) for a C2H6/O2 mixture on a Pt/Sn catalyst (5 slpm feed flow rate and 30 vol % N2 dilution): ethane conversion and C-atom selectivity for C2H4 (a), C-atom selectivity for CO and CO2 (b), C-atom selectivity for CH4 and C2H2 (c), and H-atom selectivity (d) at varying C2H6/O2 ratios.

Figure 6. Simulation results (lines) and experimental data (black symbols,34 white symbols7) for a Pt/Sn catalyst with H2 addition (C2H6/O2 ) 2/1, variable feed flow rate and N2 dilution): ethane conversion and C-atom selectivity for C2H4 (a), C-atom selectivity for CO and CO2 (b), C-atom selectivity for CH4 and C2H2 (c), and moles of H2 formed per mole of O2 fed (d) at varying H2/O2 ratios.

instead of 5 slpm).8,46 The front face catalyst temperature and the heat losses from the experimental reactor were not measured in experiments and were estimated instead as fitting parameters. An inlet temperature of

350 °C and heat losses 25% higher than in the ethane ODH reactor were assumed. For a CH4/O2 feed ranging from 1.6 to 2, which are typical conditions for syngas production, model simula-

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Figure 7. Simulation results (lines) and experimental data (black symbols,46 white symbols8) for a CH4/O2 mixture on a Pt catalyst (4 slpm feed flow rate and 10% N2 dilution): methane conversion (a) and C-atom selectivity for CO and H-atom selectivity for H2 and H2O (b) at varying CH4/O2 ratios.

tions agree well with experimental data (Figure 7). Methane conversion is well reproduced, and predicted

CO selectivity agrees well with experimental data. Predicted H2 selectivity is lower than the experimental values, especially at a low CH4/O2 ratio, due to the onset of gas-phase reactions partially consuming CH4 in radical reactions leading to coupling products. 3.5. Temperature and Species Profiles for Pt. In this section, the computed profiles of temperature and major species conversion and yield are reported along the axial coordinate for the Pt heterogeneous mechanism together with the heterogeneous and homogeneous reaction rates for a feed with and without H2 addition. In order to have comparable values between heterogeneous and homogeneous reaction rates, surface reaction rates were converted from an area basis [mol/(cm2 s)] to a volume basis [mol/(cm3 s)] by using the monolith’s specific geometric surface area (∼8000 m2/m3 based on the volume of the entire monolith29). 3.5.1. C2H6/O2 Feed Mixtures. For a feed with a C2H6/O2 ratio of 2, the computed profiles of temperature, ethane and oxygen conversion, and the carbon and hydrogen yields of the main products are shown in Figure 8. The net rates of both gas-phase and surface chemistry for major species along the axial coordinate are shown in Figure 9. Experimental temperature data are also presented in Figure 8a, and good qualitative agreement between simulated and experimental temperatures is observed. However, direct quantitative comparison between experimental and predicted simulation temperatures is inappropriate since there are fundamental differences between the model and experimental geometries and heat transfer behavior. For example, temperature data shown for variable bed length experiments13 (black circles) correspond to a fixed bed of spheres and a variable catalyst length instead of

Figure 8. Simulated axial species and temperature profiles for a Pt catalyst (C2H6/O2 ) 2/1, 5 slpm feed flow rate, 30 vol % N2 dilution): (a) C2H6 and O2 conversion and temperature; (b) yield to C2H4, CO, and CO2; (c) yield to CH4 and C2H2; and (d) H-atom yields to H2, H2O, and hydrocarbons (HC). Experimental temperature data (black circles/gray triangles13 and square at 1 cm axial distance7) are also presented in panel a for qualitative comparison with the simulations. Direct quantitative comparison between experimental and predicted simulation temperatures is inappropriate since there are differences between model and experimental geometries and heat transfer properties.


Figure 9. Predicted net rates of homogeneous (dashed lines) and heterogeneous (solid lines) chemistry of the major species from simulations shown in Figure 8.

a 1 cm foam monolith in simulations. From Figure 8a, it is evident that C2H6 conversion increases with a more gradual slope in the first 2 mm of the catalyst than in the remaining length. The model predicts that only surface reactions of C2H6 and O2 consumption and CO2, H2O, and CO formation take place in the first 2 mm (Figure 9). In agreement with experiments,12,14 CO formation occurs at higher temperature than CO2 (Figure 8b). The formation of oxidation products, such as CO2, some CO (Figure 8b), and H2O (Figure 8d) with the consumption of about 10% of ethane and 50% of oxygen in the first 2 mm effects a strong temperature increase from the inlet temperature (350 °C) to above 800 °C (Figure 8a), which can be considered the approximate threshold temperature of gas-phase reactions.11,12 Above 800 °C, a change in slope of ethane conversion can be

observed (Figure 8a), as ethane consumption in the gas phase is much faster than on the surface (Figure 9). The onset of gas-phase reactions after 2 mm is also revealed by the formation of products, such as ethylene (Figure 8b), CH4 (Figure 8c), and H2 (Figure 8d). C2H2 (Figure 8c) is also formed, but for longer residence times and in lower amounts. Once ignited in the presence of oxygen, gas-phase reactions become much faster than surface reactions, and the catalyst does not seem to play a crucial role except favoring the formation of some CO and H2O. About 85% of the converted C2H6 is reacted in the gas phase toward the formation of C2H4, H2, and CO (Figure 9). A reaction path analysis (Figure 10a) shows that ethane is consumed mainly via gas-phase cracking reactions (∼80%) and to a minor extent through the gasphase O2-assisted dehydrogenation (∼20%). All C2H4 is

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Figure 10. Major gas-phase reactions contributing to net rates of species production shown in Figure 9 for (a) C2H6 and (b) C2H4. The main path leading to ethylene from ethane goes through mainly dehydrogenation steps not involving oxygen. Steps involving oxygenated compounds account for ∼20% of gas-phase ethane consumption and less than 10% of ethylene formation.

formed by homogeneous reactions, while heterogeneous reactions consume it to a small extent. Homogeneous steps involving oxygenated compounds account for less than 10% of ethylene formation (Figure 10b). H2 is formed exclusively in the gas phase. CO is formed mainly on the surface (∼80%) under conditions of higher temperature and lower oxygen content than at the reactor inlet, where CO2 is preferentially produced. Moreover, simulations suggest that very little CO2 is formed in the gas phase and that CH4 is formed only at high temperatures both in the gas phase and on the surface when a large surface coverage of C and H is attained. Because of endothermic reactions and heat losses along the reactor, temperature rapidly decreases after the first 2 mm, quenching other possible reactions and preserving ethylene (Figure 8a). Hence, all the products reach stable concentration within 1 cm from the inlet. 3.5.2. C2H6/O2/H2 Feed Mixtures. The results from H2 addition for a feed mixture with C2H6/O2/H2 equal to 2/1/2 are shown in Figure 11, where axial species profiles are reported, and in Figure 12, where axial profiles of homogeneous and heterogeneous reactions rates are shown. Upon H2 addition, heat production occurs mainly from oxidation of H2 to H2O on the catalyst surface (Figures 11d and 12). H2 consumption and H2O formation (up to 70% of the final value) in the first 3 mm of the reactor correspond to an O2 conversion

of ∼60% and an extremely low ethane conversion (very close to 0%). As temperature rises to above 900 °C, effected by H2 conversion, homogeneous reactions are ignited that lead to ethylene formation (Figure 11b) and the re-formation of H2 (Figure 11d). At the ignition of homogeneous reactions, C2H6 is consumed primarily through homogeneous chemistry (Figure 12). C2H4 formation occurs exclusively in the gas phase (Figure 12) and is accompanied by formation of H2, CO, and H2O. H2 is consumed by surface reactions and is subsequently formed through homogeneous chemistry. CO2 yield is extremely low because little O is available for deep C oxidation on the catalyst surface for the high fuel-to-oxygen ratios investigated, and C is instead oxidized to CO at temperatures higher than the inlet temperature (Figure 11b). Methane formation occurs in the gas phase and on the catalyst surface at high temperatures with a slightly higher yield than in the absence of H2. A reaction path analysis for the H2 addition case indicates very similar results for C2H6 consumption and C2H4 formation as the previous case with no hydrogen addition (Figure 10). C2H6 is consumed and C2H4 is formed mainly through nonoxidative gas-phase cracking (data not shown). 3.6. System Performance at Higher Flow Rates and Pressure. In addition to gaining a better understanding of the kinetics involved in ethane ODH at short contact times, this mechanism was also used as a tool to predict system performance under experimental conditions not easily achieved. Specifically, the predicted effects of flow rate and pressure were explored. In both cases, a reaction mixture with C2H6/O2 of 2/1 over a Pt catalyst was investigated with 30 vol % N2 dilution. The heat transfer coefficient of the reactor was assumed not to change significantly with varying flow rate or pressure, since it is mainly dependent on the system geometry. Thus, the same heat loss function was implemented as in the previous simulations. It should be noted that the simulations used to explore higher pressures should not be considered exact quantitative predictions for two main reasons: (1) catalytic reactions and adsorption/desorption steps of most gasphase radicals and intermediates are not considered in the present work, and (2) some rate constants in the homogeneous mechanism for chemically activated reactions and hydrocarbon radical oxidation are strictly valid only at 1 atm.42 Instead, results from higher pressure simulations are meant only as a qualitative guide to understand the product distribution from ethane ODH at high vs low pressure. 3.6.1. Effect of Flow Rate. Increasing flow rate has little effect on simulated ethane conversion over a range of space velocities (0.7-3.4 × 105 h-1). This result can be explained by considering that very little of the catalytic channel (2 mm) is required to ignite and sustain gas-phase reactions. Increasing flow rate mainly moves the reaction front downstream and increases peak temperature (data not shown). However, the increase in the axial temperature profile does not significantly affect product distribution. C2H4 selectivity is constant, and CO and CO2 selectivities change by less than 1%. Other species selectivities are not significantly affected by an increase in flow rate. Therefore, larger flow rates can be safely fed to the reactor without a decrease in ethylene selectivity, until the reaction front is pushed outside of the reactor leading to extinction (in agreement with the experimental findings on Pt13).


Figure 11. Simulated axial species and temperature profiles for a Pt catalyst (C2H6/O2/H2 ) 2/1/2, 7.33 slpm feed flow rate, 20.5 vol % N2 dilution): (a) C2H6 and O2 conversion and temperature; (b) yield to C2H4 and CO; (c) yield to CH4, C2H2, and CO2; (d) moles of H2 and H2O per mole of H2 fed. Experimental temperature data (black circles/gray triangles13 and square at 1 cm axial distance7) are also presented in panel a for qualitative comparison with the simulations. Direct quantitative comparison between experimental and predicted simulation temperatures is inappropriate since there are differences between model and experimental geometries and heat transfer properties.

3.6.2. Effect of Pressure. An increase in pressure of about 1 order of magnitude (from 1.2 to 10 atm) gives rise to an increase in ethane conversion of about 10% and a concurrent decrease in ethylene selectivity of about 3% (Figure 13a). In this case, the increased system heat formation due to increased partial pressure of the reactants leads to a higher system temperature and higher ethane conversion. At the catalyst back face, the temperature at 10 atm is about 50 °C higher than at 1 atm, which has a negative effect on ethylene formation. Above 875 °C, ethylene is further decomposed to CH4 (CH4 selectivity is increased from ∼4 to ∼8%) and to C2H2 (Figure 13c). While CO2 selectivity remains constant, pressure increase has a negative effect on CO selectivity, which decreases ∼8% over the pressure range considered in these simulations (Figure 13b). H2 and H2O selectivities slightly decrease (Figure 13d), since larger amounts of CH4 and C2H2 and other C3C4 hydrocarbons (not shown) are formed at higher temperatures. These predictions agree qualitatively with experimental H2 adition data on Pt that show increasing the pressure from 2 to 4 bar does not significantly influence catalytic combustion but lowers cracking selectivity to ethylene.47 4. Discussion Previous kinetic models for ethane ODH on Pt monoliths are limited in that they either predict that ethylene formation occurs primarily on the catalyst surface9,26 or cannot properly simulate co-feeding of H2.18 To address these issues, a surface mechanism was derived for Pt-based catalysts from published kinetic data for H2 and CO oxidation on Pt combined with lumped steps for ethane decomposition and combined with a pub-

lished homogeneous C1-C2 reaction mechanism for rich oxidation conditions.42 Through minor modifications, the surface mechanism (although not parametrically unique) was validated by experimental sets reported in the literature5,7,34 for ethane/oxygen feeds with and without H2 addition on two different catalysts. In addition, qualitative observations concerning the threshold temperature of surface reactions and product distribution at low temperatures (before the ignition of gas-phase reactions) were validated by published experimental data.10,12 Specifically, experimental observations that only CO2 and H2O are formed on Pt at low temperatures, while larger amounts of CO are produced at higher temperatures with larger O2 consumption, are in agreement with the reaction rates reported in Figures 9 and 12. 4.1. The Role of Surface Reactions in Ethane ODH. Ethane ODH on Pt foams is a process driven mainly by gas-phase reactions; the catalyst ignites the feed mixture and sustains the homogeneous reactions leading to ethylene formation. C2H4 formation reaches completion within 1 cm past the monolith exit and occurs mostly while O2 is still present. Not all O2 is consumed on the catalyst surface. Instead, a fraction of it speeds up homogeneous reactions. As argued previously,14,48,49 oxygen is necessary in the gas phase to speed radical propagation reactions50 and achieve quick ethylene formation, even though the main paths leading to ethylene are nonoxidative. Consistently, experiments showed that at the exit of the 1 cm long catalytic monolith, O2 conversion and ethylene formation are not complete.13 These results indicate that only 2 mm of catalyst may be sufficient to sustain the ODH process at the flow rates investigated. Nevertheless, surface

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Figure 12. Predicted net rates of homogeneous (dashed lines) and heterogeneous (solid lines) chemistry of the major species from simulations shown in Figure 11.

kinetics are important and affect ethylene selectivity, as shown by the different performances obtained on various catalysts (e.g., Pt vs Pt/Sn). The importance of surface chemistry is even more evident when H2 is added to the feed. H2 is preferentially oxidized on the catalyst surface instead of ethane (Figures 11d and 12) leading to an increase in temperature and the onset of the gas-phase reactions. In terms of ethylene yield, the option of co-feeding a secondary fuel such as H2 is advantageous because the gain in ethylene selectivity compensates for the loss in ethane conversion. Remarkably, as the H2 content in the feed is increased, the extent of ethane conversion and the amount of H2 produced per mole of H2 fed decrease. The contribution of hydrogen to ethylene selectivity mainly consists of substituting the fraction of C2H6 that is catalytically oxidized, but as H2 content is increased, and especially when it is increased above the stoichio-

metric value for combustion (H2/O2 ) 2), its contribution merely acts to dilute the feed and cool the reactor. 4.2. Pt vs Pt/Sn. Numerous experimental and a few theoretical studies have documented the decreased rate of hydrogenolysis, increased rate of hydrogenation, and reduced coke formation on Pt/Sn compared to monometallic Pt catalysts for hydrocarbon reforming and isomerization in the absence of O2. These effects have been attributed to the geometric, electronic, and energetic nature of the Pt/Sn surface. Dilution of the Pt surface with Sn most likely decreases the size of large Pt ensembles (required for hydrogenolysis)51-53 and may reduce the adsorption rate of hydrocarbons.54 Density functional theory calculations studying oxygenated hydrocarbon conversion have shown that the addition of Sn to Pt increases the transition state energies for C-C and C-O bond cleavage significantly more than for hydrogenation reactions.55


Figure 13. Effect of pressure on simulated axial species and temperature profiles for a Pt catalyst (C2H6/O2 ) 2/1, 5 slpm feed flow rate, 30 vol % N2 dilution): (a) C2H6 conversion, C2H4 selectivity, and temperature at 1 cm; (b) selectivity to CO and CO2; (c) selectivity to CH4 and C2H2; and (d) H-atom selectivity.

However, very little theoretical information is available in the literature about how Sn addition to Pt affects the dehydrogenation of ethane and other hydrocarbons in the presence of O2 (e.g., ethane ODH). Most knowledge is phenomenological in nature.5,7,34 Given these considerations, modifications were made to the Pt mechanism to account for Pt/Sn performance in a simple and flexible manner. In the absence of quantitative data specifying the differences between the transition state energies for cracking and dehydrogenation reactions on the two surfaces, no energetic changes were made to the Pt mechanism. To simulate the entropic differences between the two surfaces, the relative ratio between COx and H2O produced on the catalyst surface was adjusted by increasing the preexponential factor for the highly sensitive H oxidation reaction (R19) and decreasing the preexponential for CO oxidation (R39). According to the simulations, neither Pt nor Pt/Sn effect the direct surface formation of ethylene, yet different ethylene selectivities are obtained on each catalyst because of the different extents of ethane oxidation on each catalyst. While these modifications attempt to simulate the entropic differences between the two surfaces, it should be noted that these modifications are not unique or complete, and other entropic and energetic perturbations to the mechanism would probably fit the data equally well. However, the modifications to R19 and R39 demonstrate how entropic differences between the surfaces can lead to significant differences in product yields. Ethyl decomposition steps were not found to be rate determining in the relative formation of oxidation products (sensitivities for these reactions were always lower than 10-3), and thus R29 and R30 were described through lumped steps and not considered for adjustment in the Pt/Sn mechanism.

4.3. Mechanism Development with the PlugFlow Model. A plug-flow model was used to generate the results presented in this work as preliminary considerations concerning transport inside foam monoliths imply that the model was sufficient for initial mechanism development based on experimental data from the monolith outlet. Indeed, the complexity of fluid transport in foams (where a certain degree of turbulence is produced by the irregular geometry) suggests that resistances to heat transfer are concentrated on the external side (insulation side) of the reactor. In addition, previous experimental and numerical studies for methane partial oxidation on foams propose that the system is flux limited by methane adsorption rather than by external mass transfer through a boundary layer.31,39 In contrast, for reactive flows in honeycomb monoliths, where fast exothermic wall reactions take place, surface reactions are controlled by mass transfer making a detailed model of surface reactions unnecessary.56 However, for intracatalyst study the plug-flow model neglects any concentration and temperature gradients that develop over the short catalyst entrance length where the flowing gases experience very mild tortuosity in the 45 pore per linear inch foam. To study these gradients over the entrance length, the catalyst foam was simulated by a straight channel geometry and a 2D boundary-layer model. It should be noted that the validity of this transport treatment is limited after the first few millimeters of axial distance since critical aspects of the foam geometry are ignored with the straight channel approximation. Figure 14 shows a comparison between mass-averaged temperature, conversion, and yield profiles of an adiabatic channel for a boundary-layer model (reasonable for catalytic oxidation reactions in honeycomb

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Figure 14. Axial comparison between radially mass-averaged boundary-layer (solid lines) and plug-flow simulation (dashed lines) results: (a) temperature; (b) O2 and C2H6 conversion and C-atom yield to C2H4; (c) C-atom yield to C2H4, CH4, and C2H2; (d) C-atom yield to CO and CO2; and (e) H-atom yield.

monoliths28) and plug-flow model. Not surprisingly, heat is better maintained near the wall in the boundary-layer model, thus speeding surface reactions and accelerating heat production. Ignition is faster in the boundary-layer model. Nevertheless, after the first few millimeters, when homogeneous reactions become dominant, the values of temperature, conversion, and selectivity predicted by the two models converge. Temperature differences between wall and centerline are always below 100 °C (not shown), suggesting that the interphase resistances to heat and mass transfer are small. Some small differences persist, especially in CO, CH4, and C2H2 yields, which are higher in the 2D model. This may be because the plug-flow model is unable to describe local hot spots formed on the catalyst walls, which promote the local formation of species favored at high temperature, such as CH4 and C2H2. Nevertheless, the plugflow model is sufficient for surface mechanism tuning in this work since the objective function is composed of experimental data collected at the monolith outlet where values predicted by the boundary-layer and plug-flow models approximately converge. 5. Summary A kinetic model for ethane ODH was presented and composed of a published gas-phase mechanism42 and a surface mechanism that combined well-established elementary kinetic steps accounting for carbon and hydrogen oxidation and lumped steps describing ethane decomposition on the catalyst surface. Simulation results are in good agreement with experimental results obtained on Pt foam catalysts in short contact time reactors. Ethylene formation occurs predominantly in the gas phase, while the catalyst oxidizes a fraction of the fuel to produce heat on the surface to drive homo-

geneous reactions. The simplicity of the Pt surface mechanism allowed its extension to a Pt/Sn catalyst by simply regulating the H oxidation step. Agreement of the Pt/Sn simulation results with experimental data is excellent. Changing flow rate or pressure is found to have little effect on ethylene selectivity for a Pt-based catalyst. Acknowledgment F.D. acknowledges the International Exchange Short Mobility Program of the Universita` di Napoli Federico II for funding of his stay at the University of Minnesota. K.A.W. acknowledges funding through a National Science Foundation Graduate Research Fellowship. Nomenclature ai ) internal surface area of channel per unit length, m2/m ae ) external surface area of the channel per unit length, m2/m A ) channel cross-sectional area, m2 Ae ) external surface area of the entire monolith, m2 cp ) mass average specific heat of mixture, J/(kg K) cp,i ) specific heat of species i in mixture, J/(kg K) dc ) channel diameter, m Di,m ) mixture average diffusion coefficient between species i and mixture, m2/s hi ) specific enthalpy of species i, J/kg Ji,r ) radial mass flux of species i, kg/(m2 s) Kg ) number of gas-phase species L ) axial length of the monolith, m P ) pressure, Pa q(z) ) heat flux from the external surface of the monolith, J/(m2 s) Q ) heat losses from the reactor, J/s

Ind. Eng. Chem. Res., Vol. 44, No. 10, 2005 3469 Qe ) heat flux from the surroundings to the channel wall, J/(m2 s) r ) radial coordinate, m s˘ i ) production rate of species i by all surface reactions, mol/(m2 s) T ) temperature, K Tadiab ) adiabatic temperature, K Text ) external temperature, K Tmeas ) measured temperature at monolith exit, K U ) average heat transfer coefficient, J/(s m2 K) vr ) radial velocity, m/s vst ) Stefan velocity, m/s vz ) axial velocity, m/s wi ) mass fraction of species i Wi ) molecular weight of species i, kg/mol W h ) mean molecular weight, kg/mol Yi ) mole fraction of species i z ) axial coordinate, m Greek Letters λ ) thermal conductivity, W/(m K) µ ) viscosity, Pa s F ) density, kg/m3 φ ) any intensive variable φav ) mass-weighted radial average of any intensive variable ω˘ i ) production rate of species i by all homogeneous reactions, mol/(m3 s)

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Received for review July 26, 2004 Revised manuscript received February 4, 2005 Accepted March 1, 2005 IE0493356

A Multistep Surface Mechanism for Ethane Oxidative Dehydrogenation

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