Heat Capacities of Fluids: The Performance of Various Equations of State
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Abstract
Heat capacities, including both isochoric and isobaric, are one of the most important thermophysical properties of fluids.
They are related to the temperature derivatives of fundamental thermodynamic functions and they can, therefore, be used to derive other properties such as enthalpy and entropy.
In the context of equations of state (EoS), heat capacities are of interest also for testing the performance limits of such models.
Variations in heat capacities are also required to understand the changes of fluid structures and heat transfer processes in industrial applications.
In terms of the calculations of heat capacities, many models have been proposed in the literature.
Equations of state represent one of the most promising methods, but their performance has not been systematically studied and extensively reviewed.
In this work, the calculations and performance of various equations of state for heat capacities are reviewed, and the different contributions to heat capacities are also discussed.
The accuracy of the calculated heat capacities, as presented in literature, is also compared for some specific compounds, and the effects of different parametrization strategies as well as association schemes are analyzed.
Then, to carry out a fair and comprehensive comparison of two of the most well-known models, we consider CPA and PC-SAFT and perform in this work a systematic evaluation of them for heat capacities and other relevant properties for several associating and non-associating compounds.
These calculations are performed at both compressed condition and critical isotherms. Then based on the knowledge of the contribution of heat capacity from the ideal gas, the performance of two equations at different conditions are discussed.
After that, the performance of the polar CPA with Jog and Chapman’s dipole term is tested for several polar fluids, which are divided into associating fluids, polar non-associating fluids and ionic liquids.
For polar non-associating fluids and ionic liquids, different parametrization strategies are used in this work, and the ionic liquids are considered as both pure associating compounds and pure polar compounds. The contributions of derivative properties from different terms for polar fluids are also calculated, and this gives us a clear knowledge of dominant term for these properties and compounds.
At last, some conclusions with three aspects of performance of EoS, contribution to heat capacity and accuracy in different regions are given.