Packed-bed Catalytic Membrane Reactor Research Articles

Modeling & experimental studies on enhancement of H2S conversion using catalytic membrane reactor for hydrogen production

Abstract Acidic gases such as H2S and CO2 are generated in the refineries during coal gasification. These pollutant gases need to be treated before releasing to the atmosphere. Conventionally, H2S gas is treated by Claus process in which H2S is converted to elemental S and H2O, in presence of O2 at ∼1,200 K. In the present energy scenarios, hydrogen has got importance as a source of clean fuel for industrial application. The H2S generated in the refineries can be a potential precursor of hydrogen. If this hydrogen can be obtained by thermal decomposition of H2S, that can increase the overall economic value of the H2S containing streams and the produced hydrogen can be used for hydrocracking and hydro-treating in refineries, as well as to synthesize value-added products for chemical industries. However, H2S thermal decomposition is an endothermic and equilibrium limited reaction (equilibrium conversion is only ∼15 % at 1,000 K), requiring a very high temperature (∼1,500 K) to achieve even a conversion of greater than 40 %. Packed Bed Catalytic Membrane Reactor (PBCMR) for thermal decomposition of H2S can be a potential technology augmentation and process intensification to increase this equilibrium conversion. In this work, modeling and experimental studies of H2S thermal decomposition using in-house designed & developed PBCMR have been carried out. The modeling studies were validated with experimental data. Clay-alumina ceramic tubular membrane (L: 250 mm; OD: 12 mm; thickness: 2 mm) was fabricated using extrusion process having an average membrane pore size of ∼1 μm and with a porosity of ∼20 %. Pt (2 %) coated alumina extrudes were used as catalyst to accelerate the reaction kinetics. Experimental studies showed that H2S to hydrogen conversion of ∼90 % is achieved using PBCMR at ∼1,523 K, compared to only ∼40 % conversion in a conventional packed bed tubular reactor (without membrane). Modelling studies were carried out to study the influence of operating parameters such as, reactor wall temperature, feed temperature, pressure and feed velocity. Studies showed that reactor wall temperature is having the most dominant effect on H2S conversion, which is confirmed by experimental findings. The studies offer useful insights into the application of PBCMR technology for management of waste gas stream containing H2S and recovery of hydrogen.

Ammonia decomposition for hydrogen production using packed bed catalytic membrane reactor

Ammonia can be used for hydrogen storage as it is relatively easy to transport, and can be generated at the site by decomposition of ammonia. In this work, the authors report studies on ammonia decomposition using packed-bed catalytic membrane reactor (PBCMR) based on Pd–Ag alloy membrane. The studies were carried out at different operating conditions in single tube PBCMR as well as in packed bed catalytic reactor without membrane (PBCR). A conversion of ∼93% was achieved in PBCMR at a temperature of 773 K and 1 bar, as against a conversion of ∼80% in PBCR. A CFD model was also developed and validated using experimental results. The model was further extrapolated to carry out 3D simulations of multi-tube PBCMR for parametric optimisation. This is a first of its kind study wherein a computational approach is used for studies on ammonia decomposition using multi-tube PBCMR.

Tantalum membrane reactor for enhanced HI decomposition in Iodine–Sulphur (IS) thermochemical process of hydrogen production

HI decomposition reaction in Iodine–Sulphur (IS) thermochemical process for hydrogen production is one of the critical steps, which suffers from low equilibrium conversion. Metallic membrane reactor is proposed to be a process intensification tool that can enable efficient HI decomposition by overcoming the equilibrium limitation of the reaction. In this study, we report the fabrication and characterization of clay alumina supported tantalum (Ta) membrane and its application in packed bed catalytic membrane reactor for HI decomposition section of IS process for the first time. The alumina tubes were coated with thin film of Ta metal of thickness 2.5 μm using DC sputter deposition technique under the optimum set of parameters. The membranes were found to remain stable upto 200 Hrs of exposure under HI-H2O environment at 125 °C. Modeling of a packed bed Ta membrane reactor for HI decomposition was carried out to find out the optimum operating parameters of the membrane reactor to get a conversion of at least 95%. A packed bed membrane reactor (PBMR) assembly was fabricated with integration of the in-house synthesized Ta membrane and Pt-alumina catalyst for carrying out HI decomposition studies at 450 °C. The tube side was chosen as permeation zone and the shell side as the reaction zone. A single-pass conversion of 94.7% of HI to hydrogen was obtained by employing Ta metal membrane reactor against the theoretical equilibrium conversion value of 22%. The findings showed that Ta-based PBMR shall offer a potential benefit to reach higher efficiency of IS thermochemical process.

Catalysis-spillover-membrane. III: The effect of hydrogen spillover on the palladium membrane reactor in the steam reforming reactions

In this paper, the effect of varying the distance between a palladium membrane and the steam reforming catalyst in a packed bed catalytic membrane reactor is studied. A set of the specially structured palladium membrane tube is prepared and used for the steam reforming reaction of n-hexane and of methanol. This specially designed palladium membrane structure has a long tube at the upstream-end of membrane tube, and it is electroplated with thin nickel or palladium film. This nickel or palladium plated metal surface serves as a bridge for the surface diffusion of the nascent hydrogen atoms over a 10 cm distance to the membrane surface for permeation out of the system after they are formed on the catalyst site and spillover from the catalyst surface to the diffusion bridge. As a result, the conversion level of steam reforming reaction is increased while byproduct formation of methane is suppressed. Furthermore, it is believed that the direct migration of the nascent hydrogen atom from the catalyst site to the membrane surface via direct spillover or via surface diffusion brings about a stronger positive contribution to the steam reforming reaction than does the vapor phase diffusion of the molecular hydrogen on the reaction. This extension tube allows the membrane and the catalyst section to operate under mutually optimized temperatures in a single reaction chamber while preserving high membrane efficiency; thus the use of this new membrane structure makes design of a membrane reactor more flexible to maximize mutual efficiency of catalyst and the membrane.

Epoxidation of propylene with H 2 and O 2 in the explosive regime in a packed-bed catalytic membrane reactor

The vapor-phase epoxidation of propylene with H 2 and O 2 over a highly dispersed Au/TS-1 catalyst was carried out in a packed-bed catalytic membrane reactor. The membrane reactor allowed the use of high concentrations of H 2 and O 2 (40% each) that were well within the explosive regime, and the production rate of propylene oxide was improved by 100–200% compared with that from a conventional packed-bed reactor. At 180 °C, the C 3H 6 conversion was 10% and the PO selectivity was 80%, corresponding to a space–time yield of 150 g PO kg cat −1 h −1 , whereas at 212 °C, the yield increased to 200 g PO kg cat −1 h −1 . The kinetic dependency, r PO = k (H 2) 0.53 (O 2) 0.26 (C 3H 6) 0.18, obtained free from mass transport and heat transfer limitations at these high concentrations was in good agreement with that reported at low concentrations.

Design Aspects of Hybrid Adsorbent−Membrane Reactors for Hydrogen Production

We present a detailed investigation of the design characteristics and performance of a novel reactor system, termed the hybrid adsorbent−membrane reactor (HAMR), for hydrogen production. The HAMR concept, originally proposed by our group1,2 for esterification reactions, couples the reaction and membrane separation steps with adsorption on the membrane feed or permeate side. The HAMR system investigated previously involved a hybrid pervaporation membrane reactor and integrated the reaction and pervaporation steps through a membrane with water adsorption. Coupling reaction, pervaporation, and adsorption significantly improved the performance. In this paper, we investigate a new HAMR system involving a hybrid packed-bed catalytic membrane reactor coupling the methane-steam-reforming reaction through a porous ceramic membrane with a CO2 adsorption system. The present HAMR system is of potential interest to pure hydrogen production for proton exchange membrane (PEM) fuel cells for various mobile and stationary applications. The reactor characteristics have been investigated for a range of temperature and pressure conditions relevant to the aforementioned applications. The HAMR system exhibits enhanced methane conversion, hydrogen yield, and product purity and shows good promise for reducing the hostile operating conditions of conventional methane−steam reformers and for meeting the product purity requirements for PEM operation.

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.

Comparison of different catalysts in the membrane-supported dehydrogenation of propane

Dehydrogenation of propane is studied in a high temperature packed bed catalytic membrane reactor with a hydrogen-selective silica membrane. The silica membrane is prepared by a two-step sol–gel process. The removal of hydrogen in the membrane reactor results in higher propane conversion and higher propene yields in comparison to an equivalent fixed-bed reactor. Unfortunately, as a result of the H 2 removal coking is favoured in the membrane reactor. Therefore, the higher propene yields are found only for the first 100–120 min time on stream. However, the lower selectivity of the membrane reactor due to coking is compensated to some extend by a reduced hydrogenolysis. Two commercial dehydrogenation catalysts of different activity were tested in the membrane reactor: Cr 2O 3/Al 2O 3 and Pt–Sn/Al 2O 3. The two catalysts show a different activity, coking, and regeneration behaviour in the membrane-supported propane dehydrogenation.

Dense Palladium and Perovskite Membranes and Membrane Reactors

The development of high-temperature processes and tighter environmental regulations requires utilization of efficient gas-separation processes that will provide high fluxes, high selectivity of separation, and the ability to operate at elevated temperatures. Dense inorganic membranes and membrane reactors are especially well suited for high-temperature reactions and separations, due in part to their thermal stability and high separation selectivity (in theory, infinite). Furthermore, membrane reactors offer an inherent advantage of combining reaction, product concentration, and separation in a single-unit operation for the improvement of process economics and waste minimization.The classification of membrane reactors can either be by membrane material and geometry or by the configuration of the reactor. Porous and dense membranes in both tubular and disk forms have been used for membrane reactors. The membrane can either be catalytically active (catalytic membrane reactor [CMR]) or simply act as a separation medium. In the latter case, the catalyst is packed in the reactor, whose walls are formed by the membrane (packed-bed membrane reactor [PBMR]). In addition, if the membrane is also catalytically active, the reactor is called a packed-bed catalytic membrane reactor (PBCMR).The principal materials from which porous inorganic (ceramic) membranes are made are alumina, zirconia, and glass. Alumina and zirconia membranes are usually asymmetric and composite, with a porous support (0.5–2.0 mm thick) for mechanical strength and one or more thin layers for carrying out separations.On the other hand, glass membranes, such as Vycor and microporous glass, have symmetric pores. Materials commonly used as the porous support are alumina, granular carbon, sintered metal, and silicon carbide.

Design of Membranal Dehydrogenation Reactors: The Fast Reaction Asymptote

This study presents a systematic analysis of certain design aspects of a packed-bed catalytic membrane reactor for simple and fast dehydrogenation reactions. For sufficiently fast reactions, local chemical equilibrium can be assumed everywhere and conversion is transport limited. Under these assumptions, in a plug-flow isothermal reactor with a permselective membrane, analytical results can be derived showing the conversion dependence on reactor length and equilibrium coefficient. These results suggest that significant conversions should be expected in such a reactor with a reasonable membrane area-to-feed flow rate ratio (e.g., at 450 °C during butane dehydrogenation, with membrane flux of 1 cm2/cm3/min). High pressures lead to lower equilibrium conversions and to higher diffusive fluxes, resulting in a marginal change in overall conversion. Shell-side hydrogen partial pressure affects the conversion significantly, and the shell-to-tube flow rate ratio should be sufficiently large (10 to 100). Catalyst l...

Isobutane dehydrogenation reaction in a packed bed catalytic membrane reactor

A packed-bed catalytic ceramic membrane reactor (PBCMR) was used for the isobutane dehydrogenation reaction. The experimental results have shown that through the use of the membrane reactor one can attain better conversions and yields than in a conventional reactor operating under the same outlet pressure and temperature, and feed composition conditions.