268156 Extending the Capabilities of Molecular-Based Equations of State: Simultaneous Calculations of Phase Equilibria and Transport Properties
The description of the fluid phase equilibria of complex systems by means of an equation of state (EoS) has been a subject of study since several years ago, and accurate representations of the phase envelope of pure fluids and binary mixtures are achievable today thanks to these efforts. In this sense, the use of the Statistical Associating Fluid Theory (SAFT), proposed by Gubbins and co-workers , and their variants have allowed a better description of complex systems, overcoming some of the limitations of classical equations for complex fluids. Less effort has been devoted to the exploration of other thermodynamic properties with the same approach, including, for instance, heat capacity, compressibility and the speed of sound; although in the recent years an increasing number of publications have arisen. However, the calculation of transport properties still remains a challenge, due to the difficulties in describing the behavior of properties like the viscosity or the diffusivity of fluids and fluid mixtures with a similar approach and degree of accuracy. Although some models like the Free-Volume-Theory , the Friction Theory [2,3], kinetic-based approaches  or relations obtained from molecular-dynamic simulations  have been proposed in the literature, the amount of publications on the topic and its implementation in conjunction with equations of state still very scarce.
The purpose of this contribution is to advance in this field, showing the practical applicability of the Free Volume Theory (FVT) coupled to the soft-SAFT equation  to simultaneously describe thermodynamic and transport properties of different fluids of industrial interest over a wide range of temperatures and pressures . The approach has been applied to n-alkanes, hydrofluorocarbons, 1-alkanoIs, water and ionic liquids. The procedure is as follows: first, a molecular model for each fluid has been proposed within the framework of soft-SAFT. The molecular parameters are, as usual, fitted to liquid-density (and vapor pressure, if available) experimental data, and are tested by calculating several thermodynamic properties. Then, the FVT is applied to these systems, and 3 additional parameters related to the viscosity are fitted to viscosity data of the pure fluid at several isotherms or isobars. The effect of these parameters has been investigated in order to better understand their impact on the final results. When possible, several trends with the molecular weight and the number of carbons are proposed, in order to increase the predictive ability of the methodology and to reduce the parameters dependency. The work is completed by the calculation of the vapor-liquid equilibria and the viscosity of some mixtures. In some cases, experimental information on the mixture has been used to discriminate among different sets of molecular viscosity parameters of the pure fluid. In general, good agreement is obtained with the experimental data, assessing the reliability of the approach for modeling processes in which such reliable data is needed.
F. Llovell acknowledges a TALENT fellowship from the Generalitat de Catalunya. This work was partially financed by the Spanish Government under projects CTQ2008-05370/PPQ and CENIT SOST-CO2, CEN-2008-1027. Additional support from Carburos Metálicos, Air Products Group, and the Catalan Government was also provided (2009SGR-666).
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