Aspen Physical Property System - Physical Property Models

NRTL-SAC Reference States
The NRTL-SAC activity coefficient model for component I is composed of the
local composition term, ln �I lc , the Pitzer-Debye-Hückel long-range interaction
term, ln �I PDH , and the Flory-Huggins term, �I FH :
This equation needs to be normalized based on the reference states of
molecular and ionic components.
Reference state for molecular components
The reference state for a molecular component is defined as follows:
This definition is the so-called standard state of pure liquids for molecular
components and it is also called the symmetric reference state for molecular
components.
Reference state for ionic components
The standard state of pure liquids is hypothetical for ionic components in
electrolyte systems. The symmetric reference state is defined as the pure
fused salt state of each electrolyte component in the system.
However, the conventional reference state for ionic components is the
infinite-dilution activity coefficient in pure water; it is also called the
unsymmetric reference state for ionic components. In NRTL-SAC model, we
will consider both of these reference states; the unsymmetric state is the
default.
Pure fused salt state of an electrolyte component
For an electrolyte component CA, the pure fused salt state can be defined as
follows:
where �± is the mean ionic activity coefficient of the electrolyte component
and is related to the corresponding cationic and anionic activity coefficients �C
and �A by this expression:
where �C is the cationic stoichiometric coefficient and �A is the anionic
stoichiometric coefficient, and �=�C+�A (one mole of salt releases � moles of
ions in solution). They are given by the chemical equation describing the
dissociation of the electrolyte. Therefore Eq. 5 can be written in terms of
charge numbers zC and zA:
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