Thermodynamic Properties Of Phase Equilibria Research Articles

Implementation of the UNIQUAC model in the OpenCalphad software

The UNIQUAC model is often used, for example in engineering, to obtain activity coefficients in multicomponent systems, while the CALPHAD method is known for its capability in phase stability assessment and equilibrium calculations. In this work, we combine them by representing the UNIQUAC model according to the CALPHAD method and implementing it in the OpenCalphad software. We explain the harmonization of nomenclature, the handling of the model parameters and the equations and partial derivatives needed for the implementation. The successful implementation is demonstrated with binary and multicomponent phase equilibrium calculations and comparisons with literature data. Additionally we show that the implementation of the UNIQUAC model in the OpenCalphad software allows for the calculation of various thermodynamic properties of the systems considered. The combination provides a convenient way to assess interaction parameters and calculate thermodynamic properties of phase equilibria.

Monte Carlo simulations of vapor–liquid equilibria of neon using an accurate ab initio pair potential

Gibbs ensemble Monte Carlo (GEMC) simulations are reported for the vapor–liquid phase coexistence of neon using an accurate ab initio pair potential (NE3) and also a quantum effective Wigner–Kirkwood potential (NE3-WK) based on the NE3 potential. It is noticed that the results from simulations for the thermodynamic properties of phase equilibria deviate from the experimental values (the simulated curve is outside the experimental temperature–density phase envelope). With an ab initio three-body potential of limited quality, the thermodynamic properties of phase equilibria are estimated in good agreement with experiment. It should be stressed that in contrast to most other work no empirical data are used in the present work.

Phase Equilibria in Sugar Solutions Using the A-UNIFAC Model

In this work, a modified UNIFAC model that explicitly takes into account association effects is used to describe the thermodynamic properties of phase equilibria of mixtures containing common sugars, alcohols, and water. Three main groups were defined to represent the sugars family: the sugar ring (pyranose and furanose), the osidic bond (−O−), and the hydroxyl ring group (OHring). For the association term, a general two-site OH associating group is used to represent association effects in these solutions, allowing a straightforward extension to multicomponent mixtures. Correlation of both solvent properties (osmotic coefficients, water activities, vapor pressures, and boiling and freezing points of binary aqueous sugar solutions) and sugar solubility in water and alcohols gives very accurate results. Good predictions are obtained for vapor−liquid equilibrium and solid−liquid equilibrium of ternary and quaternary mixtures of sugars in mixed solvents.

Monte Carlo simulations of neon and argon using ab initio potentials

Gibbs ensemble Monte Carlo simulations of neon and argon have been performed with pair potentials taken from literature as well as with new ab initio potentials from just above the triple point to close to the critical point. The densities of the coexisting phases, their pair correlation functions, the vapour pressure and the enthalpy and entropy of vaporization have been calculated. The influence of the potential choice and of the addition of the Axilrod-Teller (AT) three-body potential on the above mentioned properties have been investigated. It turns out that an accurate ab initio two-body potential in connection with the AT potential yields very good results for thermodynamic properties of phase equilibria.