Aspen Physical Property System - Physical Property Models

Where is the liquid Gibbs free energy of mixing; it is defined as the
difference between the Gibbs free energy of the mixture and that of the pure
component and is the ideal Gibbs free energy of mixing. Once the
excess liquid functions are known, the thermodynamic properties of liquid
mixtures can be computed as follows:
2 Thermodynamic Property Models 167
(95)
(96)
(97)
where Hi l and Hi ig are the enthalpy and ideal gas enthalpy of component i at
the system conditions. Similarly, Gi l and Gi ig are the Gibbs free energy and
ideal gas Gibbs free energy of component i at the system conditions. In Aspen
Plus, both Hi ig and Gi ig are computed by the expressions:
where is the standard enthalpy of formation of ideal gas at
, is the ideal gas heat capacity, and is the
standard Gibbs free energy of formation of ideal gas at .
However, the above equations are directly not applicable to mixtures
containing ionic components because the ideal gas model becomes invalid for
ionic components.
The formulation to calculate the enthalpy and Gibbs free energy for
electrolyte systems can be carried out as follows:
(100)
(101)
(102)
where indexes s, h, and ca are meant to represent the contributions from
solvents, Henry components and ionic components, respectively.
(98)
(99)