Pure H2 production through hollow fiber hydrogen‐selective MFI zeolite membranes using steam as sweep gas

Hollow fiber MFI zeolite membranes were modified by catalytic cracking deposition of methyldiethoxysilane to enhance their H₂/CO₂ separation performance and further used in high temperature water gas shift membrane reactor. Steam was used as the sweep gas in the MR for the production of pure H₂. Extensive investigations were conducted on MR performance by variations of temperature, feed pressure, sweep steam flow rate, and steam‐to‐CO ratio. CO conversion was obviously enhanced in the MR as compared with conventional packed‐bed reactor (PBR) due to the coupled effects of H₂ removal as well as counter‐diffusion of sweep steam. Significant increment in CO conversion for MR vs. PBR was obtained at relatively low temperature and steam‐to‐CO ratio. A high H₂ permeate purity of 98.2% could be achieved in the MR swept by steam. Moreover, the MR exhibited an excellent long‐term operating stability for 100 h in despite of the membrane quality. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3459–3469, 2015     

Effect of Reaction and Permeation Rates on the Performance of a Catalytic Membrane Reactor for Methylcyclohexane Dehydrogenation

Abstract

The effect of the relative rates of reaction and H₂ permeation through palladium-silver (Pd-Ag) membranes upon the performance of a catalytic membrane reactor (CMR) for methylcyclohexane dehydrogenation has been investigated. Mathematical models have been used to identify the conditions at which a membrane reactor gives yields of toluene (TOL) and H₂ significantly in excess of equilibrium values at throughputs of industrial interest. The simulation shows that a catalyst with no product TOL inhibition performs exceptionally well in a CMR, giving conversions considerably above the equilibrium values at favorable operating conditions. Using a membrane unit between two conventional packed-bed reactors to separate the H₂ ex-situ gives significant improvement in performance over the shell-and-tube type CMR, resulting in conversions substantially higher than equilibrium at 633 K, 1.5 MPa, and liquid hourly space velocities of 3–10 volume feed/h/catalyst volumes.

     

Theoretical investigation of a water‐gas‐shift catalytic membrane for diesel reformate purification

The novel application of a catalytic water‐gas‐shift membrane reactor for selective removal of CO from H₂‐rich reformate mixtures for achieving gas purification solely via manipulation of reaction and diffusion phenomena, assuming Knudsen diffusion regime and the absence of hydrogen permselective materials, is described. An isothermal, two‐dimensional model is developed to describe a tube‐and‐shell membrane reactor supplied with a typical reformate mixture (9% CO, 3% CO₂, 28% H₂, and 15% H₂O) to the retentate volume and steam supplied to the permeate volume such that the overall H₂O:CO ratio within the system is 9:1. Simulations indicate that apparent CO:H₂ selectivities of 90:1 to >200:1 at H₂ recoveries of 20% to upwards of 40% may be achieved through appropriate design of the catalytic membrane and selection of operating conditions. Under these conditions, simulations predict an apparent hydrogen permeability of 2.3 × 10^?10 mol m^?1 Pa, which compares favorably against that of competing hydrogen‐permselective membranes. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4334–4345, 2013     

Design and operational considerations of catalytic membrane reactors for ammonia synthesis

Production of ammonia using hydrogen derived from renewable electricity instead of hydrocarbon reforming would dramatically reduce the carbon footprint of this commodity chemical. Novel technologies such as catalytic membrane reactors (CMRs) may potentially be more compatible with distributed ammonia production than the conventional Haber–Bosch process. A reactor model is developed based on integrating a standard industrial iron catalyst into a CMR equipped with an inorganic membrane that is selective to NH₃ over N₂/H₂. CMR performance is studied as functions of wide ranges of membrane properties and operating conditions. Conversion and ammonia recovery are dictated principally by the ammonia permeance, and the benefits by using membranes become significant above 100 GPU = 3.4 × 10⁻⁸ mol m⁻² s⁻¹ Pa⁻¹. To be effective, the CMR requires a minimum selectivity for ammonia of 10 over both nitrogen and hydrogen and purity scales with the effective selectivity. Increasing the pressure of operation significantly improves all metrics, and at P = 30 bar with a quality membrane, ammonia is almost completely recovered, enabling direct recycle of unreacted hydrogen and nitrogen without need for recompression. Temperature drives conversion and scales monotonically without thermodynamic limitations in a CMR. Alternatively, the temperature may be reduced as low as 300°C while achieving conversion levels surpassing equilibrium limits at T = 400°C in a conventional reactor.     

Upgrading of a syngas mixture for pure hydrogen production in a Pd-Ag membrane reactor

Water gas shift (WGS) is a thermodynamics limited reaction and CO equilibrium conversion of a traditional reactor is furthermore reduced owing to the presence of H₂ (ca. 50%) in the feed stream coming from a reformer.The upgrading of a simulated reformate stream was experimentally investigated as a function of temperature (280-320 °C), feed pressure (up to 600 kPa), gas hourly space velocity (GHSV), etc. using a Pd-alloy membrane reactor (MR) packed with a commercial catalyst CuO/CeO₂/Al₂O₃; no sweep gas was used. The MR performance was also evaluated using new parameters such as conversion index, H₂ recovery and extraction index, etc., which evidence the advantages with respect to a traditional reactor.A Pd-based MR operated successfully overcoming the thermodynamic constraints of a traditional reactor and, specifically, the drawback introduced by the hydrogen presence. In fact, a CO conversion of 90% significantly exceeded (three times) the thermodynamics upper limit (<36%) of a traditional reactor owing to ca. 80% of hydrogen permeated through the membrane.The overall process performance was significantly improved by the presence of the Pd-based membrane and, thus, by the high reaction pressure which allowed and drove the hydrogen permeation.     

A robust mixed‐conducting multichannel hollow fiber membrane reactor

To accelerate the commercial application of mixed‐conducting membrane reactor for catalytic reaction processes, a robust mixed‐conducting multichannel hollow fiber (MCMHF) membrane reactor was constructed and characterized in this work. The MCMHF membrane based on reduction‐tolerant and CO₂‐stable SrFe_0.8Nb_0.2O_3‐δ (SFN) oxide not only possesses a good mechanical strength but also has a high oxygen permeation flux under air/He gradient, which is about four times that of SFN disk membrane. When partial oxidation of methane (POM) was performed in the MCMHF membrane reactor, excellent reaction performance (oxygen flux of 19.2 mL min^?1 cm^?2, hydrogen production rate of 54.7 mL min^?1 cm^?2, methane conversion of 94.6% and the CO selectivity of 99%) was achieved at 1173 K. And also, the MCMHF membrane reactor for POM reaction was operated stably for 120 h without obvious degradation of reaction performance. © 2015 American Institute of Chemical Engineers AIChE J, 61: 2592–2599, 2015     

Steam reforming of methanol over a CuO/ZnO/Al2O3 catalyst part II: A carbon membrane reactor

The reaction of methanol steam reforming was studied in a carbon membrane reactor over a commercial CuO/ZnO/Al₂O₃ catalyst (Süd-Chemie, G66 MR). Carbon molecular sieve membranes supplied by Carbon Membranes Ltd. were tested at 150 °C and 200 °C. The carbon membrane reactor was operated at atmospheric pressure and with vacuum at the permeate side, at 200 °C. High methanol conversion and hydrogen recovery were obtained with low carbon monoxide permeate concentrations. A sweep gas configuration was simulated with a one-dimensional model. The experimental mixed-gas permeance values at 200 °C were used in a mathematical model that showed a good agreement with the experimental data. The advantages of using water as sweep gas were investigated in what concerns methanol conversion and hydrogen recovery. The concentration of carbon monoxide at the permeate side was under 20 ppm in all simulation runs. These results indicate that the permeate stream can be used to feed a polymer electrolyte membrane fuel cell.     

Analysis of Membrane Reactors for Integrated Coupling of Oxidative and Thermal Dehydrogenation of Propane

This study presents strategies capable to intensify the thermal dehydrogenation of propane (TDH) using integrated reactor concepts. An inert packed bed membrane reactor for distributed dosing of oxygen to realize the oxidative dehydrogenation (ODH) was studied and compared to a reactor with catalytically active membrane. The latter concept allows to combine TDH and ODH in one apparatus to overcome the chemical equilibrium by in situ conversion of the by‐product H₂ using O₂ or in a reverse water‐gas shift reaction by CO₂. If CO₂ is used as active sweep gas the reactor offered better performance regarding yield and selectivity. Strategies for further thermal integration are discussed.     

Performance of Pd composite membrane prepared by organic–inorganic method in WGS membrane reactor

H₂ production via water–gas shift (WGS) reaction in a Pd membrane reactor prepared by the electroless plating technique (ELP) “organic–inorganic” method was investigated. Pd nanoparticles embedded polyethylene glycol (PEG) was used as a polymer template during the activation step. Gas permeation results showed an infinite selectivity for H₂/N₂ with a H₂ flux of 0.004–0.016 mol/m²·s depending on operating conditions while it decreased until 0.0005 mol/m²·s for gas mixtures. Furthermore, WGS membrane reactor experiments showed a maximum CO conversion of 98.5% with a H₂ recovery of 16% at 450°C. The membrane performance was consistent during WGS catalytic membrane reactors (CMR) tests, thereby confirming the stability of the obtained membrane.     

Amorphous alloy/ceramic composite membrane: preparation, characterization and reaction studies

A Ni-P amorphous alloy/ceramic membrane with high selectivity and permeability for hydrogen was prepared by a novel technique of local electroless Ni-plating with metal-activated paste. The separation factor obtained is higher for H₂/Ar compared with γ-Al₂O₃/ceramic composite membrane. In addition, two kinds of Ni-P alloy/ceramic composite membranes, as-prepared and crystallized ones, were applied to the membrane reactor of ethanol dehydrogenation, and the effect of the reaction temperature, argon sweeping rate and space time on ethanol conversion and yield of acetaldehyde was investigated. The results demonstrated that ethanol conversion in an as-prepared Ni-P amorphous alloy membrane reactor was significantly higher than that in a Ni-P alloy membrane reactor after crystallization, owing to an original structure of as-prepared Ni-P membrane. Meanwhile, the morphology of the membrane was observed by SEM. The crystallized process of non-supported Ni-P alloy membrane was detected by XRD. The surface composition and valence state of the membrane before and after reaction was investigated by XPS. This revised version was published online in July 2006 with corrections to the Cover Date.     

Ammonia decomposition in catalytic membrane reactors: Simulation and experimental studies

Catalytic decomposition of NH₃ with H₂‐selective microporous silica membranes for CO_x‐free hydrogen production was studied theoretically and experimentally. The simulation study shows that NH₃ conversion, H₂ yield and H₂ purity increase with the Damköhler number (Da), and their improvement is affected by the effect of H₂ extraction as well as NH₃ and N₂ permeation through the membranes. The experimental study of NH₃ decomposition was carried out in a bimodal catalytic membrane reactor (BCMR), consisting of a bimodal catalytic support and a H₂‐selective silica layer. Catalytic membranes showed H₂ permeances of 6.2–9.8 × 10^?7 mol m^?2 s^?1 Pa^?1, with H₂/NH₃ and H₂/N₂ permeance ratios of 110–200 and 200–700, respectively, at 773 K. The effect of operating conditions on membrane reactor performance with respect to NH₃ conversion, H₂ yield and H₂ purity was investigated, and the results were in agreement with those calculated by the proposed simulation model. © 2012 American Institute of Chemical Engineers AIChE J, 59: 168–179, 2013     

Material properties and operating configurations of membrane reactors for propane dehydrogenation

A modeling‐based approach is presented to understand physically realistic and technologically interesting material properties and operating configurations of packed‐bed membrane reactors (PBMRs) for propane dehydrogenation (PDH). PBMRs composed of microporous or mesoporous membranes combined with a PDH catalyst are considered. The influence of reaction and membrane transport parameters, as well as operating parameters such as sweep flow and catalyst placement, are investigated to determine desired “operating windows” for isothermal and nonisothermal operation. Higher Damköhler (Da) and lower Péclet (Pe) numbers are generally helpful, but are much more beneficial with highly H₂‐selective membranes rather than higher‐flux, lower‐selectivity membranes. H₂‐selective membranes show a plateau region of conversion that can be overcome by a large sweep flow or countercurrent operation. The latter shows a complex trade‐off between kinetics and permeation, and is effective only in a limited window. H₂‐selective PBMRs will greatly benefit from the fabrication of thin (～1 µm or less) membranes. © 2014 American Institute of Chemical Engineers AIChE J, 61: 922–935, 2015     

A comparison of co-current and counter-current modes of operation for a novel hydrogen-permselective membrane dual-type FTS reactor in GTL technology

In this work, a comparison of co-current and counter-current modes of operation for a novel hydrogen-permselective membrane reactor for Fischer-Tropsch Synthesis (FTS) has been carried out. In both modes of operations, a system with two-catalyst bed instead of one single catalyst bed is developed for FTS reactions. In the first catalytic reactor, the synthesis gas is partly converted to products in a conventional water-cooled fixed-bed reactor, while in the second reactor which is a membrane fixed-bed reactor, the FTS reactions are completed and heat of reaction is used to preheat the feed synthesis gas to the first reactor. In the co-current mode, feed gas is entered into the tubes of the second reactor in the same direction with the reacting gas stream in shell side while in the counter-current mode the gas streams are in the opposite direction. Simulation results for both co-current and counter-current modes have been compared in terms of temperature, gasoline and CO₂ yields, H₂ and CO conversion, selectivity of components as well as permeation rate of hydrogen through the membrane. The results showed that the reactor in the co-current configuration operates with lower conversion and lower permeation rate of hydrogen, but it has more favorable profile of temperature. The counter-current mode of operation decreases undesired products such as CO₂ and CH₄ and also produces more gasoline.     

Integrated membrane system for pure hydrogen production: A Pd-Ag membrane reactor and a PEMFC

In this work, an integrated system consisting of single stage hydrogen production and a commercial PEMFC was investigated experimentally. The CO-free hydrogen fed to the PEMFC was produced in a Pd-Ag membrane reactor (MR), upgrading a syngas stream with a composition similar to that coming out of a reformer (CO 45%; H₂ 50%; CO₂ 4%; N₂ balance, on dry basis). The performance of the MR was evaluated in terms of CO conversion and H₂ recovery as a function of the feed pressure (up to 600 kPa) and space velocity; no sweep gas was used for promoting the H₂ permeation, since this role was assigned exclusively to the feed pressure.Special attention was paid to the analysis of the integrated system, focusing on the influence of the Pd-Ag MR operating conditions on the electrical performance of the PEMFC. The PEMFC internal crossover was also considered to have an effect on the electrical performance and this was taken into account estimating the PEMFC actual efficiency. Furthermore, the chemical efficiency of the integrated membrane plant was evaluated, considering the H₂ converted into electricity with respect to the total amount of H₂ contained in the feed mixture. An interesting performance was shown by the integrated system since the PEMFC performance was close to the power nominal value.     

Dehydrogenation of propane with selective hydrogen combustion: A mechanistic study by transient analysis of products

The role of lattice and adsorbed oxygen species in propane dehydrogenation in a perovskite hollow fiber membrane reactor containing a Pt–Sn dehydrogenation catalyst was elucidated by transient analysis of products with a sub-millisecond time resolution. Propane is mainly dehydrogenated non-oxidatively to propene and hydrogen over the catalyst, while lattice oxygen of the perovskite oxidizes preferentially hydrogen to water. For achieving high propene selectivity at high propane conversions, the formation of gas phase O₂ on the shell side of the membrane reactor should be avoided. Otherwise, oxygen species adsorbed over the Pt–Sn catalyst participate in non-selective C₃H₈/C₃H₆ transformations to C₂H₄ and CO_x.     

Pure hydrogen generation in a fluidized-bed membrane reactor: Experimental findings

A pilot-scale fluidized-bed membrane reactor was tested for the production of hydrogen. The prototype reactor operated under steam methane reforming (SMR) and autothermal reforming (ATR) conditions, without membranes and with membranes of different total areas. Heat was added either externally or via direct air addition. Hydrogen permeate purity of up to 99.995+% as well as a pure-H₂-to-natural-gas yield of 2.07 were achieved with only half of the full complement of membrane panels active under SMR conditions. A permeate-H₂-to reactor natural gas feed molar ratio >3 was achieved when all of the membrane panels were installed under SMR conditions. Experimental tests investigated the influence of such parameters as reactor pressure, hydrogen permeate pressure (vacuum vs atmospheric pressure), air top/bottom split, feed flowrate and membrane area. Reactor performance was strongly dependent on the active membrane surface area.     

Pure Hydrogen and Propylene Coproduction in Catalytic Membrane Reactor-Assisted Propane Dehydrogenation

Propane dehydrogenation on a commercial Pt-Sn/Al₂O₃ catalyst in a Pd-Ag membrane reactor is considered. A mathematical model is developed to evaluate the performance of the catalytic membrane reactor for the process of propane dehydrogenation. Design and operating conditions are systematically evaluated for key performance metrics such as propane conversion, propylene selectivity, hydrogen selectivity, and hydrogen recovery under different operating conditions. The results confirm that the high performance of the membrane reactor is related to the continuous removal of hydrogen from the reaction zone to shift the reaction equilibrium towards the formation of more propylene and hydrogen.     

Experimental and modeling studies on the low-temperature water-gas shift reaction in a dense Pd–Ag packed-bed membrane reactor

In this work, an experimental and modeling study is described, focusing on the performance of a Pd–Ag membrane reactor recently proposed and suitable for the production of ultra-pure hydrogen. A packed-bed membrane reactor (MR) with a “finger-like” membrane configuration has been used for carrying out the water-gas shift reaction (WGS) in the region of low temperature operation using a simulated reformate feed.The experiments were performed under a broad range of operating conditions of temperature (200–300 °C) and space velocity (1200–10,800 L_N kg_cat⁻¹ h⁻¹); the effect of feed pressure (1–2 bar) was also analyzed, as well as the operating mode at the permeate side: vacuum (30 mbar) or sweep gas (1.0 bar; nitrogen at 1 L_N min⁻¹). A one-dimensional, isothermal and steady-state model is proposed, which assumes axially dispersed plug flow pattern and pressure drop in the retentate side and plug flow with constant pressure in the permeate side. An innovative composed kinetic model was also used to describe the catalytic activity of the catalyst for the WGS reaction. In general, the simulation results showed a good agreement to the experimental data, in terms of carbon monoxide conversion and hydrogen recovery (and also outlet retentate composition) using only two fitting parameters related to the decline of H₂ permeability due to the presence of CO. Both simulation and experimental runs showed that the MR achieves high performances, for some operating conditions clearly above the maximum limit for conventional packed bed reactors. The performance reached is particularly relevant when hydrogen is recovered via sweep gas mode (a high sweep flow rate was employed), because a lower partial pressure could be reached than using vacuum pumping. In the first case, almost complete CO conversion and H₂ recovery could be reached.     

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.     

Vapor phase dehydrogenation of methanol to methyl formate in the catalytic membrane reactor with Cu/SiO2/ ceramic composite membrane

The Cu/SiO₂/ceramic composite membrane was prepared on the SiO₂/ceramic mesoporous membrane by an ion exchange method, and vapor phase dehydrogenation of methanol to methyl formate in the catalytic membrane reactor was investigated. It showed much better performance in the catalytic membrane reactor than that in the fixed-bed reactor under the same reaction conditions. At 240 °C, 57.3% conversion of methanol and 50.0% yield of methyl formate were achieved in the catalytic membrane reactor and only 43.1% conversion of methanol and 36.9% yield of methyl formate were achieved in the fixed-bed reactor.