Quantitative Kinetic Model of CO Absorption in Aqueous Tertiary Amine Solvents.

Aqueous tertiary amine solutions are increasingly used in industrial CO capture operations because they are more energy-efficient than primary or secondary amines and demonstrate higher CO absorption capacity. Yet, tertiary amine solutions have a significant drawback in that they tend to have lower CO absorption rates. To identify tertiary amines that absorb CO 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 CO with OH forming HCO. The performance of the resulting model is demonstrated for a consistent experimental data set of the absorption rates of CO 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 CO with OH is evaluated from the differences among the solvation free energies of CO, OH, and HCO, while the p 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 CO with pure water is combined with information about measured rates of absorption of CO 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 for the free energies of activation for CO absorption in aqueous amine solutions and 0.07 g L min for the absorption rate of CO. Such high accuracies are necessary to predict the correct experimental ranking of CO absorption rates, thus providing a quantitative approach of practical interest.