Theories of permeation processes

The permeation of gases through polymer membranes depends upon whether the membrane is porous or dense. If the membrane is porous, the gas flow is predominantly controlled by the mean free path of the gas molecules and the pore size. The mechanism of flow through porous membranes has been discussed elsewhere. In short, the gas flow regime is contributed to by Poiseuille flow and Knudsen flow, the amount of each contribution being defined by pore size, pressure, viscosity and the molecular weight of the gas involved. However, microporous membranes exhibit low gas selectivity, as shown earlier in the resistance model approach. 

For more efficient separations, dense polymer membranes are used, although they will usually have a high microporous substructure. In dense polymer membranes, gas flow across the membrane is via a solution diffusion mechanism. In simple terms, the gas is sorbed onto the polymer, then diffuses down a concentration gradient via Pick s law (Eq. (11.1)) and finally desorbs from the low pressure side of the membrane. A large number of publications have been written on the subject of the mechanism of gas permeation, and some key arguments have been described elsewhere. 

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