Mass transfer in Direct Air Capture sorbents

Abstract

The severe rise in global surface temperature and its associated damages to the ecosystem are primarily attributed to anthropogenic carbon dioxide emissions. The present CO2 levels in the air today and the continuing emissions necessitate even the most ambitious development scenarios to rely on net negative CO2 emissions to limit the global temperature rise to 1.5 °C until the year 2100. Direct Air Capture (DAC) refers to the direct extraction of carbon dioxide from the ambient air and addresses the need for net negative emissions. As the technology is still in its infancy, understanding and modeling the occurring processes is crucial.
This thesis addresses the challenges in modeling adsorption and desorption processes in solid-sorbent Direct Air Capture, focusing on the complexities introduced by the porous structures of sorbent materials and the effects of humidity. Current models fail to incorporate the full spectrum of mass transfer processes, or do not consider the significant effects of humidity on the process. To bridge this gap, a detailed mass transfer and reaction kinetics model at the particle scale was developed, aiming to provide a more accurate framework for predicting the performance of amine-functionalized sorbents in DAC applications. The model was developed on a particle scale, and subsequently included in contactor models of a packed-bed and a monolith. The study considered all relevant transport mechanisms, finding that pore diffusion and reaction kinetics are critical in governing sorbent uptake dynamics. The contactor model effectively compares the performance of packed-bed and monolith reactors, highlighting a tradeoff between productivity and energy efficiency, with significant potentials for energy savings offered by the monolith.