Kinetic separation of methane/carbon dioxide by molecular sieve carbons

The bulk separation of CH₄/CO₂ mixture by pressure swing adsorption (PSA) on two different types of carbon molecular sieves (CMS) is analyzed. The two CMS are Bergbau-Forschung (BF) CMS and Takeda 3A CMS. Two different PSA cycles are considered, differing by whether a feed step is used. For a 50/50 feed mixture, it is shown that the separation is feasible with both sorbents to produce a methane product at over 90% purity and at reasonably high product recoveries and sorbent productivities. It is shown in this work that temperature is an important factor to consider for kinetics-based separations, and that a higher temperature is favorable for CH₄/CO₂ separation on molecular sieve where diffusion is slow. At 25°C, BF CMS yields better separation because the diffusion of both CO₂ and CH₄ are slow in the Takeda 3A CMS. Increasing the temperature decreases the equilibrium adsorption amounts, but increases the diffusivities. The separation results with Takeda 3A are substantially improved when the temperature is increased to 70°C, and the separation is better than that of BF CMS at 25°C. Therefore, the rate of diffusion (rather than equilibrium) is the dominating factor in kinetics-based separations. The effects of the sorbent working capacity (defined as the difference in the amounts adsorbed of the fast diffusing component between the ends of the adsorption step and the desorption step) on the separation are illustrated. For the kinetics-based separation, a high-pressure feed step is desirable while a low-pressure purge step is not desirable. The reasons are seen clearly from the bed profiles.