Modelling of equilibrium working capacity of PSA, TSA and TVSA processes for CO₂ adsorption under direct air capture conditions

In this study, direct air capture performance of polymeric adsorbent was studied by determining CO₂-isotherms in very dilute conditions (CO₂ partial pressure up to 5 mbar). The effect of cold conditions (0 °C and −10 °C) and humidity on the CO₂ capacity was also studied. The experimental isotherms were used in temperature-dependent equilibrium modelling to simulate equilibrium CO₂ working capacities in pressure-swing adsorption (PSA), temperature-swing adsorption (TSA) and temperature-vacuum swing adsorption (TVSA). Experimental adsorption capacities of 0.80 mmol_CO2/g_sorbent and 0.89 mmol_CO2/g_sorbent were obtained from 400 ppmv CO₂ in dry cold conditions and at 25 °C in humid conditions, respectively. The highest experimental capacity gained from 400 ppmv CO₂ was 1.06 mmol_CO2/g_sorbent in humid cold conditions. In terms of the working capacity, PSA was found not to be a viable process option. Humidity promoted TSA working capacity up to by 0.36 mmol_CO2/g_sorbent (78%). TSA could produce larger than 0.5 mmol_CO2/g_sorbent working capacity levels even with very low regeneration temperatures (50–60 °C) when adsorbing either in dry cold conditions or humid warm conditions. Such EWC levels with dry TVSA could only be achieved using 90 °C regeneration temperature with adsorption in either cold conditions or from compressed air. Based on these results, TVSA should only be considered in PtX applications requiring high-purity CO₂.