Gas Separation

Gas separation only became a major industrial application of membrane technology in the past 30 years, but the study of gas permeation through membranes has a long history. Systematic studies began with Thomas Graham who, over a period of 20 years, measured the permeation rates of all the gases then known, through every diaphragm available to him [1]. This was no small task because his experiments had to start with synthesis of the gas. Graham gave the first description of the solution-diffusion model, and his work on porous membranes led to Graham’s law of diffusion. Through the remainder of the nineteenth and the early twentieth centuries, the ability of gases to permeate membranes selectively had no industrial or commercial use. The concept of the perfectly selective membrane was, however, used as a theoretical tool to develop physical and chemical theories, such as Maxwell’s kinetic theory of gases. From 1943 to 1945, Graham’s law of diffusion was exploited for the first time, to separate UF6 from U F6 as part of the Manhattan project. Finely microporous metal membranes were used. The separation plant, constructed in Knoxville, Tennessee, represented the first large-scale use of gas separation membranes and remained the world’s largest membrane separation plant for the next 40 years. However, this application was unique and so secret that it had essentially no impact on the long-term development of gas separation. In the 1940s to 1950s, Barrer [2], van Amerongen [3], Stern [4], Meares [5], and others laid the foundation of the modern theories of gas permeation. The solution-diffusion model of gas permeation developed then is still the accepted model for gas transport through membranes. However, despite the availability of interesting polymer materials, membrane fabrication technology was not sufficiently advanced, at that time, to make useful membrane systems from these polymers. The development of high-flux anisotropic membranes and large-surface-area membrane modules for reverse osmosis applications in the late 1960s and early 1970s provided