The transport of gases in a mesoporous γ-alumina supported membrane

We investigate the low pressure diffusion of several gases in a disordered mesoporous γ-alumina membrane of pore diameter 10.4 nm, coated on a macroporous α-alumina tubular support. Transport data for the uncoated support is also separately obtained and interpreted using the conventional slip flow correlation with a single representative pore size, and with effective medium theory while considering the entire pore size distribution. It is shown that the conventional correlation based on a single pore size yields significant anomalies in the tortuosity and pressure profile for all gases, which are eliminated on using the effective medium theory, demonstrating the importance of considering the pore size distribution (PSD). These anomalies lead to failure of the correlation for the membrane layer. The effective medium theory is also extended to the disordered membrane layer, using the classical slip flow model and a version corrected for finite molecular size, as well as the Oscillator Model developed in this laboratory. All the diffusion models fitted the experimental data accurately, without the artifacts observed with the commonly used single pore size model. The results make evident the importance of correctly taking the pressure profile into account, in any investigation for a multi-layered supported membrane (i.e. with a macroporous support and a mesoporous membrane layer). It is seen that the slip flow model leads to significantly higher apparent diffusivities than the Oscillator flow model at small pore sizes. Nevertheless, the performance of all three diffusion models is comparable, and it is not possible to distinguish between them for the large pore diameter of 10.4 nm of the mesoporous γ-alumina membrane layer. The results unequivocally show that it is critical to consider the full PSD for each layer rather than using a single representative pore size in modeling membrane transport, and in interpreting experimental permeability data.