Equilibrium measurements of the NH3–CO2–H2O system – measurement and evaluation of vapor–liquid equilibrium data at low temperatures

The equilibrium behavior of carbon dioxide (CO2) in aqueous ammonia at low temperatures was studied by means of experiments and modeling. The low-temperature conditions of this system are of interest for the development of ammonia-based CO2 capture. This study includes experimental analyses of the vapor and liquid phases to determine the fate of CO2 in aqueous ammonia. The partial pressure of CO2 at equilibrium was measured in a gas chromatograph. The vapor phase was assessed with ammonia concentrations at 5 and 10 wt%, CO2-loadings in the range of 0.15–0.75, and at the temperatures of 10 °C, 20 °C, and 40 °C. The liquid-phase carbon distribution was determined based on Raman spectroscopy and partial least-squares regression modeling. Speciation of the liquid phase was determined at 5 wt% and 10 wt%, CO2-loadings in the range of 0–0.6, and room temperature (25 °C). Two thermodynamic models of the NH3–CO2–H2O system were evaluated with respect to the measured data. For the prediction of the partial pressure of CO2, the model devised by Que and Chen [6] proved to be the most accurate, while for the liquid-phase predictions, there was no significant difference between the models in terms of accuracy.