Process-performance of solid sorbents for Direct Air Capture (DAC) of CO2 in optimized temperature-vacuum swing adsorption (TVSA) cycles

The process performance of three amine-functionalized chemisorbents (TRI-PE-MCM-41, SI-AEATPMS, APDES-NFC-FD-S) and two physisorbents (SIFSIX-18-Ni-β and NbOFFIVE-1-Ni) was evaluated for direct air capture (DAC) of CO2 in temperature-vacuum swing adsorption (TVSA) and steam-assisted temperature-vacuum swing adsorption (s-TVSA) cycles. Rigorous process optimizations were performed to evaluate the trade-off between energy consumption and productivity. Thermal energy consumption, with main contributions arising from desorption of CO2 and H2O, sensible heat of solids, is higher than electrical energy. While steam purge generally improves productivity at the cost of energy, an exception was noticed where it was also found to reduce energy. For the sorbents studied, minimum energy and maximum productivity ranged between 6.25–30.4 MJth/kgCO2, and 0.01–0.15 TPD of CO2/m3 of sorbent, respectively. For all the sorbents, while target CO2 purity (≥95%) can be achieved with moderate vacuum pressure (≈0.30 bar), low regeneration pressures (≈0.050 bar) can lower energy demand and increase productivity. A new ambient pressure temperature swing adsorption cycle is introduced. This cycle achieves target purities, albeit at higher energy consumption compared to those that use vacuum. The study showed that physisorbents, generally not studied for DAC, can be promising. Parametric studies revealed that the lack of multi-component thermodynamic and kinetic data impedes the objective evaluation of DAC processes.