Abstract:
A method and apparatus for the separation and recovery of SF 6  from a gas mixture consisting essentially of SF 6 , CF 4 , and N 2 . The method and apparatus involve membrane separation to separate N 2  from SF 6  and CF 4 , and liquefaction to separate SF 6  from CF 4 .

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a non-provisional application of Provisional Application No. 60/157,730, filed on Oct. 5, 1999. The benefit of that filing date is hereby claimed under 35 U.S.C. § 119. The entire content of the provisional application is hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention generally relates to the separation and recovery of SF 6  from a gas stream comprising CF 4  and at least one of air and nitrogen. The invention specifically relates to a method and apparatus for the separation and recovery of SF 6  from a gas stream comprising CF 4  and at least one of air and nitrogen using a combination of membrane and liquefaction separation techniques. 
     BACKGROUND OF THE INVENTION 
     Sulfur hexafluoride (SF 6 ) is widely used in the electric power distribution industry. In particular, SF 6  is used as an insulator or dielectric gas in power distribution equipment such as transformers, switch boxes, gas insulated lines, and substations. Under high voltage conditions, SF 6  decomposes into various polar components including HF, F + , SO 2 , and the like. These by-products degrade the insulating qualities of the gas. As a result, the gas has to be replaced or refined periodically, In addition to the formation of these polar by-products, carbon tetrafluoride (CF 4 ) is also generated during arcing in the presence of carbon containing insulators such as the materials known under the trade designations Teflon® and Megelit®. 
     Currently, there are several known methods for purifying and recycling SF 6  used as an insulator in electrical equipment. These methods are based upon adsorption and liquefaction. The SF 6  polar decomposition by-products are removed by soda lime, activated alumina, or molecular sieves. The refined SF 6  is then refilled into the circuit breakers, substations, or transformers. 
     Due to both technical and environmental concerns, the use of SF 6 /N 2  (or SF 6 /air) mixtures has been suggested to replace pure SF 6  as a gaseous dielectric in the electric power distribution industry. However, there is no known method or apparatus that can economically and efficiently recover and refined SF 6  from a gas mixture containing N 2  or air and CF 4 . Thus, there is a need in the industry for an economical and efficient method and apparatus for the capture and recycle of SF 6 /N 2  mixture containing CF 4  and the polar by-products. The present invention is intended to address this need in the art. 
     SUMMARY OF THE INVENTION 
     Briefly, in one aspect, the present invention relates to a method for the paration of SF 6  from a gas mixture consisting essentially of SF 6 , CF 4 , and N 2 . 
     The method comprises the steps of: 
     (a) contacting a gas mixture consisting essentially of SF 6 , CF 4 , and N 2  with a membrane at conditions effective to obtain a permeate stream rich in N  2  and a retentate stream rich in SF 6  and CF 4 ; and 
     (b) liquefying the retentate stream at conditions effective to obtain liquid SF 6  and gaseous CF 4 . 
     In another aspect, the present invention relates to an apparatus for the separation of SF 6  from a gas mixture consisting essentially of SF 6 , CF 4 , and N 2 . 
     The apparatus comprises: 
     (a) at least one membrane separation unit which permeates N 2  faster than SF 6  and CF 4 ; and 
     (b) means for liquefying a retentate stream comprising SF 6  and CF 4  from the at least one membrane separation unit to form liquid SF 6  and gaseous CF 4 . 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be described in greater detail with reference to the accompanying drawings in which like elements bear like reference numerals, and wherein: 
     FIG. 1 is a schematic drawing of one preferred method and apparatus according to the invention; and 
     FIG. 2 is a schematic drawing of another preferred method and apparatus according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a schematic drawing of one preferred method and apparatus according to the invention. Electric power distribution equipment (“EPDE”)  10  can be any type of device used in the electric power industry for distributing power that contains an insulating or dielectric gas comprising a mixture of SF 6  and N 2  (or air). Such EPDE  10  include, for example, transformers, switch boxes, gas insulated lines, and substations. 
     Under high voltage conditions, SF 6  in the insulating gas mixture decomposes into various polar components including HF, F + , SO 2 , and the like as well as non-polar components including CF 4 . The composition of such a gas mixture can vary over a wide range. For example, the insulating gas mixture can contain from about 10 vol % to about 90 vol % SF 6 , from about 90 vol % to about 10 vol % N 2 , from about 5 vol % to about 0.005 vol % CF 4 , and from about 0.5 vol % to about 0.001 vol % polar by-products. 
     The insulating gas mixture is transferred from EPDE  10  to at least one scrubber/filter unit  12  via conduit  11 . The scrubber/filter unit  12  is designed to remove the SF 6  polar decomposition by-products from the insulating gas mixture to produce a gas mixture  13  depleted in the SF 6  polar decomposition by-products. As used herein, the term “depleted” means that the concentration of a specified component(s) in the effluent stream of a particular separation step or unit is less than the concentration of the same component(s) in the feed stream to that particular separation step or unit. 
     The scrubber/filter may be conducted at a pressure ranging from 1 to 50 bar. The temperature for carrying out this step can vary from 20° C. to 100° C. The flow rate per unit scrubber/filter unit (i.e., space velocity) can vary from 0.1 to 20 min −1 . 
     Any scrubber/filter material can be used in the present invention so long as the material can selectively remove the SF 6  polar decomposition by-products from the gas mixture in line  11 . Suitable scrubber/filter materials include molecular sieves, soda lime, and activated alumina. 
     The amount of scrubber/filter material used, of course, varies depending on the amount of polar by-products to be removed and the desired purity of the product gas  13 . Such a determination is within the scope of one skilled in the art. Generally, a weight of scrubber/filter material corresponding to 10% of the weight of the gas to be separated is used. 
     The gas mixture  13  existing the scrubber/filter unit  12  and depleted in the SF 6  polar decomposition by-products is compressed in compressor  14  to form a compressed gas mixture  15  having a pressure ranging from about 3 to about 10 bar. Preferred compressors are sealed and oil-free, such as the compressors sold under the tradename POWEREX, available from the Powerex Harrison Company of Ohio. 
     The compressed gas mixture  15  is then passed to at least one membrane separation unit  16  at conditions effective to obtain a permeate stream  17  rich in N 2  and a retentate stream  18  rich in SF 6  and CF 4 . As used herein, the term “rich” means that the concentration of a specified component(s) in the effluent stream of a particular separation step or unit is greater than the concentration of the same component(s) in the feed stream to that particular separation step or unit. 
     Any membrane can be used in the present invention so long as the membrane can selectively retain SF 6  and CF 4  while passing the other components, such as N 2 , in the compressed gas stream  15  through. The membrane should also be substantially non-reactive with the gaseous components to be separated. 
     In accordance with the foregoing, membranes most useful in the invention are preferably glassy membranes, such as polymer membranes made preferably from polyimides, polyamides, polyamide-imides, polyesters, polycarbonates, polysulfones, polyethersulfone, polyetherketone, alkyl substituted aromatic polyesters, blends of polyethersulfone, aromatic polyimides, aromatic polyamides, polyamidesimides, fluorinated aromatic polyamide, polyamide and polyamideimides, glassy polymeric membranes such as disclosed in U.S. Ser. No. 08/247,125 filed May 20, 1994 and incorporated herein by reference, cellulose acetates, and blends thereof, copolymers thereof, substituted polymers (e.g. alkyl, aryl) thereof and the like. Also sulfonated polymers as taught by U.S. Pat. No. 5,364,454 are within the scope of membranes useful in carrying out the present invention. 
     Asymmetric membranes are prepared by the precipitation of polymer solutions in solvent-miscible non-solvents. Such membranes are typified by a dense separating layer supported on an anisotropic substrate of a graded porosity and are generally prepared in one step. Examples of such membranes and their methods of manufacture are disclosed in U.S. Pat. Nos. 4,113,628; 4,378,324; 4,460,526; 4,474,662; 4,485,056; 4,512,893; 5,085,676; and 4,717,394; all incorporated herein by reference. Particularly preferred membranes are polyimide asymmetric gas separation membranes as disclosed in U.S. Pat. No. 5,085,676. 
     Some non-polymeric media fit the criteria for size-selection of gaseous and vapor components, and may be utilized in the practice of the present invention. Two such media which have been described for membrane application are carbon sieve and zeolite membranes. Both of these media separate species by a molecular sieving mechanism. Because of the highly discriminate nature of this process, very high selectivities can be achieved even between molecules of very similar size. For instance, a typical upper bound for O 2 /N 2  selectivity for polymeric media is 8-10 while carbon sieve membranes have exhibited selectivities on the order of 12-14. 
     The most successful means of producing carbon sieve membranes has been performed by pyrolysis of a polymeric membrane precursor. Means of producing such membranes and characterization for separation of gaseous materials are described in: 
     A. Soffer, J. Koresh and S. Saggy, U.S. Pat. No. 4,685,940 (1987); H. yoneyama and Y. Nishihara, U.S. Pat. No. 5,089,135 (1992); and C. W. Jones and W. J. Koros,  Carbon , Vol. 32, p. 1419 (1994), all incorporated herein by reference. 
     Zeolite coated or filled membranes have also been shown to offer benefits for gaseous and vapor components, and are described in: 
     K. Kusakabe, S. Yoneshige, A. Murata and S. Morooka,  J. Membrane Science , Vol. 116, p. 39 (1996); S. Morooka, S. Yan, K. Kusakabe and Y. Akiyama,  J. Membrane Science , Vol. 101, p. 89 (1995); E. R. Geus, H. van Vekkum, W. J. W. Bakker and J. A. Moulijn,  Microporous Mater. , Vol. 1, p. 131 (1993); and M. G. Suer, N. Bac and L. Yilmaz,  J. Membrane Science , Vol. 9, p. 77 (1994), all incorporated herein by reference. 
     Such zeolite coated or filled membranes may be useful in the practice of the present invention. 
     In a pressure driven gas membrane separation process, one side of the gas separation membrane is contacted with a complex multicomponent gas mixture and certain of the gases of the mixture permeate through the membrane faster than the other gases. Gas separation membranes thereby allow some gases to permeate through them while serving as a barrier to other cases in a relative sense. The relative gas permeation rate through the membrane is a property of the membrane material composition and its morphology. It is believed that the intrinsic permeability of a polymer membrane is a combination of gas diffusion through the membrane, controlled in part by the packing and molecular free volume of the material, and gas solubility within the material. Selectivity is the ratio of the relative permeability of two gases being separated by a material. It is also highly desirable to form defect-free dense separating layers in order to retain high gas selectivity. 
     Composite gas separation membranes typically have a dense separating layer on a preformed microporous substrate. The separating layer and the substrate are usually different in composition. Composite gas separation membranes have evolved to a structure of an ultrathin, dense separating layer supported on an anisotropic, microporous substrate. Composite membrane structures can be prepared by laminating a preformed ultrathin dense separating layer on top of a preformed anisotropic support membrane. Examples of such membranes and their methods of manufacture are disclosed in U.S. Pat. Nos. 4,664,669; 4,689,267; 4,741,829; 2,947,687; 2,953,502; 3,616,607; 4,714,481; 4,602,922; 2,970,106; 2,960,462; 4,713,292; 4,086,310; 4,132,824; 4,192,824; 4,155,793; and 4,156,597, all incorporated herein by reference. 
     Alternatively, composite gas separation membranes may be prepared by multistep fabrication processes, wherein first an anisotropic, porous substrate is formed, followed by contacting the substrate with a membrane-forming solution. Examples of such methods are described in U.S. Pat. Nos. 4,826,599; 3,648,845; and 3,508,994, all incorporated herein by reference. 
     U.S. Pat. No. 4,756,932 (incorporated herein by reference) describes how composite hollow-fiber membranes may also be prepared by co-extrusion of multiple polymer solution layers, followed by precipitation in a solvent-miscible non-solvent. 
     According to one embodiment of the present invention, the membrane can be post-treated with, or coated by, or co-extruded with, a fluorinated or perfluorinated polymer layer in order to increase its ability to withstand harmful constituents in the gas mixture from which SF 6  and CF 4  are to be separate, at low levels or temporary contact with such components. 
     The hollow-fiber spinning process depends on many variables which may affect the morphology and properties of the hollow-fiber membrane. These variables include the composition of the polymer solution employed to form the fiber, the composition of fluid injected into the bore of the hollow-fiber extrudate during spinning, the temperature of the spinneret, the coagulation medium employed to treat the hollow-fiber extrudate, the temperature of the coagulation medium, the rapidity of coagulation of the polymer, the rate of extrusion of the fiber, take-up speed of the fiber onto the take-up roll, and the like. It may be preferable to modify the membrane morphology to enhance the separation efficiency. One such method is taught by U.S. Pat. No. 5,468,430. 
     The temperature of the compressed gas stream  15  and/or the membrane during the contacting step in each membrane separation unit  16  can vary from about −10° C. to about 100° C. Preferably, the temperature is between about 10° C. and 80° C. More preferably, the temperature ranges from ambient, i.e., from about 20° C. to 25° C., to about 60° C. 
     The flowrate of the compressed gas stream  15  across the membrane in each membrane separation unit  16  can vary from about 0 to about 10 5  Nm 3 /h per square meter of membrane available for separation. Preferably, the flowrate ranges from about 10 −4  to about 10 Nm 3 /h-m 2 . More preferably, the flowrate ranges from about 0.1 to about 0.5 Nm 3 /h-m 2 . 
     Of course, the particular contacting conditions in each membrane separation unit  16  may be the same or different, depending on various factors including the type of membrane employed as well as the degree of separation or purity desired. The selection of such parameters is within the level of skill of the ordinary worker in this art. 
     Both the permeate stream  17  and the retentate stream  18  may be contacted with additional membrane separation units (not shown) in order to improve the purity of those streams. Such a modification is within the scope of this invention. 
     The retentate stream  18  is then passed to a condensation unit  20  where CF 4  is separated from SF 6 . In condensation unit  20 , a nitrogen (“LN 2 ”) through line  21  and discards, typically, a mixture of LN 2 /gaseous nitrogen (“GN 2 ”) in line  22 . The condensation unit  20  is operated at a temperature ranging from about −10° C. to about −60° C. so as to produce a liquid SF 6  stream  24  and a gaseous CF 4  stream  23 . 
     The liquid SF 6  stream  24  is then introduced into a heater/vaporizer  25  where the liquid SF 6  stream  24  is converted into a purified SF 6  gas stream  26 . The purified SF 6  gas stream  26  is optionally mixed with make-up N 2  via line  28  and/or make-up SF 6  via line  29  in a mixer  27  to form a SF 6 /N 2  recycle stream  30 . The SF 6 /N 2  recycle stream  30  can contain from about 5 vol % to about 95 vol % SF 6 , and from about 95 vol % to about 5 vol % N 2 . This stream  30  is recycled to EPDE  10 . Optionally, the N 2  in permeate stream  17  may be transferred through line  19  to the mixer  27  and used as part of the make-up N 2  stream  28 . Also optionally, the GN 2  in line  22  may also be used as part of the make-up N 2  stream  28  (not shown). A mixer suitable for use in this process is described in Applicant&#39;s Ser. No. 09/470,977, filed Dec. 23, 1999, incorporated herein by reference. 
     FIG. 2 is a schematic drawing of another preferred method and apparatus according to the invention. The method and apparatus shown in FIG. 2 is the same shown in FIG. 1, except that condenser  20  has been replaced with compressor  31  and gas-liquid separator  33 . In particular, the retentate stream  18  is compressed in compressor  21  to a pressure ranging from about 20 bar to about 50 bar to form a compressed retentate stream  32 . The pressure provided by compressor  31  is sufficient to form liquid SF 6  while leaving CF 4  in gaseous form. Stream  32  is then introduced into the gas-liquid separator  33  wherein CF 4  gas is removed via line  34  and liquid SF 6  is withdrawn in line  35 . 
     Prior to introduction into the mixer  27 , the liquid SF 6    35  is converted into a purified SF 6  gas stream  26  by heater/vaporizer  25 . 
     While the invention has been described with reference to preferred embodiments, it is to be understood that variations and modifications may be resorted to as will be apparent to those skilled in the art. Such variations and modifications are to be considered within the purview and the scope of the claims appended hereto.