Cs-Bentonite Clay for Biogas Upgrading: A Numerical Assessment.

Biogas upgrading by vacuum-pressure swing adsorption involves the selective adsorption of CO over CH 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 (CO mole fraction of 0.35 or 0.45, balance CH), and the process operating parameters. Specific constraints on CH purity and CH 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 Nm feed biogas (CO mole fraction of 0.45, balance CH) per kg sorbent per h to a bio-CH product with a purity of 0.906 and with a CH recovery of 0.967 at a comparatively low specific energy consumption of only 0.072 kWh per produced Nm of CH. Using more columns and pressure equalization steps further enhances the CH recovery. The low bentonite cost, the comparatively low specific energy consumption due to the favorable linear CO adsorption isotherms, and the high recovery due to the high CO/CH selectivity make Cs-bentonite an excellent alternative for conventional sorbent materials.