Aspen Physical Property System - Physical Property Models

Aspen Plus and Aspen Properties. �Hi and �Gi are the new contributions from
the symmetric reference state.
The unsymmetric enthalpy for an ionic component is calculated from
the infinite dilution aqueous phase heat capacity as follows:
170 2 Thermodynamic Property Models
(124)
where T ref = 298.15K. By default, is calculated from the aqueous
infinite dilution heat capacity polynomial. If the polynomial model parameters
are not available, is calculated from the Criss-Cobble correlation.
The unsymmetric Gibbs free energy for an ionic component is
calculated from the infinite dilution aqueous phase heat capacity as
follows:
(125)
(126)
where the term RT ln (1000/Mw) is added because and
are based on a molality scale, and is based on a mole fraction scale.
�Hi and �Gi for the symmetric reference state
The reference state for ionic components in the symmetric electrolyte NRTL
model is the pure fused salts containing these ions, so the enthalpy or Gibbs
free energy of the ionic components at the symmetric reference state is the
enthalpy contributions or the Gibbs free energy contributions of these ions to
the system of the pure fused salts. The condition of the pure fused salts can
be defined as follows:
which applies to all molecular components in the solution.
(127)
Given that the calculated total enthalpy or total Gibbs free energy of an
electrolyte solution by any reference state should be the same, the
formulation of enthalpy or Gibbs free energy of the ionic components at the
symmetric reference state can be derived from the unsymmetric electrolyte
NRTL enthalpy or Gibbs free energy calculations at the condition that all
molecular components (solvents and Henry components) approach zero, i.e.