Membrane based separation of nitrogen, tetrafluoromethane and hexafluoropropylene
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.