Synthesis of hexafluorobenzene through batch reactive distillation.
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Date
2018
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Abstract
South Africa is home to the second largest fluorspar (CaF2) reserves in the world,
outside China; and it is consequently integral to the international supply of CaF2 to
hydrogen fluoride producers. This is demonstrated by South Africa’s position as the
third largest exporter of CaF2. But while it produces 5% of the world’s total
requirements (ca 4.55 million tons per annum), South Africa earns less than 10% of its
final output value. As a result, a Fluorochemical Expansion Initiative (FEI) was
established to increase the beneficiation of South African mined CaF2.
Under the auspices of FEI, this dissertation describes the research and development of a
method for the synthesis of a perfluorinated aromatic compound, hexafluorobenzene
(C6F6). C6F6 is a product within the CaF2 beneficiation value chain that has utility as a
heat exchange fluid, lubricant, solvent and within the pharmaceutical industry.
The investigation was broken up into two phases: Firstly the equipment was validated
by replicating the experiments and comparing results to the biodiesel reactive
distillation experiments found in literature. Once the equipment was validated, the main
experiment for improving the yield of hexafluorobenzene was carried out. This was
undertaken by reacting hexachlorobenzene, dissolved in sulfolane under the action of
various alkali metal fluorides (potassium fluoride (KF) and caesium fluoride (CsF)) and
through variations in the ratio of the alkali fluoride to hexachlorobenzene. For both
phases, the experiments were carried out in a glass, batch reactive distillation system.
The quantities of the various products formed were determined via quantitative analysis
using a gas chromatograph equipped with a flame ionization detector.
It was observed that the use of caesium fluoride increased the molar yield of C6F6 to
0.59% as to the 0.27% that was produced when potassium fluoride was used. The effect
of temperature on the yield of C6F6 was also investigated and the results depicted that
higher temperatures favoured higher yields of C6F6. The effect of varying molar
quantity of KF on the molar selectivity of all fluorinated products was additionally
examined and it was concluded that a change in amount of KF did not significantly
affect the molar selectivity of the products. On the other hand, for increasing amounts of
CsF an increase in molar selectivity’s of the higher fluorinated compounds were
observed. It was further noted that an increase in temperature resulted in an increase in
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molar selectivity of fluorinated products. The opposite was observed for the lower
fluorinated species.
A simplified kinetic model was developed for the KF and CsF system. The model
comprised six reactions and the experimental data was used together with a nonlinear
regression technique implemented in MATLAB® to identify the kinetic parameters.
Using the kinetic parameters, a simulation was then performed to determine the effect of
time on the moles of products and consumption of hexachlorobenzene using either KF
or CsF as the fluorinating agent. It was observed that using KF resulted in a better
conversion but poorer selectivity towards the highly fluorinated products as compared to
using CsF. The better conversion may be due to the mixing efficiency due to the lower
actual mass of solid KF in the reaction mixture than solid CsF. The poorer selectivity
may be due to the slightly superior solubility of the CsF which promoted fluorination in
the sulfolane medium.
Description
Masters Degree. University of KwaZulu- Natal, Durban.