Reference & Ideal-Gas Properties

Equations of state always describe residual thermodynamic properties with respect to the ideal gas state. To arrive at a complete description of the fluid, it is necessary to complement the equation of state with the ideal gas heat capacity of the fluid in question and a reference value of the enthalpy and entropy and some specified temperature and pressure. Having the perfect EoS is of no use if the ideal gas properties are inaccurate.

To emphasize the importance of this issue, we have evaluated isentropic compression of pure CO$_2$ from 1 to 2 and 5 bar respectively. The first row in the table below shows that the standard ideal gas heat capacity used in a different, established process modelling software results in an outlet temperature that is 1 K and 1.5 K lower than the most accurate correlation used in TP-Cloud¹ after a compression to 2 and 5 bar respectively.

Using an inaccurate ideal gas capacity can give errors of several degrees of Kelvin on the outlet temperature from valves, compressors or heat exchangers.

Furthermore, in order to correctly capture both the equilibrium compositions and the enthalpy of reaction, it is crucial to have accurate and consistent reference enthalpies and entropies for the components involved. Many EoS, such as cubic EoS, use the critical temperature and pressure as input.

In TP-Cloud, we use the most accurate values from literature for all of the above properties, regardless of the choice of EoS. We use the standard enthalpy and entropy of formation of the pure components at a pressure of one bar and a temperature of 298.15K as a reference, unless otherwise specified.

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References

Footnotes

1. Neumann, Tobias, et al. "EOS-CG-2021: A Mixture Model for the Calculation of Thermodynamic Properties of CCS Mixtures." International Journal of Thermophysics 44.12 (2023): 178. ↩