Surface Flow:

Permeation rates of gaseous molecules by the membranes might be enhanced in some cases through surface diffusion mechanism. In surface diffusion, gas molecules adsorb to the surface of the pores of the membranes and then diffuse along the pore walls. High selectivity could be achieved within cases whereas preferential adsorption of one of the elements occurs. Surface flow is the result of solution diffusion mechanism and is used to define the preferential permeation rate of a few gaseous elements by membranes from a given feed combination and is same to preferential sorption capillary flow mechanism described previously for liquid feed systems. Separation of condensible gases and vapours from noncondensible permanent gases by membranes is frequent explained through surface flow. Such a separation is carried out at a suitable temperature and pressure near or below the critical temperature of the gas. At high pressures, multilayer adsorption of gas molecules occurs at the pore walls reducing the size of the pores. At a particular pressure, the whole pore volume is filled along with the permeating gas and the gas molecules are said to have condensed within the membrane pores, efficiently blocking the passage of permanent gases. Like a phenomenon is known as capillary condensation.