Understanding the effect of H2O on CO2 adsorption capture: mechanism explanation, quantitative approach and application

Abstract

The presence of water vapor in realistic flue gas should be emphasized in the development of adsorbents for post-combustion CO₂ capture. However, the co-adsorption mechanisms of H₂O and CO₂ molecules on adsorbents are still unclear. In this paper, the adsorption performance of CO₂ under wet flue gas was investigated through molecular simulation. The mechanism of H₂O being preferentially adsorbed at low temperature and CO₂ being preferentially adsorbed at high temperature has been clarified. In order to further provide a quantitative criterion, a calculation method was introduced. The Gibbs free adsorption energy was applied to explain the competitive adsorption behavior of CO₂ and H₂O. The adsorption entropy and the adsorption enthalpy were derived according to the statistical thermodynamics. Molecular dynamics simulation is the main research method in this paper. A proper force field for H₂O was identified among SPC/E, TIP4P, and TIP5P models. The adsorption isotherms of CO₂, H₂O, and CO₂/H₂O were obtained by Grand Canonical Monte Carlo (GCMC) simulations from 300 K to 400 K. A case study for CO₂/H₂O mixture adsorption on zeolite MFI is presented to demonstrate the feasibility. The obtained equilibrium adsorption isotherms from the case study showed that the presence of a small amount of H₂O in the gas mixture had a significant impact on the adsorption loads of CO₂. A detailed thermodynamics analysis demonstrated that the adsorption entropies and enthalpies were temperature dependent and they would switch the relative order of Gibbs free adsorption energy between adsorbed CO₂ and H₂O. Moreover, the CO₂ working capacity and energy consumption of regeneration in the process of temperature swing adsorption (TSA) were presented.