Separation in the Knudsen and Transition Regions

As discussed above, the ideal separation factor a in the case of pure Knudsen diffusion is given by Eq. (9.37) and is equal to the permselectivity provided that surface diffusion is not present (high temperature). As can be seen from (9.37) the highest ideal separation factors are obtained for mixtures of light and heavy gases. Back-diffusion effects are taken into account by Eq. (9.38) to give the real separation factor. 

The support can have a considerable influence on the separation factor of the membrane consisting of separation layer and support when its flow resistance is not negligible and the gases in the support pores are in the transition or viscous flow regime [20]. This point will be discussed in Section 9.5. 

In the transition region intermolecular momentum transfer decreases the separation factor considerably. The effects of the pressure ratio P, with feed pressure as a parameter of temperature of pore size and of concentration, are analysed by Wu et al. [18] and by Eichmcinn and Werner [19]. 

Wu et al. used Eq. (9.34) to simulate the permeate composition and separation factor for H2/N2, H2/C0 and He/O2 gas mixtures and compared them with experimental results obtained on a Membralox asymmetric membrane system. 

9 — TRANSPORT AND SEPARATION PROPERTIES OF MEMBRANES WITH GASES AND VAPOURS 

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