Humidity-driven swelling and mass transfer in amine-functionalised resins for direct air CO₂ capture

Direct air capture (DAC) using amine-functionalised adsorbents is a promising route to negative emissions. Here, we investigate how humidity influences the swelling, CO2 uptake, and mass transfer of two amine-functionalised polymeric resins, Lewatit VP OC 1065 and Purolite A110, using a combined experimental and modelling approach. We quantify humidity induced swelling and introduce a simplified diffusion model to explore how swelling, humidity, and effective diffusivities affect water and CO2 transport within a bead. Swelling is found to slow water transport to the bead centre, an effect amplified at higher humidities, where the effective diffusivity is 10-100 times slower. Our analysis further shows that CO2 saturation times are of the same magnitude at both low and high humidity, supporting the use of linear driving force (LDF) constants to describe lumped mass transfer. Based on these findings, we provide guidance for optimising both sorbent design and process modelling for direct air capture.