Adsorption Equilibrium and Transport of CO2, N2, and H2O in CALF–20

Abstract

CALF–20 is a hydrophobic MOF adsorbent demonstrated at a pilot scale for the capture of CO₂ from wet flue gas using a direct steam heating TSA cycle. It has been synthesized following a published protocol, and its XRD structure matches known results. Both crystals and particles are used to study single-component adsorption and the diffusion of CO₂, N₂, and H₂O by using gravimetric, volumetric, and dynamic column breakthrough methods. Temperature and relative humidity ranges explored are 25–150 °C and 0–95% in helium, respectively, up to 1 bar pressure. A steam–helium mixture is used above 100 °C. Small pressure steps are used to determine (approximately) locally constant transport parameters. The Sips–Henry isotherm is the best-fit model, which correctly captures the dependence of the isosteric heat of adsorption on adsorbent loading, especially the complex shape for H₂O. The pore diffusion model captures crystal uptakes. The micropore diffusivity is an increasing function of the adsorbed-phase concentration up to a certain level before showing reversal, which is consistent with the Darken equation, a function of isotherm curvature. Gas/moisture transport in CALF–20 particles is controlled by Knudsen diffusion in the macropores. The key features observed from the single-component adsorption and diffusion studies and their impact on process studies are demonstrated by applying them to predict breakthrough results.