Ceramic membranes characteristics

The interest in ceramic membranes grew, together with the interest in membrane separation processes, due to their specific properties. They are chemically stable, can withstand high temperatures and are noncompressible. These characteristics made them the only materials available, which could withstand the harsh environment in the isotope separation. On the other hand, the brittleness of most materials is a problem and so is the selectivity. 

The latter two modes circumscribe the role of ceramic membranes in membrane reactors. In the case of membrane reactor applications, the ceramic membranes must be crack-free. In addition, it is desirable for the membrane to exhibit good permselectivity characteristics. 

Ceramic membrane is the nanoporous membrane which has the comparatively higher permeability and lower separation fector. And in the case of mixed gases, separation mechanism is mainly concerned with the permeate velocity. The velocity properties of gas flow in nanoporous membranes depend on the ratio of the number of molecule-molecule collisions to that of the molecule-wall collision. The Knudsen number Kn Xydp is characteristic parameter defining different permeate mechanisms. The value of the mean free path depends on the length of the gas molecule and the characteristic pore diameter. The diffusion of inert and adsorbable gases through porous membrane is concerned with the contributions of gas phase diffusion and sur u e diffusion. 

Because of the unique characteristics of inorganic membranes mentioned above, the search for inorganic membranes of practical significance has been continuing for several decades. With the advent of ceramic membranes with superior stabilities coming to the separation markets, the potentials for inorganic membranes as separators and/or reactors are being explored at an accelerated rate never wimessed before. 

A membrane can be defined as a thin and selective barrier that enables the transport or the retention of compounds between two media. In the case of ceramic membranes, the usual driving force for transport is a pressure gradient between the feed and strip compartments (transmembrane pressure). The treated phases can be liquid or gas. For porous membranes, the pore size mainly manages the cutoff of the membrane. However, for retention of the smallest entities by the smallest pores, the transport mechanisms are more complex than simple sieving. Specific physical and chemical interactions (electrostatic repulsion, physisorp-tion, capillary condensation, etc.) become preponderant and determine the membrane selectivity. Table 25.1 summarizes the characteristics of the main processes in which ceramic membranes are involved. 

Characteristics of the Main Processes in which Ceramic Membranes Can Be Used CD O... 

Silver membranes are permeable to oxygen. Metal membranes have been extensively studied in the countries of the former Soviet Union (Gryaznov and co-workers are world pioneers in the field of dense-membrane reactors), the United States, and Japan, but, except in the former Soviet countries, they have not been widely used in industry (although fine chemistry processes were reported). This is due to their low permeability, as compared to microporous metal or ceramic membranes, and their easy clogging. Bend Research, Inc. reported the use of Pd-composite membranes for the water-gas shift reaction. Those membranes are resistant to H2S poisoning. The properties and performance characteristics of metal membranes are presented in Chapter 16 of this book. 

Since their early development, the geometrical and structural characteristics of ceramic membrane elements have readily changed (Figure 6.2). Originally, they were prepared as single tubes with an inside diameter ranging from 6 to 15 mm and a wall thickness of about 2 mm. These ceramic tubes are still available from some suppliers, but the main handicaps with such... 

Characteristics of Sunflower CeRam Inside Ceramic Membrane... 

C. J. Brinker and G.W. Scherer, Sol-Gel Science. Academic Press, New York, 1990. A.F.M. Leenaars, Preparation, structure and separation characteristics of ceramic membranes, PhD Thesis, 1984, University of Twente, Enschede, The Netherlands. 

Characteristics of commercially available monolith ceramic membranes... 

Several characteristics of ceramic membranes, resulting from metal oxide grains sintering, are worth underlining ... 

Typical characteristics of both porous and dense inorganic membranes are given in Table 14.1. Only applications with porous ceramic membranes will be dealt with in this chapter. 

Permeation characteristics of Knudsen diffusion membranes, consisting of a support and two consecutive layers, have been used to calculate the performance of the ceramic membrane reactor, see also Section 14.2.1 [17,31]. The pore size of the separation layer of these membranes is 4 nm in diameter [31,38]. Ideal membranes which remove all the hydrogen formed do not exist (possible Pd-based membranes will come close to the required characteristics), but are used as a basis for calculating the maximum possible increase in conversion and selectivity. 

C. B. Ersu and S. K. Ong, Operating Characteristics and Treatment Performance of a Membrane Bioreactor Using Tubular Ceramic Membrane, IWA Environmental Biotechnology Advancement on Water and Wastewater Applications in the Tropics, Kuala Lumpur, Malaysia, December 9-10, 2003. 

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