Gas permeation through composite membranes

Composite gas separation membranes generally consist of a selective, ultrathin top layer backed by a nonselective porous support. The top layer performs the separation and the support provides mechanical strength. A composite membrane will possess a selectivity close to the intrinsic selectivity of the top layer only if most of the permeation resistance lies within this top layer. To make high flux composite membranes, it is necessary to minimize the thickness of the selective layer. This, in turn, means the porous support must be very permeable. A simple model shows that the minimum thickness of a defect-free selective layer in a composite membrane having the intrinsic selectivity of the permselective layer is limited by the resistance of the porous support membrane. The model is supported by experimental gas and vapor permeation data for polysulfone-silicone rubber composites.