Exploring Geometric\nProperties and Cycle Design in\nPacked Bed and Monolith Contactors Using Temperature-Vacuum Swing\nAdsorption Modeling for Direct Air Capture

This study presents a comprehensive comparison between\nthe packed\nbed and monolith contactor configurations for direct air capture (DAC)\nvia process modeling of a temperature-vacuum swing adsorption (TVSA)\nprocess. We investigate various design parameters to optimize performance\nacross different contactor geometries, including pellet size, monolith\nwall thickness, active sorbent content in monoliths, and packed bed\nstructure configurations, considering both a traditional long column\n(PB<sub>40</sub>) and multiple shorter columns configured in parallel\n(PB<sub>5</sub>). Our parametric analysis assesses specific exergy\nconsumption, sorbent, and volume requirements across different operating\nconditions of a five-step TVSA cycle. For minimizing sorbent requirements,\nPB<sub>5</sub> and monoliths with over 80% sorbent loading were the\nbest-performing contactor designs with overlapping performance in\nthe low-exergy region. Beyond this region, PB<sub>5</sub> faced limitations\nin reducing sorbent requirements further and was constrained by a\nmaximum velocity at which it is sensible to operate without substantially\nincreasing the exergy demand. In contrast, monoliths decreased sorbent\nrequirements with minimal exergy increase due to reduced mass transfer\nresistances and lower pressure drop associated with their thin walls.\nThe analysis of volume requirement-specific exergy Pareto fronts revealed\nthat PB<sub>5</sub> was less competitive with this metric due to the\nrequirements for additional void space in the contactor configuration.\nThe study also revealed that optimal sorbent loading for reducing\nvolume requirements in monoliths differed from those minimizing sorbent\nusage, with the most effective loading being below 100%. Thus, the\noptimal contactor design varies depending on the goals of minimizing\nsorbent and volume requirements, and the choice and design of the\ncontactor will depend on the relative costs of these factors. Lastly,\nour findings challenge the assumption that higher velocities are always\npreferable for direct air capture, suggesting instead that the operating\nvelocity depends on the contactor configuration.