High Temperature Catalytic Membrane Reactors Research Articles

Pilot scale fabrication of lanthanum tungstate supports for H2 separation membranes

This paper reports the fabrication of asymmetric tubular geometry lanthanum tungstate-based membranes for high temperature H2 separation applications. As starting materials, powders with the compositions La28-yW4+yO54+δ (y = 0.848), La28-yW4+yO54+δ (y = 1) and La28-y(W1-xMox)4+yO54+δ (x = 0.3, y = 1) were synthesized by solid state reaction method. Porous, thick-wall tubes made of lanthanum tungstate-based powders with high gas permeability (7–8 x10-15 m2 at 2 bar overpressure) and homogeneous porous microstructure were made by combining a cost-effective solid-state synthesis and thermoplastic extrusion method. Deposition of lanthanum tungstate-based dense thin film (∼20 μm) was achieved on top of porous supports through dip coating technique. The influence of conformation routes, thermal debindering and sintering processes were investigated in detail to achieve defect free H2 transport membranes. Asymmetric geometry membranes with ∼20 μm dense layer thickness supported on porous tubular (∼ 7.5 mm diameter and ∼0.8 mm thick) supports.As a result, a procedure for proton conducting ceramics manufacture was established which can be deployed to fabricate high temperature catalytic membrane reactors for synthetic fuel from gas-to-liquid (GTL) processes.

On the use of supported ceria membranes for oxyfuel process/syngas production

Ceramic oxygen transport membranes (OTMs) enable selective oxygen separation from air at high temperatures. Among several potential applications for OTMs, the use in (1) oxygen production for oxyfuel power plants and (2) the integration in high-temperature catalytic membrane reactors for alkane upgrading through selective oxidative reactions are of special interest. Nevertheless, these applications involve the direct contact of the membrane surface with carbon-rich atmospheres. Most state-of-the-art permeable membranes are based on perovskites, which are prone to carbonation under operation in CO 2-rich environments and/or decomposition in reducing gas environments. The oxygen flux through supported thin film membranes of Ce 0.9Gd 0.1O 1.95− δ (CGO) with 2 mol.% of cobalt was measured for oxygen separation in oxyfuel processes and in syngas production and degradation was compared to perovskite membranes. The CGO membranes consist of a 27 μm-thick gastight CGO layer supported on a porous CGO substrate. The flat surface of the membrane was coated using two different porous catalytic layers aiming to improve the oxygen activation rate on the permeate side while the porous substrate was infiltrated with an oxygen reduction catalyst. Oxygen separation was studied using air as feed and argon/CO 2 or argon/CH 4 mixtures as sweep gas in the temperature range 750–1000 °C. The supported membrane exhibited a maximum oxygen flux of ca. 5 ml min −1 cm −2 at 1000 °C when diluted methane was used as sweep gas. The CGO membrane showed high stability in CO 2 (in contrast to tests on La 0.6Sr 0.4Co 0.2Fe 0.8O 3− δ (LSCF) membranes) and no detrimental effect on the oxygen flux is observed when CO 2 is present in the sweep gas even at temperatures below 800 °C. Moreover, the SEM analysis showed that membrane integrity remained stable after the permeation tests using CO 2.

Preparation and Characterization of Nanocrystalline Mixed Proton−Electronic Conducting Materials Based on the System Ln6WO12

The tungstates Ln6WO12 are proton-conducting materials exhibiting sufficient electronic conductivity to consider them as potential candidates for the separation of hydrogen at high temperature. Hydrogen-permeable membranes will find application in power plants applying precombustion strategies, process intensification using high-temperature catalytic membrane reactors, and in components for electrochemical systems as proton conducting fuel cells (PCFCs) and electrolyzers. This work presents the preparation and characterization of nanocrystalline mixed conducting materials with three different nominal compositions (Nd6WO12−Eu6WO12−Er6WO12) using a sol−gel complexation synthesis method. The evolution of the crystalline structure and crystallite size is studied as a function of the sintering temperature. Generally, the nanosized oxides show a (pseudo)-cubic crystalline fluorite structure which evolves into the most stable fluorite symmetry (tetragonal for Nd and rhombohedral for Er) with increasing sintering...

Computational Fluid Dynamics Study on the Decomposition of Ammonia in a Selective Porous Membrane

The development of alternative technologies for the removal of gas pollutants is an important aspect for the environmental friendliness of energy production. During coal gasification, N2 contained in coal is converted to NH3 and, as much as 50% of the ammonia in the fuel gas can be converted to nitrogen oxides (??x). At these conditions, decomposition seems to be the only applicable solution for the removal of NH3. The application of a high temperature catalytic membrane reactor process appears to offer an efficient and cost effective method of removing the NH3 from coal gasification gas streams.The present work examines the operation of such a selective membrane, used for the decomposition of NH3, under a 2-D axissymetric CFD approach where the flow field, the chemical reactions and the selective porous membrane behavior are being modeled and computed. The main target of this effort was to obtain a more detailed view of the flow field and to investigate the decomposition of ammonia in comparison with a simpler 1-D modeling approach and, thus, to evaluate the advantages and disadvantages of each method.

Zeolite based films, membranes and membrane reactors: Progress and prospects

The integration of reaction and separation in catalytic membrane reactors has received increasing attention during the past 30 years. The combination promises to deliver more compact and less capital-intensive processes with substantial savings in energy consumption. With the advent of new inorganic materials and processing techniques, there has been renewed interest in exploiting the benefits of membranes in many industrial applications. Zeolite membranes, however, have only recently been considered for catalytic membrane reactor applications. Despite the significant recent interest in these types of membranes there are relatively few reports of the application of such membranes in high-temperature catalytic membrane reactor applications. This can be attributed to a number of limitations that still need to be addressed such as the relatively high price of membrane units, the difficulty of controlling the membrane thickness, permeance, high-temperature sealing, reproducibility and the dilemma of upscaling. A number of research efforts, with some degree of success have been directed to finding solutions to the remaining challenges. This review makes a critical assessment of what has been achieved in the past few years in terms of hurdles that still stand in the way of the successful implementation of zeolite membrane reactors in industry.

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.

Equilibrium-shift in alkane dehydrogenation using a high-temperature catalytic membrane reactor

This study involves the application of a tubular fixed-bed catalytic membrane reactor to effect equilibrium-shift during the catalytic dehydrogenation of ethane to ethylene and hydrogen. The specific characteristic behaviour of the reactor to shift the equilibrium was analysed using a two-dimensional mathematical model which was solved using the orthogonal collocation method. An important extrinsic variable, the time factor (W/F Ao) was manipulated to yield results which were then used to explain reactor performance. Experimental results showed that under optimal conditions an eight-fold shift in the equilibrium conversion could be attained. Generally, good agreement was obtained between model and experimental predictions for reactor operation using pure nitrogen as sweep gas. When air was employed as sweep, the agreement was not as expected; possibly due to oxidation of the Pd surface.

Ethane dehydrogenation using a high-temperature catalytic membrane reactor

Experiments have been conducted for the dehydrogenation of ethane to ethylene using a high-temperature catalytic membrane reactor under isothermal conditions. Typically, conversions of up to 7 times (cocurrent mode) and 8 times (countercurrent mode) higher than the equilibrium value achievable in conventional fixed-bed reactors have been attained at high sweep flow rates. The membrane used in this study was a thin layer of Pd-23 wt% Ag on porous Vycor glass. The significant improvement in the ethane conversion is attributed to the exclusive and continuous permeation of hydrogen through the membrane.

A high temperature catalytic membrane reactor for propane dehydrogenation

High temperature catalytic membrane reactors are attracting renewed interest. This surge in research activity is motivated by the development of good quality ceramic membranes. In this paper, results are presented of a study of the propane dehydrogenation reaction in a membrane reactor utilizing a sol-gel alumina membrane for feed mixtures containing significant amounts of propylene and hydrogen. Yields to propylene are reported which are higher than the corresponding equilibrium yields at the same temperature and pressure conditions.

A high temperature catalytic membrane reactor for propane dehydrogenation

High temperature catalytic membrane reactors are attracting renewed interest. This surge in research activity is motivated by the development of good quality ceramic membranes. In this paper, results are presented of a study of the propane dehydrogenation reaction in a membrane reactor utilizing a sol-gel alumina membrane for feed mixtures containing significant amounts of propylene and hydrogen. Yields to propylene are reported which are higher than the corresponding equilibrium yields at the same temperature and pressure conditions.

A high temperature catalytic membrane reactor for ethane dehydrogenation

A high temperature catalytic membrane reactor, containing a Pt impregnated alumina ceramic membrane tube in a shell-and-tube configuration, was used to study dehydrogenation reactions. Experiments in this membrane reactor in the temperature range of 450–600° C, with the ethane dehydrogenation reaction to produce ethylene, show reactor conversions up to 6 times higher than equilibrium conversions. This shift in equilibrium is due to the selective permeation of one of the reaction products, i.e. hydrogen, according to Knudsen diffusion. In the experiments we have utilized a trans-membrane pressure difference and an inert sweep gas on the low pressure side of the membrane.