| PATENT ASSIGNEE'S COUNTRY | USA |
| UPDATE | 12.00 |
| PATENT NUMBER | This data is not available for free |
| PATENT GRANT DATE | 05.12.00 |
| PATENT TITLE |
CO.sub.2 removable from fluorocarbons by semipermeable membrane |
| PATENT ABSTRACT |
A process for removing carbon dioxide from a fluorocarbon carbon dioxide mixture in which the fluorocarbon carbon dioxide mixture is contacted with a semipermeable polyimide 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. |
| 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 removing carbon dioxide from a fluorocarbon carbon dioxide mixture comprising one or more stages in which a feed stream of said fluorocarbon 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 fluorocarbon present in said feed stream. 2. The process of claim 1 wherein said fluorocarbon of said fluorocarbon carbon dioxide mixture consists essentially of non-chlorine containing fluorocarbons. 3. The process of claim 1 wherein said fluorocarbon has at most one hydrogen atom. 4. The process of claim 1 wherein said fluorocarbon carbon dioxide mixture comprises a fluorocarbon selected from the group consisting of trifluoromethane (HFC-23), hexafluoroethane (FC-116), tetrafluoroethylene, hexafluoropropylene, perfluoro(alkyl vinyl ethers) wherein the alkyl group contains 1-3 carbon atoms, and hexafluoropropylene oxide (HFPO). 5. The process of claim 1 wherein said fluorocarbon comprises tetrafluoroethylene. 6. The process of claim 1 wherein said semipermeable membrane comprises a membrane selected from the group consisting of polyimide membranes and polyaramid membranes. 7. The process of claim 1 in which the semipermeable membrane comprises a polyimide membrane having phenylindane residues incorporated in the polyimide backbone chain. 8. The process of claim 1 wherein less than 3 weight % carbon dioxide is present in said fluorocarbon carbon dioxide mixture. 9. The process of claim 1 wherein less than 0.1 weight % carbon dioxide is present in said fluorocarbon carbon dioxide mixture. 10. The process of claim 1 wherein said process in at least one stage causes at least 50% by weight of said carbon dioxide present in said feed stream to be removed in said stage. 11. The process of claim 1 wherein said exit stream with increased concentration of carbon dioxide contain less than about 10% by weight of the fluorocarbon present in said fluorocarbon carbon dioxide mixture prior to said contacting with said semipermeable membrane. 12. The process of claim 1 wherein said exit stream with increased concentration of carbon dioxide contain less than about 5% by weight of the fluorocarbon present in said fluorocarbon carbon dioxide mixture prior to said contacting with said semipermeable membrane. -------------------------------------------------------------------------------- |
| PATENT DESCRIPTION |
FIELD OF THE INVENTION This invention relates to a process for removing quantities of carbon dioxide from fluorocarbons. BACKGROUND OF THE INVENTION Conventional methods of manufacturing and purifying non-chlorine-containing fluorocarbons typically result in a product containing at least a small amount of undesired impurities. These fluorocarbons are useful as refrigerants, blowing agents, cleaning agents and many other applications. When any of these compounds are used as an etchant in electronics applications, purity requirements are unusually high. Even trace impurities can markedly affect the reject rate for miniaturized electronic circuits or optical disks, in some cases affecting the reject rate a thousand-fold. As a result, highly purified fluorocarbons used for such applications typically require unusually stringent purification procedures and command a premium market position and price. Two of the compounds so used are trifluoromethane (HFC-23) and hexafluoroethane (FC-116). It is known that HFC-23 can be made in a mixture with chlorofluoromethanes by intensive fluorination of chloromethane compounds under special conditions, or by disproportionation of chlorodifluoromethane (HCFC-22). These reactions are costly to carry out on a small scale because of the small volume of HFC-23 required, and the reaction products require extensive purification to obtain a product of sufficient purity for the etchant market. It is also known that commercial processes to make the important refrigerant chlorodifluoromethane (HCFC-22) on a large scale by catalytic reaction of chloroform with HF typically also produce several percent of HFC-23 as a byproduct. Depending on the size and design of the plant, this HFC-23 can be vented to the atmosphere or recovered. The environmentally desirable recovery of HFC-23 for sale to the etchant market is costly because of the small percentage produced and the number of impurities which must be removed. It is also known that FC-116 can be made by similar catalytic processes involving the reaction of HF or fluorine with perchloroethylene or other two-carbon halocarbons. Again, these processes yield a product that requires extensive purification for the etchant market. Other fluorocarbons such as perfluoroepoxides are useful in preparation of various fluoropolymers. One of the most important perfluoroepoxides is hexafluoropropylene oxide, or HFPO, which is used to make a variety of specialty fluorochemical polymers with complex structures. The presence of small amounts of impurities interferes with many of the subsequent processing steps, particularly in polymerization, where low levels of impurities can have a serious limiting effect on achieving the desired molecular weight polymer. Hexafluoropropylene oxide (HFPO) is typically manufactured by oxidation of hexafluoropropylene (HFP) using oxidizing agents such as hydrogen peroxide, sodium hypochlorite, oxygen or ozone. Most of the bulk impurities from the above reactions to make HFC-23, FC-116 or HFPO can be readily removed from the desired fluorocarbon by careful fractional distillation and/or scrubbing to remove acids, followed by drying by passing through a silica gel bed. When HFPO is manufactured by oxidation of HFP, HFP can be present in the stream to be purified but would usually not be considered to be an impurity. However, even after careful purification, these compounds typically contain small amounts of carbon dioxide (CO.sub.2). This may result from the presence of CO.sub.2 in the water used for scrubbing acidic impurities, as a byproduct of the reaction, or from other sources. For generally non-reactive fluorocarbons such as HFC-23 and FC-116, the amount of CO.sub.2 can be reduced by scrubbing the fluorocarbon with an excess of caustic solution (relative to the CO.sub.2), or by passing it through a fixed bed of soda-lime pellets, also present in excess relative to the amount of CO.sub.2. The reactions involved are shown below: 2 NaOH+CO.sub.2 .fwdarw.Na.sub.2 CO.sub.3 +H.sub.2 O Ca(OH).sub.2 +CO.sub.2 .fwdarw.CaCO.sub.3 +H.sub.2 O However it is difficult to achieve reliably low levels of CO.sub.2 with either of these approaches because the needed excess of alkali results in an alkali-alkali carbonate mixture, the composition of which must be carefully and continually monitored for maximum effective removal of the CO.sub.2. That is, if the proportion of alkali carbonate in the resulting alkali-alkali carbonate mixture becomes too high, the mixture becomes less effective in removing CO.sub.2, and the product no longer meets specifications [a current goal is 50 parts per million (ppm) of CO.sub.2 on a molecular or volume basis, with a future goal of 10 ppm]. If the alkali-alkali carbonate mixture is replaced with fresh alkali while the proportion of alkali carbonate is too low, the cost of the operation becomes excessive. In addition, either approach creates an alkali-alkali carbonate mixture which must be disposed of. Furthermore, either of these steps introduces some water (from the scrubbing solution and/or as neutralization byproduct) into the dry fluorocarbon which must then be removed in an additional step. For more reactive fluorocarbons such as HFPO, scrubbing with an alkali may give rise to unwanted side reactions and yield losses. Processes have been proposed for removing trace quantities of impurities from etchant gases by contacting them at high temperatures with special Zr-V-Fe alloys as disclosed in EP 501 933 A2, or by contacting with hydrogenated Ni-NiO catalysts as disclosed in JP 06116180 A2, in order to react with and remove the impurities. These methods of treatment are costly. 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. There is a need for a process to remove low or trace quantities of CO.sub.2 from fluorocarbons such as HFC-23, FC-116 or HFPO in a reliable manner without contacting them with other chemicals which can introduce water or other impurities and create waste disposal problems or problems in polymerization for polymerizable monomers, or require costly alloy or catalyst reaction treatments. DESCRIPTION OF THE RELATED ART The use of semipermeable membranes to separate gases other than CO.sub.2 and fluorocarbons 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. Many other such references describe 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. The problems inherent in removing low concentrations of impurities by gas permeation techniques are discussed in some detail in the Membrane Handbook, written by W. S. Winston Ho and K. K. Sirkar, published by Van Nostrand Reinhold, 1992. On pages 22 and 23 of this reference, it is noted that an externally applied field such as an electrical or magnetic field may be used to provide an additional driving force across the membrane for such cases, and states: "This makes it feasible to separate electrochemically gases that have a low feed concentration. Carbon dioxide, oxygen, and sulfur oxides have been separated in the laboratory by this technique." This laboratory method of removing low concentrations of CO.sub.2 has the disadvantage of requiring special equipment which would be expensive and not easily available on a commercial scale. SUMMARY OF THE INVENTION In accordance with the invention, a process is provided for removing carbon dioxide from a fluorocarbon carbon dioxide mixture in which the fluorocarbon 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. In a preferred form of the invention, the fluorocarbon in the fluorocarbon carbon dioxide mixture consists essentially of non-chlorine containing fluorocarbons. It is also preferred for the fluorocarbon to have at most one hydrogen atom. More preferably, the fluorocarbon carbon dioxide mixture contains a fluorocarbon selected from the group consisting of trifluoromethane (HFC-23), hexafluoroethane (FC-116), tetrafluoroethylene (TFE), hexafluoropropylene (HFP), perfluoro(alkyl vinyl ethers) (PAVE) wherein the alkyl group contains 1-3 carbon atoms, and hexafluoropropylene oxide (HFPO). A particularly preferred fluorocarbon is tetrafluoroethylene. In a preferred form of the invention, the semipermeable membrane is a polyimide membrane having phenylindane residues incorporated in the polyimide backbone chain. In another preferred mode, the semipermeable membrane is a polyaramid membrane. The invention is advantageously used to remove low quantities of CO.sub.2, i.e., less than 3 weight % carbon dioxide being present in the fluorocarbon carbon dioxide mixture. The invention is also advantageously used to remove trace quantities of CO.sub.2, i.e., less than 0.1 weight % carbon dioxide being present in the fluorocarbon carbon dioxide mixture. In a preferred form of the invention, the exit stream with increased concentration of carbon dioxide contains less than about 10% by weight of the fluorocarbon present in the original fluorocarbon carbon dioxide mixture. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatical view of laboratory scale apparatus illustrating an embodiment of the present invention. FIG. 2 is a diagrammatical view of alternate laboratory scale apparatus illustrating an embodiment of the present invention. DETAILED DESCRIPTION In the context of the present invention, "fluorocarbon" means an organic compound containing carbon and fluorine. Fluorocarbons in the fluorocarbon carbon dioxide mixture for the practice of the present invention may also contain hydrogen, oxygen and/or other halogens. Preferably, the fluorocarbon consists essentially of non-chlorine containing fluorocarbons. "Consisting essentially of non-chlorine containing fluorocarbons" means that the chlorine content from chlorinated impurities of the starting fluorocarbon carbon dioxide mixture is less than about 0.1 weight %. Preferably, in addition, the fluorocarbon contains at most one hydrogen atom per molecule. More preferably, the fluorocarbon is selected from the group consisting of trifluoromethane (HFC-23), hexafluoroethane (FC-116), tetrafluoroethylene (TFE), hexafluoropropylene (HFP), perfluoro(alkyl vinyl ethers) (PAVE) wherein the alkyl group contains 1-3 carbon atoms, and hexafluoropropylene oxide (HFPO). A particularly preferred fluorocarbon is tetrafluoroethylene. The fluorocarbon component of the fluorocarbon carbon dioxide mixture from which CO.sub.2 is removed by the process of the present invention is comprised of at least one fluorocarbon compound. Thus, the fluorocarbon component of the fluorocarbon carbon dioxide mixture can be a mixture of fluorocarbon compounds. Such mixtures include, for example, HFP/HFPO, TFE/IHFP, TFE/PAVE, and TFE/HFP/PAVE. 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 (chlorine-free, at most one hydrogen) 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. The invention is advantageously applied to process streams which contain low quantities of CO.sub.2. By "low quantities of CO.sub.2 " is meant quantities below about 3 weight %. The invention is also advantageously applied to process streams which contain trace quantities of CO.sub.2. By "trace quantities of CO.sub.2 " is meant quantities less than about 0.1 weight %, i.e., less than about 1000 parts per million (ppm) on a weight basis. In the present process, the fluorocarbon gas containing CO.sub.2 is contacted with a selected semipermeable membrane to form two exit streams, one of which is fluorocarbon depleted in CO.sub.2 and the other is fluorocarbon enriched in CO.sub.2. 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 farther processed by conventional means to achieve additional separation or recovery/removal of a desired component. The fluorocarbon enriched in CO.sub.2 can be recycled to earlier stages in the purification process, subjected to further purification before recycling, blended with fluorocarbon used for less demanding markets, or disposed of by incineration or other means as permitted by environmental regulations. The process of the invention can also be used to obtain purified fluorinated compounds from mixtures with CO.sub.2 such as the TFE/CO.sub.2 shipping mixture disclosed in U.S. Pat. No. 5,345,013. 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 preferably 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 fluorocarbon. That is, the ratio of permeation rates of the CO.sub.2 to that of the fluorocarbon 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 described 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 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. When low or trace levels of CO.sub.2 are present, removal to a few ppm can be achieved in a one or two stage process. Preferably, at least about 50%, more preferably at least about 75%, by weight of the CO.sub.2 present is removed in each stage. 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 fluorocarbon. "Loss" is determined from the weight of the fluorocarbon in the stream with increased carbon dioxide concentration (usually the permeate stream) in relation to the weight of the fluorocarbon present in the original fluorocarbon 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 fluorocarbon present in the original fluorocarbon carbon dioxide mixture. The aforesaid low losses can be achieved in multiple stage processes, but preferably are achieved in a single stage. The following examples are presented for illustrative purposes only and in no way are intended to limit the present inventive process. |
| PATENT EXAMPLES | This data is not available for free |
| PATENT PHOTOCOPY | Available on request |
|
Want more information ? Interested in the hidden information ? Click here and do your request. |