Mass Transfer of CO₂ in Amine-Functionalized Structured Contactors in Ultra-Dilute Conditions

Extracting CO₂ from the atmosphere via direct air capture (DAC) provides a pathway to counteract the rising CO₂ concentration in the atmosphere. Processes using amine functionalized solid sorbents have attracted considerable attention, as they exhibit high affinity toward CO₂ at atmospheric concentrations. The process is significantly influenced by the mass transfer kinetics of adsorption, and accurate quantification is crucial for improving process models and DAC systems. In this study, we addressed this critical issue by quantifying the mass transfer kinetics of three amine functionalized structured sorbents: two alumina pellets with unimodal (TRI@unimodal) and bimodal (TRI@bimodal) pore size distributions, and a honeycomb mullite/alumina monolith (TRI@monolith). A modeling framework was developed to enable the use of a commercial volumetric sorption device to measure sorbent mass transfer kinetics, and to distinguish them from resistances within the device. The measurements revealed distinct mass transfer regimes, with pore diffusion playing a significant role in the bimodal pellets, whereas a surface resistance introduced by the functionalization procedure dominated in the unimodal pellets. The device was unable to capture the pore diffusion in the monolith due to instrument resistances limiting this regime. A self-limiting diffusion behavior previously reported in literature was identified in the amine layer, which decreased diffusion with increasing CO₂ uptake. We estimate kinetic parameters for all three sorbent materials for use in a widely used linear driving force (LDF) model adapted for amine functionalized sorbents. The parameter describing the mass transfer in the gas phase is nearly five times larger for TRI@bimodal than for TRI@unimodal. For the mass transfer in the amine layer, the parameter increases progressively from TRI@monolith to TRI@unimodal to TRI@bimodal. The results highlight the importance of pore structure and functionalization procedure to improve DAC sorbents.