A STATISTICAL-MECHANICS APPROACH TO THE SEPARATION OF METHANE AND NITROGEN USING CAPILLARY CONDENSATION IN A MICROPOROUS MEMBRANE

A statistical mechanics model (Local Density Approximation, LDA) is introduced and used to study the separation of a nitrogen and methane gas mixture by permeation through a microporous membrane. The essential mechanism for the separation is that one gas preferentially condenses. The LDA was used to determine the conditions of capillary condensation because it provides a more accurate description of capillary condensation than the classical Kelvin equation, especially for very small (< 10 nm) pores. Separation factors of 12 to 2,400 were calcualted which appears encouraging insofar as the nitrogen-methane separation is concerned. Permeation rates were also calculated and are in accordance with prior experimental results for other condensable gas systems.