Aspen Physical Property System - Physical Property Models

The pure component dielectric constant coefficients of nonaqueous solvents
and Born radius of ionic species are required only for mixed-solvent
electrolyte systems.
The temperature dependency relations of these parameters are given in
Electrolyte NRTL Activity Coefficient Model.
Heat of mixing is calculated from temperature derivatives of activity
coefficients. Heat capacity is calculated from secondary temperature
derivative of the activity coefficient. As a result, the temperature dependent
parameters are critical for modeling enthalpy correctly. It is recommended
that enthalpy data and heat capacity data be used to obtain these
temperature dependency parameters. See also Electrolyte NRTL Enthalpy and
Electrolyte NRTL Gibbs Energy.
Obtaining Parameters
In the absence of electrolytes, the electrolyte NRTL model reduces to the
NRTL equation which is widely used for non-electrolyte systems. Therefore,
molecule-molecule binary parameters can be obtained from binary
nonelectrolyte systems.
Electrolyte-molecule pair parameters can be obtained from data regression of
apparent single electrolyte systems.
Electrolyte-electrolyte pair parameters are required only for mixed
electrolytes with a common ion. Electrolyte-electrolyte pair parameters can
affect trace ionic activity precipitation. Electrolyte-electrolyte pair parameters
can be obtained by regressing solubility data of multiple component
electrolyte systems.
When the electrolyte-molecule and electrolyte-electrolyte pair parameters are
zero, the electrolyte NRTL model reduces to the Debye-Hückel limiting law.
Calculation results with electrolyte-molecule and electrolyte-electrolyte pair
parameters fixed to zero should be adequate for very dilute weak electrolyte
systems; however, for concentrated systems, pair parameters are required
for accurate representation.
See Physical Property Data, Chapter 1, for the pair parameters available from
the electrolyte NRTL model databank. The table contains pair parameters for
some electrolytes in aqueous solution at 100�C. These values were obtained
by using the Aspen Physical Property Data Regression System (DRS) to
regress vapor pressure and mole fraction data at T=100�C with SYSOP15S
(Handbook of Chemistry and Physics, 56th Edition, CRC Press, 1975, p. E-1).
In running the DRS, standard deviations for the temperature (�C), vapor
pressure (mmHg), and mole fractions were set at 0.2, 1.0, and 0.001,
respectively. In addition, complete dissociation of the electrolyte was
assumed for all cases.
Option Codes for Electrolyte NRTL Activity
Coefficient Model (GMENRTL)
The electrolyte NRTL activity coefficient model (GMENRTL) has three option
codes and the option codes can affect the performance of this model.
2 Thermodynamic Property Models 107