A New PC-SAFT Model for Pure Water, Water-Hydrocarbons, and Water-Oxygenates Systems and Subsequent Modeling of VLE, VLLE, and LLE

Accurate analytic thermodynamic modeling of water and its mixtures with hydrocarbon and oxygenates is difficult even with new and advanced equations of state such as the perturbed-chain statistical associating fluid theory (PC-SAFT). Several attempts have been made in the past by various authors to solve this issue. However, current models generally fail to describe simultaneously and accurately pure water properties (especially its liquid density) and liquid-liquid equilibria for mixtures involving water, hydrocarbons, and oxygenates. In the current work, this problem is dealt with by modification in the fundamental structure of the model. It was established that the temperature dependent diameter d(T) does not behave in the same way for water as it inscribed in the original model. Hence, a modification was proposed for d(T) of water in order to correctly represent the phase behavior of pure water and its mixtures with hydrocarbons and oxygenates. The deviations in saturated liquid densities and vapor pressure for pure water were reduced to 0.6% and 2.2%, respectively, in a large temperature range. The results for liquid-liquid equilibrium (LLE), vapor-liquid equilibrium (VLE) and vapor-liquid-liquid equilibrium (VLLE) of various water-hydrocarbons and oxygenates show the accuracy of this new model and its predictive capability when coupled with a group contribution approach. For certain oxygenated mixtures such as water with aldehydes, ketones, ethers, and esters a new contribution to the Helmholtz energy, known as the non-additive hard sphere contribution, was used. The cross-interaction parameters obtained for mixtures were validated qualitatively by calculating octanol/water partition coefficients and the Gibbs free energy of hydrogen bonding (Delta G(HB)). Results are found in good agreement with experimental data.