Polymer Membranes with Hydrogen-Selective and Hydrogen- Rejective Properties

Abstract

The first successful gas separation membrane process is hydrogen separation using a polymer membrane in 1980s. So far, polymer membranes have been commercially used to separate hydrogen from ammonia purge gas, coke oven gas, petroleum refining gas, steam reforming gas etc. The impurities include carbon monoxide, carbon dioxide, nitrogen , oxygen, and hydrocarbons. Hydrogen is the smallest molecule among gas mixtures. A hydrogen separation polymer membrane is required to have either a hydrogen-selective property or a hydrogen-rejective property. The former property is based on common size-sieving filtration. The latter one is, however, unusual and it was difficult to design a gas separation membrane for such applications. The transport of gases in a dense polymer membrane is thought to obey the solution-diffusion mechanism. The total permeability is a product of the solubility and the diffu-sivity. Hence, the permselectivity is a product of the solubility selectivity and the diffusivity selectivity. The solubility selectivity of a gas to hydrogen is larger than 1. In contrast, the diffusivity selectivity of a gas to hydrogen is always smaller than 1. The dominant factor to determine the permselectivity is the diffusivity selectivity for hydrogen-selective polymer membranes and the solubility selectivity for hydrogen-rejective polymer membranes. The control of the polymer-gas interaction and/or the size and distribution of the fractional free volume produces a wide variety of hydrogen separation properties in polymer membranes.