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A statistical associating fluid theory for potentials of variable range to model dipolar fluids is presented. The new theory, termed the SAFT-VR+D equation (the statistical associating fluid theory for potentials of variable range plus dipole), explicitly accounts for dipolar interactions and their effect on the structure of the fluid. This is achieved through the use of the generalized mean spherical approximation (GMSA) to describe a reference fluid of dipolar square-well segments. Isothermal-isobaric and Gibbs ensemble Monte Carlo simulations have been performed in order to test the new theoretical approach. Predictions for the thermodynamic properties and phase behavior of dipolar square-well monomer and chain fluids, in which one or more segments are dipolar, are considered and compared with new computer simulation data. The results show that the equation of state in general provides a good description of the phase behavior of dipolar monomer and chain fluids. Some deviations are seen between the simulation data and theoretical predictions for monomer fluids with large dipole moments and for molecules composed of segments with dipoles in different orientations. This is addressed through the replacement of the GMSA by the linearized exponential approximation. [ABSTRACT FROM AUTHOR]

We extend the SAFT-VR+DE equation of state to describe 19 aqueous electrolyte solutions with both a fully dissociated and a partially dissociated model. The approach is found to predict thermodynamic properties such as the osmotic coefficient, water activity coefficient, and solution density, across different salt concentrations at room temperature and pressure in good agreement with experiment using only one or two fitted parameters. At higher temperatures and pressures, without any additional fitting, the theory is found to be in qualitative agreement with experimental mean ionic activities and osmotic coefficients. The behavior of the dielectric constant as a function of salt concentration is also reported for the first time using a statistical associating fluid theory (SAFT)-based equation of state. At high salt concentrations, the stronger electrostatic interactions between the ionic species due to the dielectric decrement, is captured through the inclusion of ion association. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3053-3072, 2015 [ABSTRACT FROM AUTHOR]

Abstract: In recent work [Peng et al. Fluid Phase Equilibr. 227(2) (2009), 131] a molecular-based group contribution SAFT-VR equation of state (GC-SAFT-VR) was proposed as a predictive tool for the study of fluid phase behavior. In the GC-SAFT-VR approach a fluid is modeled by a chain of tangentially bonded segments that each represent the different functional groups in the molecule, such as CH3, CH2 and OH, and so represents a more physically realistic model than that used in other SAFT-based approaches in which the segments of the model chain are identical i.e., have the same model parameters. The molecular parameters of the functional groups are determined by fitting to experimental vapor pressure and saturated liquid density data for selected members of different chemical families in a successive fashion, i.e., alkanes for CH3 and CH2 groups and subsequently alcohols for the OH group. The transferability of the molecular parameters can then be tested by comparing the theoretical predictions with experimental data for the phase behavior of pure fluids not included in the fitting process as well as binary mixtures of both simple fluids and polymer systems. Here, we continue the development of the GC-SAFT-VR approach, and for the first time, test its ability to predict the phase behavior of ternary mixtures. In particular, we consider binary and ternary mixtures of 1-pentanol, dibutyl ether and n-nonane. We find that the GC-SAFT-VR approach provides a good description of the phase equilibria of these binary and ternary systems without the need to fit cross interaction parameters to experimental mixture data. [Copyright &y& Elsevier]

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]

SAFT is perhaps the most versatile, fundamentally, based engineering equation of state in use today. However, in common with all analytic equations of state, SAFT exhibits classical behavior in the critical region rather than the non-analytical, singular behavior seen in real fluids. Recently, so-called crossover equations of state have been developed which solve this shortcoming by incorporating the scaling laws valid asymptotically close to the critical point while reducing to the original classical equation of state far from the critical point. We have combined the SAFT-VR equation of state with an analytical crossover technique to obtain the SAFT-VRX equation of state. The SAFT-VRX approach combines the accurate low temperature behavior of SAFT-VR with a precise representation of the critical region. Preliminary results are presented for hydrocarbon systems which illustrate the accuracy of the SAFT-VRX approach over the entire fluid phase region. [Copyright &y& Elsevier]

As part of a combined experimental and theoretical study of the thermodynamic properties of perfluoroalkylalkanes (PFAAs), the liquid density of perfluorobutylpentane (F4H5), perfluorobutylhexane (F4H6), and perfluorobutyloctane (F4H8) was measured as a function of temperature from 278.15 to 353.15 K and from atmospheric pressure to 70 MPa. The liquid densities of n-perfluoropentane, n-perfluorohexane, n-perfluorooctane, and n-perfluorononane were also measured at room pressure over the same temperature range. The PVT behavior of the PFAAs was also studied using the SAFT-VR equation of state. The PFAA molecules were modeled as heterosegmented diblock chains, using different parameters for the alkyl and perfluoroalkyl segments, that were developed in earlier work. Through this simple approach, we are able to predict the thermodynamic behavior of the perfluoroalkylalkanes, without fitting to any experimental data for the systems being studied. Molecular dynamics simulations have also been performed and used to calculate the densities of the perfluoroalkylalkanes studied. [ABSTRACT FROM AUTHOR]

Abstract: Partial molar volumes at infinite dilution have been obtained for 3 perfluoroalkylalkanes (PFAA) in n-octane at 298.15K from experimental apparent molal volumes. The results were interpreted estimating the contributions of the hydrogenated and perfluorinated segments to the partial molar volume. For this reason, partial molar volumes at infinite dilution for 3 perfluoroalkanes (PFA) in n-octane at 298.15K have also been obtained. The results were further interpreted using the hetero-SAFT-VR equation of state, which models the perfluoroalkylalkanes as heterosegmented di-block chains. The cross interactions, both intra and intermolecular, were characterised using parameters developed in earlier studies for (alkane+perfluoroalkane) mixtures: the calculations are thus fully predictive. The theory is able to accurately predict the volumetric behaviour of the solutions of perfluoroalkylalkanes, without fitting to any experimental data. [Copyright &y& Elsevier]

The statistical associating fluid theory is a widely used molecular-based equation of state that has been successfully applied to study a broad range of fluid systems. It provides a framework in which the effects of molecular shape and interactions on the thermodynamics and phase behavior of fluids can be separated and quantified. In the original approach, molecules were modeled as chains composed of identical segments; the heterogeneity of molecules in terms of structure and functional groups was described implicitly through effective parameters. To overcome this limitation, in recent works [Peng et al. Fluid Phase Equilib. 227(2), 131 (2009); Ind. Eng. Chem. Res. 49(3), 1378 (2010)] the GC-SAFT-VR approach has been developed to extend the theory to model chains composed of segments of different size and/or energy of interaction and enable the development of a group-contribution approach within the SAFT-VR framework in which molecular heterogeneity and connectivity is explicitly accounted for. The parameters for several key functional groups (CH3, CH2, CH, CH2=CH, C=O, C6H5, esters, ethers, cis-alkenes and trans-alkenes groups) were determined by fitting to experimental vapor pressure and saturated liquid density data for a number of small molecules containing the functional groups of interest and transferability of the parameters tested by comparing the theoretical predictions with experimental data for pure fluids not included in the fitting process and binary mixtures of both simple fluids and the VLE and LLE of small molecules in polymer systems. In this work, we further extend the applicability of the GC-SAFT-VR approach through the study of the vapor–liquid phase behavior of associating systems, such as linear and branched alcohols, primary and secondary amines, aldehydes, and carboxylic acids, and their mixtures. In the study of these new molecules several new functional groups (OH (linear and branched), HC=O, NH2, NH and C=OOH) are defined and their molecular parameters characterized. The transferability of the parameters is again tested by comparing the theoretical predictions with experimental data for pure fluids and binary mixtures not included in the fitting process. The GC-SAFT-VR approach is found to predict the phase behavior of the systems studied in most cases in good agreement with experimental data and accurately captures the effects of changes in structure and molecular composition on phase behavior. [Copyright &y& Elsevier]

The recently developed heteronuclear group contribution SAFT-VR equation (GC-SAFT-VR) [Penget al. Fluid Phase Equilib.2009, 227(2), 131−144] enables the predictive study of the thermodynamic properties of fluids and their mixtures by using a molecular-based model in which the molecules are described by heterosegmented chains in which each type of segment represents a functional group present in the molecule. Given the success of the GC-SAFT-VR approach in predicting the fluid phase equilibria of mixtures without fitting to any mixture data, and the known difficulties in determining equation-of-state parameters for polymers because of the lack of coexistance data, in this work, we extend the GC-SAFT-VR equation to study the phase equilibria of small molecules in polymer systems. The results demonstrate the ability of the GC-SAFT-VR equation of state to predict the vapor−liquid and liquid−liquid equilibria of polymer solutions and accurately capture the effects of polymer molecular weight and molecular topology on phase behavior. [ABSTRACT FROM AUTHOR]

Abstract: The hetero-segmented version of the statistical associating fluid theory for potentials of variable range (hetero-SAFT-VR) is used to develop a predictive molecular-based group-contribution SAFT-VR equation of state (GC-SAFT-VR). The hetero-SAFT-VR approach models molecules composed of segments of different size and/or energies of interaction enabling an accurate description of real molecules composed of different functional groups. The differences in interactions between functional groups are maintained throughout the theory in contrast to other approaches in which the parameters for functional groups are averaged in order to model a molecule as a homonuclear chain with “group-averaged” parameters. Through the GC-SAFT-VR approach we can study the effect of molecular functionality and topology on the thermodynamic properties of real fluid systems, since parameters are determined for each functional group and chain connectivity is explicitly specified. In this initial study GC-SAFT-VR parameters are developed for key organic functional groups (CH3, CH2, CH2 glyph name="dbnd" />O, C6H5, OCH3 and OCH2) by fitting to experimental vapor pressure and saturated liquid density data for a selected group of compounds that contain these functional groups. The transferability of the parameters obtained is tested by comparing theoretical predictions with experimental data for pure fluids not included in the fitting process and binary mixtures. Using the GC-SAFT-VR approach good agreement is obtained between experimental data and the theoretical predictions for pure substances, including isomers, and their mixtures. The GC-SAFT-VR approach is able to accurately predict the effect of molecular functionality on mixture phase behavior without fitting to any experimental data for the system being studied. [Copyright &y& Elsevier]