Online NMR spectroscopic study of species distribution in MEA–H2O–CO2 and DEA–H2O–CO2

Quantitative online NMR spectroscopy was used for studying the species distribution in solutions of carbon dioxide in aqueous monoethanolamine (MEA) and diethanolamine (DEA). The mass fraction of the amine in the unloaded solution was 0.2 and 0.3 g/g, respectively, the carbon dioxide loading was up to 1.1   molCO2/molamine1.1   molCO2/molamine, temperatures were between 293 and 353 K. A special apparatus was designed that allows preparing the mixtures gravimetrically and applying pressures up to 25 bar to keep the carbon dioxide in solution. It was coupled to a 400 MHz NMR spectrometer by heated capillaries. By using both 1H and 13C NMR spectroscopy quantitative information on the concentrations of the following species were obtained: amine, carbamate, bicarbonate, and carbon dioxide. Due to the fast proton transfer between molecular and protonated amine, only the sum of their concentrations can be determined. Furthermore, a byproduct, 2-oxazolidone, was observed and quantified. The experimental data were used for developing a thermodynamic model of the studied electrolyte solutions based on the extended Pitzer GE-model. In the model development, also vapor–liquid equilibrium data from the literature were included. The model gives reliable results both for the species distribution and the vapor–liquid equilibrium of the studied mixtures.