NMR Studies of Amine Species in MEA-CO2-H2O System: Modification of the Model of Vapor-Liquid Equilibrium (VLE)

Speciation studies in aqueous monoethanolamine (MEA) solution at different CO2 loadings have been conducted via nuclear magnetic resonance (NMR) spectroscopy. The results showed that the diagnostic peaks corresponding to the MEA and the protonated amine (MEAH(+)) observed by H-1 NMR spectroscopy were gradually shifted downfield, whereas the corresponding C-13 peaks were shifted upfield as the CO2 loading increased. Quantitatively, the molar fraction of the carbamate was obtained by integration of the relevant peak areas in the H-1 NMR spectra while the molar fractions of free MEA and MEAH(+) were determined according to the MEAH(+) dissociation constant and pH value of the solution. Furthermore, as the CO2 loading increased, the fraction of free MEA dropped steadily, while the ratio of carbamate and MEAH(+) increased monotonically. The molar ratio of the carbamate reached its maximum at a CO2 molar loading of 0.5. The data obtained from this study have led us to propose a modification to the current vapor-liquid equilibrium (VLE) model for this system.