Aspen Physical Property System - Physical Property Models

Parameter Symbol No. of
Name
Elements
Dielectric Constant Unary Parameters
2 Thermodynamic Property Models 97
Default MDS Units
E ca'',ca' 1 0 x —
E c'a,c''a 1 0 x —
E c''a,c'a 1 0 x —
GMELCN � ca',ca'' = � ca'',ca' 1 .2 x —
� c'a,c''a = � c''a,c'a 1 .2 x —
† Certain Electrolyte NRTL activity coefficient model parameters are used with
reciprocal temperature terms:
� CPDIEC
� NRTL/2
� GMELCD for electrolyte-electrolyte or electrolyte-molecule pairs
When any of these parameters is specified, absolute temperature units are
used for the calculations in this model.
Reference: H. Renon, and J.M. Prausnitz, "Local Compositions in
Thermodynamic Excess Functions for Liquid Mixtures", AIChE J., Vol. 14, No.
1, (1968), pp. 135-144.
Theoretical Basis and Working Equations
In this section, the theoretical basis of the model is explained and the working
equations are given. The different ways parameters can be obtained are
discussed with references to the databank directories and the Data
Regression System (DRS). The parameter requirements of the model are
given in Electrolyte NRTL Activity Coefficient Model.
Development of the Model
The Electrolyte NRTL model was originally proposed by Chen et al., for
aqueous electrolyte systems. It was later extended to mixed solvent
electrolyte systems (Mock et al., 1984, 1986). The model is based on two
fundamental assumptions:
� The like-ion repulsion assumption: states that the local composition of
cations around cations is zero (and likewise for anions around anions).
This is based on the assumption that the repulsive forces between ions of
like charge are extremely large. This assumption may be justified on the
basis that repulsive forces between ions of the same sign are very strong
for neighboring species. For example, in salt crystal lattices the immediate
neighbors of any central ion are always ions of opposite charge.
� The local electroneutrality assumption: states that the distribution of
cations and anions around a central molecular species is such that the net
local ionic charge is zero. Local electroneutrality has been observed for
interstitial molecules in salt crystals.
Chen proposed an excess Gibbs energy expression which contains two
contributions: one contribution for the long-range ion-ion interactions that
exist beyond the immediate neighborhood of a central ionic species, and the