Vacuum pressure swing adsorption process with carbon molecular sieve for CO2 separation from biogas

A 3-bed 6-step vacuum pressure swing adsorption (VPSA) process for biogas upgrading using carbon molecular sieve (CMS) adsorbent is here studied both experimentally and numerically. The selected CMS adsorbent, with micropore size distribution in the range of 0.3-0.65 nm, obtained a high kinetic selectivity of 163 but a low equilibrium selectivity of 2.4 for CO2/CH4. A biogas mixture containing 55 % CH4 balanced with CO2 was effectively upgraded to a 98 % CH4 purity product with a CH4 recovery higher than 83 % using a desorption pressure at 0.1 bar and a light product purge-to-feed (P/F) ratio of 0.1. Meanwhile, VPSA process experiments indicated that a lower feed velocity with a longer cycle time achieved much higher CH4 purity with a slight decrease of CH4 recovery. A lower desorption pressure more efficiently improved the degree of regeneration of the CMS adsorbent than increasing the P/F ratio. A non-isothermal mathematical process model, deployed a macropore-micropore bi-linear driving force mass transfer model, was validated by comparison to both dynamic column breakthrough and VPSA process experiments.