Overview of Important Points

In systems consisting of a macro-and/or mesoporous support and a meso-or microporous separation (top) layer, the permeation is a system property and the driving force for transport is distributed over the system components. In studying the permeation and separation properties of the top layer, corrections must be made on the permeation of the total system to find that of the top layer, unless it is shown that the flow resistance of the support is negligible compared to that of the top layer. Even when the permeation of the support is much larger 

Almost all physical models use simple pore geometries. Practical pore systems are, however, very complicated and contain parameters which are difficult to measure or which have a wide distribution of their characteristic parameters. The applicability of a rigorous treatment and of very refined models and physical expressions is therefore doubtful. The treatment in this chapter will make use mainly of phenomenological equations which allow description of data, data reduction and some extrapolation and which rely on experimentally determined parameter values. Gas kinetic theory and expressions based on the microscopic (atomic) level will be used only to estimate some parameter values and to predict trends. 

For practical applications a combination of high selectivity and high permeation is required. As will be shown below, these two requirements are more or less contradictory and so an optimal compromise has to be sought. In this chapter a certain focus will be given to mechanisms with a large potential for high separation factors and at least reasonable permeation values. This leads to microporous systems or capillary condensation type of phenomena. 

Complete membrane systems can be operated in a variety of modes with e.g. CO- or counter flow of feed (high pressure side) and permeate (low pressure side) streams and with membrane modules coupled in different ways. Permeation and separation in these complex engineering systems will not be treated in this chapter. Heat and mass transfer limitations on the gas-membrane surfaces or interfaces can be important with high fluxes and/or strongly adsorbing gases as well as in membrane reactors. These effects will not be treated explicitly but are introduced in experimental results, e.g., by variation of sweep rates of permeated gases. 

---