METHOD FOR THE SEPARATION OF PHOSPHORUS PENTAFLUORIDE FROM HYDROGEN CHLORIDE

Abstract

The present invention relates to a process of separating a feed stream comprising HCl and PF.sub.5 into a plurality of streams, wherein a first stream is enriched in PFS and a second stream is enriched in HCl, the process comprising the feed stream entering one or more permeable membrane separation modules, wherein the membrane separation module comprises a permeable membrane which is selectively permeable to one of HCl or PF.sub.5.

Claims

1. A process of separating a feed stream comprising HCl and PF.sub.5 into a plurality of streams, wherein a first stream is enriched in PF.sub.5 and a second stream is enriched in HCl, the process comprising the feed stream entering one or more permeable membrane separation modules, wherein the membrane separation module comprises a permeable membrane which is selectively permeable to one of HCl or PF.sub.5.

2. The process according to claim 1 wherein the feed stream is separated into two streams: (i) the first stream enriched in PF.sub.5; and (ii) the second stream enriched in HCl.

3. The process according to claim 1 wherein the feed stream comprises HCl:PF.sub.5 in a molar ratio of greater than 1:1 up to about 15:1, or in a molar ratio of greater than 1:1 up to about 10:1, or in a molar ratio of greater than 1:1 up to about 5:1.

4. The process according to claim 1, wherein the process further comprises a step of purifying the first stream and/or second stream so as to produce a purified stream and a waste stream, wherein the purified stream comprises PF.sub.5 and/or HCl in an increased impurity compared to the first and/or second stream, respectively.

5. The process according to claim 1, wherein the first stream is enriched with PF.sub.5 so that the molar ratio of PF.sub.5:HCl is at least 1:1, at least 2:1, at least 5:1, or at least 10:1.

6. The process according to claim 4 wherein when the first stream is enriched with PF.sub.5 so that the molar ratio of PF.sub.5:HCl is at least 1:1, the purification step is carried out by distillation.

7. The process according to claim 4 wherein the process further comprises a step of recycling the waste stream back into the one or more membrane separation modules.

8. The process according to claim 1, wherein the first stream and/or the second stream is passed through a further permeable membrane separation module in order to further enrich the amount of PF.sub.5 or HCl in the first or second stream respectively, before the purification step, if present.

9. The process according to claim 1 wherein the membrane is a microporous, inert polymeric material.

10. The process according to claim 9 wherein the microporous, inert polymeric material is selected from the group consisting of poly tetrafluoroethene (PTFE), polyvinylidene fluoride (PVDF), fluorinated ethylene-propylene (FEP), sulfonated perfluorovinyl ether-tetrafluoroethene copolymers or a combination thereof.

11. The process according to claim 9 wherein the pressure gradient across the membrane in one or more of the permeable membrane separation modules is from about 0.1 bar to about 20 bar, from about 0.25 bar to about 15 bar, or from about 0.5 bar to about 10 bar.

12. The process according to claim 9 wherein the process is carried out in one or more of the permeable membrane separation modules at a temperature of from about 50 C. to about 80 C., from about 30 C. to about 50 C., or from about 20 C. to about 40 C.

13. The process according to claim 1 wherein the membrane material is selected from a rubbery or glassy polymeric material wherein the solubility of one of PF.sub.5 or HCl is enhanced relative to the other component so that the component with a higher solubility permeates through the membrane at a faster rate than the component with a lower solubility.

14. The process according to claim 13 wherein the membrane material is selected from the group consisting of fluorinated polymers, polyvinyl chloride, polysiloxanes, poly-methyl-pentene, polysulfones, polyimides, partially fluorinated or substituted polyimides, or a combination thereof.

15. The process according to claim 14 wherein the polysiloxane is poly dimethyl siloxane.

16. The process according to claim 14 wherein the polyimide is a fluorinated polyimide, preferably 6FDA-6FpDA.

17. The process according to claim 13 wherein the pressure gradient across the membrane in one or more of the permeable membrane separation modules is from about 0.1 bar to about 20 bar, from about 0.25 bar to about 15 bar, or from about 0.25 bar to about 10 bar.

18. The process according to claim 13 wherein the process is carried out in one or more of the permeable membrane separation modules at a temperature of from about 80 C. to about 120 C., from about 60 C. to about 80 C., or from about 40 C. to about 40 C.

19. The process according to claim 13 wherein the membrane is made of a material which has a Hildebrand solubility parameter closer in value to that of HCl (21 (MPa).sup.1/2 in the temperature range 60 C. to +20 C.) compared to that of PF.sub.5 (11 (MPa).sup.1/2 in the temperature range 60 C. to +20 C.).

20. The process according to claim 13 wherein the membrane is made of a material which has a Hildebrand parameter closer in value to that of PF.sub.5 (11 (MPa).sup.1/2 in the temperature range 60 C. to +20 C.) compared to that of HCl (21 (MPa).sup.1/2 in the temperature range 60 C. to +20 C.).

21. The process according to claim 1 wherein the membrane is a flat sheet membrane.

22. The process according to claim 1 wherein the membrane is a hollow fiber membrane.

23. The process according to claim 1 wherein the process is carried out in a batchwise or continuous operation.

24. The process according to claim 1 wherein the one or more streams entering the membrane separation module is in the gaseous or vapour state.

25. The process according to claim 5 wherein the PF.sub.5 produced in this process is used in a process to synthesis LiPF.sub.6.

Description

DESCRIPTION OF THE INVENTION

[0021] In a first aspect, the present invention provides a process for separating a feed stream comprising HCl and PF.sub.5 into a plurality of streams, wherein a first stream is enriched in PF.sub.5 and a second stream is enriched in HCl, the process comprising the feed stream entering one or more permeable membrane separation modules, wherein the permeable membrane separation module comprises a membrane which is selectively permeable to one of HCl or PF.sub.5.

[0022] In preferred embodiments the phase to be separated is gaseous and the resulting product streams are also gaseous on leaving the separation module.

[0023] In some embodiments, the feed stream may be separated into only two streams with the first stream being enriched in PF.sub.5 and the second stream being enriched in HCl.

[0024] In some preferred embodiments, the feed stream may be in the vapour phase and may comprise HCl:PF.sub.5 in a molar ratio of greater than 1:1 up to about 15:1, preferably in a molar ratio of greater than 1:1 up to about 10:1, more preferably in a molar ratio of greater than 1:1 up to about 5:1.

[0025] By a stream being enriched in a certain compound we mean that the composition of the product stream contains a higher molar percentage of said product that the feed stream.

[0026] For example, the separation process of the present invention may increase the concentration of PF.sub.5 from about 16 mol. % in HCl in the feed stream to a concentration of greater than 50 mol. %.

[0027] In some embodiments, the process of the present invention may further comprise a step of purifying the first stream and/or second stream so as to produce a purified stream and a waste stream, wherein the purified stream comprises PF.sub.5 and/or HCl in an increased impurity compared to the first and/or second stream, respectively.

[0028] In some embodiments, the process of the present invention further comprises a step of purifying the first stream and/or second stream so as to produce a purified stream and a waste stream, wherein the purified stream comprises PF.sub.5 and/or HCl in an increased impurity compared to the first and/or second stream, respectively.

[0029] In some embodiments, the first stream is enriched with PF.sub.5 so that the molar ratio of PF.sub.5:HCl is at least 1:1, preferably 2:1, more preferably 5:1, most preferably 10:1.

[0030] When the first stream is enriched with PF.sub.5 so that the molar ratio of PF.sub.5:HCl is at least 1:1 the first stream is considered to be sufficiently enriched so that the first stream may be used in LIPF.sub.6 synthesis without further processing (e.g., further purification).

[0031] In some embodiments when the first stream is enriched with PF.sub.5 so that the molar ratio of PF.sub.5:HCl is at least 1:1 the purification step may be carried out by distillation.

[0032] Achieving an enriched stream with greater than 50 mol. % PF.sub.5 allows the distillation of the enriched stream to yield a bottom product of high purity PF.sub.5 and a top product of mixed gas at or near the azeotropic concentration of 46% molar PF.sub.5.

[0033] After distillation, the purity of PF.sub.5 in the purified stream is at least 90 mol % PF.sub.5, more preferably 95 mol. % PF.sub.5, even more preferably 97 mol. % PF.sub.5, even more preferably 99 mol. % PF.sub.5.

[0034] Advantageously, the purified stream comprises at least 50 mol. % PF.sub.5 when used in LIPF.sub.6 synthesis.

[0035] A further example of a described separation process falling within the scope of the claims may be one in which an initial separation step generates a partially enriched stream whose composition is greater than 46% molar PF.sub.5 (advantageously greater than 50% PF.sub.5), followed by a distillative separation of the partially enriched stream to give a stream at the desired purity of PF.sub.5 (for example, 90 mol % PF.sub.5, more preferably 95 mol. % PF.sub.5, even more preferably 97 mol. % PF.sub.5, even more preferably 99 mol. % PF.sub.5).

[0036] In some embodiments, the process of the present invention further comprises a step of recycling the waste stream back into the one or more membrane separation modules. In such a process the waste stream comprises an azeotropic or near-azeotropic mixture of PF.sub.5 and HCl. The waste stream comprises the remainder of the material fed into the distillation step that does not exit the distillation phase as the purified stream. The waste stream is fed back to the one or more permeable membrane separation modules, preferably the first or the primary permeable membrane separation module. Accordingly, the composition of the waste stream goes through the separation process again.

[0037] In some embodiments, the first stream and/or the second stream may be passed through a further permeable membrane separation module in order to further enrich the amount of PF.sub.5 or HCl in the first or second stream respectively, before the purification step.

[0038] For the avoidance of doubt, the term permeable membrane takes the conventional definition known in the art. Namely, permeable membranes relate to membranes with allow a permeating fluid to diffuse through the membrane material as a consequence of the pressure difference over the membrane. The skilled person will appreciate that such membranes may be selective (i.e., have higher permeation rates) towards certain fluid. Factors which affect the selectivity of a membrane include, but are not limited to, the size of the pores of the membrane, the size of the molecules, the diffusivity of the molecules and the solubility of the permeate in the membrane.

[0039] Without wishing to be bound by theory, the membrane separation process relies on permeation of fluids at different rates through a membrane from a region of higher pressure to a region of lower pressure. The pressure gradient may be controlled by creating a lower pressure environment one side of the membrane within the membrane separation module by using, for example, a vacuum pump or other device. The lower pressure may therefore be created by removing fluid as it is passed through the membrane.

[0040] The higher pressure side of the membrane within the membrane separation module is fed with fluid so as to maintain the higher pressure at or close to the supply pressure.

[0041] Alternatively, or additionally, an intermediate booster compressor may be used to elevate the feed fluid pressure prior to its admission to the separation module.

[0042] Preferred processes of this invention are those wherein the feed and product fluids entering the permeation module are in the gaseous or vapour state.

[0043] A single (e.g., primary) membrane separation module may be used in the process of the claimed invention. In this case, one method of operation may be to increase the amount of PF.sub.5 in the retentate so that the amount of PF.sub.5 moves to be greater than the amount of PF.sub.5 present in the PF.sub.5/HCl azeotrope composition. Both of the retentate and permeate streams may then be further purified to yield PF.sub.5 and HCl streams of desired purity, with unwanted material from each distillation stage being boosted in pressure and returned as recycle to an inlet of the membrane separation module.

[0044] Additional membrane separation units may be employed (e.g., one, two, three, four, five, or more additional membrane separation modules may be used) on either stream (i.e., the retentate or permeate stream) arising from the primary membrane separation module as desired to further enrich the PF.sub.5 or HCl as needed before distillation is carried out

[0045] When more than one membrane separation module is used, the membrane separation units may be connected in series. The waste streams resulting from the process at each membrane separation module may also be cojoined so as to form a single waste stream that is recycled back into the primary membrane separation module.

[0046] In one embodiment of the present invention the membrane found in the permeable membrane separation module is a microporous, inert polymeric material.

[0047] As used herein, the term microporous material means a material containing pores with diameters less than 2 nm.

[0048] In some embodiments the microporous, inert polymeric material is selected from the group consisting of poly tetrafluoroethene (PTFE), polyvinylidene fluoride (PVDF), fluorinated ethylene-propylene (FEP), sulfonated perfluorovinyl ether-tetrafluoroethene copolymers (e.g., Nafion) or a combination thereof. Naflon means polymer materials sold under the Naflon trademark by the Chemours Corporation.

[0049] In embodiments where the permeable membrane separation module is a microporous, inert polymeric material, the pressure gradient across said membrane in the one or more permeable membrane separation modules is from about 0.1 bar to about 20 bar, preferably from about 0.25 bar to about 15 bar, more preferably from about 0.5 bar to about 10 bar.

[0050] In embodiments where the permeable membrane separation module is a microporous, inert polymeric material, the process is carried out in the relevant one or more permeable membrane separation modules at a temperature of from about 50 C. to about 80 C., preferably from about 30 C. to about 50 C., more preferably from about 20 C. to about 40 C.

[0051] Alternatively, in other embodiments, the membrane material may be selected from a rubbery or glassy polymeric material.

[0052] As used herein, glassy polymers relate to polymers which have a glass transition temperature (Tg) greater than room temperature.

[0053] As used herein, rubbery polymers relate to polymers which have a glass transition temperature (Tg) below than room temperature.

[0054] In these embodiments the solubility of one of PF.sub.5 or HCl is enhanced relative to the other component so that the component with a higher solubility permeates through the membrane at a faster rate than the component with a lower solubility.

[0055] In some relevant embodiments, the membrane material may be selected from the group consisting of fluorinated polymers, polyvinyl chloride, polysiloxanes, poly-methyl-pentene, polysulfones, polyimides, partially fluorinated or substituted polyimides, or a combination thereof.

[0056] In some embodiments, preferably the polysiloxane is poly dimethyl siloxane.

[0057] In some embodiments, preferably the polyimide is a fluorinated polyimide, preferably 6FDA-6FpDA. For the avoidance of doubt, 6FDA-6FpDA has the following structure:

##STR00001##

[0058] The Hildebrand solubility parameter (6) provides a numerical estimate of the degree of interaction between materials and can be a good indication of solubility, particularly P for nonpolar materials such as many polymers. In other words, the Hildebrand solubility parameter provides a measure of the affinity of a solvent for a solute.

[0059] If the value of the Hildebrand solubility parameter is similar for a solvent (e.g., the membrane) and solute (e.g., the relevant molecules) then a reasonable degree of solvency of solute in the bulk material may be anticipated.

[0060] For example, the Hildebrand parameter for HCl is about 21 (MPa).sup.1/2 in the temperature range 60 C. to +20 C. and the Hildebrand parameter for PF.sub.5 is about 11 (MPa).sup.1/2 in the temperature range 60 C. to +20 C. The fluorinated polyimide 6FDA-6FpDA has a Hildebrand parameter of about 21 (MPa).sup.1/2 and so it may be anticipated that HCl will exhibit high solubility in 6FDA-6FpDA. The solubility parameter for PTFE is about 13(MPa).sup.1/2 and the solubility parameter of PDMS is about 16 (MPa).sup.1/2 so it may be anticipated that the solubility of PF.sub.5 in these materials may be higher than the solubility of HCl.

[0061] If a separation process is desired in which PF.sub.5 is enriched in the permeate stream then the membrane may be selected so that PF.sub.5 has a higher preferential solubility in the material than does HCl. Therefore, the membrane may be made of a material which has Hildebrand parameter of the membrane is closer in value to that of PF.sub.5 (e.g., 11 (MPa).sup.1/2 in the temperature range 60 C. to +20 C.) compared to that of HCl (21 (MPa).sup.1/2 in the temperature range 60 C. to +20 C.).

[0062] Alternatively, if a separation process is desired in which HCl is enriched in the permeate stream then the membrane may be selected so that HCl has a higher preferential solubility in the material than does PF.sub.5.

[0063] In embodiments where the permeable membrane separation module is a rubbery or glassy polymeric material, the pressure gradient across said membrane in the one or more permeable membrane separation modules is from about 0.1 bar to about 20 bar, preferably from about 0.25 bar to about 15 bar, more preferably from about 0.25 bar to about 10 bar.

[0064] In embodiments where the permeable membrane separation module is a rubbery or glassy polymeric material, the process is carried out in the relevant one or more permeable membrane separation modules at a temperature of from about 80 C. to about 120 C., preferably from about 60 C. to about 80 C., more preferably from about 40 C. to about 40 C.

[0065] In any of the above embodiments, the membrane may be a flat sheet membrane. Alternatively, the membrane may be a hollow fiber membrane.

[0066] Hollow-fiber membrane systems are those where the membrane has been formed into small-diameter hollow tubes. Assemblies of these hollow tubes are conventionally made with bundles of these tubes encased inside a pressure-tight tubular shell fitted with gas-tight header plates so that the assembly resembles a shell-and-tube heat exchanger. The direction of permeation may be either from inside the hollow fibers to the shell, or the other way round.

[0067] Flat sheet membranes are typically formed of the separation membrane itself bonded to one or more inert support membrane materials, whose pores do not represent a significant impediment to separation. These are conventionally wound in a spiral pattern around spacer materials then the whole assembly is placed inside a pressure-tight cylindrical shell. The annular arrangement means that the internal space is divided into a high pressure and low pressure region and so fluid may be contacted with a large surface area in a compact overall enclosure size.