CO2 mass transfer and solubility in aqueous primary and secondary amine

Abstract

Three amines were tested as solvents for CO2 capture: monoisopropanolamine (MIPA) and 3 amino 1 propanol (MPA), which are primary amines, and diethanolamine (DEA), which is a secondary amine. The three amines were tested at 7 molal in water, which corresponds to 30 wt % monoethanolamine (MEA). The CO2 absorption rate and solubility in the solvents was measured using a bench scale wetted wall column (WWC) at 20-100 °C and over the range of CO2 loading expected for coal flue gas. The experimental data are analyzed to suggest performance in a real process. The performance of the three new amines are compared against literature results for 7 m MEA and other solvents with amines of related structures, such as 10 m Diglycolamine (DGA)® and 4.8 m 2-amino-2-methyl-1-propanol (AMP). The mass transfer performance and the CO2 carrying ability of the amine are related to the structural hindrance and the pKa. The CO2 absorption rate of 7 m MIPA and 7 m DEA are competitive with 7 m MEA. For 7 m MPA, the measured liquid film mass transfer coefficient is about 20% lower than 7 m MEA. The CO2 solubility data are used to generate a semi-empirical model for each of the new solvents. The results for CO2 solubility at 40 °C suggest that MIPA and MPA have similar CO2 capacity to MEA, whereas DEA has double the CO2 capacity of the three primary amines. Based on the comparison of data collected in this work and literature results, it is generalized for unhindered amines that the overall CO2 mass transfer performance of the solvent is inversely related to the pKa of the amine. Also, the CO2 capacity of the amine increases with the relative hindrance of the nitrogen group in the amine molecule.