Vapor–liquid equilibrium of CO2 in NH3–CO2–SO2–H2O system

Vapor–liquid equilibrium (VLE) of CO2 in CO2 and SO2 loaded aqueous ammonia (NH3–CO2–SO2–H2O system) was investigated in this study. The effect of SO2 loading (mol SO2/mol NH3 in solvent) on VLE of CO2 was measured in the range of 0.1–0.3 mol SO2/mol NH3 in 2.5–7.5 wt.% aqueous ammonia at 20 °C, 40 °C and 60 °C, using a Fourier transform infrared (FT-IR) gas analysis method with a pressure determination stirred tank apparatus. The equilibrium vapor pressures of CO2 in the aqueous ammonia with no SO2 content were compared with experimental data in the public domain. The electrolyte NRTL model was carried out to predict the VLE data using Aspen Plus. The comparison between experimental results and model predictions showed good agreement. The total equilibrium pressure, NH3 equilibrium partial pressure and liquid speciation in the NH3–CO2–SO2–H2O system were calculated and analyzed. It was found that the total equilibrium pressure decrease slightly at lower CO2 loading (mol CO2/mol NH3), but increase sharply at higher CO2 loading with SO2 loading. The NH3 equilibrium partial pressure, and the NH3 and NH2COO− molar concentrations decrease with SO2 loading. However, the NH4+ and HCO3− molar concentrations increase with SO2 loading. Further CO2 equilibrium partial pressure analysis shows that the driving force and solvent capacity of CO2 absorption decrease with SO2 loading, while the desorption capacity and driving force of CO2 at high temperature significantly increases with SO2 loading, which has the potential to reduce the regeneration energy of CO2.