Evaluation of hydrogen permselective separation from “synthesis gas” components based on single gas permeability measurements on anodic alumina membranes

Anodic alumina membranes were prepared by anodizing aluminum followed by chemical and hydrothermal treatments. Anodization was performed on both tube surfaces. External anodization was restricted to selected spots leaving an aluminum mechanically strong frame. Nitrogen sorption data analysed with the CPSM method (Corrugated Pore Structure Model) detected a mesopore structure (i.e. D_mean;15–20) and surface areas of 2–20 m²/g. SEM microscopy images revealed a regular pore structure of independent pores with densities of N_pore ~ 5.6 × 10¹⁴ pores/m². Single gas permeances (Π) for X: H₂, CH₄, CO and CO₂ were measured on a Wicke–Kallenbach apparatus at varying mean transmembrane pressure P_m by the “dead-end” method. The observed slight dependence of Π on Ρ_m is indicative of strong Knudsen diffusion and weak viscous flow contributions. By correlating the experimental data with a linear Π vs Ρ_m relationship, Knudsen contribution evaluation was enabled, and found to vary in the range K_C ≈ 0.7–1.0. The Knudsen number criterion for flow regime discrimination is critically discussed and a realistic dual Knudsen number approach is proposed. Experimental permselectivities α_Η2Χ = Π_H2/Π_X (X ≠ H₂) approach by 70–100% their Knudsen selectivity counterparts. Anodic alumina membranes exhibit pore structure and gas permeability characteristics useful in designing integrated gas separation and catalytic membrane reactor systems.