Aspen Physical Property System - Physical Property Models

For a molecular segment, the activity coefficient can be carried out as follows:
2 Thermodynamic Property Models 125
(47)
For the univalent cation or anion segment, the activity coefficient can be
carried out as follows:
(48)
The original Debye-Hückel theory is based on a single electrolyte with water
as the solvent. The molar volume v and the dielectric constant � for the
single solvent water need to be extended for mixed-solvents based on the
molecular solvent properties; a simple composition average mixing rule is
proposed to calculate them as follows:
where S is a solvent component, MS is the solvent molecular weight, and each
sum is over all solvent components in the solution.
The long range interaction term for the logarithm of the activity coefficient of
component I is computed as the sum of the individual segment contributions.
Molecular components
(49)
(50)
(51)
For molecular components, the activity coefficients are given as follows:
(52)
From Eq. 47, it is easy to show that the PDH term activity coefficients for all
molecular components are normalized; that is
where I applies to all molecular components in the system.
(53)