Direct capture of CO2 from ambient air is technically feasible today, with com mercial units already in operation. A demonstrated technology for achieving
direct air capture (DAC) is chemical separation of CO2 in a steam-assisted
temperature-vacuum swing adsorption (S-TVSA) process. However, the poten tial to develop scalable solutions remains high, requiring a detailed understanding
of the impact of both process design and operation on the performance of the
DAC unit. Here, we address this knowledge gap by presenting a state-of-the-art
process simulation tool for the purification of CO2 from ambient air by a 5-step
S-TVSA process. By considering the benchmark adsorbent APDES-NFC, we con duct multi-objective productivity/energy usage optimization of the DAC unit,
subject to the requirement of producing a high purity CO2 product (≥95%).
For the base case scenario, we find a maximum productivity of Prmax = 6.20
kg/m3
/day and a minimum specific equivalent work of WEQ,min = 1.66 MJ/kg.
While in reasonable agreement with published data, our results indicate that
the description of both competitive adsorption and adsorption kinetics are key
factors in introducing uncertainty in process model predictions. We also demon strate that the application of formal optimization techniques, rather than design
heuristics, is central to reliably assess the process performance limits. We identity
that system designs employing moderate CO2 sorption kinetics and contactors
with low length-to-radius ratios yield the best performance in terms of system
productivity. Finally, we find that moderate-high ambient relative humidities (50-
75%) offer significantly favourable performance, and that a wide range of feed temperatures (5-30oC) can be accommodated via process optimization without
a significant impact on performance.