Permeation Time Lag and the Concentration Dependence of the Diffusion Coefficient of CO₂ in a Carbon Molecular Sieve Membrane

The CO 2 permeation experiments were performed on a microporous carbon molecular sieve membrane (CMSM) at 25 °C and over a wide pressure range. The permeation time lag of the penetrant was employed to study the concentration dependence of the diffusion coefficient. The analytical solutions are derived for permeation systems with a nonlinear sorption isotherm (Langmuir) and a concentration-dependent diffusivity [with the functional form of D μ ( c ) = 1/(1 − θ) n, where θ is the ratio of surface coverage and n ≥ 1]. By comparison of the experimental data with the theoretical models, it is found that, in this CMSM, the apparent diffusivity of CO 2 takes a concentration dependence stronger than that described by the traditional HIO model [i.e., D μ ( c )/ = 1/(1 − θ)].