Processes for separation of aromatic compounds using a thin film composite membrane
This invention discloses a method for separation of an aromatic compound from a mixture comprising an alkane using an improved thin-film composite membrane. The membrane is particularly useful for separation of benzene from cyclohexane, which have similar boiling points. The membrane comprises a more mechanically durable and defect-free separation layer as a result of its fabrication from an ionomer solution that is substantially free of dissolved ionic species not associated with the ionomer.
1. A method of separating an aromatic compound from a mixture comprising an alkane using a thin-film composite membrane; the method comprising:
a) providing a thin-film composite membrane having a feed side and a permeate side; the thin-film composite membrane comprising:
i) a substrate comprising a porous-layer support; and
ii) a separation layer comprising an ionomer comprising sulfonate groups, selected from: silver sulfonate, sulfonic acid, or non-silver sulfonate; said separation layer is predominantly laminar, in direct contact to said substrate, and substantially free of ionic species not associated with the ionomer;
b) exposing the thin-film composite membrane to water or water vapor; and
c) exposing the feed side to a flowing first mixture comprising an aromatic compound and an alkane; and
d) providing a driving force and producing a flowing second mixture at the permeate side having a higher concentration of aromatic compound to alkane than the flowing first mixture at the feed side.
2. The method of claim 1 wherein the aromatic compound is selected from a group consisting of: benzene, toluene, or xylene.
3. The method of claim 1 wherein the flowing first mixture comprises an alkane selected from a group consisting of: ethane, propane, pentane, or cyclohexane.
4. The method of claim 1 wherein the cation of the non-silver sulfonate group is selected from: Li + , Na + , K + , ammonium, alkyl ammonium, or mixtures therefrom.
5. The method of claim 1 wherein the ionomer is comprised of repeat units A and B in which A is a polymerized derivative of a fluorinated monomer and B comprises silver sulfonate groups.
6. The method of claim 5 wherein repeat unit A is selected from a group consisting of: tetrafluoroethylene, chlorotrifluoroethylene, trifluoroethylene, vinyl fluoride, or vinylidene fluoride.
7. The method of claim 5 wherein a precursor to the repeat unit B is a monomer of structure CF 2 ═CFOR f SO 2 F wherein R f is a perfluoroalkylene group of 1 to 10 carbon atoms.
8. The method of claim 7 , wherein the perfluoroalkylene group comprises ether oxygens.
9. The method of claim 1 wherein a precursor to the ionomer in the separation layer is a copolymer comprising tetrafluoroethylene and CF 2 ═CFOCF 2 CF 2 SO 2 F.
10. The method of claim 1 wherein a precursor to the ionomer in the separation layer is a copolymer comprising tetrafluoroethylene and CF 2 ═CFOCF 2 CF(CF 3 )OCF 2 CF 2 SO 2 F.
11. The method of claim 1 wherein the ionomer has an equivalent weight that is less than 800-grams/mole.
12. The method of claim 1 wherein the separation layer has a thickness that is less than 2-μm and is predominantly laminar wherein at least 50% of the gas-separation layer extends on top of the substrate comprising a porous-layer support and is not interpenetrating into the substrate comprising a porous-layer support.