High-temperature Membrane Reactor Research Articles

Inhibiting in situ phase transition in Ruddlesden-Popper perovskite via tailoring bond hybridization and its application in oxygen permeation

Inhibiting in situ phase transition in Ruddlesden-Popper perovskite via tailoring bond hybridization and its application in oxygen permeation

The Effect of Bond Phase Additive and Sintering Temperature on the Properties of Mullite Bonded Porous SiC Ceramics

Currently, porous SiC ceramics have been a focus of interesting research in the field of porous materials due to their excellent structural properties, high strength, high hardness, and superb mechanical and chemical stabilities even at high temperatures and hostile atmospheres. Porous SiC ceramics have been considered as suitable candidate materials for catalyst supports [1-2], hot gas or molten metal filters [3], high temperature membrane reactors [4], thermal insulating materials [5], gas sensors [6] etc. Porous SiC ceramics are fabricated by various methods including partial sintering [7], carbothermal reduction [8-9], replication or pyrolysis of polymeric sponge [10-12], reaction bonding [13] etc. In all these methods SiC needs to be sintered which requires a very high temperature due to the strong covalent nature of the Si-C bond, selective sintering additives, expensive atmosphere, costly and delicate instrumentation. Processing of porous SiC ceramics at low temperature using a simple technique thus becomes necessary. Bonding of SiC can be done at low temperatures by use of different oxide and non-oxide secondary phases. They include silica, mullite, cordierite, silicon nitride, etc. Various sintering additives are used for the formation of variety of secondary oxide bond phases for formations for porous SiC [14-19] Choice of mullite as a bond for SiC has many advantages. Mullite possesses a high melting point (Tm= 1850°C) and a low oxygen diffusion coefficient (5.6 x 10-14 m2/sec at 50°C). It has a matching thermal expansion coefficient with SiC (CTEmullite= 5.3 ×10-6/K; CTESiC = 4.7 ×10-6/K at RT-1000 °C) and a high strength that can be retained up to a very high temperature. Different sources of aluminum, such as Al2O3, Al, AlN, and Al (OH)3 powders were used for the formation of mullite bonded porous SiC ceramics (MBSC) [20-21]. However, the mullitization temperature of 1550o C is still necessary. In this work, mullite bonded porous SiC ceramics were fabricated by an in situ reaction-bonding process; the mixture of clay and CaCO3 were chosen as sintering additives to lower the mullitization reaction between Al2O3 and oxidation-derived SiO2. The effect amount of alumina, sintering temperature and other sintering aids on material property such as porosity/pore size distribution mechanical and micro structural properties of porous oxide bonded SiC ceramics were studied.

Channelized Substrates Made from BaZr0.75Ce0.05Y0.2O3-d Proton-Conducting Ceramic Polymer Clay.

A novel process for producing thick protonic ceramics for use in hydrogen separation membrane reactors is demonstrated. Polymer clay bodies based on polyvinyl acetate (PVA) and mineral oil were formulated, and they permitted parts with complex architectures to be prepared by simple, low-pressure molding in the unfired, “green” state. Ceramic proton conductors based on doped barium zirconate/cerate, made by solid-state reactive sintering, are particularly well-suited for the polymer clay process. In this work, the ceramic proton conductor, BZCY755 (BaZr0.75Ce0.05Y0.2O3−d) was fabricated into a variety of shapes and sizes. Test coupons were produced to confirm that the polymer clay route leads to a high-quality ceramic material suitable for the demanding environment of high-temperature membrane reactors. It has been demonstrated that protonic ceramic specimens with the requisite properties are easily prepared at the laboratory scale. The polymer clay fabrication route opens up the possibility of high-volume, low-cost manufacturing at a commercial scale, by a process similar to how dinnerware and sanitary porcelain are produced today.

(Invited) High-Temperature Mixed Conducting Membranes and Reactors for Direct CO2 Capture and Conversion

Direct CO2 capture and conversion have been an active research area in recent years due to its importance in addressing the current global warming/climate change and future depletion of fossil fuels and creating a carbon neutral ecosystem. The benchmark technology for CO2 capture is the post-combustion amine absorption, which is energy intensive and cost prohibitive to large-scale commercial implementation. In comparison, the CO2 conversion technologies are still at their infancy with many technical challenges in efficiency and selectivity to be overcome, but primarily being explored in low-temperature solution-based and high-temperature solid-oxide-based electrochemical cells with solar or electrical energy as the input. Here we present a new type of high-temperature membrane reactors operated on mixed conducting chemistry for direct CO2 separation and instant conversion. In the presentation, we will first discuss the chemistries of three basic types of mixed ionic/electronic conductors designed for CO2 separation and conversion. We will then present materials and synthesis of the membranes, followed by testing the performance of membrane reactors with mock-up flue gas as the feeding gas and methane/ethane as the sweeping gases. The performances of oxidative conversions of methane and ethane are characterized by conversion rate of the reactants, selectivity and yield of the products and flux density of both reactant and product species. Finally, the fundamental separation and conversion mechanisms will be discussed in detail.

Experimental study on combustion characteristics and lean blow-out limits of non-premixed oxy-methane flames in a porous-plate reactor

The combustion characteristics and lean blow-out (LBO) limits of non-premixed oxy-methane flames in a face-to-face two-porous-plates reactor are experimentally investigated in this study. The reactor mimics the process of oxygen permeation and oxy-fuel combustion inside high-temperature membrane reactors (HTMRs). The fuel (CH4) was supplied together with CO2 in the main longitudinal stream while the oxygen was supplied via the upper and lower rectangular porous plates. The experiments were conducted over ranges of equivalence ratio (0.5, 0.7 and 1.0), CO2 percentage in the oxidizer mixture (0, 27, 35, 55 and 70%), and fuel firing rate (0.5, 0.75, 1.0, 1.5, 2, 2.5 and 3 kW). At fixed CO2%, there was insignificant increase in flame length while increasing the equivalence ratio. The flame luminous intensity was found to be decreasing while increasing CO2%. Increase in the LBO limit was obtained with the increase in the equivalence ratio at fixed fuel firing rate. The minimum limit of O2% to allow for a visible flame was found to be about 21% (79% CO2) at stoichiometric condition. The increase in fuel firing rate decreased CO2% at blow-out condition due to attainment of the required flow speed that overwhelmed the increase in flame speed.

Experimental and numerical investigation of flow field and oxy-methane combustion characteristics in a low-power porous-plate reactor

This study investigates experimentally and numerically the laminar flow field and oxy-methane combustion characteristics in a face-to-face two-porous-plates reactor over wide range of operating global equivalence ratio. The reactor is of low power to mimic the operation of high-temperature membrane reactors (HTMRs), but under sufficient oxygen permeation flux for combustion. This is achieved by using porous plates instead of low oxygen separation rate membranes. Mixture of methane and CO2 is introduced to the reactor in the channel between the two porous plates and the oxygen is permeated through the plates. The cold flow (non-reacting) characteristics under oxygen permeation are investigated experimentally using particle imaging velocimetry (PIV) system over a range of equivalence ratio, typically 0.4, 0.5 and 0.6. The data are compared with the numerical results and showed good agreement. Reacting flow field and oxy-methane combustion characteristics, in terms of flow mixing, flame location with respect to the porous plate and species distributions, are investigated numerically over a range of equivalence ratio, from 0.4 to 1.0. The results support the operation of the reactor under stoichiometric condition from performance point of view, especially when the case is projected on the HTMR operation.

On the effects of fuel type, fuel mixing and sulphur content on the performance of a high-temperature membrane reactor adapting liquid fuel: A numerical study

Abstract The study investigates numerically oxygen permeation and combustion and emission characteristics of a button-cell high-temperature membrane reactor (HTMR) handling different grades of liquid fuels including methanol (CH3OH), ethanol (C2H5OH), benzene (C6H6), n-octane (C8H18) and gas oil (C16H29). Effects of mixing different grades of fuels on the HTMR performance are also investigated. A disc-shaped La2NiO4 (LNO) ceramic membrane is used. A plain orifice atomization model is used for numerical calculations of liquid fuel atomization. Fuel is introduced along with CO2 at sweep inlet and air is introduced at fed inlet of the membrane reactor. 2-D axisymmetric mesh was constructed, and simulations were performed using Ansys-Fluent Workbench 15.0. All calculations were performed at fixed inlet gas temperature of 1173 K, fixed inlet fuel temperature of 300 K and fixed molar fuel concentration of 5% in the vapor phase. For single fuel operation, the highly reactive light oxygenated alcohol fuels resulted in stoichiometric combustion with better oxygen flux compared to rich combustion and low oxygen flux for heavier fuels. Gas oil fuel showed poor fuel evaporation, poor mixing with the sweeping gas, and low reaction rate. Trying to improve the performance of the HTMR while burning heavy fuels, three mixtures of three fuels containing gas oil fuel were tested. The compositions of the mixtures 1, 2 and 3 are 2%C6H6/2%C8H18/1% C16H29, 2%CH3OH/2%C8H18/1%C16H29 and 2%CH3OH/2%C6H6/1%C16H29. The results showed better performance of the HTMR when burning gas oil fuel in a mixture than burning it purely. Mixtures 1 and 2 resulted in about three times and mixture 3 resulted in about six times the oxygen flux value when using pure gas oil fuel. The maximum temperature with the reactor has been improved from 1208 K for gas oil fuel to 1383 K, 1372 K and 1557 K for mixtures 1, 2 and 3. Using methanol as fuel additive significantly improve the performance of the HTMR, even better than burning it alone, and enables the use of heavier fuels in HTMR applications. Based on that, the performance of the HTMR was evaluated while burning mixtures of two fuels, one of them is methanol. Three mixtures were considered (mixtures 4, 5 and 6) with the compositions of 4%CH3OH/1%C6H6, 4%CH3OH/1%C8H18 and 4%CH3OH/1%C16H29. Mixing methanol with benzene (mixture 4) resulted in the highest obtained oxygen permeation flux among the tested ranges of pure and mixed fuels. Locations where sulfates compounds are created within the HTMR with respect to the membrane are calculated for proper control of membrane operation while utilizing heavy gas oil fuel with Sulphur content.

Membrane reactors for low temperature applications: An overview

Today, low temperature membrane reactors are used at various levels of development in biotechnology and environmental protection, the chemical and petrochemical industry, energy production, etc. The aim of this review is, first at all, to provide the reader with a clear understanding of the unified approach that can be followed in the design of a membrane reactor and of the various aspects that can unite both high and low temperature membrane reactors, taking into account the specificity of the case proposed. Afterwards the role of the membrane in a membrane reactor and the various functions that a membrane reactor can cover for the development of a process are described, mentioning also the most recent membrane bioreactors and photo-catalytic membrane reactors, which will be included in the more general concept of a membrane reactor. Finally, an overview of the main uses to which low temperature membrane reactors can be applied is also provided.

H2/CH4/CO2-tolerant properties of SrCo0.8Fe0.1Ga0.1O3 − δ hollow fiber membrane reactors for methane partial oxidation to syngas

Oxygen permeable ceramic membranes have potential applications as the high temperature membrane reactors for cost-effective syngas production from methane partial oxidation. The prerequisite to realize this potential is that the membrane must be chemically stable at high temperatures in atmospheres containing reducing or acidic gases. To improve the stability of conventional perovskite (SrCo0.8Fe0.2O3−δ) ceramic membranes, a doping strategy has been adopted. For this purpose, an asymmetric SrCo0.8Fe0.1Ga0.1O3−δ (SCFG) hollow fiber membrane has been fabricated. Its respective oxygen permeability is tested using helium, carbon dioxide, hydrogen and methane as sweep gases, which are the main gas components during the partial oxidation of methane (POM) to syngas. The oxygen permeability of the as-prepared SCFG hollow fiber membrane is up to 3.75mL·min−1·cm−2 at 1000°C when supplied with air as the feed and swept by He, while it declines to 1.85mL·min−1·cm−2 when swept by CO2. Whereas, the oxygen permeation fluxes increase to 3.98 and 3.81mL·min−1·cm−2 at 1000°C, respectively, when the membrane is swept by the reducing gases such as 4% H2-96% He and 20% CH4-80% He, keeping a stable operation for 220h. During the membrane reactor (an integrated separation-reaction process) performance for the POM reaction using the SCFG hollow fiber packed with Ni–Al2O3 catalysts, the ratio of H2/CO, the CH4 conversion, the selectivity of CO and the oxygen permeability are 2.08, 100%, 33%, and 4.14mlmin−1cm−2, respectively upon steady operation for 23h at 800°C. The promising stability of SCFG membranes during POM reactions has been attributed to the stabilizing effect provided by the doped gallium oxide, which acts as an anchor for the perovskite structure to maintain the integrity of the crystal lattice structure in the reducing atmosphere.

Application of a Pd–Ru composite membrane to hydrogen production in a high temperature membrane reactor

In this work, a ∼5.0μm thick Pd–Ru membrane (area∼13.3cm2), supported on a porous yttria-stabilized-zirconia/stainless steel substrate (ZrOD AccuSep®, Pall Corp.) was used to carry out steam methane reforming (SMR) over a commercial Ni-based reforming catalyst at a temperature of ∼580°C and pressures up to 2.9MPa. Methane conversions in the membrane reactor ranged from 65 to over 80%, depending on the reactor’s space velocity. The conversions obtained were significantly higher than the thermodynamic equilibrium predicted for the feed composition and process variables. The effect of parameters such as space velocity and reactor pressure on methane conversion and hydrogen recovery were investigated.The long term operation on this membrane revealed the potential suitability of this Pd-alloy to be a candidate for use in SMR membrane reactors at temperature as high as 580°C. The permeate hydrogen flux was very stable at 580°C, 2.9MPa feed pressure, and a steam-to-carbon ratio (SCR) of 3 for more than 1000h of continuous testing. The hydrogen permeate purity remained >93% over the course of testing. Stable methane conversions as high as ∼80% were obtained for the duration of the long term test which may be further improved by optimizing the fluid mechanics of the reactor design or geometry. 1-Dimensional process simulations were carried out and the results suggested that the methane conversion and hydrogen flux during the membrane reactor experiments were reduced by gas phase mass transfer due to concentration polarization effects. Ongoing work will focus on further optimization and testing of scaled-up Pd–Ru membranes in SMR reactor environments.

High Temperature Membrane Reactor System for Hydrogen Permeation Measurements and Validation with Pd Based Membranes

Hydrogen separation membranes are under development for integration with a coal gasifier or natural gas reformer for pre-combustion separation of hydrogen and carbon dioxide. Because of the high operating temperatures and pressures, a robust reactor and associated control systems are required for fast screening of membrane materials with a strong emphasis on operator and plant safety. In this paper, the design, construction and commissioning of a reliable membrane reactor and a versatile test station for evaluation of hydrogen permeation membrane materials (metals, ceramics or cermets) at high temperatures and high differential pressures has been described. The membrane reactor system has been designed to operate at temperatures up to 800oC and pressure differentials across the membrane to 1.0MPa. The system has multiple levels of safety redundancy built-in which include a range of controls and monitors for both operator and system safety. A number of Pd and Pd-Ag alloys of nominal thicknesses in the 20 and 140µm range were sourced and alumina based porous ceramic support structure were fabricated for evaluation of metal membranes. The test station has been validated with Pd and Pd-Ag alloys of different thicknesses. The data obtained from the reactor for various membrane types and thicknesses are in agreement with those reported in the literature.

Pd/Al2O3 Composite Membranes for the Production of Pure Hydrogen

In recent years, the application of high-temperature membrane reactors, especially for the catalytic dehydrogenation of hydrocarbons, has received growing attention. It is well known that membranes have the potential to surpass the equilibrium conversion by selective removal of one of the reaction products, usually hydrogen. Palladium-based membranes have the potential to separate hydrogen from gas streams due to the unique permeation properties of palladium metal to hydrogen. However, thin films of the metal are required in order to obtain the necessary flux; this is usually achieved by deposition onto a porous substrate. Because of its simplicity and low cost, electroless plating was used in this work to deposit paladium as a thin film onto a porous Al2O3 substrate. The prepared membranes were characterized by SEM method. Their thickness was found to be 9 μm. Permeation experiments were performed at 573–648 K temperature range. Hydrogen fluxes approximately 0.35 mol m−2 s and hydrogen/nitrogen selectivities of 5,500 were obtained. Measurements of the temperature coefficient for hydrogen permeation through the final repaired membrane gave a value of 10.25 kJ mol−1 in good agreement with previous reports. The pressure exponent value of 0.5 was obtained. The prepared membranes could be used in the membrane reactor in order to perform dehydrogenation of methylcyclohexane reaction, which was investigated in a fixed bed catalytic reactor previously.

Hydrogen Affinity of Silica-based Nanocomposite for High Temperature Hydrogen Separation Membranes

AbstractHydrogen Affinity of Silica-based Nanocomposite for High Temperature Hydrogen Separation Membranes Japan Fine Ceramics Center, 2-4-1, Mutsuno, Nagoya, 456-8587, Japan Yumi H. Ikuhara, Tomohiro Saito, Koji Hataya, Yuji Iwamoto and Seiji Takahashi Because of concerns about global warming, increasing attention is being directed to find an alternative to fossil hydrocarbon fuels and hydrogen is rapidly becoming one of the leading candidates. For hydrogen production, high temperature membrane reactor is applicable by simplify the process of producing hydrogen from natural gas and purifying it by combining these process into single step. Among the materials, ceramic membranes with molecular sieve-like properties have been expected for application in membrane reactors for conversion enhancement in dehydrogenation and methane reforming reactions. Amorphous silica (Si-O) membranes prepared by sol-gel method have been intensively studied as molecular sieve membranes for gas separation at high temperature. To enhance the hydrogen permselectivity, we have developed Ni nanoparticle-dispersed amorphous Si-O based composite membrane through the precursor solution method and achieved higher hydrogen permeance compared to helium and nitrogen at 573K to 773K. In order to understand the phenomenon of the high hydrogen permeance of the novel nanocomposite membrane, it is important to clarify the expected high temperature hydrogen affinity, i.e., hydrogen adsorption properties. Here, the relationship between microstructure and hydrogen affinity of the nanocomposite was intensively studied from the view point of concentration of Ni nanoparticle in the amorphous Si-O matrix and reversible hydrogen adsorption property. Ni nanoparticles with about 3 to 5 nm in size were homogeneously dispersed in the amorphous Si-O matrix, and the Ni nanoparticles reached to saturate in the Si-O matrix with Ni/(Si+Ni) ratio of 0.2. The reversibly adsorbed hydrogen was hardly detected on the amorphous Si-O and Ni at 573 K, while Ni nanoparticle-dispersed amorphous silica apparently exhibited reversible hydrogen adsorption property. There was appreciable pressure dependence of the reversible hydrogen adsorption on the composite. Further study of the relationship between the increase amount of the reversibly adsorbed hydrogen (Vr) and Ni content on the composite powders revealed that the Vr gradually increased with increasing the Ni content and the highest Vr was ascertained for the composite with the Ni/(Si+Ni) ratio of 0.2. Combining the results of the unique hydrogen permeance through the composite membrane and the hydrogen affinity in the composite powder, we conclude that the existence of reversibly adsorbed hydrogen due to the extensive dispersion of Ni nanoparticles in the Si-O-based membrane involve the enlargement of the number of solubility site for hydrogen, which resulted in the selective enhancement in the hydrogen permeance of the nanocomposite membrane. Contact persons: Y

Integration of H 2-separating membrane technology in gas turbine processes for CO 2 capture

The paper describes the promising possibility to capture CO 2 in natural gas fired power cycles through the use of a high-temperature membrane reactor of the methane-steam reforming type with an integrated H 2-separating membrane. Two types of membranes are investigated: Pd membranes, which could allow for zero-emission power cycles, and microporous membranes, the use of which in the present work means that 20% of the generated CO 2 is emitted to the atmosphere. The membrane reactor is a complex component and much further work is required to find optimum power cycle operating conditions. Combined cycles with an integrated membrane reactor were investigated and the thermal efficiency in some of these non-optimized cases was superior at 47%.

Thermal Evolution of the Structure of a Mg−Al−CO3 Layered Double Hydroxide: Sorption Reversibility Aspects

In our prior study, we employed several in-situ techniques to investigate the thermal evolution of the structure of a Mg−Al−CO3 layered double hydroxide (LDH) under an inert atmosphere and proposed a model to describe the structural evolution of the Mg−Al−CO3 LDH structure. In this paper we present results of our ongoing investigations with these materials pertaining to their sorption characteristics and thermal reversibility under both inert and reactive atmospheres. The experimental observations are shown to be consistent with the structural model for the LDH previously proposed. The structure, sorption characteristics, and thermal reversibility of these materials are of importance in their use for the preparation of CO2-permselective membranes for high-temperature membrane reactor applications.

Reactor and Technical Feasibility Aspects of a CO2 Decomposition-Based Power Generation Cycle, Utilizing a High-Temperature Membrane Reactor

The technical feasibility of separating and decomposing CO2 in the context of a novel energy generation cycle during the operation of a glass furnace was investigated. The concept envisions CO2 decomposition to take place in a membrane reactor utilizing high-temperature solid-oxide membranes with the aid of oxygen chemical pumping. The glass furnace environment is well suited for such an application because it provides the high-temperature conditions needed for the operation of these membranes. As part of this effort, solid-oxide membranes were prepared and their oxygen permeation was measured in a membrane reactor system. The O2 permeabilities in the presence of a reactive sweep were significantly higher than the corresponding permeabilities measured in the presence of He as a sweep gas. These membranes were utilized for CO2 decomposition in a membrane reactor. Conversions significantly higher than the expected thermodynamic equilibrium values were attained. Engineering calculations of the overall glass furnace efficiency were also carried out, and the conditions for optimal performance were determined. The effects of the application of this novel cycle on glass furnace operation in terms of combustion stability (quantified by the extinction strain rates of the relevant combustion mixtures) and pollutant (NOx) emissions were also determined. Generally, improvements in combustion intensity and stability can be achieved but at a cost of increased NOx emissions and vice versa. A range of conditions also exists for which the cycle allows for effective waste heat utilization without undue impact on pollutant emissions.

High temperature membrane reactors for clean productions

The coupling of reaction and separation in the same device (membrane reactor) has been demonstrated to be an interesting way to enhance the performance of several reactions. In particular, by-products can be separated, side-reactions can be avoided and higher yields and conversions can be achieved. This means that the same conversions of traditional reactors can be obtained at more mild conditions (e.g. lower temperatures and pressures) with consequent energy saving and, thus, environmental benefits. Low temperatures are preferred also because they increase the life-time of the catalyst (it can be used for much longer time before it has to be regenerated) and reduce the tendency of both products and reactants to degrade (to avoid undesired side-reactions), which might lead to a loss of yield. Membrane reactors, then, seem to constitute a possible means for reducing waste production. In this paper the potential role of high temperature membrane reactors in clean productions is presented and discussed.

Membrane performance: the key issues for dehydrogenation reactions in a catalytic membrane reactor

In a high-temperature membrane reactor, one of the reaction products is selectively removed from the reaction mixture, thus preventing the mixture from reaching equilibrium. In a previous study [1], a CVI-silica membrane was used for the direct dehydrogenation of propane in a high-temperature catalytic membrane reactor. This H 2 selective membrane had only a moderate permeation (∼140 × 10 −9 mol/m 2Pa s) and a limited H 2/C 3H 8 permselectivity ( α 0 ≈ 70–90 at 500°C). These experiments proved that (at 500°C) the propane conversion could be improved from the equilibrium value (∼18%) to a value which is about twice as high. The increase was however only significant for relatively small values of the propane feed stream ≤16.5 μmol/s. This is because at high propane feed, the hydrogen cannot be removed fast enough through the membrane and conversion is again limited by the thermodynamic equilibrium. In this study, the comparison is made between the performance of the CVI-silica membrane and a Pd/Ag membrane when used as the H 2 selective membrane. The performance of the Pd/Ag membrane is far superior to the performance of the SiO 2 membrane. H 2 fluxes of more than 0.1 mol/m 2s were measured and the H 2/Ar permselectivity exceeds 4500. When it is run under comparable conditions, the performance of the Pd/Ag membrane reactor is much better. The increase in propane conversion persists at values of the propane feed stream that are about six times higher (105 μmol/s). Since the H 2 is selectively removed from the reaction mixture, it is not available for any competitive side reactions. The production of methane, which limits the propene selectivity of the reaction in a conventional plug-flow reactor, is much less in a catalytic membrane reactor. This means that the selectivity in the membrane reactor is higher than in the plug-flow reactor when they are run under similar conditions.

High-temperature membrane reactors: potential and problems

The most recent literature in the field of membrane reactors is reviewed, four years after an analogous effort of ours (Saracco et al., 1994), describing shortly the potentials of these reactors, which now seem to be well established, and focusing mostly on problems towards practical exploitation. Since 1994, progress has been achieved in several areas (sol–gel deposition of defect free sol–gel derived membranes, reduction in thickness of Pd membranes, synthesis of zeolite membranes) whereas stagnation was noticed in some others (high-temperature sealing of membranes into modules, scaling-up of membrane reactor, etc.). As a result, despite the still increasing research efforts, industrial application does not seem to be round the corner, yet. However, several non-permselective membrane reactor opportunities with currently available membranes might pave the way for more sophisticated applications.

A COMPARATIVE EVALUATION OF HIGH-TEMPERATURE MEMBRANE SYSTEMS FOR CATALYTIC PROCESSING

An investigation has been carried out using mathematical simulation to evaluate the relative performance of various composite membranes incorporated in high-temperature membrane reactors. Two membrane categories (porous and dense) and six types of composite membrane systems (Pd/Ag, polyimide, silica, inert porous and Ru-dispersed porous silicalite zeolites and carbon molecular sieve) have been compared. Also considered is the special case of the Pd/Ag composite system having imperfections (pinholes and cracks). The industrially important reaction of ethylbenzene dehydrogenation over promoted iron oxide catalyst using rate constant values from the literature has been studied as the model reaction. Important characteristic results have been obtained which show that while the dense systems possess higher performance levels at lower membrane thicknesses, the porous composite systems especially those containing highly dispersed active metal particles within their micropores can have advantages due to their significantly higher contact surface to volume ratio. The study has also confirmed that high hydrogen permselectivity is a key factor in determining reactor performance in terms of conversion enhancement.