Fluid-membrane separation

A process for the separation of mixture components uses a supercritical fluid that is fresh to at least one side of a nonporous membrane after which the fluid is either recycled to the opposite side of the membrane or removed from the separation process. Fluids and membranes, including modified membranes, are provided to facilitate passage of one or more components of a mixture through the membrane while retarding passage of other components of the mixture. The process is useful for the separation of mixtures with similar components including axeotropic mixtures and pharmaceutical and biological products.