Aspen Physical Property System - Physical Property Models

meaningless and therefore the pure liquid state at the temperature and
pressure of the system cannot serve as the reference state. The reference
state for a Henry component is redefined to be at infinite dilution (that is, at
xi�0) and at the temperature and pressure of the system.
The liquid phase reference fugacity, fi *,l , becomes the Henry’s constant for
Henry components in the solution, Hi, and the activity coefficient, �i, is
converted to the infinite dilution reference state through the relationship:
164 2 Thermodynamic Property Models
(76)
�
where �i is the infinite dilution activity coefficient of Henry component i
(xi�0) in the solution.
By this definition �i * approaches unity as xi approaches zero. The phase
equilibrium relationship for Henry components becomes:
(77)
The Henry’s Law is available in all activity coefficient property methods. The
model calculates the Henry’s constant for a dissolved gas component in all
solvent components in the mixture:
(78)
(79)
�
where His and �is are the Henry’s constant and the infinite dilution activity
coefficient of the dissolved gas component i in the solvent component s (xi�0
and xs�1), respectively.
Since ionic species exist only in the liquid phase and therefore do not
participate directly in vapor-liquid equilibria, the activities of Henry
components are mainly through the local interactions with solvents. We can
calculate the activity coefficients for Henry components as follows:
(80)
(81)
(82)
where xh�0 applies to all Henry components in the solution.