Reaction kinetics of carbon dioxide absorption in aqueous solutions of piperazine, N-(2-aminoethyl) ethanolamine and their blends

Amine blends have been extensively studied as advanced solvents modification for CO2 absorption. Due to the lack of knowledge on mechanism of amines interaction, reaction in blends was traditionally regarded as parallel reactions between CO2 and different amines. In this work, the kinetics of CO2 reaction with blends of two promising diamines, piperazine (PZ), N-(2-aminoethyl) ethanolamine (AEEA), were studied in a wetted wall column. The reaction rate constants of single amine were derived from kinetic data based on the zwitterion and termolecular mechanism for PZ and AEEA, respectively. It is interesting to find that the overall reaction rate of the blend is 12.0-28.1% lower than the calculated value through parallel reaction mechanism, which indicates the strong interaction between PZ and AEEA. A 'zwitterion bridge' reaction pathway was proposed as the microscopic mechanism of the interaction in the blend. Based on the kinetic models and experimental data, the interaction that PZ can promote the reaction in amines blend by transferring CO2 to AEEA through the PZ-CO2 intermediate was revealed. Besides, the interaction was proved to be a fast reaction with the reaction rate of same magnitude compared with the zwitterion hydrolysis reaction. The mechanisms of interaction between the two amines are further investigated through density functional theory (DFT). Results of simulation, in terms of activation energy and molecular geometry deformation, indicate that the PZ prefers a direct reaction with CO2 rather than the reaction with the AEEA-CO2 intermediate.