A multicolumn vacuum pressure swing adsorption biogas upgrading process for simultaneous CO2 and N2 separation from methane: Exergy and energy analysis

Adsorption is commonly used as an industrial process for biogas upgrading through elimination of unwanted species to obtain high quality biomethane. While several studies have addressed individual removal of CO2 and N2 from raw biogas, limited attention has been paid to their simultaneous removal due to the complexities involved. This works presents the results of design, modeling, and numerical simulation of a novel vacuum pressure swing adsorption (VPSA) process for upgrading a model raw biogas containing 40 and 10 vol% CO2 and N2, respectively, to increase CH4 purity to 98% and with less than 2 % N2. The proposed process is comprised of 4 columns including two columns of carbon molecular sieves (CMS) and two others with zeolite Sr-ETS-4 as the selective adsorbents for removal of CO2 and N2, respectively. Two configurations are evaluated and compared in terms of CH4 purity, CH4 recovery, productivity, energy consumption, and 2nd law efficiency. For the best configuration, an energy consumption of 24.9 kJ/mol CH4, productivity of 3.38 mol h−1 kg−1, CH4 purity of 98.2 mol%, and CH4 recovery of 73.8 mol% were achieved. 15% improvement in CH4 recovery in comparison to the basic configuration is obtained. The findings highlight the importance of adsorbent selection and process configuration for achieving high separation performance.