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Tuck Foundation Chair for Biofuels
Models for mixed-solvent strong electrolytes, using an equation of state (EoS) are reviewed in this work. Through the example of ePPC-SAFT (that includes a Born term and ionic association), the meaning and the effect of each contribution to the solvation energy and the mean ionic activity coefficient are investigated. The importance of the dielectric constant is critically reviewed, with a focus on the use of a salt-concentration dependent function. The parameterization is performed using two adjustable parameters for each ion: a minimum approach distance (σMSA) and an association energy (εAB). These two parameters are optimized by fitting experimental activity coefficient and liquid density data, for all alkali halide salts simultaneously, in the range 298 K–423 K. The model is subsequently tested on a large number of available experimental data, including salting out of Methane/Ethane/CO2/H2S. In all cases the deviations in bubble pressures were below 20% AADP. Predictions of vapor-liquid equilibrium of mixed solvent electrolyte systems containing methanol, ethanol are also made where deviations in bubble pressures were found to be below 10% (AADP).
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 (ΔGHB). Results are found in good agreement with experimental data. (Figure Presented).