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
(2-102)
The partial fugacity coefficients
(2-103)
The ideal gas enthalpy for mixture
(2-104)
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
(2-105)
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
(2-106)
Equation of State
The equation of state determines V at given x, T, and P. Density can be expressed as Equation 2-102. The partial fugacity coefficients are derived from
(2-107)
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:
(2-108)
(2-109)
(2-110)
Heat Capacity
 
The heat capacity at constant pressure is calculated from
(2-111)
It is available when the enthalpy is available. The heat capacity at constant volume is defined by
(2-112)
The relationship between the heat capacity at constant pressure and constant volume can be expressed as
(2-113)
were v is a function of T and P. A Taylor expansion of vat constant composition gives
(2-114)
Rearranging equations above gives
(2-115)
Cv is available if Cp and volume are available and fluid is compressible by means of volume is pressure dependent. For ideal gas Equation 2-113 express as
(2-116)
Specific heat capacity ratio is defined as
(2-117)
Activity Coefficient Models
The enthalpy, entropy, and Gibbs free energy follow from the activity coefficients and the ideal gas contributions in Equation 2-104Equation 2-106. The activity coefficients describe the deviation of chemical potentials from the ideal liquid phase, so heats of vaporization need to be accounted for
(2-118)
(2-119)
(2-120)
Note that if the vapor phase is ideal, then the saturated fugacity, , contribution can be ignored.
Other Properties
Partial fugacity is calculated from
(2-121)
Internal energy is calculated from
(2-122)
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
(2-123)
The K-values for phases p and q are taken from
(2-124)
If only liquid phases are defined, the K-value calculation is reduced to
(2-125)
(2-126)