Kinetics and new mechanism study of CO₂ absorption into water and tertiary amine solutions by stopped‐Flow technique

The objective of the present work was to find the accurate kinetic models and mechanism for CO 2 absorption into tertiary amine solution, aiming at understanding the contribution of the CO 2 reaction with H 2 O, OH − , and tertiary amines on the overall reaction rate. First, the kinetics of CO 2 absorption into water instead of a buffer solution were studied using the stopped‐flow technique at 293–313 K, with initial CO 2 molar concentration of 1.1–37.3 mM. The experimental first‐order reaction rate constant ( ) was determined to be about 1000 times larger than the value for CO 2 absorption into buffer solution reported in the reference. The was then correlated by a proposed semiempirical model and a simplified theoretical model, giving the activation energy for CO 2 reacting with H 2 O as fitted by the simplified theoretical model in good agreement with the value of previous research. Also, the pH values and hydroxyl ion concentrations of aqueous Diethylaminoethanol (DEEA) solutions were determined at 293–313 K, with DEEA molar concentration of 0.1–0.4 M and CO 2 loading of 0–0.626 mol/mol. In addition, the observed first‐order reaction rate constant ( k 0_DEEA ) of binary DEEA‐H 2 O solution with DEEA molar concentration of 0.1–0.4 M reacting with CO 2 was determined at 293–313 K. It should be pointed out that the kinetic experiments of CO 2 absorption into DEEA solution was done with the molar ratio of DEEA to CO 2 fixed at 20. The values of k 0_DEEA were then fitted and predicted by four models (i.e., termolecular model, base‐catalyzed model, the improved model, and Khalifah model). The results show the improved model and Khalifah model can predict k 0_DEEA well with an average absolute relative difference (AARD) <5%. The predicted results indicate that the contribution of OH − to k 0_DEEA cannot be ignored for the absorption of CO 2 into tertiary amine solutions, and could be responsible for 50–70% of the total absorption reaction rate. Furthermore, the k 0 value of CO 2 absorption into aqueous triethanolamine and CO 2 ‐loaded DEEA solution were further investigated and comprehensively discussed, suggesting that both pK a and the CO 2 solubility affect k 0 , with pK a having a much more significant effect.