Application of the statistical associating fluid theory for potentials of variable range (SAFT-VR) coupled with renormalisation-group (RG) theory to model the phase equilibria and second-derivative properties of pure fluids

Abstract

In earlier work, a methodology to couple SAFT-VR with an RG treatment (SAFT-VR+RG) for square-well chain and associating fluids was presented [E. Forte, F. Llovell, L.F. Vega, J.P.M. Trusler, A. Galindo, J. Chem. Phys. 134 (15) (2011) 154102]. The approach is based on a recursive procedure which begins with an initial free energy incorporating only contributions from short-wavelength density fluctuations, which are treated locally. The contribution from long-wavelength fluctuations is incorporated through the iterative procedure based on attractive interactions that incorporate the structure of the fluid following the ideas of perturbation theories and using a mapping that allows integration of the radial distribution function. The resulting SAFT-VR+RG theory provides an equation of state based on the classical SAFT-VR equation and yields the correct critical behaviour without the need for additional adjustable parameters. In this work, the application of this equation to real fluids is presented. The ability of the methodology to reproduce the phase behaviour close to and far from criticality solely through the standard SAFT-VR molecular parameters is demonstrated here. Molecular model parameters for a range of fluids, both non-associating and associating are presented, including the homologous series of n-alkanes, benzene, carbon dioxide, water, light alcohols, ammonia, hydrogen fluoride and hydrogen sulphide. These were determined by comparison with experimental data for vapour pressure and saturated liquid density. Furthermore, the models presented are used to predict second derivative properties for these compounds. Deviations compared with the original SAFT-VR equation are discussed in order to assess further the improvements introduced by the coupled theory to predict these properties. The critical exponents α, β, γ and δ have also been obtained for a number of these fluids and found to be in good agreement with the universal values of the three-dimensional Ising model.