Biogas upgrading by CO₂ removal with a highly selective natural amino acid salt in gas-liquid membrane contactor

For biogas upgrading, a natural amino acid salt, potassium l-argininate (PA) is studied in a membrane contactor to capture CO₂ from biogas. CO₂ removal performance in terms of the overall volumetric gas phase mass transfer coefficient, membrane selectivity towards CO₂ and the economic cost factor is systematically investigated. It is shown that PA is a highly CO₂ selective absorbent and has a better affinity towards CO₂ than monoethanolamine (MEA). The highest CH₄ content in the upgraded biogas can reach about 99.15vol% by using PA, fully meeting the requirement of biogas upgrading. Furthermore, lower solvent concentration, lower liquid velocity and higher reaction temperature may be adopted when using PA in comparison to MEA. PA also has a better flexibility to the change of CO₂ partial pressure and biogas flow rate than MEA. Regarding the economic cost factor of membrane process, CO₂ loading of the lean PA solution can be optimized to 0.69-0.78mol/mol as the suitable range. Moreover, CO₂ removal performance of l-arginine (ARG) is also explored. Due to the large amounts of bicarbonate other than carbamate formed in CO₂-rich ARG solution, ARG has a lower biogas upgrading capability than diethanolamine (DEA) but higher than triethanolamine (TEA).