(54g) Insights into the Solubility of Hydrocarbons in Water by the Soft-Saft Equation of State

We present here the application of the soft-SAFT equation of state [Blas, F.J. and Vega, L.F., Mol. Phys. 1997, 92, 135] and the crossover soft-SAFT equation of state [Llovell, F. et al., J. Chem. Phys. 2004, 121, 10715; Llovell, F. and Vega, L.F. J. Phys. Chem. B 2006, 110, 1350) to describe the phase behavior of water as well as the mutual solubility of water with hydrocarbons, as compared to experimental data and other modeling approaches. The purpose of this work is twofold: (1) to provide an accurate molecular model for water within the soft-SAFT equation of state and (2) to check the capability of this molecular-based equation of state for capturing the solubility minima experimentally found at room temperature for these mixtures. Water is modeled as a Lennard-Jones sphere with four associating sites, with parameters obtained by fitting to experimental vapor-liquid equilibrium data. An available correlation is used for the molecular parameters of the n-alkane series [Llovell et al. J. Chem. Phys 2004, 121, 10715]. The crossover soft-SAFT equation is able to accurately describe the phase behavior of the pure compounds near to and far from the critical point. A single energy binary parameter, independent of temperature and chain length, is used to describe the water/methane to water/n-decane mixtures. The equation describes the mutual solubilities in almost quantitative agreement with experimental data, including the presence of the solubility minima at ambient temperature. Comparisons for soft-SAFT results with published calculations [Economou, I.G. et al. AIChE J. 1997, 43, 535] with the Huang and Radosz version of the SAFT equation shows clear improvements, due to the more refined reference term and the more accurate radial distribution function used in the chain and association terms. This is the first time a SAFT-type equation is shown to capture this minima.