South Africa is a land blessed with an abundance of mineral deposits. Yet, despite this, very
little value adding of minerals exists. Most of the mined minerals are exported, where it is
reworked into valued items. The country subsequently imports the valuable items at a much
higher cost. In the 2006/7 financial year, the government made the decision to support
several projects aimed at adding value to the mined minerals and by so doing, creating job
opportunities. One such project was identified for the mineral Fluorite (CaF2). Fluorite is
exposed to a controlled burn in a plasma reactor, producing an array of different fluorocarbon
gases used in the electronics industry and for commercial polymers like Teflon®. Currently,
fluorocarbon gases are separated using a series of cryogenic distillation columns. Although
this technique has proven to be successful, it has several negative aspects such as the high
cost involved when operating at cryogenic conditions as well as difficulty handling the gases
at these sub–zero temperatures.
It was proposed to study the possibility of using membranes to separate fluorocarbon gases at
ambient conditions. Several membranes were screened to determine which one is best suited
for this application. Two Teflon® based membranes were selected from this data. One of
the membranes had a PAN support, while the other had a PEI support.
Pure gas data for both membranes showed promising results. It yielded the highest flux for
C3F6, followed by N2 and CF4. c–C4F8 was not used because it was demonstrated that the gas
tends to condensate at low pressures. It is recommended to rather use pressure swing
condensation to remove this gas from the mixture before the remainder is purified using
membranes. Both membranes behaved similarly, with selectivity between C3F6 and CF4, and
N2 and CF4; all above 10. By including the permeate pressure in the Solution–diffusion
model, it was possible to model the pure gas data
Binary feed gas mixture experiments showed a large amount of coupling existing between the
feed gas mixtures. The result is a decrease in the selectivity as well as the total flux of the gas
mixture. Partial fluxes were modelled by introducing a thermodynamic factor that was
shown to follow a power law equation. The PAN–supported membrane outperformed the
PEI–supported one; it was decided to use this membrane from this point onwards.
Analysis of the ternary feed mixtures showed a strong selectivity towards the gas abundant in
the feed blend. The existence of convective diffusion was proven, and included in the
modelling, as well as a breakthrough pressure constant. This is indicative of strong
interaction between the different gases and the membrane. Throughout the study it became
clear that the difference in surface charge between the gases and the membrane were
decisive. Opposite charges between a gas (C3F6) and the membrane aided in gas permeation.
Membrane separation of fluorocarbon gases at ambient conditions is possible. Teflon® based
membranes are recommended. It will be advantageous to study the effect of elevated
temperatures on the separation efficiency of such a system. / Thesis (M.Ing. (Chemical Engineering))--North-West University, Potchefstroom Campus, 2012.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:nwu/oai:dspace.nwu.ac.za:10394/7629 |
| Date | January 2011 |
| Creators | Le Roux, Marco |
| Publisher | North-West University |
| Source Sets | South African National ETD Portal |
| Detected Language | English |
| Type | Thesis |
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