Liquid-Phase Composition Determination in CO₂−H₂O−Alkanolamine Systems: An NMR Study

Abstract

NMR studies were performed investigating the liquid-phase composition in samples where various amounts of CO2 were dissolved in aqueous alkanolamines (butyl-ethanolamine (BEA), methyl-di-ethanolamine (MDEA), and monoethanolamine (MEA)) at various temperatures. Chemical shifts of functional groups present in the systems were determined, and the liquid-phase compositions were calculated for 20 and 40 °C with an estimated error of ∼5−10%, largely dependent on the temperature. The obtained speciation was based on the NMR spectra only and represents, thereby, additional and independent information on the systems that could be used for VLE model refinement. The experimental speciation was compared with the speciation predicted by a thermodynamic model with the activity coefficients calculated by the extended UNIFAC model (group contribution method UNIFAC combined with the Debye−Hückel electrolyte theory). The comparisons showed qualitative agreement and also quantitative agreement for the main species at low and medium loadings. However, for the minor components, and in particular for CO2, the agreement was not satisfactory. Spectra acquired at temperatures above 40 °C (70 and 90 °C), were broadened by fast chemical exchange between the species. The dynamic nature of the system complicated the quantitative evaluation of the spectra; because of the broadening of the peaks, the integration was inaccurate. These spectra could also provide quantitative information and information on the kinetics, but they were not evaluated within the frame of this work. More advanced software for dynamic NMR simulation and regression of kinetic parameters is needed.