Biogas Upgrading Using Cation-Exchanged Bentonite Clay

The separation of CH4 and CO2 in biogas, known as biogas upgrading, can produce two valuable gas products and prevent greenhouse gas emissions. One method hereto is (vacuum-)pressure swing adsorption that involves the selective adsorption of CO2 on a sorbent. This work evaluates three montmorillonite-rich bentonite clay sorbents (cesium-, monomethylammonium-, and tetramethylammonium-exchanged). These layered materials were specifically selected for their interlayer spacing that nearly matches the molecular size of CO2 and hence can exclude the larger CH4 molecule. Adsorption isotherms of CO2 and CH4 and breakthrough measurements of the gas mixture determined for powders and particles demonstrate unambiguously the ability to separate both gases. The sorbents show a CO2/CH4 selectivity at 20-30 °C and PCO2 = PCH4 = 0.5 bar in the range of ∼35 to 7, decreasing with increasing cation size. While diffusional transport inside cesium- and tetramethylammonium-bentonite particles is fast, it is severely hindered in the case of monomethylammonium-bentonite particles. Desorption with nitrogen-purge, at room temperature and without external heating, demonstrates fast regeneration, typically within several minutes for cesium- and tetramethylammonium-bentonite particles. Based on the fast kinetics and their good selectivity, cation-exchanged clays are considered a promising alternative for conventional sorbents that often suffer from a trade-off between adsorption and desorption kinetics and selectivity.</p