Surface Effects on Permeation in Microporous Membranes

In the preceding discussion it was assumed that the transfer from molecules from the gas phase to the solid (porous) membrane was not the rate-determining step in the permeation. This assumption will be evaluated in this section because in oxygen permeation of dense oxidic membranes surface reactions become clearly rate determining for several groups of materials (see also Chapter 10). 

De Lange [63] used a gas kinetic expression to estimate the total number of molecules colliding per second with the walls of a volume  

At 1 atm and 300 K the calculated value of Z aii is approximately 1.8 mol cm s . A typical hydrogen flux through the microporous membranes is 10x10 mol cm s (calculated from a permeation of 10x10 mol m s Pa at a pressure difference of 1 bar). 

Not every collision leads to penetration of the molecule into the membrane. This is expressed by the sticking factor t as defined by Eq. (9.65)  

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

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