Introduction and Important Concepts

4 PERMEATION AND SEPARATION IN MICROPOROUS MEMBRANES 9,4.1 Introduction and Important Concepts 

Existing ceramic, mesoporous membranes (with a 4 nm pore diameter) perform most gas separations according to Knudsen diffusion. The obtainable separation factors (Section 9.3.2.) are usually not economical for most gas separations and provide incremental but limited conversion enhancement in catalytic membrane reactor applications. Capillary condensation and preceding surface flow yield economically interesting separation factors but this mechanism is limited to easily condensable gases and is limited to rather low pressure drops due to stability problems (Sections 9.2.3. and 9.3.3.). 

To enhance the separation factor the average pore diameter should be decreased considerably. According to Eqs. (9.9a) and (9.15) the contribution to the total gas flux of the gas (Knudsen) diffusion decreases and at the same time that of surface flow (diffusion) increases with decreasing pore radius. In recent years modification of existing membranes for improving their separation efficiency has been actively pursued especially by attempts to decrease the pore size of membranes. This resulted in different types of microporous membranes. According to lUPAC convention these are porous systems with a pore diameter below 2 nm. In the literature the name microporous is frequently misused and this should be avoided. 

An overview of microporous membrane types is given in Table 9.4. The oldest microporous membranes are based on carbon and are reported by Koresh and Softer in a series of papers from 1980 to 1987 (see overviews in Refs. [6,42]). They are made by pyrolysis of a suitable polymer (hollow fibre) as reviewed by Burggraaf and Keizer [9]. More recently Rao and Sircar [42] developed a new technique. A macroporous graphite sheet was coated with a suitable polymer (latex) which was pyrolysed subsequently. This process was repeated 4—5 times and resulted in a total carbon layer thickness of 2.5 pm with an average pore diameter between 0.5 and 0.6 nm. The membrane has interesting properties (see Section 9.4.3). 

Finally, very recently Linkov and Sanderson et al. [55] modified and improved the method reported by Koresh and Softer and produced flat sheets as well as hollow-fibre systems. 

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