Commercial separation of binary mixtures by gas permeation is common. In binary mixtures we can use the substitution
where y without the subscript A or B is understood to refer to the mole frac of the faster permeating species A. In a binary system, the flux of the slower moving component B becomes
Since pressure and mole fracs can vary along the membrane, Eqs. (17-2a), (17-2c), and (17-3b) are applied point by point along the membrane. In almost all systems there is no concentration gradient ...