Quantitative Kinetic Model of CO<sub>2</sub> Absorption in Aqueous Tertiary Amine Solvents
Xavier Rozanska, E. Wimmer, Frédérick de Meyer
Abstract
Aqueous tertiary amine solutions are increasingly used in industrial CO2 capture operations because they are more energy-efficient than primary or secondary amines and demonstrate higher CO2 absorption capacity. Yet, tertiary amine solutions have a significant drawback in that they tend to have lower CO2 absorption rates. To identify tertiary amines that absorb CO2 faster, it would be efficacious to have a quantitative and predictive model of the rate-controlling processes. Despite numerous attempts to date, this goal has been elusive. The present computational approach achieves this goal by focusing on the reaction of CO2 with OH– forming HCO3–. The performance of the resulting model is demonstrated for a consistent experimental data set of the absorption rates of CO2 for 24 different aqueous tertiary amine solvents. The key to the new model’s success is the manner in which the free energy barrier for the reaction of CO2 with OH– is evaluated from the differences among the solvation free energies of CO2, OH–, and HCO3–, while the pKa of the amines controls the concentration of OH–. These solvation energies are obtained from molecular dynamics simulations. The experimental value of the free energy of reaction of CO2 with pure water is combined with information about measured rates of absorption of CO2 in an aqueous amine solvent in order to calibrate the absorption rate model. This model achieves a relative accuracy better than 0.1 kJ mol–1 for the free energies of activation for CO2 absorption in aqueous amine solutions and 0.07 g L–1 min–1 for the absorption rate of CO2. Such high accuracies are necessary to predict the correct experimental ranking of CO2 absorption rates, thus providing a quantitative approach of practical interest.