Gas separation by thin porous silica membrane of ultra fine pores at high temperature.

Abstract

For the purpose of gas separation at high temperature, a thin porous silica membrane of ultra fine pores has been prepared by the sol-gel method, where silica sol was coated near the outer surface of a porous cylindrical substrate of α-alumina, then it was dried and fired at around 450°C.Permeability measurements for H2, He and N2 through the silica membrane were carried out at various temperatures (30-270°C) to show that the permeabilities of H2 and He, which increased with the increase of temperature, were 30-300 times larger than that of N2, and that the permeability of He was appreciably larger than that of H2, These results clearly show that the activated diffusion or permeation prevails instead of the Knudsen diffusion through the silica membrane, while the permeability of N2 showed a negative temperature dependency, which is probably due to the Knudsen diffusion through some pin holes still left in the membrane. The activation energy for HZ and He were estimated to be 8.3kJ/mol and 4.0kJ/mol respectively.Separation results for H2/N2 and He/N2 mixtures at 270°C showed that large separation ratio could be obtained, and also that each gas component permeated independently without any molecular interactions among permeating molecules in the fine pores of the silica membrane, Which makes possible an estimation of separation performance from the permeation results for pure gases.