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A statistical associating fluid theory to model electrolyte fluids that explicitly accounts for solvent molecules by modeling water as a dipolar square-well associating fluid is presented. Specifically the statistical associating fluid theory for potentials of variable range (SAFT-VR) is combined with integral equation theory and the generalized mean spherical approximation using the nonprimitive model to describe the long-range ion-ion, ion-dipole, and dipole-dipole interactions. Isothermal-isobaric ensemble Monte Carlo simulations have been performed in order to test the new theoretical approach. In particular, simulations are performed for different ion concentrations and different ratios of the cation, anion, and solvent segment diameters. Predictions for the thermodynamic properties from the new equation of state are compared with the computer simulation data. Additionally, results from a combination of the SAFT-VR approach with Debye-Hückel theory and the primitive model are also presented and compared to those obtained with the nonprimitive model to illustrate the advantages of the new statistical associating fluid theory for potentials of variable range plus dipole and electrolytes (SAFT-VR+DE) approach. The results show that the proposed equation of state provides a good description of the PVT properties of electrolyte fluids with different sizes of ions and solvent. [ABSTRACT FROM AUTHOR]

A statistical associating fluid theory for potential of variable range is developed to model dipolar fluids (SAFT-VR+D) and the high-pressure phase diagram of hydrogen sulfide + n-alkane binary mixtures is examined. The SAFT-VR+D theory has explored and predicted the effect of temperature and the molecular weight of the hydrocarbon component on the solubility of [H.sub.2]S in the n-alkanes.

Abstract: The high-pressure phase diagram and other thermodynamic properties of the binary systems of water+hydrogen sulphide have been examined using the SAFT-VR+D equation of state [Zhao and McCabe, J. Chem. Phys. 125 (2006) 104504]. The SAFT-VR+D approach is based on a version of the statistical associating fluid theory that was developed to model dipolar fluids by explicitly accounting for dipolar interactions and their effect on the thermodynamics and structure of a fluid. In the SAFT-VR+D equation this is achieved through the use of the generalized mean spherical approximation (GSMA) to describe a monomer fluid of dipolar square-well segments. In the present work, we further develop the SAFT-VR+D equation to study mixtures of dipolar associating fluids of arbitrary size using the analytical solution of the Ornstein–Zernike equation due to Cummings and Blum [J. Chem. Phys. 84, 1833 (1986)]. The high-pressure phase diagram and other thermodynamic properties of the water+hydrogen sulphide binary mixture are then examined. Both components, hydrogen sulphide and water, are naturally modeled as associating spherical molecules with four off-centre sites to mimic hydrogen bonding and an embedded dipole moment (μ) to describe their polarity. Using simple Lorentz–Berthelot combining rules, the theory is able to predict the phase behavior of the H2O+H2S system from pure component parameters. Specifically, type III phase behavior according to the classification of Scott and van Konynenburg is observed, with a three-phase line at low temperatures that terminates at an upper critical end point and two separate critical lines, one at high temperatures that runs continuously from a gas–liquid critical line to a liquid–liquid critical line at high pressures and a second gas–liquid critical line located at low temperatures. The phase behavior predicted by the SAFT-VR+D approach is in excellent agreement with the experimental data, without requiring any fitted binary interaction parameters. In addition, the theory is also able to describe the excess molar enthalpies and the most important features of the Henry''s constants as a function of temperature. [Copyright &y& Elsevier]

A statistical associating fluid theory for potential of variable range has been recently developed to model dipolar fluids (SAFT-VR+D) [Zhao and McCabe, J. Chem. Phys. 2006, 125, 104504]. The SAFT-VR+D equation explicitly accounts for dipolar interactions and their effect on the thermodynamics and structure of a fluid by using the generalized mean spherical approximation (GMSA) to describe a reference fluid of dipolar square-well segments. In this work, we apply the SAFT-VR+D approach to real mixtures of dipolar fluids. In particular, we examine the high-pressure phase diagram of hydrogen sulfide+n-alkane binary mixtures. Hydrogen sulfide is modeled as an associating spherical molecule with four off-center sites to mimic hydrogen bonding and an embedded dipole moment (micro) to describe the polarity of H2S. The n-alkane molecules are modeled as spherical segments tangentially bonded together to form chains of length m, as in the original SAFT-VR approach. By using simple Lorentz-Berthelot combining rules, the theoretical predictions from the SAFT-VR+D equation are found to be in excellent overall agreement with experimental data. In particular, the theory is able to accurately describe the different types of phase behavior observed for these mixtures as the molecular weight of the alkane is varied: type III phase behavior, according to the scheme of classification by Scott and Konynenburg, for the H2S+methane system, type IIA (with the presence of azeotropy) for the H2S+ethane and+propane mixtures; and type I phase behavior for mixtures of H2S and longer n-alkanes up to n-decane. The theory is also able to predict in a qualitative manner the solubility of hydrogen sulfide in heavy n-alkanes.