Gas permeability in rarefied flow conditions for characterization of mineral membrane support

Abstract : Gas Permeability Measurement Technique (GPMT) has the advantage of being a non-destructive method, which is efficient in characterizing filtration membranes. Ceramic filtration membranes consist of successive layers of micro (support) to nano size (skin) pores. When gas flows through such a small scale structure, the molecular mean free path becomes comparable to the pore size. The Slip flow model, validated to describe the gas transport properties under rarefied flow conditions in a microchannel, is extended to porous media. The porous structure is modeled as a cluster of several identical cylindrical channels. By measuring the pressure drop ∆P at several different mean pressures, the pore radius and the porosity on square tortuosity ratio /τ 2 of the porous model structure that have the same flow property were estimated. Nomenclature G Geometric factor (m −1) H Effective length of hollow fiber (m) J Mass flux density (kg.s −1 .m −2) K Hydraulic conductivity (s) Kn Knudsen number L p Channel length / Gas path length through the porous media (m) N Channel number P Gas pressure (Pa) P 0 Pressure upstream the nozzle (bar) P 1 Pressure upstream the porous sample (bar) P 2 Pressure downstream the porous sample (bar) P c Nozzle critical pressure (bar) P m Mean pressure (Pa)