ABSTRACT
In principle, gas separations can be performed using membranes functioning on one of three general transport mechanisms: Knudsen diffusion, solution-diffusion, or molecular sieving. However, solution-diffusion-based gas transport through polymeric membranes is used exclusively in current commercial devices. Properly formed high-performance membranes that function based on the solution-diffusion mechanism provide highly efficient separations of a chosen component from a feed gas stream. The five principal types of membrane structures are used to find application for gas separations: integrally skinned asymmetric, multicomponent (or "caulked"), single-layer composite, multilayer composite, and asymmetric composite membranes. Phase separation in a membrane-forming system consisting of a polymer, solvent(s), and nonsolvent(s) can be induced by a change in temperature, solvent evaporation, or solvent depletion and/or nonsolvent addition during a quench step. The J. Gibbs free energy–composition diagram shows the stability limits, i.e., binodal and spinodal, for a binary polymer solution having an upper critical solution temperature as a function of the polymer volume fraction.
