Phase Equilibria, Excess Properties, and Henry's Constants of the Water + Carbon Dioxide Binary Mixture

Abstract

The high-pressure phase diagram and other thermodynamic properties of the water + carbon dioxide binary mixture are examined using the SAFT-VR approach. The carbon dioxide molecule is modeled as two spherical segments tangentially bonded. The water molecule is modeled as a spherical segment with four associating sites to represent the hydrogen bonding. Dispersive interactions are modeled using the square-well intermolecular potential. The polar and quadrupolar interactions present in water and carbon dioxide are treated in an effective way via square-well potentials of variable range. The optimized intermolecular parameters are taken from the works of Galindo and Blas (Fluid Phase Equilib. 2002, 194−197, 502; J. Phys. Chem. B 2002, 106, 4503) and Clark et al. (Mol. Phys. 2006, 22−24, 3561) for carbon dioxide and water, respectively. The phase diagram of the mixture exhibits a number of interesting features: type-III phase behavior according to the classification of Scott and Konynenburg, three-phase behavior at low temperatures with its corresponding upper critical end point, a gas−liquid critical line at high temperatures and pressures that continuously changes from gas−liquid to liquid−liquid as the pressure is increased and gas−gas immiscibility of second kind. Only one unlike interaction parameter is fitted to give the best possible representation of the temperature minimum of the gas−liquid critical line of the mixture. This unlike parameter is then used in a transferable manner to study the complete pressure−temperature−composition phase diagram. The phase diagram calculated with SAFT-VR is in excellent agreement with the experimental data taken from the literature in a wide range of thermodynamic conditions. The theory is also able to predict a good qualitative description of the excess molar volume and enthalpy of the mixture as well as the most important features of the Henry's constants at different temperatures.