Diffusion of CH4 and N2 in Barium-Exchanged Reduced Pore Zorite (Ba-RPZ) and Zeolite 4A

Barium-exchanged reduced pore zorite (Ba-RPZ) is a titanosilicate molecular sieve that separates CH4 from N2 based on their relative molecular sizes. A detailed study of N2 and CH4 adsorption equilibrium and diffusion on Ba-RPZ was completed using low and high-pressure volumetry. Adsorption equilibrium data for Ba-RPZ from vacuum to 1.2 bar were measured at 30, 40, and 50 °C for CH4 and at 30, 50, and 70 °C for N2. Constant volume uptake experiments were conducted to estimate the diffusivities of CH4 at 30, 40, and 50 °C and N2 at −20, −10, and 0 °C. Similar experiments were carried out with zeolite 4A to validate the methods used in this study. On the one hand, the transport of N2 in Ba-RPZ was found to be controlled by diffusion in the micropores. On the other hand, the transport of CH4 in Ba-RPZ was described by a dual-resistance model, including a barrier resistance and micropore diffusional resistance. Both the barrier and micropore diffusion coefficients demonstrated concentration dependence. While the micropore diffusion constant followed Darken’s relationship, the barrier resistance did not. The activation energies of the micropore diffusion and barrier resistance for CH4 on Ba-RPZ were calculated to be 30.46 and 60.19 kJ/mol, while that of micropore diffusion for N2 on Ba-RPZ was calculated to be 25.77 kJ/mol. A concentration-dependent dual-resistance diffusion model for methane was constructed and validated using experimental data across a range of pressures and temperatures. The concentration-dependent dual-resistance model was able to describe the complex diffusion behavior methane displays as it progressed from the dual-resistance controlled region to the micropore-controlled region of the isotherm. The calculated limiting N2/CH4 kinetic selectivity of Ba-RPZ was shown to be ∼3 orders of magnitude larger than the current benchmark material for CH4/N2 separation (Sr-ETS-4).