Relationship between Substructure Resistance and Gas Separation Properties of Defect-Free Integrally Skinned Asymmetric Membranes

Integrally skinned asymmetric membranes consist of an ultrathin skin layer supported by a microporous substructure. Optimum membrane properties are obtained if the skin layer is defect-free and its thickness is minimized. A series resistance model shows that the minimum desirable skin thickness of defect-free integrally skinned asymmetric membranes having the intrinsic selectivity of the membrane material is limited by the resistance of the microporous substructure. Asymmetric membrane substructures show selectivities essentially equal to those predicted by Knudsen flow. However, pressure-normalized fluxes of substructures can vary by several orders of magnitude. Model calculations reveal that the pressure-normalized flux of the fast-permeating gas component through the substructure has to be ~ 10 times higher than that of the component through the skin layer to achieve at least 90% of the intrinsic selectivity of the membrane material. The validity of the model is demonstrated for defect-free asymmetric polycarbonate and polyimide membranes. These considerations become increasingly important as higher performance polymeric materials are employed in membrane separation applications.