Quantitative online nuclear magnetic resonance (NMR) spectroscopy was used to study the species distribution in solutions of carbon dioxide (CO2) in aqueous N-methyldiethanolamine (MDEA), and MDEA + piperazine (PIP). The mass fraction of MDEA in the unloaded ternary solution was 0.2, 0.3, and 0.4 g/g. In quaternary solutions, the mass fraction of MDEA was 0.3 g/g, and that of PIP was 0.1 g/g. The temperature ranged from 293 K to 333 K, and the overall CO2 loading was up to 1.4 molCO2
/molamine. For the measurements, a special apparatus was used that allowed the mixtures to be prepared gravimetrically and applied pressures up to 25 bar to keep the CO2 in solution. It was coupled to a 400 MHz NMR spectrometer by heated capillaries. Using both 1H and 13C NMR spectroscopy, quantitative information on the concentrations of the following species was obtained: amines, carbamates, bicarbonate, and CO2. Because of the fast proton transfer between molecular and protonated amines, only the sum of their concentrations can be determined. Furthermore, a byproduct was observed and quantified. The experimental data were used to develop a thermodynamic model of the studied electrolyte solutions, based on the extended Pitzer G
E-model. In the model development, vapor−liquid equilibrium (VLE) data from the literature also were included. The model describes both the species distribution and the VLE of the studied mixtures. The properties of the quaternary system are predicted from information on the subsystems.
In this work, the monoalkylcarbonate ((Nhydroxyethyl)(N-methyl)(2-aminoethyl) hydrogen carbonate) formation in the system methyldiethanolamine (MDEA)−water (H 2 O)− carbon dioxide (CO 2 ) is investigated by nuclear magnetic resonance (NMR) spectroscopy. Aqueous solutions containing 0.4 g/g of MDEA were loaded with CO 2 in valved NMR tubes, and the composition of the liquid phase in equilibrium was determined in situ at 298 K at pressures up to 11 bar. By two-dimensional NMR, the presence of monoalkylcarbonate was verified, which has been widely overlooked in the literature so far. The experimental data of this work and reevaluated NMR data obtained in previous work of our group were used to calculate chemical equilibrium constants of the proposed monoalkylcarbonate formation. A model taken from the literature that describes the solubility of CO 2 in aqueous solution of MDEA and the corresponding species distribution is extended so that it can account for the monoalkylcarbonate in the liquid phase as well. The extended model is validated using NMR data in the temperature range 273−333 K. The study shows that more than 10 mol % of the absorbed CO 2 is bound as monoalkylcarbonate under conditions relevant for technical applications.
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