posted on 2024-02-29, 09:29authored byAntonio Marcilla, Inés Oliver, Liam Moncur, Paloma Carbonell-Hermida, María del Mar Olaya
Experimental vapor–liquid equilibrium (VLE) data,
and the
activity coefficient model parameters obtained by their correlation,
are frequently used in separation processes design. The inspection
of this information for ternary systems has revealed some problems
that are not widely known and could lead to serious mistakes. A small
sampling of ten ternary systems, with VLE data sets at constant pressure,
arbitrarily selected has been considered to illustrate these problems.
The experimental VLE data and the NRTL model parameters for these
systems have been taken from different sources, one database and two
commercial simulation packages. A MATLAB code has been developed to
prepare 3D graphs where the T/x and T/y surfaces calculated by the activity
coefficient model (NRTL) are represented along with the experimental
VLE data. The results of the analysis of this small sample are very
worrying. In fact, half of the selected systems present severe inconsistences
between the experimental VLE data and the T/x,y surfaces calculated using the correlation
results published or implemented in the simulation software. These
inconsistences are of different natures, but the most notable is the
existence of parameter sets that do not correlate at all the VLE behavior
of the system that they supposedly represent. These irregularities
may occur only in the ternary region, only in the binary subsystems,
or, in some cases, in both. These problems go frequently unnoticed
for different reasons. As discussed in this work, the typical 2D graphs
and deviations in T or P and vapor
composition are not enough in some cases so as to have an adequate
idea of both the quality of the data and the suitability of the model
to represent them. This is particularly true when considering, as
is strongly advisible, the complete set of data, i.e. the ternary
and binary VLE data simultaneously. Some recommendations are presented
in this work to overcome the problems detected both when planning
the experimental design to determine VLE data and to correlate them
with a model.