Transport and separation of gases in ceramic membranes

The diffusing flux through different membranes can be adequately described by Tick s law (Equation [8.4]), indicating that gas transport through porous membranes is driven by a cross membrane pressure gradient. Based on the differences in partial pressures, gas diffusivities, molecular sizes and shapes, gases can be separated when they flow through a membrane. 

In principle, the separation properties of a multilayer porous ceramic membrane, such as permselectivity, should be dependent only on the pore size distribution of the top separation layer. However, they can be compromised if resistances in the intermediate layers and the macroporous support become significant. For transport through macro- and meso-pores, molecular diffusion, Knudsen diffusion and viscous flow all contribute to the total transport, while the activated surface flow of the adsorbed phase will affect microporous transport. Therefore, any theoretical models used in analysing the transport data of gases through a porous ceramic membrane with a distributed pore size must take the following contributions into consideration (1) viscous flow, (2) Knudsen flow, (3) surface flow and (4) molecular sieving 

Knudsen diffusion is relevant when the mean free path of the gas molecules is greater than the pore size of the membrane, that is, (r /A) 0.05 (Liepmann, 1961). In this case, the permeating molecule is more likely to collide with the pore wall than with another molecule inside the pore, and the Knudsen diffusivity D can be described by  

Surface flow occurs when the permeating species exhibit a strong affinity for the membrane surface and adsorb along the pore walls at sufficiently low temperature and/or high pressure. This flow regime is generally only 

Flux through microporous membranes incorporates both adsorption and diffusion characteristics and as such the equations developed are modified based on the membrane material and pore structure. For example, the following expression (Equation [8.8]) for permeating flux through microporous silica membranes is accepted as an appropriate description of molecular sieving or activated transport (de Lange et al, 1995c)  

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