Gas Permeability Measurement Methods in Membrane-Based Gas Separations: Analysis of Transport Properties of Pure Gases Through Dense Polymeric Membranes

The permeation behavior of pure gases, CO₂, CH₄ and N₂ through poly (dimethylsiloxane) (PDMS) membrane was investigated using three different permeation techniques, which include the conventional methods, such as constant volume (CV) or time-lag technique, and constant pressure (CP) system; and a new permeation apparatus based on continuous-flow technique.  The continuous-flow apparatus is capable of making on-line measurements of both flux transient and permeate composition in gas permeation, and this transient measurement allows the simultaneous determination of permeation characteristics, such as permeability, diffusion and solubility coefficients, and activation energies from just one experiment. Unlike the conventional time-lag (CV) system that is associated with such drawbacks as vacuum leaks and system outgassing, permeate adsorption on the walls of downstream system, unsuitable for gases having large permeabilities and non-constant diffusivity through permeation process, the new apparatus avoids all these drawbacks associated with the conventional system and provides accurate and quick determination of permeation characteristics. The effect of testing protocols, in particularly the effect of transmembrane pressure on permeation behavior of the pure gases in all techniques was studied. Permeabilities for similar PDMS films obtained by the three testing techniques were compared and their reliability was assessed. In all the three systems, the flux of CO₂ increase almost linearly with increase in transmembrane pressure, while CH₄ and N₂ fluxes were observed to be slightly increased with pressure. The CO₂ permeability values obtained in all the three systems were found to be slightly increased with pressure, while CH₄ and N₂ permeabilities were observed to be independent of pressure. The permeabilities of CO₂, CH₄ and N₂; and CO₂/N₂ and CH₄/N₂ selectivities with respect to pressure ranges, obtained with the continuous-flow apparatus were all found to be comparable, similar and showed close agreement with the respective values obtained using the most widely applied conventional system, i.e., time-lag or CV technique. This thus indicates and verifies the validity and accuracy of the apparatus.  Pure gas experiments using the continuous-flow apparatus showed that, increasing transmembrane pressure increases permeability and diffusion coefficients of CO₂, while increase in transmembrane pressure does not affect or slightly decreases those of CH₄ and N₂. Increasing gas temperature increased permeabilities of CH₄ and N₂ and decreased permeability of CO₂. Diffusion coefficients of all gases increased with increasing gas temperature, while solubility coefficients of gases decreased with increasing gas temperature. It should be noted that, the variation rates of permeation properties with temperature were different for various gases with regard to their activation energies. Van’t Hoff-Arrhenius type temperature dependence (activation energies E_p, DH_s and E_d) of permeability, solubility and diffusivity coefficients in the pure gas experiments were estimated and discussed thoroughly.