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

III.3. Application to Pure Compounds
III.3.1. soft-SAFT without crossover
Modeling
Values for the molecular parameters of pure compounds are usually adjusted by
minimizing deviations from the theory with respect to VLE experimental data.
Nevertheless, whenever possible, it is important to use physical information in order to
minimize the number of parameters to be optimised. In this way, xenon was, as in other
works (Filipe et al., 2000), modelled as a single sphere. In the case of radon, experimental
VLE data was difficult to find. Vapor pressure data was available at NIST Chemistry
WebBook but only one value for the liquid density at 211 K was found (Herreman, 1980).
This value was used as a reference to calculate radon liquid densities for a broader
temperature range following the methods described in Poling et al. (2001). For
fluorocarbons, experimental studies indicate that C-C bond lengths for crystalline
poly(tetrafluoroethylene) and polyethylene are equivalent (Cui et al., 1998) and so, the
values of the m parameter for n-perfluoalkanes were set equal to those optimised for the
n-alkanes in a previous work (Pàmies and Vega, 2001). Also, the values of σ for
perfluoroalkanes are greater than in the corresponding alkanes as would be expected once
CF3 and CF2 groups are bigger than the corresponding CH2 and CH3 for alkanes. In the
case of the substituted perfluorooctanes, the m parameter was set equal to the saturated
perfluoro-n-octane once the size of the molecules should be essentially the same.
The remaining molecular parameters of the nonassociating model for the pure
compounds were calculated by fitting vapour pressures and saturated liquid densities to
experimental data away from the critical region, and they are listed in Table III.1. They
were adjusted using, whenever available, experimental data up to about Tr = T/Tc = 0,8.
The references of the experimental vapour pressure and density data used for the
optimisation of the parameters are also presented in the table.
Very limited experimental data are available in the literature for the complete
vapour pressure curve or saturated liquid density of most of the fluorinated compounds
studied here and in some cases there are obvious inconsistencies between the available
sources as discussed before in Part II. It is believed that an improvement in the accuracy of
the soft-SAFT results could be obtained if reliable data were available for the entire phase
diagram as in the case of the alkanes.
- 110 -