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.

Experimental and Molecular Modeling Study of the Three-Phase Behavior of (n-Decane + Carbon Dioxide + Water) at Reservoir Conditions

“…Later studies of the same system with the SAFT-VR equation have resulted in similar findings: the properties of (carbon dioxide + water) can be predicted accurately without incorporating association between the two species. [67][68][69] More rigourous models for CO 2 can additionally be considered by incorporating the contribution of the molecular quadrupole into the equation of state. 70,71 Regarding the study of ternaries and, in general multicomponent mixtures, a question arises.…”

“…We compare our experimental findings with the predictions of SAFT-VR. 37,38 We use this method as it has already been shown in a number of studies [67][68][69][72][73][74] to provide an accurate description of the phase behaviour of mixtures consisting of alkanes, CO 2 and water, for wide ranges of thermodynamic conditions. Finally, where possible, further comparisons with data for the binary subsystems are also presented.…”

Theoretical investigation of the phase behaviour of model ternary mixtures containingn-alkanes, perfluoro-n-alkanes, and perfluoroalkylalkane diblock surfactants

“…The SAFT approach has been extended to deal with different intermolecular potentials, such as the SAFT-VR extension developed by Gil Villegas et al [32,33] that describes the behaviour of chains formed by spherical segment interacting through a potential of variable range, typically the square-well potential. The SAFT-VR approach, as other versions of SAFT, can be considered as one of the most powerful predicting equations of state nowadays for determining the thermodynamic properties of systems as complex as chain-like, including the n-alkane series, the perfluoro-n-alkanes and their mixtures [33][34][35][36][37][38], systems with refrigerants [39][40][41], surfactants [42][43][44][45][46], polymeric [47][48][49][50] and aqueous systems [51][52][53], among others. In recent works [43][44][45][46], in a collaboration with other research groups, we have used a modification of the SAFT-VR approach, called hetero-SAFT-VR [54][55][56], where molecules are composed of different segments, to predict some thermodynamic properties, including the phase equilibrium of two different perfluoroalkylalkane molecules, namely the perfluorohexylhexane and perfluorohexyloctane, without the need of experimental data from these surfactants.…”

Experimental and molecular modelling study of the three-phase behaviour of (propane+carbon dioxide+water) at reservoir conditions

“…We compare our experimental findings with the predictions of SAFT-VR [74,75]. We use this method as it has already been shown in a number of studies [4,66,68,69,71,72,[76][77][78] to provide an accurate description of the phase behaviour of mixtures consisting of alkanes, CO 2 and water, for wide ranges of thermodynamic conditions. Finally, where possible, the agreement or deviations from published binary experimental data are further studied to analyse the effect of adding a third component and a third coexisting phase in the phase equilibria of the system.…”

“…Associating theories such as the statistical associating fluid theory (SAFT) [52,53] prove advantageous for modeling these highly non-ideal mixtures. Previous applications of associating theories to model (alkane + water) systems [54][55][56][57][58][59][60][61][62][63][64][65] and (carbon dioxide + water) [66][67][68][69][70][71] were reviewed in detail in our previous manuscript [4], to which the reader is referred for a complete analysis. Certain challenges were highlighted in these systems, such as the difficulty of modelling mutual solubilities of phases of very different nature, as represented by large variations between the dielectric constant of each phase and/or dipole moment of the molecules.…”