Aspen Physical Property System - Physical Property Models

Reference state for ionic components
The standard state of pure liquids is hypothetical for ionic components in
electrolyte systems. Instead, 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 coefficients; it is also called the unsymmetric
reference state for ionic components.
GMENRTLS uses the symmetric reference state, while GMENRTLQ uses the
unsymmetric reference state.
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:
with
2 Thermodynamic Property Models 155
(6)
(7)
(8)
(9)
(10)
Therefore Eq. 4 can be written in terms of charge numbers zc and za:
(11)
At the pure fused salt state, the total moles of ionic components (for one
mole of salt) are:
�=�c+�a
therefore
(12)
(13)