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 CO 2 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 1 H NMR spectroscopy were gradually shifted downfield, whereas the corresponding 13 C peaks were shifted upfield as the CO 2 loading increased. Quantitatively, the molar fraction of the carbamate was obtained by integration of the relevant peak areas in the 1 H 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 CO 2 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 CO 2 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.