Cs-Bentonite Clay for Biogas Upgrading: A Numerical Assessment

Biogas upgrading by vacuum-pressure swing adsorption involves the selective adsorption of CO2 over CH4 on a sorbent material to separate both components. This work assesses numerically the performance of the previously characterized Cs-exchanged bentonite clay for this separation. This benchmarking study includes the effect of the process cycle configuration (seven different configurations using one stage and up to three columns), the ambient temperature (15 or 25 °C), the feed biogas composition (CO2 mole fraction of 0.35 or 0.45, balance CH4), and the process operating parameters. Specific constraints on CH4 purity and CH4 recovery provide Pareto fronts for maximum productivity and minimum specific energy consumption. A two-column unit operated at ambient feed pressure can upgrade 0.097 Nm3 feed biogas (CO2 mole fraction of 0.45, balance CH4) per kg sorbent per h to a bio-CH4 product with a purity of 0.906 and with a CH4 recovery of 0.967 at a comparatively low specific energy consumption of only 0.072 kWh per produced Nm3 of CH4. Using more columns and pressure equalization steps further enhances the CH4 recovery. The low bentonite cost, the comparatively low specific energy consumption due to the favorable linear CO2 adsorption isotherms, and the high recovery due to the high CO2/CH4 selectivity make Cs-bentonite an excellent alternative for conventional sorbent materials