Gas Separation:

The concept of separating gases using membranes is more than 100 years old, but the widespread use of gas separation membranes has occurred only within the last decade. Membranes separate gases just if some elements pass by the membranes more readily as compared to others. A classic membrane procedure for gas separation operates along with hydrostatic or partial pressure difference across the membrane as a driving force. The feed gas combination is fed to the membrane at an elevated pressure, whereas it permeates across the membranes. Another side of the membranes is held at a lower pressure. Separation is achieved since of differences in selective permeation rates of the feed gas elements. Components which permeate more rapidly across the membranes become enriched within the permeate stream although the slower permeating elements are concentrated within the residual or the retentate at high pressure.

The degree to which gaseous components are separated is governed by the ability of the membranes to discriminate between the two gases as well as the relative driving force for each component. The solubility of the gases within the membrane matrix and the diffusivity of the dissolved gas molecules are the major governing factors within the selectivity. Several gas separating membranes are considered to be dense membranes. The pore size of the membranes for use within gas separation membranes requires being much smaller than the mean free path of the permeating gas molecules. Details regarding the mechanism included within gas separation by membranes and the relation among mean free path of the gas molecules and membrane pore size will be dealt along with in the next section. Significant applications of gas separation using membranes are in the production of high purity nitrogen from air, oxygen enrichment from air and recovery of helium from natural gas.