Thermodynamics of the carbon dioxide plus argon (CO 2 + Ar) system: An improved reference mixture model and measurements of vapor-liquid, vapor-solid, liquid-solid and vapor-liquid-solid phase equilibrium data at the temperatures 213–299 K and pressures up to 16 MPa

Abstract Phase equilibrium behavior of the carbon dioxide and argon system has been investigated at the temperatures 213, 223, 243, 263, 273, 283, and 299 K. The full vapor-liquid equilibria phase envelope has been measured at all temperatures using an analytical technique where the compositions of both the liquid and vapor phase have been measured. In addition, the fluid compositions at the three-phase line and phase equilibria involving solids at 213 K have been measured. The three-phase line was determined at 213 K with an estimated uncertainty of 2 kPa. Otherwise, the estimated uncertainty is better than 13 mK for the temperature measurements, 3.2 kPa for the pressure measurements, and 0.12% in total combined uncertainty in terms of mole fraction for all the 107 measured data points. The new data have been compared with existing models, and estimates for the critical points of the 7 isotherms have been made. Together with recently established experimental results for homogeneous density, speed of sound, and dew-point pressure, the vapor-liquid-equilibrium data were used to develop an improved Helmholtz-energy-explicit mixture model. This new model enables the calculation of highly-accurate data for all types of thermodynamic properties. Its development is part of the ongoing work on setting up an extended multi-fluid mixture model for the description of carbon-dioxide-rich mixtures with various impurities as relevant for CCS applications.