| PATENT ASSIGNEE'S COUNTRY | USA |
| UPDATE | 09.00 |
| PATENT NUMBER | This data is not available for free |
| PATENT GRANT DATE | 26.09.00 |
| PATENT TITLE |
Separation of CO.sub.2 from unsaturated fluorinated compounds by semipermeable membrane |
| PATENT ABSTRACT |
A process for separating carbon dioxide from an unsaturated fluorinated compound carbon dioxide mixture comprising contacting the unsaturated fluorinated compound carbon dioxide mixture with a semipermeable membrane to form at least one exit stream having an increased concentration of carbon dioxide and at least one other exit stream having a reduced concentration of carbon dioxide. |
| PATENT INVENTORS | This data is not available for free |
| PATENT ASSIGNEE | This data is not available for free |
| PATENT FILE DATE | 01.05.98 |
| PATENT REFERENCES CITED |
Membranes can Efficiently Separate CO.sub.2 From Mixtures, Schell et al., Oil & Gas Journal, Aug. 15, 1983, p. 83. Relationship Between Gas Separation Properties and Chemical Structure in a Series of Aromatic Polyimides, Kim et al., Journal of Membrane Science, 37 (1988), 45-62. |
| PATENT PARENT CASE TEXT | This data is not available for free |
| PATENT CLAIMS |
What is claimed is: 1. A process for separating carbon dioxide from an unsaturated fluorinated compound carbon dioxide mixture comprising one or more stages in which a feed stream of said unsaturated fluorinated compound carbon dioxide mixture is contacted with a semipermeable membrane to form at least one exit stream having an increased concentration of carbon dioxide and at least one exit stream having a reduced concentration of carbon dioxide, said process in at least one of said one or more stages causing said exit stream with increased concentration of carbon dioxide to contain less than about 5% by weight of the unsaturated fluorinated compound present in said feed stream. 2. The process of claim 1 wherein said unsaturated fluorinated compound comprises a compound of the formula R--CF.dbd.CF.sub.2, wherein R is F, Cl, R.sub.f or O--Rf and R.sub.f is perfluoroalkyl containing 1-5 carbon atoms. 3. The process of claim 1 wherein said unsaturated fluorinated compound comprises tetrafluoroethylene. 4. The process of claim 1 wherein said semipermeable membrane comprises a membrane selected from the group consisting of polyimide membranes and polyaramid membranes. 5. The process of claim 1 wherein said semipermeable membrane comprises a polyimide membrane having phenylindane residues incorporated in the polyimide backbone chain. -------------------------------------------------------------------------------- |
| PATENT DESCRIPTION |
FIELD OF THE INVENTION The present invention relates to a process for separating carbon dioxide from unsaturated fluorinated compounds such as fluoroalkyl perfluorovinyl ethers and other fluorinated compounds having a double bond. BACKGROUND OF THE INVENTION Fluoroalkyl perfluorovinyl ethers of the formula R.sub.f --CF.sub.2 --O--CF.dbd.CF.sub.2, wherein R.sub.f is fluorine or a fluorine-containing organic radical, have found extensive use as co-monomers for preparation of fluoroplastics and fluoroelastomers. Fluoroalkyl perfluorovinyl ethers are known to co-polymerize with alkenes such as ethylene, tetrafluoroethylene, chlorotrifluoroethylene, vinylidene fluoride, vinyl fluoride, propene and hexafluoropropene. Of particular interest and significant application are co-polymers formed by co-polymerization of perfluoroalkyl perfluorinated ethers with tetrafluoroethylene and/or hexafluoropropylene. These co-polymers are often referred to as perfluoroalkoxy co-polymers, and are useful in producing high-quality electrical insulation and molded components. A general review of perfluoroalkoxy co-polymers occurs in a review article titled "Organic Fluoropolymers" by Carlson et al, found in Ullman's Encyclopedia of Industrial Chemistry, Fifth Edition, p. 393. The prevalent method found in the art for preparation of fluoroalkyl perfluorovinyl ethers (see Kirk-Othmer Encyclopedia of Chemical Technology, Fourth Edition, page 672) involves firstly the reaction of a fluoroalkyl carboxylic acid fluoride with hexafluoropropylene oxide to form an intermediate fluoroalkyl 2-alkoxypropionic acid fluoride as shown by the following equation: ##STR1## wherein R.sub.f is fluorine or a fluorine-containing organic radical. This intermediate fluoroalkyl 2-alkoxypropionic acid fluoride, after purification by distillation or other means, is then defluorocarbonylated by treatment with a dry alkali carbonate to form a fluoroalkyl perfluorovinyl ether as shown by the following equation: R.sub.f --CF.sub.2 --O--CF(CF.sub.3)--COF+Na.sub.2 CO.sub.3 .fwdarw.Rf--CF.sub.2 --O--CF.dbd.CF.sub.2 +2CO.sub.2 +2NaF where Na.sub.2 CO.sub.3 is used as an example of the alkali carbonate. In a typical process, the solid impurities such as NaF and unreacted Na.sub.2 CO.sub.3 are then removed by filtration as by passing through a bag filter, and the CO.sub.2 is removed by scrubbing with an alkali solution such as NaOH or KOH solution to form the corresponding alkali carbonate. KOH is preferred because of improved solubility. Water may then be removed as by passage through molecular sieves or by other means, and the dried crude fluoroalkyl perfluorovinyl ether is further purified as required for its intended use, typically by distillation to remove high and low-boiling impurities. The removal of CO.sub.2 by alkali scrubbing is costly for a number of reasons. Since there is a large amount of CO.sub.2 produced, there is a large consumption of alkali and a large waste disposal problem for the alkali carbonate solution which is contaminated with fluorine-containing organic impurities. In addition, there is a significant yield loss, perhaps 1 to 5%, of expensive fluorochemical product caused by a combination of solubility in the waste scrubbing solution and some reactions of the fluoroalkyl perfluorovinyl ether with the alkali. There is a need for a method for separation of part or essentially all of the CO.sub.2 from the fluoroalkyl perfluorovinyl ethers reaction product with a minimum or elimination of alkali scrubbing, and without introducing new chemicals into the system. It is also known to carry out polymerizations of fluorinated monomers in media comprising CO.sub.2. See, for example, U.S. Pat. No. 5,674,957. Unreacted monomers from such processes are desirably recovered from mixtures with CO.sub.2 for recycle to the polymerization reaction. DESCRIPTION OF THE RELATED ART The use of semipermeable membranes to separate gases other than CO.sub.2 and fluorinated compounds, e.g., fluoroalkyl perfluoro vinyl ethers, is well known. Many of the separations disclosed in the literature are based on polyimide membranes. For example, Kim et al., "Relationship between Gas Separation Properties and Chemical Structure in a Series of Aromatic Polyimides", Journal of Membrane Science, 37 (1988), 45-62, discloses various polyimide structures tested for a number of gas separations. The present invention is not limited to a particular polyimide structure. Many other such references describe suitable polyimide structures for gas permeation. U.S. Pat. No. 5,015,270 discloses a process for separation of gases using a polyimide membrane having phenylindane residues incorporated in the polyimide backbone chain. A preferred polyimide is "MATRIMID" 5218 polyimide resin, made by Ciba-Geigy and based on 5(6)-amino-1-(4'-aminophenyl)-1,3-trimethylindane. Examples to demonstrate selectivity were made with common atmospheric gases (O.sub.2, N.sub.2, CO.sub.2, He). U.S. Pat. No. 5,085,676 discloses a process for preparing a multicomponent membrane comprising a porous polymeric substrate and a separating layer of various polyimide or other structures. Example 40 utilizes "MATRIMID" 5218 as the separating layer and "ULTEM" 1000, a polyetherimide made by GE as substrate. Its selectivity was measured with O.sub.2 /N.sub.2 mixtures. U.S. Pat. No. 5,042,992 discloses a novel class of polyimides based on a partially fluorinated polyimide. It is said to be useful for making semipermeable membranes which have a high permeability and acceptable selectivity for CO.sub.2 from mixtures of CO.sub.2 and methane. The examples used to determine selectivity were either made using pure CO.sub.2 and methane, a mixture of 30% CO.sub.2 and 70% methane, or of 10% CO.sub.2 and 90% methane. U.S. Pat. No. 5,120,329 discloses a method for providing a controlled atmosphere in a food storage facility using a semipermeable membrane which has a higher permeability to CO.sub.2 than to nitrogen. Typical CO.sub.2 levels are given as about 0 to 20%, with 2% CO.sub.2 used as the dividing line between low and high concentrations for various applications. Polyimide membranes are cited as examples of suitable membranes for this application. In an article by Schell et al, "Membranes can Efficiently Separate CO.sub.2 from Mixtures", Oil & Gas Journal, Aug. 15, 1983, page 83, an example is given of removing low concentrations of CO.sub.2 from a refinery off-gas containing hydrogen by using a commercially available but unspecified membrane that allows CO.sub.2 to permeate more rapidly than hydrogen. A two-stage membrane system was required to reduce the CO.sub.2 concentration from 6% to 0.2% (60,000 ppm to 2000 ppm), with 50% of the hydrogen still retained in the non-permeate stream. SUMMARY OF THE INVENTION In accordance with the invention, a process is provided for separating carbon dioxide from an unsaturated fluorinated compound carbon dioxide mixture comprising contacting the unsaturated fluorinated compound carbon dioxide mixture with a semipermeable membrane to form at least one exit stream having an increased concentration of carbon dioxide and at least one other exit stream having a reduced concentration of carbon dioxide. Preferably, the membrane is a polyimide or a polyaramid membrane. The process is preferably employed for separating carbon dioxide from mixtures comprising a compound selected from the group consisting of CF.sub.2 .dbd.CF--R wherein R is F, Cl, R.sub.f or O--R.sub.f and R.sub.f is perfluoroalkyl containing 1-5 carbon atoms. When R is O--R.sub.f, CF.sub.2 .dbd.CF--R is preferably selected from perfluoropropyl perfluorovinyl ether (PPVE), perfluoromethyl perfluorovinyl ether (PMVE) and perfluoroethyl perfluorovinyl ether (PEVE). Another preferred unsaturated fluorinated compound is tetrafluoroethylene. In a preferred form of the invention, the semipermeable is a polyimide membrane and has phenylindane residues incorporated in the polyimide backbone chain. In a preferred process in accordance with the invention, the exit stream with increased concentration of carbon dioxide contains less than about 10% by weight of the unsaturated fluorinated compound present in the original unsaturated fluorinated compound carbon dioxide mixture. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatical view of laboratory scale apparatus useful for illustrating an embodiment of the present invention. DETAILED DESCRIPTION In this application, "unsaturated fluorinated compound" refers to unsaturated perhalogenated compounds containing fluorine, and preferably to compounds of the formula R--CF.dbd.CF.sub.2, wherein R is F, Cl, R.sub.f or O--R.sub.f and R.sub.f is perfluoroalkyl containing 1-5 carbon atoms. Preferred R in compounds in the carbon dioxide mixtures for the practice of the present invention are R, R.sub.f and O--R.sub.f. When R is O--R.sub.f, preferred fluorinated compounds include perfluoropropyl perfluorovinyl ether (PPVE), perfluoromethyl perfluorovinyl ether (PMVE) and perfluoroethyl perfluorovinyl ether (PEVE). Another preferred fluorinated compound is tetrafluoroethylene (TFE). Another preferred fluorinated compound is hexafluoropropylene (HFP). The fluorinated compound component of the unsaturated fluorinated compound carbon dioxide mixture from which CO.sub.2 is removed by the process of the present invention is comprised of at least one fluorinated compound. Thus, the fluorinated compound component of the unsaturated fluorinated compound carbon dioxide mixture can be a mixture of unsaturated fluorinated compounds. Such mixtures include, for example, TFE/HFP, TFE/PAVE, and TFE/HFP/PAVE, wherein PAVE is perfluoro(alkyl vinyl ether). In the foregoing mixtures, PAVE can be a single compound or a PAVE mixture, e.g., a mixture of perfluoro(methyl vinyl ether) and perfluoro(propyl vinyl ether), or, e.g., a mixture of perfluoro(ethyl vinyl ether) and perfluoro(propyl vinyl ether). One skilled in the art will recognize that compounds other than those defined above, i.e., CF.sub.2 .dbd.CF--R, can be present in the mixture. Such other compounds can contain fluorine or can be fluorine-free, and may or may not be separated from CO.sub.2 by the process of the present invention. In accordance with the present invention, a mixture of unsaturated fluorinated compound and carbon dioxide is contacted with a semipermeable membrane to form two exit streams, one of which has an increased concentration of carbon dioxide (CO.sub.2) and the other exit stream has a reduced concentration of carbon dioxide. Usually, the reduced CO.sub.2 stream is the "non-permeate" stream, often called the "reject" stream, and does not pass through the membranes whereas the "permeate" stream passes through the membrane and has increased CO.sub.2 content. Typically, the stream with reduced CO.sub.2 content is recovered, i.e., shipped or sold in the form recovered, processed by contact with other semipermeable membranes, or further processed by conventional means to achieve additional separation or recovery/removal of a desired component. The fluorinated compound, e.g., vinylether, enriched in CO.sub.2 can be recycled to earlier stages in the purification process, subjected to further purification before recycling, blended with fluorinated compounds of the same type to be used for less demanding markets, or disposed of by incineration or other means as permitted by environmental regulations. When the process of the invention is used in conjunction with polymerizations of fluorinated monomers in CO.sub.2, the monomer concentration in the reactor discharge stream can be increased for recycle to polymerization, a step that usually would not require a very high monomer concentration. Other components, organic or inorganic, may be present during the contacting step of the instant invention. The membrane separation device useful in the present invention can be any such device as known in the art and may be in any shape which has a feed side and a permeate side. Included in this description are membranes which can be formed into films (with or without support), tubular devices, spiral wound devices, hollow fibers and the like. The semipermeable membrane useful in the instant invention prefereably is polyimide membrane or polyaramid membrane. Such membrane may be made of any polyimide or polyaramid material capable of preferentially passing the CO.sub.2 relative to the unsaturated fluorinated compounds. That is, the ratio of permeation rates of the CO.sub.2 to that of the unsaturated fluorinated compounds should be greater than 1. Obviously, the higher the ratio, the more efficient will be the separation process. Polyimide membranes typically used in commercial use for conventional gas separations may be used. Preferably, the polyimide has phenylindane residues incorporated in the polyimide backbone chain. One membrane of this type is "MATRIMID" 5218 polyimide resin, manufactured by Ciba-Geigy and based on 5(6)-amino-1-(4'-aminophenyl)-1,3-trimethylindane. The membrane may be a composite of a porous substrate and the polyimide resin. For example, hollow fibers of "Ultem" 1000, a polyetherimide made by General Electric, are a particularly suitable support for "Matrimid" 5218. Such membranes and their manufacture are disclosed in U.S. Pat. No. 5,085,676. Polyaramid membranes that can be used include those of the types disclosed in U.S. Pat. No. 5,085,774. As in known permeation separation process, parameters which are usually considered as variables to enhance the separation process are the temperature, the pressure differential and average pressure ratio between the feed side of the membrane and the permeate side of the membrane, and the residence time of the feed stream on the feed side of the membrane and the residence time of the permeate on the permeate side of the membrane. In the instant invention, these parameters may be varied to enhance the separation so long as the values selected are not damaging to the membrane material. Temperature can be any convenient temperature, usually from about -50 to 150.degree. C. The primary temperature limitations are that the temperature should be below any temperature at which the membrane is affected adversely and above the dew point of the fluorocarbon. Preferably, the temperature range will be from about 0 to about 75.degree. C. The pressure differential between the feed side of the membrane and the permeate side is preferably at least about 0.1 atmosphere (10 kPa). The process may be operated at a lesser pressure differential but the separation process will be slower. The pressure differential can be the result of higher pressure on the feed side of the semipermeable membrane or the result of reduced pressure on the permeate side of the membrane or a combination of both. Useful feed pressures can vary substantially with the mode in which the membrane device is employed and with the materials being separated. For hollow fiber membranes, for example, feed pressure might be as high as 1000 psig (7 MPa) for feed to the outside of the fibers (shell-side feed) but might be limited to 200-250 psig (1.5-1.8 MPa) for bore-side feed. Additionally, choice of pressure should be consistent with safe handling of the various streams. The present process can be carried out as a batch process or as a continuous process. Since this permeation separation process is a differential process producing a substantial reduction in CO.sub.2, multiple pass or multiple stage processes may be the most efficient system to achieve very high purity fluorocarbons. In such multiple stage arrangements, an output stream from one stage can be fed to another stage either as the primary feed to that other stage or as a recycle stream. The term "stage" as used in the present application is intended to encompass a stage in which gases are fed to a separate membrane separation device or a pass in which gases are returned to the same device. Preferably, at least about 50%, more preferably at least about 75%, by weight of the CO.sub.2 present is removed in a stage of the process. When low or trace levels of CO.sub.2 are present, removal to less than a few ppm can be achieved in a one or two pass process. Because of the efficient separation of CO.sub.2 in preferred processes in accordance with the invention, the process is particularly advantageous for applications in which the unsaturated fluorinated compound mixture contains significant levels of CO.sub.2, especially mixtures containing at least about 10% by weight CO.sub.2. The present invention provides separation without the purchase and addition of extraneous materials and without creating additional waste disposal problems. Preferred processes in accordance with the invention can provide low "losses" of the unsaturated fluorinated compounds. "Loss" is determined from the weight of the unsaturated fluorinated compound in the stream with increased carbon dioxide concentration (usually the permeate stream) in relation to the weight of the unsaturated fluorinated compound present in the original unsaturated fluorinated compound carbon dioxide mixture. Preferably, the exit stream with increased concentration of carbon dioxide contains less than about 10% by weight, more preferably less than about 5 percent, and most preferably less than about 2%, of the unsaturated fluorinated compound present in the original unsaturated fluorinated compound carbon dioxide mixture. The aforesaid low losses can be achieved in multiple stage processes, but preferably are achieved in a single stage. Compared to known processes which employ caustic scrubbers for removal of CO.sub.2, the process in accordance with the invention not only eliminates the need for a caustic scrubber, eliminating its capital cost, but also eliminates the required consumption of caustic and its substantial annual cost. The process also eliminates the need to remove water from the product after scrubbing. From an environmental standpoint, the amount of waste products for disposal is also substantially reduced. Compared to known processes for the manufacture of PPVE, a yield improvement for the PPVE of perhaps 1 to 5% is also achievable because of the elimination of caustic scrubber solubility and reaction losses. The following examples are presented for illustrative purposes only, and in no way are intended to limit the inventive process. |
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| PATENT PHOTOCOPY | Available on request |
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