n - PATh :.: Process and Product Applied Thermodynamics research ...

Modeling
Binary perfluoroalkane + alkane mixtures belong to the type II phase behavior in
the classification of Konynenburg and Scott (1990), characterized by a continuous vapor-
liquid critical line and the presence of liquid-liquid phase separation, when the difference
in chain length between the two components is not very large (symmetric or close to
symmetric systems). Because of the nonideal interactions in these mixtures, the unlike
energy parameter was treated as adjustable, and it was set at the optimum value
ξ = 0.9146 for the correct prediction of the experimental azeotrope of the perfluoro-n-
hexane + n-hexane mixture at 298.15 K. The unlike size parameter was not adjusted
(η = 1), because it was verified that the simple Lorentz combination rule provided
satisfactory results. It is important to notice that the value of the unlike energy parameter
agrees well with the values found by other authors using different models to describe these
systems. In a transferable manner, the same optimized energy parameter value was used to
predict the rest of the mixtures at different thermodynamic conditions.
Composition diagrams of binary mixtures of perfluoro-n-hexane + n-alkane from
(C5 - C8) and of n-hexane + n-perfluoroalkane from (C5 - C8) modeled in this work are
presented in Figures III.25 and III.26, respectively. Figure III.27 and III.28 depicts P-x-y
diagrams of the systems shown in Figure III.25 and III.26. Very good agreement is
obtained between soft-SAFT predictions and experimental data in all cases, considering
that the average uncertainty of the experimental compositions is 4%. In all cases the
azeotropic point is correctly predicted and deviations from experimental data are lower
than 5%, except in the case of the perfluoro-n-hexane + octane mixture. According to the
authors (Duce et al., 2002), at 313.15 K the mixture was not completely miscible in the
entire studied composition range and for perfluorohexane mole fractions between 0.189
and 0.709 they obtained demixed mixtures indicating that they were very close to a liquidliquid
region as predicted in fact by the soft-SAFT EoS (Figure III.25b). The UCST
calculated by the authors for this mixture is 324 K.
- 155 -