Thermodynamic Properties Definitions
The thermodynamic properties provided are listed as species and mixture property in a Property package.
Ideal Gas
The ideal gas law is independent of composition and determines V at given T and P. Density can be calculated from
(6-102)
The partial fugacity coefficients
(6-103)
The ideal gas enthalpy for mixture
(6-104)
where Hi,ig,Tref relates the enthalpy of an ideal gas to the enthalpy at the selected reference state for species i.
The ideal gas entropy for species i
(6-105)
where Si,ig,Tref is the entropy of an ideal gas to the entropy of species at the selected reference state.
The Gibbs free energy follows from
(6-106)
Equation of State
The equation of state determines V at given x, T, and P. Density can be expressed as Equation 6-102. The partial fugacity coefficients are derived from
(6-107)
where Vi is the partial molar volume. The enthalpy, entropy, and Gibbs free energy follow from the partial fugacity coefficients and the ideal gas contributions as:
(6-108)
(6-109)
(6-110)
Heat Capacity
 
Heat capacity at constant pressure is calculated from
(6-111)
It is available when enthalpy is available. Heat capacity at constant volume is defined by
(6-112)
The relationship between heat capacity at constant pressure and constant volume can be expressed as:
(6-113)
were v is a function of T and P. A Taylor expansion of vat constant composition gives
(6-114)
Rearranging equations above gives
(6-115)
Cv is available if Cp and volume are available and fluid is compressible by means of volume is pressure dependent. For ideal gas Equation 6-113 express as
(6-116)
Specific heat capacity ratio is defined as
(6-117)
Activity Coefficient Models
The enthalpy, entropy, and Gibbs free energy follow from the activity coefficients and the ideal gas contributions in Equation 6-104Equation 6-106. The activity coefficients describe the deviation of chemical potentials from the ideal liquid phase, so heats of vaporization need to be accounted for
(6-118)
(6-119)
(6-120)
Note that if the vapor phase is ideal, then the saturated fugacity, , contribution can be ignored.
Other Properties
Partial fugacity is calculated from
(6-121)
Internal energy is calculated from
(6-122)
where Ui,ig,ref is the enthalpy of an ideal gas to the species enthalpy at the selected reference state.
Helmholtz energy is calculated from
(6-123)
K-values for phases p and q are taken from
(6-124)
If only liquid phases are defined, the K-value calculation is reduced to
(6-125)
(6-126)