Engineering design of direct-air-capture contactors composed of sorbent particles using numerical simulations

We study the transport of CO2 molecules inside direct-air-capture (DAC) contactors made of packed beds of solid-sorbent-particles separated by airgaps using numerical solution of the convection-diffusion and first-order adsorption kinetics equations. We quantify an apparent slowdown in the CO2 uptake in such contactors relative to the CO2 uptake of a high-capacity sorbent particle making the bed. We derive two naturally occurring length-scales associated with the adsorption process: (1) thickness of the sorbent lambda related to the sorbent chemistry (capacity and adsorption rate constants), packing density, and diffusion coefficient of CO2 inside bed; and (2) extent of the bed in the air-flow direction lambda (2) related to convective air-flow velocity, sorbent chemistry, packed bed thickness, and packing density. DAC designs that have a bed thickness of O(lambda ) and streamwise extent O(lambda (2)) are shown to have faster CO2 uptake and more economical capture costs ($/ton-CO2) compared with designs further away from this design set-point. Our work should provide guidance to engineering design of DAC contactors with packed bed sorbents and related porous materials.