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Molecular thermodynamic analyses for the multicomponent phase equilibrium using equations of state and lattice based models

Molecular thermodynamic analyses for the multicomponent phase equilibrium using equations of state and lattice based models
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In this thesis, phase equilibria of multicomponent systems are investigated. In addition to the experimental measurement of the phase equilibrium points, the molecular thermodynamic modeling using equations of state and lattice based models has been performed. Accurate prediction of phase behavior for multicomponent systems is essential to optimizing existing processes or designing new processes. In particular, it plays a key role in setting process operating conditions for separation of mixtures that occur during product production, and solving problems caused by phase changes in the working fluid. Therefore, this study is expected to make a contribution to various chemical engineering industries related to material production. This paper contains the following four specific contents. At first, an experimental method for measuring the liquid-liquid equilibrium temperature of a mixed solution using thermo-optical analysis technique was introduced. The cloud points of binary and ternary polymer solutions were detected, and it was possible to confirm the changing phase according to the molecular weight of the polymer and the type of solvents. Secondly, a new equation of state was proposed considering the effects of different chain lengths of molecules. The equation of state accurately described the vapor-liquid equilibria of fluids far from the critical region. Then, it was combined with a renormalization group theory to consider the contribution of density fluctuations in the near critical region. As a result, accurate predictions of the vapor-liquid equilibrium were obtained for small molecules including alkanes, carbon dioxide, and other hydrocarbons and their binary mixtures. In addition, the expression for chain length dependency was newly adapted to describe phase equilibrium for polymers. The modified model showed higher accuracy than other existing models in various phase behaviors ranging from vapor-liquid, liquid-liquid, and solid-liquid equilibrium of polymer solutions. Thirdly, the lattice based molecular thermodynamic models were applied to describe the liquid-liquid equilibrium for various types of binary and ternary polymer solutions. By using the lattice based model with a relatively simple form and calculation method, it has been shown that various types of ternary liquid-liquid phase behaviors can be effectively described, including UCST, LCST, and other types that appear in sub-binary systems. Finally, prediction of the interfacial tension of the interface existing between the two separated phases was performed. The interfacial tension can be expressed by using the density gradient theory that shows the free energy for an inhomogeneous phase in consideration of the density profile distributed at the interface. The phase equilibrium results calculated with the molecular thermodynamic models were combined with density gradient theory to accurately calculate the interfacial tension at the equilibrium interface. Through the prediction of interfacial tension, it was confirmed that the suggested models can be applied not only to the description of phase behavior but also to the prediction of the important thermodynamic properties. The models developed by applying molecular thermodynamic theories were used to predict and interpret various phase behaviors and interfacial tensions of mixtures. In the case of the developed equation of state, it has a relatively complex mathematical form, but shows a high agreement with experimental data. The model using a lattice-based theory with the advantage of simple form have been shown to be applicable for variety of multicomponent liquid-liquid behaviors.
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