Polymer Membranes for Hydrogen Separations

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Polymer Membranes for Hydrogen Separations John D. Perry, Kazukiyo Nagai, and William J. Koros Abstract The development of a hydrogen-based economy would generate a substantial necessity for efficient means of collecting hydrogen with a relatively high purity. Membrane separations play a major role in the separation of hydrogen gas from various gas mixtures, and this article discusses the use of polymeric materials to produce these membranes. After a review of the historical use of polymeric membranes and some background information regarding mechanisms of gas transport in membranes, this article will review the work that has been done in the two major classes of hydrogen separation membranes: hydrogen-selective membranes and hydrogen-rejective membranes. In hydrogen-selective membranes, the very small size of the hydrogen molecule is exploited to allow rapid diffusion of hydrogen through the membrane while excluding other gases. Hydrogen-rejective membranes use the significantly higher sorption of other gases to overcome the advantages of the small size of the hydrogen molecule. The discussion of these two types of membranes will be followed by a presentation of the current state of the art with regard to polymeric membranes for hydrogen separation and a discussion of the predictions for future applications and advancements in this area. Keywords: hydrogen, membrane, polymer.

Introduction As early as 1950, membranes were considered for a variety of gas separations such as the removal of oxygen from air, helium from natural gas, and hydrogen from petroleum refinery gas.1 Still, it was nearly 25 years later before commercial membranes that could economically perform these separations were developed,2 with hydrogen recovery being one of the first widespread applications of gas separation membranes.3 These hydrogen separations include adjustment of the H2/CO ratio in synthesis gas (a mixture of hydrogen and carbon monoxide that is often obtained from the steam reforming of natural gas or the gasification of coal), removal of hydrogen from various hydrocarbon streams, and the removal of hydrogen from purge gases in ammonia production and other petrochemical processes.2–6 Membranes compete with pressure swing adsorption (PSA) and cryogenic MRS BULLETIN • VOLUME 31 • OCTOBER 2006

systems in hydrogen recovery separations. PSA uses special adsorbents, such as zeolites, to preferentially adsorb the undesired components at high pressures, thereby purifying the hydrogen gas. Cryogenic systems utilize very low temperatures to selectively condense undesired components to purify hydrogen. Since the late 1980s, membranes have been competitive with these other technologies over a wide range of operating conditions.2 This review will take a look at the work that has been done with polymeric membranes

for hydrogen separations and then suggest some desirable directions for the future development of technology in this area.

History of Polymeric Membranes for Hydrogen Separations Despite earlier considerations of the use of membran

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