Aspen Physical Property System - Physical Property Models

where and are activity coefficients at the infinite dilution
aqueous solution:
Henry Components in NRTL-SAC
Light gases (i.e. Henry components) are usually supercritical at the
temperature and pressure of the system. In that case pure component vapor
pressure is 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, xI�0) and at the temperature and pressure of the system.
2 Thermodynamic Property Models 129
(80)
(81)
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:
where 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:
(82)
(83)
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:
(84)
(85)
where HIS and 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