PELCHEM, the chemical division of NECSA, produces the fluorocarbon hexafluoropropylene (HFP) onsite.
In 2005 PELCHEM initiated research into the wet oxidation of HFP to produce the higher value
fluorocarbon hexafluoropropylene oxide (HFPO). Although successful in the conversion of HFP to HFPO,
the product stream contained both the product and the unreacted HFP. As a result, PELCHEM contracted
the Thermodynamics Research Unit at the University of KwaZulu-Natal to investigate the separation of
HFP and HFPO.
A solvent selection procedure was used to identifY potential solvents and an initial list of two hundred and
seven candidate solvents compiled. Utilising the UNIFAC group contribution method, the initial list was
narrowed down to thirty solvents using the criterion of selectivity at infinite dilution. Through the
comparison of specific solvent properties such as recoverability, safety, environmental factors and
economic considerations, a final list of ten solvents was generated. The list of ten solvents was proposed to
PELCHEM who identified four solvents for further studies. The work involving the two solvents, toluene
and hexafluoroethane (RI 16), is presented in this dissertation. The solvent toluene has been previously
used by the du Pont company for the separation of HFP and HFPO, while R116 is a novel solvent for this
application. The solvent selection procedure was performed in collaboration with a member of the
Thermodynamics Research Unit, and the work on the remaining two solvents is presented in the
dissertation of (Nelson 2008).
Experimental binary high pressure vapour liquid equilibrium data were measured for the HFP + toluene,
HFPO + toluene, R116 + HFP, and R116 + HFPO systems at two temperatures: 273.15 and 3 13.15 K. Pure
component vapour pressure data for HFPO in the temperature range of 271.90 to 318.20 K were also
measured. The HPVLE measurements were performed at the Thermodynamics Energy and Phase
Equilibria laboratories at Ecoles des Mines de Paris using two experimental techniques and equipment. The
binary systems involving toluene were measured on a static synthetic Pressure Volume Temperature
apparatus equipped with a variable volume cell. The binary systems involving RI16 were measured on a
static analytic apparatus equipped with a Rapid On-line Sampler Injector. None of the systems measured
for this project have been reported in the literature. The four binary systems and the pure component
vapour pressure measurements thus constitute new data sets.
All experimental data were modelled via the direct method using the computer software Thermopack.
Three model combinations were used to represent the data: the Peng-Robinson equation of state with the
Wong-Sandler mixing rules, the Peng-Robinson equation of state with the Modified-Huron-Vidal first
order mixing rules, and the Soave-Redlich-Kwong equation of state with the Wong-Sandler mixing rules.
The Mathias-Copeman alpha function was used in conjunction with the equation of state models, and the NRTL activity coefficient model was incorporated into the mixing rules. Due to time constraints,
experimental data for the binary system HFP + HFPO were not measured. Data for this system was
predicted at two temperatures, 273.15 and 313.15 K, via the PSRK-UNIFAC method. The critical line for
the supercritical systems R116 + HFP and R116 + HFPO were calculated in Thermopack.
PELCHEM required a commercial grade HFPO product stream of purity greater than 99 % (mole), and a
purified HFP product stream of purity greater than 95 % for the recycle and conversion of HFP into HFPO.
Using the regressed experimental high pressure vapour liquid equilibrium data, two preliminary separation
processes were designed in Aspen Plus to achieve these objectives. The first scheme involved toluene and
utilised the process of extractive distillation with toluene introduced as a liquid solvent. The toluene bonded
to the HFP and was removed as a bottoms product which allowed a purified HFPO stream to be recovered
as a distillate. The second scheme involved RI16 and utilised the process of gas stripping, with a liquid
mixture of HFP and HFPO contacted with a gaseous stream of R116. The R116 removed the HFP from the
liquid mixture, resulting in a purified HFPO stream. The toluene process resulted in an overall HFPO
product recovery of 98.46 % and HFPO product purity of99.88 % (mole). The RI16 process resulted in an
overall HFPO product recovery of96.57 % and HFPO product purity of99.71 %. For the component HFP,
the toluene process resulted in an overall HFP product recovery of 99.42 % and product purity of96.41 %.
The RI16 process resulted in an overall product recovery of99.36 % and product purity of93.45 %.
From a comparison of the preliminary design of the separation processes on the basis of patent issues,
performance, and other miscellaneous factors, it was concluded that the RI16 process compared favourably
to the process involving the solvent toluene. The preliminary process designs were presented to PELCHEM
in 2007, and pending further experimental work PELCHEM plans to patent the RI16 separation process.||en_US