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

Abstract We study the transport of CO 2 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 CO 2 uptake in such contactors relative to the CO 2 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 related to the sorbent chemistry (capacity and adsorption rate constants), packing density, and diffusion coefficient of CO 2 inside bed; and (2) extent of the bed in the air‐flow direction related to convective air‐flow velocity, sorbent chemistry, packed bed thickness, and packing density. DAC designs that have a bed thickness of O() and streamwise extent O() are shown to have faster CO 2 uptake and more economical capture costs ($/ton‐CO 2 ) 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.