Aspen Physical Property System - Physical Property Models

Activity Coefficient Basis for Henry Components
Regardless of the reference state for ionic components, there are two
�
possibilities for the basis of unsymmetric activity coefficients �h of Henry
components: aqueous and mixed-solvent. This can be specified on the Setup
| Simulation Options | Reactions sheet. Mixed-solvent is the default.
�
For mixed-solvent basis, �h is calculated as follows:
2 Thermodynamic Property Models 165
(83)
where xh�0 applies to all Henry components and xi�0 to all ionic
components in the solution.
For aqueous basis, the unsymmetric activity coefficients of Henry components
are calculated as follows:
Electrolyte Chemical Equilibria
(84)
In determining the composition of an electrolyte system, it is important to
know the equilibrium constants of the reactions taking place. An equilibrium
constant is expressed as the product of the activity of each species raised to
its stoichiometric coefficients. Two different scales are used in Aspen Plus: the
molality scale and the mole fraction scale but both are based on aqueous
electrolyte chemical equilibrium. For instance, the equilibrium constant for the
mole fraction scale in Aspen Plus is written in one of these:
Where:
K = Equilibrium constant
xw = Water mole fraction
�w = Water activity coefficient
xs = Non-water solvent mole fraction
�s = Non-water solvent activity coefficient
(85)
(86)
xi = Mole fraction of solute component (Henry or
ion)
�i * = Unsymmetric activity coefficient of solute
component
�i = Stoichiometric coefficient
The above equations are limited to aqueous electrolyte chemical equilibria
only. Therefore, the chemical constants in Aspen Plus databanks for