Analysis of gas permeability of porous alumina powder compacts

Gas permeation of alumina powder compacts with different porosity, pore size and grain size was examined at room temperature for Ar, N2, H2 and CO2 gases. The flux of Ar, N2 and CO2 gases with Knudsen numbers of 0.4–1.3 was measured above a threshold pressure difference between inlet and outlet gases, and then linearly increased with an increase in applied pressure. The H2 gas with Knudsen numbers of 1.1–2.6 showed a relatively large flux at near 0 MPa/m of pressure gradient and increased linearly with an increase in the pressure gradient. The measured gas permeability coefficients for Ar, N2 and CO2 gases were the same order as the calculated permeability coefficients based on the modified Poiseuille equation (viscous flow) and Knudsen flow equation, suggesting the mixed flow mechanisms. The slope of flux–pressure gradient plot for H2 gas permeation was also in agreement with the calculated permeability coefficients by the modified Poiseuille equation. The H2 gas flow near 0 MPa/m of pressure gradient was characterized by surface diffusion of H2 molecules adsorbed on the pore walls of alumina compacts. It is possible to separate H2 gas from the other gases at room temperature in the pressure gradient range lower than the threshold pressure for N2, Ar or CO2 gas.