Kinetics of CO2 absorption into uncatalyzed potassium carbonate–bicarbonate solutions: Effects of CO2 loading and ionic strength in the solutions

Abstract

Potassium carbonate–bicarbonate (K2CO3–KHCO3) aqueous solutions could be used as an alternative to amines as solvents in CO2 capture from the flue gas of coal-fired power plants. The rate of CO2 absorption into K2CO3–KHCO3 solutions is governed by the reaction between CO2 and OH−, which is first order with respect to both CO2 and [OH−](r=kOH[OH−][CO2]). Knowledge of the reaction kinetics and dynamic changes in kinetics along an absorber is essential for process design. However, this information, particularly the values of the rate constant of the reaction between CO2 and OH−, kOH, in K2CO3–KHCO3 solutions with different ionic strengths and CO2 loadings are seldom available in the literature. In this study, we investigated the kinetics of CO2 absorption into 5–40wt% solutions with different levels of CO2 loading at 25–80°C. The kOH values were determined by measuring rates of CO2 absorption into the solutions in a stirred cell reactor and by using the classic Danckwerts theory for interpretation of the data. The method was validated by the good agreement between the rate constants obtained for infinitely dilute solutions and those reported in the literature. The CO2 loading of K2CO3–KHCO3 solutions was found to govern the activation energy (Ea) of the absorption reaction. In K2CO3–KHCO3 solutions with the same CO2 loading, the Ea values were comparable, regardless of the concentration or total ionic strength of the solution. The Ea decreased as the CO2 loading of the solution increased. The ionic strength of the solution substantially affected the Arrhenius pre-exponential factor term [ln(A)] of kOH. Functional formulae have been developed that can be readily used to calculate kOH for K2CO3–KHCO3 solutions with different levels of CO2 loading over the applicable range. The results provide extensive information on the kinetics of the CO2 reaction in K2CO3–KHCO3 solutions, particularly in concentrated solutions (20–40wt%) with a CO2 loading up to 40% of potassium carbonate to bicarbonate conversion at temperatures up to 80°C.