Methane steam reforming modeling in a palladium membrane reactor

A mathematical model of a membrane reactor used for methane steam reforming was developed to simulate and compare the maximum yields and operating conditions in the reactor with that in a conventional fixed bed reactor. Results show that the membrane reactor resents higher methane conversion yield and can be operated under milder conditions than the fixed bed reactor, and that membrane thickness is the most important construction parameter for membrane reactor success. Control of the H₂:CO ratio is possible in the membrane reactor making this technology more suitable for production of syngas to be used in gas-to-liquid processes (GTL).     

Pure hydrogen production by methane steam reforming with hydrogen-permeable membrane reactor

Low temperature steam reforming of methane mainly to hydrogen and carbon dioxide (CH₄ + 2H₂O → 4H₂ + CO₂) has been performed at 773 and 823 K over a commercial nickel catalyst in an equilibrium-shift reactor with an 11-μm thick palladium membrane (Mem-L) on a stainless steel porous metal filter. The methane conversion with the reactor is significantly higher than its equilibrium value without membrane due to the equilibrium-shift combined with separation of pure hydrogen through the membrane. The methane conversion in a reactor with an 8-μm membrane (Mem-H) is similar to that with Mem-L, although the hydrogen permeance through Mem-H is almost double of that through Mem-L. The amount of hydrogen separated in the reaction with Mem-H is significantly large, showing that the hydrogen separation overwhelms the hydrogen production because of the insufficient catalytic activity.     

Hydrogen generation in a Pd membrane fuel processor: Productivity effects during methanol steam reforming

Performance analyses are carried out for the palladium membrane fuel processor for catalytic generation of high purity hydrogen. The reactor model includes detailed particle-scale multi-component diffusion, multiple reversible reactions, flow, and membrane transport. Using methanol steam reforming on Cu/ZnO/Al₂O₃ catalyst as the test reaction, a systematic examination of the effects of operating and reactor design parameters on key performance metrics is presented. Single particle simulations reveal a complex interplay between nonisobaric transport and the reversible reactions (methanol reforming and decomposition, and water-gas shift), which impact overall reactor performance. An analysis of characteristic times helps to identify four different productivity controlling regimes: (i) permeation control, encountered with thick membranes and/or insufficient membrane area; (ii) catalyst pore diffusion control encountered with diffusion of reacting species in larger particles; (iii) reaction control, encountered when intrinsic catalytic rates are too low because of inadequate activity or catalyst loading; and (iv) feed control, encountered when the limiting reactant feed rate is inadequate. The simulations reveal that a maximum in the hydrogen productivity occurs at an intermediate space velocity, while the hydrogen utilization is a decreasing function of space velocity, implying a trade-off between productivity and hydrogen utilization. The locus of productivity maxima itself exhibits a maximum at an intermediate membrane surface to volume ratio, the specific value of which is dependent on the particle size, membrane thickness and reaction conditions. At moderate temperature and total pressure (, 10 bar), particles smaller than 2 mm diameter, Pd membranes with thickness less than , and membrane surface to volume ratio exceeding are needed to achieve viable productivity . A comparison between the packed-bed membrane reactor and conventional packed-bed reactor indicates a modest improvement in the conversion and productivity due to in situ hydrogen removal.     

Transient modeling of combined catalytic combustion/CH4 steam reforming

This paper presents an investigation into the complex interactions between catalytic combustion and CH₄ steam reforming in a co-flow heat exchanger where the surface combustion drives the endothermic steam reforming on opposite sides of separating plates in alternating channel flows. To this end, a simplified transient model was established to assess the stability of a system combining H₂ or CH₄ combustion over a supported Pd catalyst and CH₄ steam reforming over a supported Rh catalyst. The model uses previously reported detailed surface chemistry mechanisms, and results compared favorably with experiments using a flat-plate reactor with simultaneous H₂ combustion over a γ-Al₂O₃-supported Pd catalyst and CH₄ steam reforming over a γ-Al₂O₃-supported Rh catalyst. Results indicate that stable reactor operation is achievable at relatively low inlet temperatures (400 °C) with H₂ combustion. Model results for a reactor with CH₄ combustion indicated that stable reactor operation with reforming fuel conversion to H₂ requires higher inlet temperatures. The results indicate that slow transient decay of conversion, on the order of minutes, can arise due to loss of combustion activity from high-temperature reduction of the Pd catalyst near the reactor entrance. However, model results also show that under preferred conditions, the endothermic reforming can be sustained with adequate conversion to maintain combustion catalyst temperatures within the range where activity is high. A parametric study of combustion inlet stoichiometry, temperature, and velocity reveals that higher combustion fuel/air ratios are preferred with lower inlet temperatures (≤500 °C) while lower fuel/air ratios are necessary at higher inlet temperatures (600 °C).     

CO‐Free Hydrogen Production by Ethanol Steam Reforming in a Pd–Ag Membrane Reactor

In this work, the ethanol steam reforming (ESR) reaction has been studied by using a dense Pd–Ag membrane reactor (MR) by varying the water/ethanol molar ratio between 3:1 and 9:1 in a temperature range of 300–400 °C and at 1.3 bar as reaction pressure. The MR was packed with a commercial Ru‐based catalyst and a constant sweep gas flow rate in counter current mode was used. The influence of the temperature and the feed molar ratio on different parameters such as the ethanol conversion, the hydrogen production, the hydrogen yield and the CO‐free hydrogen recovery has been evaluated.     

Optimal design conditions for dehydrogenation of cyclohexane in a membrane reactor

The design variables of a membrane reactor, such as the permeation rate through the membrane and catalyst loading in the membrane, have received little attention in comparison with the operating conditions. A non-dimensionalized model for a membrane reactor was developed to account for the effects of permeation rate and catalyst loading. The increased permeation rate did not always increase the exit conversion and there existed a maximum point of exit conversion. At isothermal conditions, the exit conversion was saturated as catalyst loading was increased; however, when the reactor was under non-isothermal condition along the axial direction, there existed an optimum catalyst loading at which the exit conversion was maximum. With this model, the optimal configuration of permeation rate and catalyst loading could be determined.     

A dense Pd/Ag membrane reactor for methanol steam reforming: Experimental study

This paper focuses on an experimental study of the methanol steam reforming (MSR) reaction. A dense Pd/Ag membrane reactor (MR) has been used, and its behaviour has been compared to the performance of a traditional reactor (TR) packed with the same catalyst type and amount. The parameters investigated are reaction time, temperature, feed ratio and sweep gas flow rate. The few papers dealing with MR applications for the MSR reaction mainly analyse the effect of temperature and pressure on the reaction system. The investigation of new parameters permitted to better understand how the fluid-dynamics of the MR influences the hydrogen separation effect on methanol conversion and product selectivity. The comparison between MR and TR in terms of methanol conversion shows that the MR gives a higher performance than the TR at each operating condition investigated. Concerning hydrogen production, the experiments have shown that the overall selectivity towards hydrogen is identical for both MR and TR. However, the MR produces a free-CO hydrogen stream, which could be useful for direct application in proton exchange membrane fuel cells. A comparison, in terms of methanol conversion versus temperature, with literature data is also included.     

Ethanol steam reforming in a membrane reactor with Pt-impregnated Knudsen membranes

An ethanol reforming membrane reactor (ERMR) with Pt-impregnated Knudsen membranes was investigated to achieve the improvement of ethanol conversion and hydrogen yield. The prepared Pt-impregnated membranes have high permeabilities and reaction activities for the water-gas shift (WGS) reaction. The ethanol reforming-membrane reactor showed ethanol conversion improvement of 7.4–14.4% in comparison with a conventional reactor (CR). Hydrogen yield improvement of 4.2–10.5% was also observed in ERMR with Pt-impregnated SKM in whole reaction temperature range. In addition, CO concentration was considerably reduced via water-gas shift reaction during the permeation.     

Hydrogen production from fluidized bed steam reforming of hydrocarbons

Steam reforming of methane, kerosene and heavy oil over a nickel/alumina commercial catalyst and other materials such as limestone, dolomite and iron ore, was studied using a 5 cm i.d. fluidized bed reactor. The effects of operating parameters on conversion, hydrogen yield, product gas composition and elutriation of fine catalysts were investigated. It was found that a fluidized bed is flexible enough to handle various feedstocks, including hydrocarbons heavier than naphtha, because it permits the addition of catalyst to, or withdrawals of, coked catalyst from the bed. The yield of hydrogen obtained from fluidized bed steam reforming of heavy oil at 800‡C over limestone was similar to that obtained over commercial nickel-based catalyst. This indicates that limestone could be a promising catalyst for the production of hydrogen from heavy oil. However, hydrogen yield decreased with reaction time in the experiments using the limestone catalyst. The main cause of the decrease in hydrogen yield was elutriation of fine catalysts from the bed during the reaction.     

Hydrogen production from methanol by oxidative steam reforming carried out in a membrane reactor

The aim of this work is to study from an experimental point of view the oxidative steam reforming of methanol by investigating the behaviour of a dense Pd/Ag membrane reactor (MR) in terms of methanol conversion as well as hydrogen production. The main parameters considered are the operating temperature and the O₂/CH₃OH feed ratio. This is a pioneer work in the application of MR to this kind of reaction, whose goal should be to produce a CO-free hydrogen stream suitable for hydrogen fuel cell applications. The experimental results show that the MR gives methanol conversions higher than traditional reactors (TRs) at each temperature investigated, confirming the good potential of the membrane reactor device for this interesting reaction system.     

Hydrogen generation and purification in a composite Pd hollow fiber membrane reactor: Experiments and modeling

“Pd nanopore” composite membranes are a novel class of H₂ permselective membranes in which a thin layer of Pd is grown within the pores of a supported nanoporous layer. In this work, Pd nanopore membranes and conventional Pd top-layer membranes were used in the generation of high-purity H₂ from the catalytic decomposition of anhydrous NH₃. An effective 4 μm thick Pd nanopore membrane and 13 μm thick Pd top-layer membrane were synthesized on 2 mm O.D. α-Al₂O₃ hollow fibers. The permeation features of the membranes were determined and the membranes were then employed in a single fiber packed-bed membrane reactor in which Ni-catalyzed NH₃ decomposition served as the test reaction, with conditions spanning a range of conditions (500–600 °C; 3–5 bar total retentate pressure; 60–1200 scc/h g cat space velocity). The NH₃ conversions in both the PBMRs were approximately 10% higher than in a packed-bed reactor (PBR) under similar conditions. The increase in conversion with the PBMR was attributed to the removal of H₂, which has an inhibitory effect on the forward kinetics of the reaction as per the Temkin-Pyzhev type rate mechanism. Reactor productivities in the range of 2 mol/s m³ (at 85% H₂ utilization) to 7 mol/s m³ (at 50% H₂ utilization) were obtained. The permeate stream purity exceeded 99.2% H₂. A two-dimensional pseudo-homogeneous model was successfully used to simulate the experimental results and to interpret the findings. Permeation and kinetic parameters were estimated in permeation and PBR experiments, respectively. Without any data fitting the PBMR model predictions demonstrated very good agreement with experimental trends. Together with an analysis of the characteristic times, the model determined that transverse transport of hydrogen in the catalyst bed limited PBMR performance. The model was used to determine the rate limiting step and to suggest ways in which the reactor productivities could be further improved.     

Highly selective supported Pd catalysts for steam reforming of methanol

Steam reforming of methanol, CH₃OH + H₂O 3H₂ + CO₂, was carried out over various Pd catalysts (Pd/SiO₂, Pd/Al₂O₃, Pd/La₂O₃, Pd/Nb₂O₅, Pd/Nd₂O₃, Pd/ZrO₂, Pd/ZnO and unsupported Pd). The reaction was greatly affected by the kind of support. The selectivity for the steam reforming was anomalously high over Pd/ZnO catalysts.     

Catalytic combustion assisted methane steam reforming in a catalytic plate reactor

A theoretical study of methane steam reforming coupled with methane catalytic combustion in a catalytic plate reactor (CPR) based on a two-dimensional model is presented. Plates with coated catalyst layers of order of micrometers at distances of order of millimetres offer a high degree of compactness and minimise heat and mass transport resistances. Choosing similar operating conditions in terms of inlet composition and temperature as in industrial reformer allows a direct comparison of CPRs with the latter. It is shown that short distance between heat source and heat sink increases the efficiency of heat exchange. Transverse temperature gradients do not exceed across the wall and across the gas-phase, in contrast to difference in temperature of outside wall and mean gas phase temperature inside the tube usually observed in conventional reformers. The effectiveness factors for the reforming chemical reactions are about one order of magnitude higher than in conventional processes. Minimisation of heat and mass transfer resistances results in reduction of reactor volume and catalyst weight by two orders of magnitude as compared to industrial reformer. Alteration of distance between plates in the range 1- does not result in significant difference in reactor performance, if made at constant inlet flowrates. However, if such modifications are made at constant inlet velocities, conversion and temperature profiles are considerably affected. Similar effects are observed when catalyst layer thicknesses are increased.     

Pure hydrogen generation in a fluidized bed membrane reactor: Application of the generalized comprehensive reactor model

A generalized comprehensive model was developed to simulate a wide variety of fluidized-bed catalytic reactors. The model characterizes multiple phases and regions (low-density phase, high-density phase, staged membranes, freeboard region) and allows for a seamless introduction of features and/or simplifications depending on the system of interest. The model is implemented here for a fluidized-bed membrane reactor generating hydrogen. A concomitant experimental program was performed to collect detailed experimental data in a pilot scale prototype reactor operated under steam methane reforming (SMR) and auto-thermal reforming (ATR) conditions, without and with membranes of different areas under diverse operating conditions. The results of this program were published in Mahecha-Botero et al. [2008a. Pure hydrogen generation in a fluidized bed membrane reactor: experimental findings. Chem. Eng. Sci. 63(10), pp. 2752-2762]. The reactor model is tested in this second paper of the series by comparing its simulation predictions against axially distributed concentration in the pilot reactor. This leads to a better understanding of phenomena along the reactor including: mass transfer, distributed selective removal of species, interphase cross-flow, flow regime variations, changes in volumetric flow, feed distribution, and fluidization hydrodynamics. The model does not use any adjustable parameters giving reasonably good predictions for the system of study.     

Fluidizable catalyst for methane reforming

Fluidizable catalysts are developed in this study for advancing an integral approach towards a new methane reforming process. With this end, catalysts constituted by nickel supported on -alumina, NaY, and USY were developed using the incipient wetness technique producing bulk nickel loadings in the 0–20 wt.% range. These catalysts were also tested under relevant conditions for industrial operation in a novel Riser Simulator. It was found that, for the case of ‘dry’ reforming of methane, nickel deposited in zeolites is a promising catalyst given that it allows for close control of metal dispersion–redispersion process. In fact, when this catalyst was exposed to repeated oxidation and reduction cycles, nickel dispersions remained stable at 25% for NaY zeolite and at 15% for USY zeolite. This catalyst offers, however, limited application for steam reforming of methane given the potential collapse of the zeolite structure under steam atmosphere. As an alternative and for cases where steam reforming of methane is preferred, nickel on -alumina catalyst was considered. In these cases, optimum catalytic activity was achieved with 2.5 wt.% of nickel on -alumina with 3–6% nickel dispersion.     

Development of a microchannel catalytic reactor system

The purpose of this article is to demonstrate the applicability of microreactors for use in catalytic reactions at elevated temperatures. Microchannels were fabricated on both sides of a silicon wafer by wet chemical etching after pattern transfer using a negative photoresist. The walls of the reactor channel were coated with a platinum layer, for use as a sample catalyst, by sputtering. A heating element was installed in the channel on the opposite surface of the reactor channel. The reactor channel was sealed gas-tight with a glass plate by using an anodic bonding technique. A small-scale palladium membrane was also prepared on the surface of a 50-Μm thick copper film. In the membrane preparation, a negative photoresist was spin-coated and solidified to serve as a protective film. A palladium layer was then electrodeposited on the other uncovered surface. After the protective film was removed, the resist was again spin-coated on the copper surface, and a pattern of microslits was transferred by photolithography. After development, the microslits were electrolitically etched away, resulting in the formation of a palladium membrane as an assemblage of thin layers formed in the microslits. The integration of the microreactor and the membrane is currently under way.     

A hybrid adsorbent-membrane reactor (HAMR) system for hydrogen production

Design characteristics and performance of a novel reactor system, termed a hybrid adsorbent-membrane reactor (HAMR), have been investigated for hydrogen production. The recently proposed HAMR concept couples reactions and membrane separation steps with adsorption on the membrane feed-side or permeate-side. Performance of conventional reactors has been significantly improved by this integrated system. In this paper, an HAMR system has been studied involving a hybrid-type packed-bed catalytic membrane reactor undergoing methane steam reforming through a porous ceramic membrane with a CO₂ adsorption system. This HAMR system is of potential interest to pure hydrogen production for fuel cells for various mobile and stationary applications. Reactor behaviors have been investigated for a range of temperature and pressure conditions. The HAMR system shows enhanced methane conversion, hydrogen yield, and product purity, and provides good promise for reducing the hostile operating conditions of conventional reformers, and for meeting the product purity requirements.     

Performance of microchannel reactor combined with combustor for methanol steam reforming

The microchannel reactor with combustor for methanol steam reforming was fabricated to produce hydrogen for onboard proton exchange membrane (PEM) fuel cell device. A commercial copper-containing catalyst (Cu/ZnO/Al₂O₃) and Pt/ZrO₂ were used as a catalyst for methanol steam reforming and combustion reaction, respectively. It was found that catalyst layer with zirconia sol solution in microchannel showed no crack on the surface of catalyst layer and an excellent adherence to stainless steel microchannel even after reaction. The temperature of combustor could be controlled between 200 and 300 °C depending on the methanol feed rate. The hydrogen flow of 3.9 l h⁻¹ hydrogen was obtained with the reforming feed flow rate of 3.65 ml h⁻¹ at 270 °C.     

A composite catalytic polymer membrane reactor using heteropolyacid-blended polymer membrane

The vapor-phase MTBE decomposition was examined in a shell and tube-type catalytic membrane reactor (CMR). 12-Tungstophosphoric acid (PW) was used as a catalyst and poly-2,6-dimethyl-1,4-phenylene oxide (PPO) was used as a polymer material. A single-phase CMR (PW-PPO/Al₂O₃, type-1) and a composite CMR (PW-PPO/ PPO/ Al₂O₃, type-2) were successfully designed and characterized. It was revealed that the single-phase PW-PPO/ Al₂O₃ showed perm-selectivities for reaction products. The selective removal of methanol through the catalytic membrane shifted the chemical equilibrium toward the favorable direction in the MTBE decomposition. The PWPPO/ PPO/ Al₂O₃ showed the better performance than PW-PPO/ Al₂O₃. The enhanced performance of PW-PPO/ PPO/ Al₂O₃ CMR was due to the intrinsic perm-selectivity of PW-PPO and the additional separation capability of sub-layered PPO membrane.