Impact of the Inter- and Intramolecular Tertiary Amino Group on the Primary Amino Group in the CO2 Absorption Process

This work has investigated CO2 absorption using 3-(diethylamino)propylamine (DEAPA), a diamine that contains one primary and one tertiary amine in the same molecule. The results are compared with those for the blended amine system of monoethanolamine and methyldiethanolamine (MEA–MDEA), a mixed amine solvent with an equal number of moles of primary and tertiary amine. The 2 M DEAPA, 2 M MEA, and 4 M MEA–MDEA (1:1 mole ratio) were tested for their CO2 absorption performance. The experimental results show that the intramolecular tertiary amino group of DEAPA can promote the CO2 absorption rate of the intramolecular primary amino group and enhance its CO2 absorption capacity. The results using 13C nuclear magnetic resonance also showed that the intermolecular tertiary amine in the MEA–MDEA system was more favored for the promotion of the primary amine in the blend to form bicarbonate at an earlier CO2 equivalent loading stage and produced less carbamate than the intramolecular tertiary amino group of DEAPA did with respect to its primary amine. Furthermore, the CO2 equilibrium solubility of DEAPA was measured at different temperatures with various CO2 partial pressures, and then an empirical model based on these experimental data was developed. The results predicted by this model were fitted well with the experimental results. The Gibbs–Helmholtz equation was used to estimate the CO2 absorption heat of the DEAPA system, and the results showed that DEAPA has a CO2 absorption heat (−36.4 kJ/mol) lower than those of MEA, MDEA, and DEA. The results demonstrate that DEAPA has the potential to be an alternative solvent with a high absorption rate, a high CO2 capacity, and a low heat of absorption in CO2 capture processes.