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.