Single component and mixed gas transport in a silica hollow fiber membrane

Abstract

The permeances of gases with kinetic diameters ranging from 2.6 to 3.9 Å were measured through silica hollow fiber membranes over a temperature range of 298 to 473 K at a feed gas pressure of 20 atm. Permeances at 298 K ranged from 10 to 2.3· 10 5 Barrer/cm for CH 4 and He, respectively, and were inversely proportional to the kinetic diameter of the penetrant. From measurements of CO 2 adsorption at low relative pressures, the silica hollow fibers are microporous with a mean pore size estimated to be between 5.9 and 8.5 Å. X-ray scattering measurements show that the orientation of the pores is completely random. Mass transfer through the silica hollow fiber membranes is an activated process. Activation energies for diffusion through the membranes were calculated from the slopes of Arrhenius plots of the permeation data. The energies of activation ranged from 4.61 to 14.0 kcal/mol and correlate well with the kinetic diameter of the penetrants. The experimental activation energies fall between literature values for zeolites 3A and 4A. Large separation factors were obtained for O 2 N 2 and CO 2 CH 4 mixtures. The O 2 N 2 mixed gas separation factors decreased from 11.3 at 298 K to 4.8 at 423 K and were up to 20% larger than the values calculated from pure gases at temperatures below 373 K. Similar differences in the separation factors were observed for CO 2 CH 4 mixtures after the membrane had been heated to at least 398 K and then cooled in an inert gas flow. The differences between the mixture and ideal separation factors is attributed to a competitive adsorption effect in which the more strongly interacting gases saturate the surface and block the transport of the weakly interacting gases. Based on Fourier transform infrared (FTIR) spectroscopy results, this unusual behavior is attributed to the removal of physically adsorbed water from the membrane surface.