Materials dense ceramic membranes

Extensively studied is oxygen permeation through dense ceramic membranes (e.g. perovskhes). Temperatures > 600 °C are applied. Here, oxygen splits at the surface and is transited as 0 . Porous membranes include porous polymer films (cellulosics, polyamides) as well as amorphous inorganic materials (alumina, silica). 

The considerations in this chapter were mainly prompted by the potential application of mixed-conducting perovskite-type oxides to be used as dense ceramic membranes for oxygen delivery applications, and lead to the following general criteria for the selection of materials... 

The complex phase diagrams and rich crystal chemistry of the transition metal-containing oxide systems, and great diversity in the defect chemistry and transport properties of mixed-conducting materials known in these systems, make it impossible to systematize all promising compositions in a brief survey. The primary attention here is therefore centered on the comparison of major families of the oxide mixed conductors used for dense ceramic membranes and porous electrodes of SOFCs and other high-temperature electrochemical devices. 

This chapter focuses on the chemical processing of ceramic membranes, which has to date constituted the major part of inorganic membrane development. Before going further into the ceramic aspect, it is important to understand the requirements for ceramic membrane materials in terms of porous structure, chemical composition, and shape. In separation technologies based on permselective membranes, the difference in filtered species ranges from micrometer-sized particles to nanometer-sized species, such as molecular solutes or gas molecules. One can see that the connected porosity of the membrane must be adapted to the class of products to be separated. For this reason, ceramic membrane manufacture is concerned with macropores above 0.1 pm in diameter for microfiltration, mesopores ranging from 0.1 pm to 2 nm for ultrafiltration, and nanopores less than 2 nm in diameter for nanofiltration, per-vaporation, or gas separation. Dense membranes are also of interest for gas... 

Kniep, J., Lin, Y. S. (2011). Oxygen- and hydrogen-permeable dense ceramic membranes. In V. V. Kharton (Ed.), Solid state electrochemistry I, fundamentals, materials and their applications (pp. 467—500). Springer (Chapter 10). 

Pecanac, G., Foghmoes, S., Lipinska-Chwalek, M., Baumann, S., Beck, T., and Malzbender, J. (2013) Strength degradation and iailure limits of dense and porous ceramic membrane materials. /. Eur. Ceram. Soc., 33, 2689-2698. 

Dong, X., Dong, F., Chen, Y., Zhao, B., Liu, S., Tade, M.O., and Shao, Z.P. (2014) Cobalt-free niobium-doped barium ferrite as potential materials of dense ceramic membranes for oxygen separation. /. Membr. Sci., 455, 75-82. 

A detailed discussion of the mathematical models of oxygen flow in ceramic membranes is given elsewhere. Typical materials employed in dense ceramic membranes have a brownmillerite or perovskite structure. The most commonly studied application for this kind of membrane is the catalytic partial oxidation of methane (POM) to obtain synthesis gas,... 

Tan, X. and Li, K. (2013) Dense ceramic membranes for membrane reactors, in Handbook of Membrane Reactors, Volume I - Fundamental Materials Science, Design and Optimisation (ed A. Basile), Woodhead Publishing Limited, Cambridge, pp. 271-297. 

There are three main types of dense ceramic membranes disk/flat sheet, tubular, and hollow fibers. The disk/flat sheet membranes are applied mostly in research work because they can be fabricated easily in laboratories with a small amount of membrane material. Comparatively, the hollow fiber membranes can provide the largest membrane area per volume but low mechanical strength, while the tubular membranes possess a satisfactory specific membrane area, high mechanical strength, and are easy to assemble in membrane reactors. Dense ceramic MRs can be constructed and operated in either packed bed MR or catalytic MR configurations. 

Dense ceramic membranes allow oxygen separation with extremely high selectivity and can be incorporated into membrane reactors for a variety of oxygen-related reactions. The applications of dense ceramic MRs will bring many economic and environmental benefits, with improved selectivity and yields. However, in order to realize the potential benefits of MRs and commercialize them successfully, there are still many challenges that have to be faced not only from membrane materials but also from engineering aspects. 

LaCoOs ceramic exhibits interesting electrical, magnetic, and catalytic properties, being used as cathode material for solid oxide fuel cells (SOFCs), catalysts for light hydrocarbon oxidation, and gas detection sensors. For other applications such as oxygen separation from air and for synthetic gas production, dense ceramic membranes with well-defined microstructure are required. 

Disc/flat-sheet-shaped membranes are mostly applied in dense ceramic membrane reactors due to the ease of the fabrication process the ceramic material powder is pressed into discs in a stainless steel mould under an isostatic or hydraulic pressure, followed by sintering at a high temperature. Such disc-shaped membranes usually have a thickness of about 1 mm so as... 

As described above, dense ceramic membranes are made of composite oxides with a large number of oxygen vacancies in the crystaUine lattice. Such materials are inherently catalytic to the oxidation and dehydrogenation reactions. Therefore, dense ceramic membrane may serve as both catalyst and separator, and catalyst is not required in the membrane reactor. As shown in Fig. 7.5a, the lattice oxygen directly takes part in the chemical reactions. Since the chemical reactions take place on the membrane surface, it is required to have a very porous membrane surface so as to contain a sufficient quantity of active sites. This can be achieved in the membrane preparation process, or by coating a porous membrane material after the preparation. The main potential problems for this are that the membrane may not have sufficient catalytic activity, and the catalytic selectivity cannot be modulated with respect to the considered reactions. 

Ceramic, Metal, and Liquid Membranes. The discussion so far implies that membrane materials are organic polymers and, in fact, the vast majority of membranes used commercially are polymer based. However, interest in membranes formed from less conventional materials has increased. Ceramic membranes, a special class of microporous membranes, are being used in ultrafHtration and microfiltration appHcations, for which solvent resistance and thermal stabHity are required. Dense metal membranes, particularly palladium membranes, are being considered for the separation of hydrogen from gas mixtures, and supported or emulsified Hquid films are being developed for coupled and facHitated transport processes. 

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