Membrane Gas Separation: A Critical Overview

Membrane gas separation was one of the earliest applications of microporous membranes, as described originally by Thomas Graham in 1866 (1). However, despite the inherent and desirable advantages of this type of membrane toward high permeability, most of the early work on synthetic membrane gas separations has been carried out with the so-called homogeneous or dense membranes which are not necessarily asymmetric. The latter type of membrane is distinct from the purely homogeneous or nonporous and chemically modified membranes, viz., the vinylidene fluoride and other commercial films reported by Heyd and McCandless (2). The obvious reason for the lack of interest in microporous membranes, which is apparently responsible for their failure to achieve commercial success until afew years ago, is the empirical view that permeability and "permselectivity" are somewhat mutually exclusive or inversely related to each other. It is interesting to note that while virtually all the important engineering and technological concepts were well developed (see, for example, Kim and Kammermeyer (3), Stern and Walawender (4), Pan and Hab Habgood (5), Hwang et al. (6), Sirkar (7) and Schulz et al. (8)), commercial realization came only in the late 1960s (9).