The capability of membranes to affect differently, both qualitatively and quantitatively, the transport rates of chemical species of dissimilar chemical structure through their interior space renders them attractive for use in many separation problems. Extensive research efforts have thus been undertaken on the preparation and characterization of membrane materials and the study of the transport processes involved in their use in separation applications. The study of the transport of gaseous species through the pore space of porous membranes and the analysis and understanding of the mechanisms that are involved in this process are a very important, if not the most important, element in the development of membranebased separation processes.
The resistance that a gaseous species encounters as it is transported through the pore space of a porous membrane is a function of its molecular properties, of its interaction with the material that makes up the walls of the pores, and of the membrane pore structure. Gaseous transport in pores can take place through various mechanisms, whose contribution to the overall transport rate of a particular species is, in general, determined by the strength of the interactions of the molecules of that species with the pore walls and by the relative magnitudes of three length scales that characterize the molecular size, the distance between pore walls, and the density of the fluid in the pore space.