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Steam Reforming of Ethanol in a Microchannel Reactor: Kinetic Study and Reactor Simulation Nageswara Rao Peela and Deepak Kunzru* Department of Chemical Engineering, Indian Institute of TechnologyKanpur, Kanpur-208016, India ABSTRACT: The kinetics of steam reforming of ethanol (SRE) was determined at atmospheric pressure in the temperature range of 450550 °C over 2% Rh/20% CeO2/Al2O3 in a microchannel reactor. The product distribution could be explained by a reaction scheme consisting of four reactions: reaction of steam with ethanol, decomposition of ethanol, methane steam reforming, and the water-gas shift reaction. The kinetic expression based on LangmuirHinshelwood kinetics could explain the experimental data satisfactorily. A two-dimensional simulation was also carried out to predict the performance of a microchannel SRE reactor in which the endothermic heat of reaction was supplied by the co-current flow of a hot gas in the adjacent channel. The performance of the coupled reactor/heat exchange system was evaluated by varying several parameters such as heating side gas velocity and temperature, width of the channel, weight of the catalyst, reactor inlet temperature, and inlet flow rate.
1. INTRODUCTION Because of the depletion of fossil fuel resources and stringent environmental constraints, focus on the generation of clean energy from renewable resources using fuel cells has increased significantly in recent years. Ethanol has been widely studied as a renewable resource for hydrogen, which, in turn, is a feed for polymer electrolyte membrane fuel cells (PEMFCs). This is due to several advantages that ethanol possesses, such as nontoxicity, easy handling, availability from renewable resources, and a closed carbon cycle. The drawbacks of conventional packed-bed reactors (PBRs) for use in space-limited applications (such as an onboard fuel reformer) are the larger size and inferior dynamic response to load changes. These problems can be effectively handled using microstructured reactors, because of their high heat- and mass-transfer rates. Steam reforming of ethanol (SRE) has been well-studied in PBRs, and several studies are also available in microreactors (MRs). However, very limited literature is available on kinetic modeling of steam reforming reaction of ethanol either in PBRs or in MRs. Only one published study on the determination of kinetics of SRE in MRs is available. Goerke et al.1 studied the kinetics of SRE in a microreactor and found that the rate-determining steps in the kinetic scheme were CO2 desorption, dissociative adsorption of ethanol, and reaction of adsorbed methane with steam. From the data obtained in a conventional PBR, Sahoo et al.2 developed a kinetic model for SRE over Co/Al2O3 using the LangmuirHinshelwood (L-H) approach. The L-H model was developed by assuming that the dehydrogenation of adsorbed ethoxy, the decomposition of an intermediate formate species, and the decomposition of acetaldehyde were the rate-determining steps for SRE, water-gas shift (WGS), and ethanol decomposition, respectively. Recently, Graschinsky et al.3 studied the SRE reaction on a MgAl2O4/Al2O3-supported Rh catalyst and proposed a reaction scheme that included four reactions, namely, ethanol decomposition, ethanol reforming, WGS, and the steam reforming of methane. The authors found surface reaction to be the rate-determining step. Simson et al.4 determined the kinetics r 2011 American Chemical Society
of SRE over a bimetallic precious metal (Rh/Pt) catalyst deposited on a ceramic monolith, using the power law model, and found that the reaction order for ethanol was 1.2 and zero for water. Ciambelli et al.5 investigated the kinetics of SRE on Pt/ CeO2 at low temperatures (573723 K), using a general form of the empirical power-function rate expression (power law), and determined the reaction orders for ethanol and water to be 0.5 and zero, respectively. The authors suggested a surface reaction mechanism involving (i) the dissociative adsorption of ethanol; (ii) decarbonylation to produce mainly H2, CH4, and CO; and (iii) WGS reaction. Mas et al.6 studied the kinetics of SRE on a Ni(II)-Al(III) lamellar double-hydroxide catalyst, using the L-H approach, and proposed two models. One was a general model that included four reactions, and the other model had two reactions, which could be used to predict the kinetic data at high temperature and high water/ethanol feed ratios. Vaidya and Rodrigues7 studied the kinetics of SRE over a Ru/Al2O3 catalyst. To derive the rate expression in their study, it was assumed that the decomposition of an activated complex formed during reaction into intermediate products was the rate-determining step. Several investigators have modeled various reactions, such as the steam reforming of methane, methanol, and other hydrocarbons, as well as hydrocarbon combustion reactions in wallcoated reactors. Zanfir and Gavriilidis8,9 studied the steam reforming of methane, coupled with the catalytic combustion of methane, and compared the results with an industrial reformer. Their results showed that a short distance between the heat source and heat sink increases the efficiency of the heat exchanger. The reactor volume and catalyst weight were 2 orders of magnitude less than those of conventional reactors. Deshmukh Special Issue: Ananth Issue Received: January 14, 2011 Accepted: September 19, 2011 Revised: September 19, 2011 Published: September 19, 2011 12881
dx.doi.org/10.1021/ie200084b | Ind. Eng. Chem. Res. 2011, 50, 12881–12894
Industrial & Engineering Chemistry Research and Vlachos10 carried out a two-dimensional (2-D) CFD simulation study of specially segregated, multifunctional, microchannel devices for hydrogen production from decomposition of ammonia. They found that the high temperature generated in the homogeneous combustion resulted in high conversions in short contact times. Arzamendi et al.11 performed a three-dimensional (3-D) simulation of a methanol steam reformer combined with a methanol combustion reactor. They investigated different flow configurations such as parallel, counter-current, and cross-flow, and found that, in parallel flow configuration, the temperature difference across the system is lowest (4 K) when compared to the other two configurations. Karakaya and Avci12 studied the CFD simulation of methanol, ethanol, and propane steam reforming in a microchannel reactor and reported that microbaffles within the channels affect the heat transfer to the reforming zone as well as the hydrogen yield. Petrachi et al.13 modeled an integrated steam reforming microchannel reactor of isooctane with catalytic and noncatalytic microchannel combustors. The simulation results showed that the heat transfer from the hot side to the cold side was very efficient, resulting in high compactness. Other relevant studies include modeling of partial oxidation of methane in a wall-coated microchannel14 and comparison of compact reformer configurations for on-board fuel processing.15 Although several simulation studies are available for reactions such as methane steam reforming, n-heptane steam reforming, etc. in a microchannel reactor, literature on the simulation of SRE reaction in a microchannel reactor is scarce. Various methods of supplying heat for the endothermic SRE reaction can be implemented, such as from combustion of the same fuel on the other side of the reformer or using flue gases at high temperature. Compared to catalytic combustion on an adjacent plate, the use of a hot gas provides for an easier control of the overall system. Moreover, with catalytic combustion, the catalyst may undergo deactivation, necessitating periodic regeneration or replacement. The objective of this study was to develop a kinetic model for the SRE reaction over 2% Rh/20% CeO2/Al2O3 in a microchannel reactor. Another objective was to model a microreactor using the developed kinetics and to ascertain the effect of important process parameters for the SRE reaction. The catalyst used in this study was chosen based on a previous study on SRE in microchannel reactors.16 This catalyst gave the best performance out of the various catalysts tested. The effect of process parameters on the microreactor was determined using a 2-D model by considering the width of the channel to be much larger than the depth. Therefore, in this model, the width/height ratio of the channel has been assumed to be >10. Several other investigators have also considered a similar geometry8,10,13 to reduce the model to being 2-D. Note that the geometry of the reactor modeled was different from the reactor used for the experimental runs.
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were coated with γ-Al2O3, using a washcoating method that has been detailed elsewhere.18 The washcoating was done by following a two-step procedure: primer coating, followed by slurry coating. The primer composition was 2% disperal + 4% poly(vinyl alcohol) (PVA), with the remainder being water. The composition of the slurry was 14% γ-Al2O3, 6% colloidal suspension of alumina (size 50 nm), and 2% PVA, with the remainder being water. The typical thickness of the catalyst layer formed was ∼30 μm, and the pore volume of the washcoat layer was ∼0.4 cm3 g1, based on nitrogen adsorption results. The washcoated plates were first impregnated with ceria (the precursor was cerium nitrate), followed by the active metal, using incipient wetness impregnation. The active metal used in this study was rhodium (the precursor was rhodium chloride). After impregnation, the catalyst was dried at room temperature for 6 h, then at 120 °C for 3 h, followed by calcination at 600 °C for 3 h. The catalyst 2% Rh/20% CeO2/Al2O3 contains 2% Rh and 20% CeO2, with the remainder being Al2O3. The catalyst-coated plate was joined to a plain stainless steel plate by laser spot-welding, and then assembled together, using two stainless steel guiding plates (12 mm thick). The guiding plates also had holes for inserting four cartridge heaters, each with a rating of 400 W, to maintain the temperature of the assembly at the desired value. The temperature of the microreactor was controlled by a thermocouple, connected to a proportionalintegraldifferential (PID) controller and inserted in the middle of the microreactor. 2.2. Catalyst Testing. A schematic of the experimental setup used for the catalytic test experiments has been presented earlier.16 It essentially consisted of a feed preparation section, a microreactor, and a product analysis section. The waterethanol mixture was fed by a micropump (Series-III, Lab Alliance, State College, PA, USA) to the preheater maintained at 573 K and then mixed with nitrogen, which was fed using a mass flow controller. Nitrogen was used as the inert and also acted as the internal standard for the subsequent gas chromatographic analysis. The mixture was then fed to the MR, which was maintained at the desired temperature. The effluent of the MR was analyzed using Porapak Q and Carbosphere columns. Ethane, ethylene, acetaldehyde, water, and ethanol were analyzed on the Porapak Q column connected online through a heated sampling valve, whereas the noncondensables (H2, CO, CH4, CO2, and N2) were analyzed online on the Carbosphere column. Before each test, the catalyst was reduced at 550 °C for 3 h, with a hydrogen flow of 20 sccm. The ethanol conversion (Xe) and the selectivity to any product i (Si) were calculated as follows: Xe ¼
Si ¼
2. EXPERIMENTAL PROCEDURE 2.1. Reactor Fabrication and Catalyst Coating. Author: The microchannels were fabricated on SS304 stainless steel sheet with a thickness of 1 mm by laser micromachining (V3+, Laservall, Italy). The plate consisted of 25 microchannels (depth = 400 μm, width = 500 μm, width of fin between channels = 300 μm, and length = 60 mm), along with inlet and outlet chambers. The design of the inlet and outlet chambers was based on the guidelines given by Commenge et al.17 Microchannels
moles of ethanol in moles of ethanol out 100 moles of ethanol in moles of i produced moles of ethanol reacted
The overall carbon balance was calculated as follows: overall carbon balance ¼ f½moles of CH4 þ moles of CO þ moles of CO2 þ 2ðmoles of CH3 CHOÞ þ 2ðmoles of C2 H4 Þ þ 2 moles of C2 H5 OH ðunreactedÞ 100 = 2 moles of C2 H5 OH ðfedÞ 12882
dx.doi.org/10.1021/ie200084b |Ind. Eng. Chem. Res. 2011, 50, 12881–12894
Industrial & Engineering Chemistry Research
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The carbon balance closure in all of the reported experimental runs was 100% ( 5%.
3. MATHEMATICAL MODEL FORMULATION The microchannel reactor geometry considered for this simulation consists of a repeating unit of two parallel channels of similar dimensions. Ethanol/water mixture is passed through one channel, upon which a thin layer of reforming catalyst is coated, and hot flue gas is passed co-currently through the other channel to supply the endothermic heat of reaction. To model this system, the following assumptions were made: steady-state operation; no heat losses to the surroundings; fully developed laminar flow with no lateral velocity components; validity of the ideal gas law; negligible pressure drop in both channels; negligible homogeneous reactions; and no diffusional effects in the catalyst layer due to the very thin catalyst layer (
