Modelling the phase equilibria and excess properties of the water plus carbon dioxide binary mixture

The high-pressure phase diagram and excess thermodynamic properties of the binary mixture of carbon dioxide and water are examined using the statistical associating fluid theory for potentials of variable range (SAFT-VR). The carbon dioxide molecule is modelled with two tangentially bonded spherical segments, while the water molecule is modelled as spherical with four associating sites to represent the hydrogen bonding. Dispersion interactions are modelled using square-well potentials. The optimised intermolecular parameters are taken from the works of Galindo and Blas [F.J. Blas, A. Galindo, Fluid Phase Equilib. 194-197 (2002) 501-509; A. Galindo, F.J. Blas, J. Phys. Chem. B 106 (2002) 4503-4515] and Clark et al. [G.N.I. Clark, A.J. Haslam, A. Galindo, G. Jackson, Mol. Phys. 104 (2006) 3561-3581] for carbon dioxide and water, respectively. The mixture exhibits type III phase behaviour in the classification of Scott and van Konynenburg, with the gas-liquid critical line continuously changing into a liquid-liquid line at high pressures. In this work one unlike intermolecular interaction parameter is fitted to give the best possible representation of the minimum temperature of the gas-liquid critical line of the mixture, and is then used in a transferable manner to study other thermodynamic conditions and properties. The phase diagrams predicted by the SAFT-VR approach are found to be in very good agreement with the experimental data at low and high pressures and temperatures. In addition, a good qualitative description of the excess molar volume and excess enthalpy and different temperatures and pressures is obtained.