Aspen Physical Property System - Physical Property Models

hydrophobic segment (x), hydrophilic segment (z), and polar segments (yand
y+). In practice, these conceptual segments become the molecular
descriptors used to represent the molecular surface characteristics of each
solute or solvent molecule. Hexane, water, and acetonitrile are selected as
the reference molecules for the hydrophobic, hydrophilic, and polar segments,
respectively. The molecular parameters for all other solvents can be
determined by regression of available VLE or LLE data for binary systems of
solvent and the reference molecules or their substitutes.
The treatment results in four component-specific molecular parameters:
hydrophobicity X, hydrophilicity Z, and polarity Y- and Y+. The two types of
polar segments, Y- and Y+, are used to reflect the wide variations of
interactions between polar molecules and water.
NRTL-SAC can also be used to model electrolyte systems. In this case, an
electrolyte segment e, corresponding to the electrolyte parameter E, is
introduced. This conceptual segment e completely dissociates to a cationic
segment (c) and an anionic segment (a), both of unit charge. All electrolytes,
organic or inorganic, symmetric or unsymmetric, univalent or multivalent, are
to be represented with this conceptual 1-1 electrolyte segment e together
with the previously defined hydrophobic segment x, polar segments y- and
y+, and hydrophilic segment z in NRTL-SAC. The reference state for ions is by
default an unsymmetric state based on infinite dilution in aqueous solution,
but an option code is available to select the symmetric state of pure fused
salts. When there are no electrolytes present, the segment e is unused and
the current model reduces to the non-electrolyte version of NRTL-SAC present
in earlier releases.
The conceptual segment contribution approach in NRTL-SAC represents a
practical alternative to the UNIFAC functional group contribution approach.
This approach is suitable for use in the industrial practice of carrying out
measurements for a few selected solvents and then using NRTL-SAC to
quickly predict other solvents or solvent mixtures and to generate a list of
suitable solvent systems.
The NRTL-SAC model calculates liquid activity coefficients.
Note: This is the updated version of NRTL-SAC, represented with property
model GMNRTLS and property method NRTL-SAC. This version does not
require the specification of components as oligomers. For the old version, see
NRTLSAC for Segments/Oligomers and ENRTL-SAC.
For the model equations, see NRTL-SAC Model Derivation.
Parameters used in NRTL-SAC
Each component can have up to five parameters, rx,I, ry-,I, ry+,I, rz,I, and re,I,
representing the equivalent number of segments of each type for the NRTL
activity coefficient model. Only one or two of these molecular parameters are
needed for most solvents in practice. These parameters are implemented
together as pure parameter XYZE with five elements representing these five
parameters. Values for this parameter are available for many common
solvents in the NRTL-SAC databank.
116 2 Thermodynamic Property Models