Thermodynamic study on two adsorption working cycles for direct air capture

•Thermodynamic performance of VPSA and TVSA cycle in DAC is explored.•Working capacity of adsorbents are determined using temperature-dependent modelling.•N2H4/Mg-MOF-74 in TVSA proves to be the best combination for DAC.•A graphical method is presented to assess the minimum separation work intuitively. Direct capture of CO2 from air, i.e. DAC, is considered as a potential solution to address anthropogenic climate change in the near future. This work provides a comparative assessment of DAC by using five adsorbents, i.e. SIFSIX-3-Ni, NbOFFIVE-1-Ni, N2H4/Mg-MOF-74, TRI/PE-MCM-41 and DETA/PPN-6, based on equilibrium working capacity and exergy efficiency analysis. Vacuum-pressure swing adsorption (VPSA) and temperature-vacuum swing adsorption (TVSA) cycles are compared by adjusting working parameters. Adsorption temperature/pressure, desorption temperature/pressure and CO2 concentration are analyzed. For working capacity, SIFSIX-3-Ni can achieve 1.08 mmol·g−1 at 6 mbar adsorption and 1 mbar desorption partial pressure in VPSA cycle. N2H4/Mg-MOF-74 performs the most outstanding working capacity in TVSA cycle. The results show that raising pressure of air is an unsatisfactory choice to increase working capacity due to the large amount of energy demand of compression process. Thus, TVSA is a promising process applied in DAC. When desorption pressure is 4 mbar and adsorption temperature is 25 °C, the maximum exergy efficiency among these adsorbents is 50.9% of DETA/PPN-6. Amine-modified adsorbents have an excellent performance above two metal–organic frameworks (MOFs) in most situations. These findings can provide helpful guides when choosing appropriate adsorbents and cycles for DAC.