Process design and phase diagrams

Phase diagrams for salt solutions

Process design, simulation, and optimization requires accurate phase diagrams. A phase diagram shows the conditions at which different phases are in thermodynamic equilibrium. Phase diagrams for pure compounds are visual tools to display the properties of pure compounds forming various phases at different temperatures and pressures. In a phase diagram for a pure component, solid-liquid, vapor-liquid, solid-vapor, and solid-solid equilibria can be shown in the same diagram. This is not possible for a binary system. Phase diagrams for binary solutions are more complicated because they have an extra dimension, composition. Because of the extra dimension in binary phase diagrams, vapor-liquid equilibria and solid-liquid equilibria are usually not shown in the same phase diagram.

Phase diagrams showing vapor-liquid equilibrium for binary systems at constant pressure or at constant temperature are the well known Txy and Pxy diagrams. Solid-liquid equilibrium phase diagrams for binary systems can be displayed as a function of temperature and composition at a pressure which is usually above the bubble point of the solutions. The formation of two or more liquid phases in equilibrium with each other in binary and in multi component solutions adds complexity to these systems. Still phase diagrams for binary, ternary, quaternary, and even quinary solutions are valuable tools for visualizing the process conditions required to achieve a specific process objective. Process design, simulation, and optimization can be made more efficiently when such phase diagrams are available.

Phase diagrams are constructed and used for all types of compounds. At this site, the focus is on phase diagrams of aqueous solutions containing salts, amines, or sour gases that create electrolytic solutions. The phase diagrams displayed on these pages are calculated with the Extended UNIQUAC thermodynamic model.

The Extended UNIQUAC model

The Extended UNIQUAC model is a thermodynamic model for aqueous solutions of electrolytes and non-electrolytes. The model has been documented through a number of publications in international scientific journals. The parameters of the model are based on a large amount of experimental data which enables the model to describe the phase behavior and the thermal properties of solutions containing electrolytes with great accuracy.

The experimental data used for determining model parameters include solid-liquid equilibrium data as well as experimentally measured activity and osmotic coefficients from the open literature. Also experimentally measured thermal properties such as heat of dilution, heat of solution, and apparent molal heat capacity for salt solutions were used for the determination of model parameters. These properties can therefore also be reproduced by the model.

The accuracy of the model is best illustrated by displaying calculated and experimental phase diagrams together. Examples of such phase diagrams are given on these pages. Calculated phase diagrams are plotted along with experimental data from the open literature to demonstrate the accuracy of the model. The phase diagrams cover both solid-liquid equilibrium (SLE), vapor-liquid equilibrium (VLE) and liquid-liquid equilibrium (LLE).

The main model parameters of the Extended UNIQUAC model are the interaction parameters, which are determined from experimental data. These parameters describe the interaction between each pair of species. In addition, the model also uses size parameters that are specific to each species. The model predicts the behavior of multi component systems based on these parameters.

The model parameters are determining for pairwise interactions between ions and species alone. The calculation of phase behavior in multi-component systems therefore represents prediction by the model. The ability of the model to predict phase behavior in multi component systems is illustrated by the calculation of quaternary phase diagrams.

Phase diagram for the sodium sulfate - sodium nitrate - water system at 30 °C
Phase diagram for the sodium sulfate - sodium nitrate - water system at 30 °C

Process simulation and optimization

The thermodynamic model makes it possible to perform process simulation and optimization of processes involving aqueous salt solutions. The model has been implemented in multi phase equilibrium calculation software. The software includes vapor-liquid-liquid-solid equilibrium (VLLSE). This software can be called from Microsoft Excel® or other Visual Basic Applications. A free demo version of the software, which only applies to solid-liquid equilibrium calculations can be downloaded from this site.

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