Comparing thermodynamic equilibrium isotherms, mechanistic kinetic models and mass transfer resistances for fixed bed CO2 direct air capture (DAC)

The dynamic behavior of CO₂ adsorption in fixed-bed systems is critical for the design and optimization of efficient adsorption-based separation processes. This study focuses on dilute CO₂ adsorption (400–2000 ppm) onto Lewatit® VP OC 1065, under ambient and sub-ambient temperature conditions (−10 to 40 °C), which are particularly relevant for direct air capture but often overlooked in the literature. A one-dimensional dynamic model incorporating convection, axial dispersion, and adsorption kinetics was developed and validated against experimental breakthrough data. A thorough evaluation of all relevant isotherms and kinetic models presented in the existing literature has been conducted, and a comprehensive comparison was performed to determine which model provides the best overall performance under the specified criteria. The dual-site Langmuir isotherm combined with a pseudo-first linear driving force approach was found to best describe the system's performance. The heat of adsorption determined from the dual-site isotherm model was calculated to be −69.8 kJ mol −1 for the high-energy sites and −50.0 kJ mol −1 for the low-energy sites. The total adsorption capacity was determined to be 3.32 mmol g −1 , closely matching the value derived from elemental analysis. Contrary to common assumptions, the Toth isotherm did not consistently provide the best fit. Simulations were conducted using MATLAB, and the results closely matched experimental data, confirming the reliability of the modeling approach. The rate constant was determined to be 0.0061 min −1 at 20 °C. The estimated barrier for activated chemisorption from fitting experimental data to the model was 61.2 kJ mol −1 , which is consistent with ab initio atomistic modeling results. We validated our findings using density functional theory (DFT), providing a multiscale perspective that links macroscopic behavior with molecular-level interactions. • Pseudo-first-order LDF model with dual-site Langmuir isotherm outperformed the Toth model. • Dual-site model reveals −69.8 and −50 kJ mol −1 heats adsorption for high and low energy sites, respectively. • Activation energy (~60 kJ mol −1 ) exceeded previously reported but was consistent with DFT calculations.