Pure fluorocarbon gases can be sold for up to 30 USD/kg, if they were manufactured
locally. Due to the absence of local demand, South Africa at present has less than
0.3 % of the fluorochemical market and most fluoro–products used in the South African
industry are currently imported. The depolymerisation of waste polytetrafluoroethylene
(PTFE or Teflon) filters in a nitrogen plasma reactor results in the mixture of gases
which includes N2, CF4 and C3F6. An existing challenge entails the separation of these
gases, which is currently attained by an energy intensive cryogenic distillation process.
Both the small energy requirements as well as the small process streams required,
make a membrane separation an ideal alternative to the current distillation process.
Based on our research groups existing expertise in the field of zeolite membranes, it
was decided to investigate the separation capability of zeolite (MFI, NaA, NaY, and
hydroxysodalite) coated tubular ceramic membranes for the separation of the above
mentioned gases. The separation study was subdivided into adsorption studies as well
as single and binary component studies.
CxFy gas adsorption on MFI zeolites. Tetrafluoromethane (CF4) and
hexafluoropropylene (C3F6) were adsorbed on zeolite ZSM–5 and silicalite–1 to help
explain permeation results through zeolite membranes. According to the obtained data,
the separation of CF4 and C3F6 would be possible using adsorption differences. The
highest ideal selectivities (~ 15) were observed at higher temperatures (373 K). While
the CF4 adsorption data did not fit any isotherm, the heat of adsorption for C3F6
adsorbed on ZSM–5 and silicalite–1 was calculated as –17 and –33 kJ/mol respectively.
Single gas permeation. A composite ceramic membrane consisting of a ceramic
support structure, a MFI intermediate zeolite layer and a Teflon AF 2400 top layer was
developed for the separation of N2, CF4 and C3F6. The adsorption properties of the
Teflon AF 2400 sealing layer was investigated. A theoretical selectivity, in terms of the
molar amount of gas adsorbed, of 26 in favour of the C3F6 vs CF4 was calculated, while
the N2 adsorption remained below the detection limit of the instrument. While the ideal
N2/CF4 and N2/C3F6 selectivities for the MFI coated support were either near or below
Knudsen, it was 5 and 8 respectively for the Teflon coated support. Ideal selectivities
improved to 86 and 71 for N2/CF4 and N2/C3F6 when using the composite ceramic membrane, while CF4/C3F6 ideal selectivities ranged from 0.9 to 2, with C3F6 permeating
faster though the composite ceramic membrane.
Zeolite based membrane separation. Inorganic membranes (?–alumina support, NaA,
NaY, hydroxysodalite, MFI) and composite membranes (Teflon layered ceramic and
composite ceramic membrane) were synthesized and characterized using the
non–condensable gases N2, CF4 and C3F6. For the inorganic membranes either near or
below Knudsen selectivities were obtained during single gas studies, while higher
selectivities were obtained for the composite membranes. Subsequently, the MFI,
hydroxysodalite and both composite membranes were chosen for binary mixture
separation studies. The membranes exhibited binary mixture permeances in the order
Teflon layered ceramic > hydroxysodalite > MFI > composite ceramic, which was
comparable to the single gas permeation results. The highest separation for N2/CF4 (4)
and N2/C3F6 (2.4) was obtained with the composite ceramic membrane indicating that
the Teflon layer was effective in sealing non–zeolitic pore in the intermediate zeolite
layer.
The aim of this project was met successfully by investigating a method of fluorocarbon
gas separation by zeolite based membranes using various inorganic and composite
membranes with single and binary mixtures. / Thesis (Ph.D. (Chemistry))--North-West University, Potchefstroom Campus, 2012.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:nwu/oai:dspace.nwu.ac.za:10394/6999 |
| Date | January 2012 |
| Creators | Bissett, Hertzog |
| Publisher | North-West University |
| Source Sets | South African National ETD Portal |
| Detected Language | English |
| Type | Thesis |
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