Measurement of concentration-dependent gas diffusion coefficients in membranes from a psuedo-steady state permeation run

Transient permeation measurements were conducted for carbon dioxide through a PDMS membrane and for carbon dioxide and methane through a ZSM-5 zeolite membrane. After pressurization the feed tank was shut off from the supply and pressure and flux measurements were conducted while the feed-side pressure gradually declined over the course of each run. A series of steady state runs were also conducted for the same range of feed-side pressures. In all runs the transient fluxes were close to the steady state fluxes at corresponding feed-side pressures indicating that the transient evolved at a pseudo-steady state. A dimensionless parameter depending on the feed tank volume, membrane geometry, and adsorption properties was defined to characterize the deviation from steady state. Concentration-dependent diffusion coefficients were estimated from (i) steady state fluxes from a sequence of runs, (ii) transient fluxes from a single run, and (iii) transient pressure measurements from a single run. The fluxes and the diffusion coefficients obtained from a single transient run, and from a sequence of steady state runs are compared and the observed differences are discussed.