A mechanistic study of molecular sieving inorganic membranes for gas separations. Final report, [August 1, 1992--July 31, 1993

Abstract

The permeability of gases with kinetic diameters of 2.89 {Angstrom} to 3.9 {Angstrom} were measured through microporous silica hollow fiber membranes at 298 K to 473 K at 20 atm feed gas pressure. Permeabilities ranged from 0.01 Barrer to 1000 Barrer and were inversely proportional to the kinetic diameter of the penetrant. Linear Arrhenius plots were obtained from permeability data activation energies ranged from 7.5 to 13.5 kcal/mole. The activation energies for the hollow Tiber membranes were consistent with literature data for diffusion of hydrocarbons in zeolites. High separation factors were obtained for O{sub 2}/N{sub 2} and CO{sub 2}/CH{sub 2} mixtures. The mixture separation factors for O{sub 2}/N{sub 2} decreased from 11.5 at 298 K to 4.6 at 423 K. Over the same temperature range, CO{sub 2}/CH{sub 2} separation factors decreased from 186 to 22.3. Separation factors for O{sub 2}/N{sub 2} mixtures were up to 20% larger than the values obtained from pure gas measurements below 373 K. A similar effect was seen for CO{sub 2}/CH{sub 2} mixtures after the membrane had been heated to at least 398 K and then cooled in inert gas flow. Differences between separation factors and ideal gas values is attributed to a competitive adsorption effect where the more strongly interacting gases (O{sub 2}, CO{sub 2}) saturate the surface and block the transport of weakly interacting gases (N{sub 2}, CH{sub 4}).