Aspen Physical Property System - Physical Property Models

Symmetric and Unsymmetric Electrolyte
NRTL Activity Coefficient Model
The Symmetric Electrolyte NRTL activity coefficient model (GMENRTLS) uses a
symmetric reference state for ions as pure fused salts, rather than infinite
dilution in aqueous solution. This basis is easier to use for nonaqueous and
mixed-solvent systems, and eliminates the need to introduce water into
otherwise water-free systems. It also allows the model to reduce to the
standard NRTL model when there are no electrolytes in the system. The
ENRTL-SR property method is based on this model.
Chemical constants, enthalpy, and Gibbs free energy are calculated with
respect to the symmetric ionic reference state.
The Unsymmetric Electrolyte NRTL activity coefficient model (GMENRTLQ)
uses the same equations as GMENRTLS, but the unsymmetric reference state
for ions (infinite dilution in aqueous solution). The ENRTL-RK property method
is based on this model. Unlike the original Electrolyte NRTL activity coefficient
model, GMENRTLQ is also used to calculate enthalpy and Gibbs free energy
from thermodynamics based on the unsymmetric ionic reference state.
These models also handle zwitterions.
Parameters
Both symmetric and unsymmetric models share the same binary and pair
parameters. The adjustable model parameters are the symmetric non-random
factor parameters, �, and the asymmetric binary interaction energy
parameters, �. These parameters exist for molecule-molecule pairs (�mm' =
�m'm while �mm' � �m'm), molecule-electrolyte pairs (�m,ca = �ca,m while �m,ca �
�ca,m where ca represents an ion pair), and electrolyte-electrolyte pairs (�ca,ca'
= �ca',ca and �ca,c'a = �c'a,ca while �ca,ca' � �ca',ca and �ca,c'a � �c'a,ca)
The parameters for ion pairs are temperature-dependent:
2 Thermodynamic Property Models 151