PROCESS FOR RECOVERING CF4 AND C2F6 FROM A GAS

Abstract: A process for recovering at least one of CF4 and C2F6 from a vent gas from an aluminum electrolysis cell. The process includes the steps of: (a) removing inorganic fluorides from a vent gas comprising inorganic florides and at least one of CF4 and C2F6 to obtain a purified vent gas; and (b) contacting the purified vent gas with a membrane at conditions effective to obtain a retentate stream rich in at least one of CF4 and C2F6, and a permeate stream depleted in at least one of CF4 and C2F6.

Description: PROCESS FOR RECOVERING CF 4 AND C 2 F 6 FROM A GAS FIELD OF THE IISTVENTION The present invention generally relates to a gas separation process. The invention particularly relates to a process for removing at least one of CF 4 and C 2 F 6 from a gas stream using a membrane. The invention also particularly relates to a method of making aluminum, which method includes removing at least one of CF 4 and C 2 F 6 from a gas stream using a membrane. BACKGROUND OF THE INVENTION Currently, aluminum metal is commercially manufactured in two steps. The first step involves extracting alumina (Al 2 0 3 ) from bauxite using a Bayer process. The second step involves reducing the alumina which is dissolved in a mixture of molten cryolite (Na 3 AlF 6 ) and aluminum trifluoride (AlF 3 ) in an electrolysis cell at about 950 to 960°C using the Hall-Heroult process. In the second step, aluminum-containing ions are reduced electrochemically to produce metallic aluminum at the metal cathode surface. During normal electrolysis, carbon anodes are consumed as oxygen-containing ions react with the carbon anodes to form carbon dioxide and aluminum metal according to the following equation: 2A1 2 0 3 3C → 4A1 + 3CO, However, at certain conditions, the electrolysis cell approaches an anode effect. This approach is characterized by the following events: (1) The alumina concentration in the bulk of the electrolyte decreases below 2 % by weight ,- (2) Higher concentrations of fluoride ions prevail near the anode as the concentration of oxygen containing ions decreases; (3) The anode polarization voltage increases significantly; (4) The critical current density of the carbon anode is exceeded for the discharge of only oxygen- containing anions ; and (5) F 2 is eventually discharged at the anode surface from the decomposition of cryolite. During anode effects, the fluorine discharged at the anode reacts with the carbon to form CF 4 and C 2 F 6 according to the following equation: 2Na,AlF fi + 2C → 2A1 + 2NaF + CF„ + C F, For a more detailed discussion of the anode effect, see Alton T. Tabereaux, Anode Effects, PFCs, Global Warming, and the Aluminum Industry, JOM, pp. 30-34 (Nov. 1994) . For a typical electrolysis cell, the emission rate of CF 4 and C 2 F 6 per day is 0.25 kg. There are normally 100 to 200 cells per plant. Therefore, the daily emission of CF 4 and C 2 F 6 per plant is about 50 kg. The emission of CF 4 and C 2 F 6 from aluminum plants has typically been vented directly into the atmosphere. However, these gases, which are 10,000 times more potent than C0 2 , have recently been classified as global warming gases. Thus, with the signing of the United Nations Framework Convention on Climate Change which is aimed at reducing the emission of global warming gases, there is a significant need in the industry for a way to minimize or eliminate the emission of these gases into the atmosphere. Accordingly, it is an object of the present invention to address this need in the aluminum industry. These and other objects of the invention will become apparent in light of the following specification, and the appended drawings and claims. SUMMARY OF THE INVENTION The present invention relates to a process for recovering at least one of CF 4 and C 2 F 6 from a vent gas from an aluminum electrolysis cell. The process comprises the steps of : (a) removing inorganic fluorides from a vent gas comprising inorganic fluorides and at least one of CF 4 and C 2 F 6 to obtain a purified vent gas; and (b) contacting the purified vent gas with a membrane at conditions effective to obtain a retentate stream rich in at least one of CF 4 and C 2 F 6 , and a permeate stream depleted in at least one of CF 4 and C 2 F 6 . In another aspect, the present invention relates to a method of making aluminum. The method includes the steps of: (a) electrolytically reducing alumina dissolved in a mixture of molten cryolite and aluminum trifluoride in an electrolysis cell to produce aluminum; (b) withdrawing a vent gas comprising F 2 , HF, and at least one of CF 4 and C 2 F 6 from the electrolysis cell; (c) contacting the vent gas with alumina at conditions effective to react F 2 and HF with the alumina to produce aluminum trifluoride and a gas stream comprising the at least one of CF 4 and C 2 F 6 ; (d) recycling at least a portion of the aluminum trifluoride from step (c) to the electrolysis cell; and (e) contacting the gas stream comprising the at least one of CF 4 and C 2 F 6 with a membrane at conditions effective to obtain a retentate stream rich in at least one of CF 4 and C 2 F 6 , and a permeate stream depleted in at least one of CF 4 and C 2 F 6 . BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 depicts a process flow diagram for recovering CF 4 and C 2 F 6 according to the present invention. Figure 2 is a graph showing the relative permeabilities of N 2 , CF 4 , and C 2 F 6 through a particular membrane at different temperatures. Figure 3 illustrates an engineering design of a membrane system that can be employed in the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In its broadest aspect, the present invention relates to a process for removing at least one of CF 4 and C 2 F 6 from a gas stream. Preferably, the present invention relates to a process for removing both CF 4 and C 2 F 6 from a gas stream containing the same. Prior to separation, the gas stream preferably contains from about 0.01 to about 20% by volume of at least one of CF 4 and C 2 F 6 . In addition to containing at least one of CF 4 and C 2 F 6 , the gas stream can include oxygen, carbon dioxide, and nitrogen as well as other gaseous components. The process according to the present invention comprises the step of contacting the gas stream with a membrane at conditions effective to obtain a retentate stream rich in at least one of CF 4 and C 2 F 6 , and a permeate stream depleted in at least one of CF 4 and C 2 F 6 . Preferably, the retentate stream is depleted in oxygen, carbon dioxide, and nitrogen while the permeate stream is rich in oxygen, carbon dioxide, and nitrogen. If both CF 4 and C 2 F 6 are present in the gas stream, then the contacting step preferably produces a retentate stream rich in both CF 4 and C 2 F 6 , and a permeate stream depleted in both CF 4 and C 2 F 6 . As used in this specification and the claims, the term "rich" means that the concentration of a particular component in that stream is greater than the concentration of the same component in the feed stream. Likewise, the term "depleted" means that the concentration of a particular component in that stream is less than the concentration of the same component in the feed stream. The retentate stream preferably comprises from about 25 to about 100% by volume of at least one of CF 4 and C 2 F 6 . The permeate stream preferably comprises from about 0 to about 0.01% by volume of at least one of CF 4 and C 2 F 6 . In a preferred embodiment, the purity of the permeate and retentate streams is improved by contacting the streams with additional membranes arranged in series. Such a process may be carried out according to the multi-step membrane separation system shown in Figure 3. As shown in Figure 3 , each of the permeate and retentate streams may be passed to another membrane contacting unit to increase the purity of the stream. The number of contacting steps "m" and "n" may vary depending on the purity desired. By using such a process, it is possible to obtain 100% recovery of at least one of CF 4 and C 2 F 6 with a purity approaching 100%. Any membrane can be used in the process according to the present invention so long as the membrane can selectively retain at least one of CF 4 and C 2 F 6 while passing the other components in the gas stream through. The membrane should also be substantially non-reactive with the gaseous components to be separated. Membranes 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, polyamides-imides, fluorinated aromatic polyimide, polyamide, and polyamide-imides; glassy polymeric membranes such as those disclosed in USSN 08/247,125 filed May 20, 1994, the content of which is hereby incorporated by reference; cellulose acetates; and blends thereof, copolymers thereof, substituted polymers (e.g. alkyl, aryl) thereof and the like. Asymmetric membranes are prepared by the precipitation of polymer solutions in solvent-miscible nonsolvents. 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. Patent 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. The '394 and '676 patents disclose preparation of asymmetric separation membranes from selected polyimides. Particularly preferred membranes are polyimide asymmetric gas separation membranes as disclosed in the '676 patent. 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 gases in a relative sense. The relative gas permeation rate through the membrane is a property of the membrane material composition and its morphology. It has been suggested in the prior art 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 permeabilities 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. These 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. Patent 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. Patent Nos. 4,826,599; 3,648,845; and 3,508,994; all incorporated herein by reference. U.S. Patent No. 4,756,932 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 nonsolvent . 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 PFCs 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, takeup speed of the fiber onto the takeup roll, and the like. The temperature of the gas mixture and/or the membrane during the contacting step 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. It is preferred, according to the present invention, to have a pressure drop across the membrane of less than about 2,000 psig. More preferably, the pressure drop ranges from about 3 to about 200 psig. Even more preferably, the pressure drop is about 20 to about 60 psig. The requisite pressure drop across the membrane can be provided in one of two ways. First, the feed gas stream can be compressed. Preferred compressors are sealed and oil -free, such as the compressors sold under the tradename POWEREX, available from the Powerex Harrison Company of Ohio. Second and more preferably, the pressure drop across the membrane can be established by lowering the pressure on the permeate side of the membrane. To create the lower pressure on the permeate side, a vacuum pump or any other suction device can be used. The flowrate of the gas stream across the membrane can vary from about 0 to about 10 5 Nm 3 /h per square meter of membrane available for separation. Preferably, the flowrate is from about 10 "4 to about 10 Nm 3 /h-m 2 . More preferably, the flowrate is from about 0.1 to about 0.5 Nm 3 /h-m 2 . In a preferred aspect, the present invention relates to a process for recovering at least one of CF 4 and C 2 F 6 from a vent gas from an aluminum electrolysis cell . The vent gas comprises (1) gaseous components such as 0 2 , C0 2 , and N 2 ; (2) inorganic fluorides such as F 2 , HF, and NaAlF 4 ; and (3) at least one of CF 4 and C 2 F 6 . The vent gas can also include some hydrocarbons and a large amount of particulates . The first step in this process involves removing the inorganic fluorides from the vent gas to produce a purified vent gas. The inorganic fluorides are preferably removed from the vent gas by using a caustic scrubber. The scrubber can be wet or dry. Dry scrubbers are usually resin-type scrubbers or soda-lime, while some dry scrubbers comprising catalysts such as Mn0 2 can also be used. Exemplary wet scrubbers that can be used in the present invention are described in the brochure entitled, "Selecting a CDO™ for Your Particular Application" from DELATECH Corporation, which brochure is hereby incorporated by reference. When various harmful constituents have to be removed, it is preferred to use a dry scrubber or scrubbers in series with a wet scrubber or scrubbers. Preferably, upstream of the scrubber or scrubbers, one or more filters are employed to remove the particulates from the vent gas. It is preferred to use a filter having a pore size diameter of less than 20 micrometers, and more preferably, less than 10 micrometers. The second step in this process involves contacting the purified vent gas with a membrane at conditions effective to obtain a retentate stream rich in at least one of CF 4 and C 2 F 6 , and a permeate stream depleted in at least one of CF 4 and C 2 F 6 . This membrane separation step can be carried out as described above. If there are remaining particulates in the purified vent gas before it is passed to the membrane separation unit, it is contemplated by the present invention to employ an additional filter or filters to remove such particulates. In another preferred aspect, the present invention relates to a method of making aluminum. The method includes the steps of: (a) electrolytically reducing alumina dissolved in a mixture of molten cryolite and aluminum trifluoride in an electrolysis cell to produce aluminum; (b) withdrawing a vent gas comprising F 2 , HF, and at least one of CF 4 and C 2 F 6 from the electrolysis cell; (c) contacting the vent gas with alumina at conditions effective to react F 2 and HF with the alumina to produce aluminum trifluoride and a gas stream comprising the at least one of CF 4 and C 2 F 6 ; (d) recycling at least a portion of the aluminum trifluoride from step (c) to the electrolysis cell; and (e) contacting the gas stream comprising the at least one of CF 4 and C 2 F 6 with a membrane at conditions effective to obtain a retentate stream rich in at least one of CF 4 and C 2 F 6 , and a permeate stream depleted in at least one of CF 4 and C 2 F 6 . The general features of this method are illustrated in Figure 1. Referring to Figure 1, a stream 1 of alumina, cryolite, and aluminum trifluoride is fed into an aluminum electrolyzing cell 2 where molten aluminum is produced by electrolytically reducing the alumina. The operating conditions and equipment necessary to carry out this step are well known to those skilled in the art. The molten aluminum is then withdrawn from the electrolyzing cell 2 through line 3. A vent gas stream 4 is also withdrawn from the electrolyzing cell 2. The vent gas stream 4 comprises 0 2 , C0 2 , N 2 , F 2 , HF, NaAlF 4 , CF 4 , C 2 F 6 , and particulates. The vent gas stream 4 is optionally passed to a filter 5 to remove the particulates therein. A filtered vent gas 6 is optionally withdrawn from filter 5 and passed to a dry scrubbing zone 7. In the scrubbing zone 7, the filtered vent gas 6 is contacted with alumina introduced through line 8 at conditions effective to remove inorganic fluorides such as F 2 and HF therefrom. These conditions for carrying out this scrubbing step are well known to those skilled in the art. During the scrubbing step, the alumina reacts with F 2 and HF in the filtered vent gas 6 to form aluminum trifluoride according to the following equations: A1 2 0 3 + F 2 A1F 3 + 0 2 A1 2 0 3 + HF → A1F, + H 2 0 A stream 9 comprising the aluminum trifluoride is then withdrawn from the dry scrubbing zone 7 and recycled to stream 1. The dry scrubbing zone 7 also yields a purified vent gas stream 10 containing CF 4 , C 2 F 6 , 0 2 , C0 2 , and N 2 . Stream 10 is passed to an additional filter 11, if necessary, to remove any remaining particulates therein. The purified vent gas stream 10 is next passed to a membrane separation system 12 such as the one described above and illustrated in Figure 3 where a retentate stream 14 and a permeate stream 13 are produced. The permeate stream 13 comprises mostly of 0 2 , C0 2 , and N 2 , while the retentate stream 14 comprises mostly of CF 4 and C 2 F 6 . The retentate stream 14 comprising CF 4 and C 2 F 6 may be further purified on or off-site to produce a CF 4 /C 2 F 6 stream suitable for use in the semiconductor industry. EXAMPLES The following examples are provided to illustrate the present invention and are not to be construed as a limitation thereof. Example 1 A gas stream comprising CF 4 , C 2 F 6 , and N 2 was contacted with a polyimide, asymmetric composite hollow fiber membrane at various temperatures to measure the relative permeabilities of CF 4 , C 2 F 6 , and N 2 . The gas stream had a constant flowrate rate of 170 seem. The pressure of the feed gas was kept constant at 3 bar. The permeabilities of each of these components are graphically shown in Figure 2. As seen from Figure 2, the selectivities of CF 4 /N 2 and C 2 F 6 /N 2 are on the order of 300. Based on the above selectivities, a computer simulation of a single stage membrane separation unit was conducted. The concentration and pressure of the feed, permeate, and retentate as well as the CF 4 /C 2 F 6 recovery are shown in Table 1 below. TABLE 1 Example 2 The procedure of Example 1 was repeated using the feed concentration and pressure listed in Table 2 below. The simulated results are also listed in Table 2. TABLE 2 Example 3 For a typical smelting plant with 100 smelting pots, the flowrate of the vent gas is about 3 Nm 3 /h. The same procedure of Example 1 was followed using such a stream. The feed concentration and pressure as well as the simulated results of such a stream are reported in Table 3 below. TABLE 3 Example 4 In an actual experiment, a feed stream comprising 1.14% CF 4 and C 2 F 6 , and the balance N 2 was contacted with a polyimide, asymmetric composite hollow fiber membrane at room temperature. The flowrate, concentration, and pressure of the feed, permeate, and retentate streams along with the percentage recovery of CF 4 and C 2 F 6 are reported in Table 4 below. TABLE 4 While the invention has been described with 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.

Claims: WHAT IS CLAIMED IS: 1. A process for recovering at least one of CF 4 and C 2 F 6 from a vent gas from an aluminum electrolysis cell, said process comprising the steps of: (a) removing inorganic fluorides from a vent gas comprising inorganic fluorides and at least one of CF 4 and C 2 F 6 to obtain a purified vent gas; and (b) contacting said purified vent gas with a membrane at conditions effective to obtain a retentate stream rich in at least one of CF 4 and C 2 F 6 , and a permeate stream depleted in at least one of CF 4 and C 2 F 6 . 2. The process according to Claim 1, wherein said inorganic fluorides are F 2 , HF, and NaAlF 4 . 3. The process according to Claim 1, wherein step (a) is carried out using a caustic scrubber. 4. The process according to Claim 1, wherein said vent gas further comprises particulates, and wherein said vent gas is contacted with a filter to remove said particulates prior to step (a) . 5. The process according to Claim 1, wherein said purified vent gas further comprises particulates, and wherein said purified vent gas is contacted with a filter to remove said particulates prior to step (b) . 6. The process according to Claim 1, wherein said vent gas comprises both CF 4 and C 2 F 6 , and said retentate stream is rich in both CF 4 and C 2 F 6 , and said permeate stream is depleted in both CF 4 and C 2 F 6 . 7. The process according to Claim 1, wherein said vent gas further comprises 0 2 , C0 2 , and N 2 . 8. The process according to Claim 7, wherein said retentate stream is depleted in 0 2 , C0 2 , and N 2 , and said permeate stream is rich in 0 2 , C0 2 , and N 2 . 9. The process according to Claim 1, wherein said vent gas comprises from about 0.01 to about 20% by volume of at least one of CF 4 and C 2 F 6 . 10. The process according to Claim 9, wherein said retentate stream comprises from about 25 to about 100% by volume of at least one of CF 4 and C 2 F 6 , and said permeate stream comprises from about 0 to about 0.01% by volume of at least one of CF 4 and C 2 F 6 . 11. The process according to Claim 1, wherein said conditions comprise a temperature between about 10 and about 80 °C, a pressure drop between about 3 and about 200 psig, and a flowrate rate between about 10 ~4 and about 10 Nm 3 /h-m 2 . 12. The process according to Claim 1, wherein said membrane is selected from the group consisting of polyimides, polyamides, polyamide-imides, polyesters, polycarbonates, polysulfones, polyethersulfone, polyetherketone, alkyl substituted aromatic polyesters, and blends of polyethersulfone, aromatic polyimides, aromatic polyamides, polyamides- imides, fluorinated aromatic polyimide, polyamide, and polyamide-imides. 13. A method of making aluminum comprising the steps of: (a) electrolytically reducing alumina dissolved in a mixture of molten cryolite and aluminum trifluoride in an electrolysis cell to produce aluminum; (b) withdrawing a vent gas comprising F 2 , HF, and at least one of CF 4 and C 2 F 6 from said electrolysis cell; (c) contacting said vent gas with alumina at conditions effective to react F 2 and HF with said alumina to produce aluminum trifluoride and a gas stream comprising said at least one of CF 4 and C 2 F 6 ; (d) recycling at least a portion of said aluminum trifluoride from step (c) to said electrolysis cell; and (e) contacting said gas stream comprising said at least one of CF 4 and C 2 F 6 with a membrane at conditions effective to obtain a retentate stream rich in at least one of CF 4 and C 2 F 6 , and a permeate stream depleted in at least one of CF 4 and C 2 F 6 . 14. The process according to Claim 13, wherein said gas stream comprises both CF 4 and C 2 F 6 , and said retentate stream is rich in both CF 4 and C 2 F 6 , and said permeate stream is depleted in both CF 4 and C 2 F 6 . 15. The process according to Claim 13, wherein said gas stream further comprises 0 2 , C0 2 , and N 2 . 16. The process according to Claim 15, wherein said retentate stream is depleted in 0 2 , C0 2 , and N 2 , and said permeate stream is rich in 0 2 , C0 2 , and N 2 . 17. The process according to Claim 13, wherein said gas stream comprises from about 0.01 to about 20% by volume of at least one of CF 4 and C 2 F 6 . 18. The process according to Claim 17, wherein said retentate stream comprises from about 20 to about 100% by volume of at least one of CF 4 and C 2 F 6 , and said permeate stream comprises from about 0 to about 0.01% by volume of at least one of CF 4 and C 2 F 6 . 19. The process according to Claim 13, wherein said conditions comprise a temperature between about 10 and about 80 °C, a pressure drop between about 3 and about 200 psig, and a flowrate rate between about 10 "4 and about 10 Nm 3 /h-m 2 . 20. The process according to Claim 13, wherein said membrane is selected from the group consisting of polyimides, polyamides, polyamide-imides, polyesters, polycarbonates, polysulfones, polyethersulfone, polyetherketone, alkyl substituted aromatic polyesters, and blends of polyethersulfone, aromatic polyimides, aromatic polyamides, polyamides-imides, fluorinated aromatic polyimide, polyamide, and polyamide-imides.

Inventors: LI YAO-EN, LI, YAO-EN
Applicants: AIR LIQUIDE, LI YAO EN, L'AIR LIQUIDE SOCIETE ANONYME POUR L'ETUDE ET, L'E, LI, YAO-EN
Application year: 1997
Main classification: B01D53
IPC codes: B01D53, C25C3