PROCESSES FOR PURIFYING PERFLUOROCYCLOBUTANE

Abstract: Disclosed is PFC-C318 containing less than 10 parts-per-million-molar of halogenated impurities and processes for producing such substantially-pure PFC-C318. In operating these processes, various PFC-C318-containing azeotropes and azeotrope-like compositions have been discovered and are of utility. These compositions comprise: perfluorocyclobutane (PFC-C318) and 2-chloro-1,1,1,2-tetrafluoroethane (HCFC-124); perfluorocyclobutane (PFC-C318) and 1,1,2,2-tetrafluoroethane (HFC-134); perfluorocyclobutane (PFC-C318) and 1,1,1,2-tetrafluoroethane (HFC-134a); and perfluorocyclobutane (PFC-C318) and 1,1-difluoroethane (HFC-152a). The processes of the present invention for producing substantially-pure PFC-C318 comprise: a) azeotropic distillation processes for separating PFC-C318 from halogenated impurities, and b) extractive distillation processes for separating PFC-C318 from halogenated impurities by employing entraining agents selected from ethers, ketones, alcohols, hydrocarbons, and hydrochlorocarbons.

Description: TITLE PROCESSES FOR PURIFYING PERFLUOROCYCLOBUTANE CROSS REFERENCE TO RELATED APPLICATIONS This application claims the priority benefit of U.S. Provisional Application 60/076923, filed March 5, 1998. FIELD OF THE INVENTION The present invention relates to azeotropic and azeotrope-like compositions containing perfluorocyclobutane, and azeotropic and extractive distillation processes for separating perfluorocyclobutane from a first mixture comprising perfluorocyclobutane and halogenated impurities, such that perfluorocyclobutane of high purity is obtained in a high-recovery efficiency. BACKGROUND OF THE INVENTION Gaseous fluorine-containing compounds are used by the electronics industry in processes to manufacture semiconductor devices. The major use of perfluorocyclobutane (PFC-C318) is in plasma etching silicon-type materials during semiconductor device fabrication. Plasma etchants such as PFC-C318 fragment under the plasma conditions, and these fragmentation products interact with the surface of the semiconductor device, modifying it so as to lay down the electrical pathways and providing for the surface functionalities that define an integrated surface. Chemicals such as PFC-C318 used as plasma etchants in semiconductor manufacturing applications are generally referred to as "electronic gases". Electronic gases having high purity are critical in this application. It is known that even very small amounts of impurities in electronic gases can result in wide line width and thus less information per semiconductor device when such impure gases are used in semiconductor device manufacturing tools. Moreover, the presence of these impurities, including particulates, metals, moisture, and halocarbon impurities, even when present only in the parts-per-million level, increases the defect rate in the production of high-density integrated circuits. As a result, there is an increasing demand by the electronics industry for very high purity etchant gases, and an increasing market value for the materials having the required purity. Identification of impurities and methods for their removal represent a significant aspect of preparing the etchant gases for these applications. 1 PFC-C318, in its pure state, exhibits properties that are valued for integrated circuit manufacturing and may be used in a variety of manufacturing steps. The desire for greater precision and consistency of the effect electronic gases such as PFC-C318 have during integrated circuit manufacture has made extremely high-purity gases critical in such applications. The presence of halogenated impurities in the PFC-C318 is objectionable for the intended uses in this field. Processes that would allow for manufacture of PFC-C318 having a purity that approaches 99.999 molar percent purity are desirable, and processes that would provide at least 99.9999 molar percent purity PFC-C318 for electronic gas applications would be preferred. PFC-C318 may be produced by the pyro lysis of chlorodifluoromethane (CHC1F 2 , HCFC-22), as is disclosed in US Patent No. 5,129,997. It is difficult to obtain a PFC-C318 product having very high purity from the product stream produced by this process because a variety of halogenated impurities are also produced that are extremely close-boiling to PFC-C318 in their separated and pure states or otherwise exhibit non-ideal behavior such that their relative volatilities compared to PFC-C318 approaches or even becomes 1.0. Impurities whose relative volatilities approach or equal 1.0 compared to PFC- C318 make their separation from PFC-C318 by conventional distillation ineffective in recovering a PFC-C318 product from which impurities have been removed. Such separation is particularly problematic where it is desired that the recovered PFC-C318 product be substantially-free of halogenated impurities and where the PFC-C318 product needs to be recovered from a mixture comprising PFC-C318 and impurities with high-recovery efficiency. Conventional methods for producing a PFC-C318 product are not capable of producing a PFC-C318 product having a 99.999 or greater molar percent purity due to their inability to remove a variety of the halogenated impurities from the PFC-C318 product. The production of PFC-C318 having 99.999 or higher molar percent purity was not known before the present invention. The present invention solves problems associated with conventional distillation methods by providing distillation processes for removing halogenated impurities from PFC-C318 so as to produce highly-purified PFC-C318 with high- recovery efficiency. SUMMARY OF THE INVENTION The present invention comprises PFC-C318 substantially free of halogenated impurities, preferably containing less than 10 parts-per-million-molar of halogenated impurities. The present invention further comprises azeotropic compositions consisting essentially of: perfluorocyclobutane (PFC-C318) and 2-chloro-l,l,l,2- tetrafluoroethane (HCFC-124); perfluorocyclobutane (PFC-C318) and 1,1,2,2- tetrafluoroethane (HFC-134);perfluorocyclobutane (PFC-C318) and 1,1,1,2- tetrafluoroethane (HFC- 134a); and perfluorocyclobutane (PFC-C318) and 1,1- difluoroethane (HFC- 152a). The present invention further comprises azeotropic and extractive distillation processes for separating perfluorocyclobutane (PFC- C318) from a first mixture comprising perfluorocyclobutane (PFC-C318) and halogenated impurities. BRIEF DESCRIPTION OF THE DRAWINGS Fig 1 is a schematic diagram of a distillation system that can be used for practicing an aspect of the present process. Fig 2 is a graphical representation of an azeotropic and azeotrope-like composition consisting essentially of PFC-C318 and HCFC- 124 at a temperature of 20°C. Fig 3 is a graphical representation of an azeotropic and azeotrope-like composition consisting essentially of PFC-C318 and HCFC- 124a at a temperature of20°C. Fig 4 is a graphical representation of an azeotropic and azeotrope-like composition consisting essentially of PFC-C318 and HFC- 134 at a temperature of 0°C. Fig 5 is a graphical representation of an azeotropic and azeotrope-like composition consisting essentially of PFC-C318 and HFC- 134a at a temperature of O°C. Fig 6 is a graphical representation of an azeotropic and azeotrope-like composition consisting essentially of PFC-C318 and HFC- 152a at a temperature of O°C. DETAILED DESCRIPTION The present invention includes PFC-C318 that is substantially-free of impurities. By impurities is meant any halogenated compounds other than PFC- C318. By substantially-free or substantially-pure is meant that the PFC-C318 produced by a process of the present invention contains less than 10 parts-per- million-molar (ppmm), preferably less than 1 ppmm, and most preferably less than 100 parts-per-billion-molar (ppbm), of halogenated impurity. Thus, included in the present invention is PFC-C318 having less than 10 ppmm, preferably less than 1 ppmm, and more preferably less than 100 ppbm, of impurities. Analytical methods that may be used to analyze such concentrations of impurities in a PFC- C318 product are disclosed in "Examining Purification and Certification Strategies for High-Purity C 2 F 6 Process Gas", Micro Magazine, April 1998, beginning page 35, hereby incorporated by reference. Processes used to produce PFC-C318 may simultaneously produce a variety of halogenated impurities in the PFC-C318 product stream. Examples of halogenated impurities that may be found in a PFC-C318 production stream include linear and cyclic, saturated and unsaturated, perfluorocarbons (PFCs), chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), and hydrochlorocarbons (HCCs). Representative examples from these classes of halogenated impurities include: PFC-31-10 (normal or iso-C 4 F 10 , perfluorobutane isomers), PFC-41-12 (C 5 F 12 , perfluoropentane isomers), PFC-1318my (cis and trans-CF 3 CF=CFCF 3 ), PFC- 1318c (CF 3 CF 2 CF=CF 2 ), PFC-1216 (HFP or CF 3 CF=CF 2 ), PFC-1114 (TFE or CF 2 =CF 2 ) perfluoroisobutene (CF 2 =C(CF 3 ) 2 ), CFC-114 (CF 2 C1CF 2 C1), CFC- 114a (CFC1 2 CF 3 ), CFC-216ba (CF 3 CFC1CF 2 C1), CFC-217ba (CF 3 CC1FCF 3 ), CFC- 1113 (CC1F=CF 2 ), HCFC-22 (CHC1F 2 ), HCFC-21 (CHC1 2 F), HCFC- 124 (CHFC1CF 3 ), HCFC- 124a (CC1F 2 CHF 2 ), HFC- 134 (CHF 2 CHF 2 ), HFC- 134a (CH 2 FCF 3 ), HFC- 152a (CH 3 CF 2 H), HFC- 125 (CF 3 CF 2 H), HFC-227ca (CF 3 CF 2 CHF 2 ), HFC-227ea (CF 3 CHFCF 3 ), HFC-1225zc (CF 3 CH=CF 2 ), HFC-236ca (CHF 2 CF 2 CHF 2 ), HFC- 236ea (CHF 2 CHFCF 3 ), HFC-236fa (CF 3 CH 2 CF 3 ), HCC-30 (CH 2 C1 2 ), HCC-40 (CH 3 C1), and HCC-160 (CH 3 CH 2 C1). Several of these impurities form azeotropic or azeotrope-like mixtures with PFC-C318, making high-recovery efficiency of substantially-pure PFC-C318 from such product streams difficult. Such impurities that form azeotropic or azeotrope-like mixtures with PFC-C318 include each of HCFC- 124, HCFC- 124a, HFC- 134, HFC- 134a and HFC- 152a. Other halogenated impurities produced during the pyrolysis of HCFC-22 that represent a significant problem in obtaining a substantially-pure PFC-C318 product include PFC-1318my, PFC-31-10, PFC- 1318c, CFC-114, and CFC-114a. By high-recovery efficiency is meant that greater than 90%, preferably greater than 95%, of the PFC-C318 in a first mixture is recovered as a result of a purification process as a PFC-C318 product substantially-free of at least one halogenated impurity. By azeotropic or an azeotrope composition is meant a constant- boiling mixture of two or more compounds that behaves as a pure compound. One way to characterize an azeotropic composition is that the vapor produced by partial evaporation or distillation of the liquid has the same composition as the liquid from which it is evaporated or distilled; i.e., the mixture distills/refluxes without compositional change. Constant-boiling compositions are characterized as azeotropic when they exhibit either a maximum or minimum boiling point, as compared with that of a non-azeotropic mixture of the same components. Azeotropic compositions are also characterized by a minimum or a maximum in the mixture vapor pressure relative to the vapor pressure of the neat components at a constant temperature. By azeotrope-like is meant a composition that has a constant boiling characteristic or a tendency not to fractionate upon boiling or evaporation. Therefore, the composition of the vapor formed is the same as, or substantially the same as, the original liquid composition. During boiling or evaporation, the liquid composition, if it changes at all, changes only to a minimal or negligible extent. An azeotrope-like composition can also be characterized by the area that is adjacent to the maximum or minimum vapor pressure in a plot of composition vapor pressure at a given temperature as a function of mole fraction of components in the composition. A composition is azeotrope-like if, after about 50 weight percent of an original composition is evaporated or boiled off to produce a remaining composition, the change between the original composition and the remaining composition is no more than about 6 weight% and typically no more than about 3 weight% relative to the original composition. By low-boiling-azeotropic or azeotrope composition is meant a composition that boils at a lower temperature at any given pressure than any one of the compounds that comprise it would separately boil at that pressure. Alternately, by low-boiling azeotropic or azeotrope composition is meant a composition that has a higher vapor pressure at any given temperature than the vapor pressure of any one of the compounds that comprise the azeotrope would separately have at that temperature. By high-boiling azeotrope is meant that an azeotropic or azeotrope- like composition boils at a higher temperature at any given pressure than any one of the compounds that comprise it would separately boil at that pressure. Alternately, by high-boiling azeotrope is meant any azeotropic or azeotrope-like composition that has a lower vapor pressure at any given temperature than any one of the compounds that comprise it would separately have at that temperature. It is possible to characterize an azeotropic or azeotrope-like composition as a substantially constant-boiling mixture that may appear under many guises, depending upon the conditions chosen, by several criteria: * The composition can be defined as an azeotrope of two compounds because the term "azeotrope" is at once both definitive and limitative, and requires effective amounts of those two or more compounds for this unique composition of matter which can be a constant-boiling composition. * It is well known by those skilled in the art, that, at different pressures, the composition of a given azeotrope or azeotrope-like composition will vary at least to some degree, as will the boiling point temperature. Thus, an azeotropic or azeotrope-like composition of two compounds represents a unique type of relationship but with a variable composition that depends on temperature and/or pressure. Therefore, compositional ranges, rather than fixed compositions, are often used to define azeotropes and azeotrope-like compositions. * An azeotrope or azeotrope-like composition of two compounds can be characterized by defining compositions characterized by a boiling point at a given pressure, thus giving identifying characteristics without unduly limiting the scope of the invention by a specific numerical composition, which is limited by and is only accurate as the analytical equipment available. It is recognized in the art that both the boiling point and the amount of each component of the azeotropic composition may change when the azeotrope liquid composition is subjected to boiling at different pressures. Thus, an azeotropic composition may be defined in terms of the unique relationship that exists among components or in terms of the exact amounts of each component of the composition characterized by a fixed boiling point at a specific pressure. It is recognized in this field that when the relative volatility of a system, for example, a mixture comprising PFC-C318 and a halogenated impurity of the present invention, approaches 1.0, such defines the system as forming an azeotrope-like composition. Should the relative volatility be equal to 1.0, such defines the system as forming an azeotropic composition. Impurities whose relative volatility to PFC-C318 approaches or equals 1.0 are extremely difficult or impossible to separate from PFC-C318 by conventional distillation. By conventional distillation is meant that the relative volatilities only of the components of the mixture to be separated are used to separate the components. To determine the relative volatility of any two compounds, a method known as the PTx method may be used. In this procedure, the total absolute pressure in a cell of known volume is measured at a constant temperature for various compositions of the two compounds. Use of the PTx Method is described in detail in "Phase Equilibrium in Process Design", Wiley-Interscience Publisher, 1970, written by Harold R. Null, on pages 124 to 126; hereby incorporated by reference. These measurements can be converted into equilibrium vapor and liquid compositions in the PTx cell by using an activity coefficient equation model, such as the Non-Random, Two-Liquid (NRTL) equation, to represent liquid phase nonidealities. Use of an activity coefficient equation, such as the NRTL equation is described in detail in "The Properties of Gases and Liquids," 4 th edition, published McGraw Hill, written by Reid, Prausnitz and Poling, on pages 241 to 387, and in "Phase Equilibria in Chemical Engineering," published by Butterworth Publishers, 1985, written by Stanley M. Walas, pages 165 to 244. Both aforementioned references are hereby incorporated by reference. Without wishing to be bound by any theory or explanation, it is believed that the NRTL equation, together with the PTx cell data, can sufficiently predict the relative volatilities of PFC-C318, halogenated impurities, and entraining agents of the present invention and can therefore predict the behavior of these mixtures in multi-stage separation equipment such as distillation columns. Surprisingly, PFC-C318 and HCFC- 124 have been found by the present inventors to form an azeotropic composition comprising 26.8 mole% PFC- C318 and 73.2 mole% HCFC- 124 at 20°C and 49 psia (pounds-per-square-inch- absolute). From these data it has been calculated that PFC-C318 and HCFC- 124 form azeotropic or azeotrope-like compositions comprising 26.8 mole% PFC- C318 and 73.2 mole% HCFC- 124 at 0°C and 24.6 psia. From these data it has been calculated that PFC-C318 and HCFC-124 form azeotropic or azeotrope-like compositions comprising 27.5 mole% PFC-C318 and 72.5 mole% HCFC-124 forms at 80°C and 234.1 psia. Accordingly, the present invention further comprises an azeotropic or azeotrope-like composition comprising from 26.8 to 27.5 mole% PFC-C318 and from 73.2 to 72.5 mole% HCFC- 124, said composition having a boiling point of from 0°C at 24.5 psia to 80°C at 234.1 psia. Surprisingly, PFC-C318 and HFC- 134 have been found by the present inventors to form an azeotropic composition comprising 25.0 mole% PFC-C318 and 75.0 mole% HFC- 134 at 0°C and 36.4 psia. From these data it has been calculated that PFC-C318 and HFC- 134 form azeotropic or azeotrope-like compositions comprising 24.6 mole% PFC-C318 and 75.4 mole% HFC-134 at -30.0° C and 10.35 psia. From these data it has been calculated that PFC-C318 and HFC- 134 form azeotropic or azeotrope-like compositions comprising 23.2 mole% PFC-C318 and 76.8 mole% HFC-134 at 80°C and 325.8 psia. Accordingly, the present invention further comprises an azeotropic or azeotrope- like composition comprising from 24.6 to 23.2 mole% PFC-C318 and from 75.4 to 76.8 mole% HFC-134, said composition having a boiling point of from -30°C at 10.35 psia to 80°C at 325.8 psia. Surprisingly, PFC-C318 and HFC- 134a have been found by the present inventors to form an azeotropic composition comprising 7.4 mole% PFC-C318 and 92.6 mole% HFC- 134a at 0°C and 43 psia. From these data it has been calculated that PFC-C318 and HFC- 134a form azeotropic or azeotrope-like compositions comprising 9.9 mole% PFC-C318 and 90.1 mole% HFC-134a at -30°C and 12 psia. From these data it has been calculated that PFC-C318 and HFC- 134a form azeotropic or azeotrope-like compositions comprising 0.6 mole% PFC-C318 and 99.4 mole% HFC- 134a at 40°C and 147 psia. Accordingly, the present invention further comprises an azeotropic or azeotrope-like composition comprising from 9.9 to 0.6 mole% PFC-C318 and from 90.1 to 99.4 mole% HFC- 134a, said composition having a boiling point of from -30°C at 12 psia to 40°C at 147 psia. Surprisingly, PFC-C318 and HFC- 152a have been found by the present inventors to form an azeotropic composition comprising 23.1 mole% PFC- C318 and 76.9 mole% HFC- 152a at 0°C and 41 psia. From these data it has been calculated that PFC-C318 and HFC- 152a form azeotropic or azeotrope-like compositions comprising 22.4 mole% PFC-C318 and 77.6 mole% HFC-152a at -20°C and 19 psia. From these data it has been calculated that PFC-C318 and HFC- 152a form azeotropic or azeotrope-like compositions comprising 21.3 mole% PFC-C318 and 78.7 mole% HFC- 152a at 80°C and 349 psia. Accordingly, the present invention further comprises an azeotropic or azeotrope- like composition comprising from 23.1 to 21.3 mole% PFC-C318 and from 76.9 to 78.7 mole% HFC- 152a, said composition having a boiling point of from -20°C at 19 psia to 80°C at 349 psia. In another aspect of the present invention, distilling mixtures comprising PFC-C318 and at least one of these halogenated impurities under conditions to form said low-boiling azeotropes comprising PFC-C318 and said impurity, and removing said azeotropic or azeotrope-like composition as the distillate product in a distillation, permits some degree of separation of PFC-C318 and said impurity. By azeotropic distillation is meant a process in which a distillation column is operated under conditions to cause an azeotropic or azeotrope-like composition to form, and the formation thereof changes the relative volatility of at least one component to another such that these components may be separated by distillation. For example, a mixture comprising PFC-C318 and HCFC- 124 can be partially purified by using the aforementioned azeotropic compositions. A distillation column can be operated at a pressure and temperature to cause a low- boiling azeotrope comprising PFC-C318 and HCFC- 124 to form, and the azeotrope can be removed as an overhead stream from the distillation column. If the concentration of PFC-C318 in the first mixture (the crude PFC-C318/HCFC- 124 mixture to be separated) is greater than that in an azeotrope of PFC- C318/HCFC-124 formed under the distillation conditions, a PFC-C318 product can be removed as a bottoms stream upon distillation of the first mixture wherein the HCFC- 124 concentration in the bottoms stream is reduced compared to the HCFC- 124 concentration in the first mixture, while the azeotropic PFC- C318/HCFC-124 composition is removed as an overhead stream from the distillation column. Conversely, if the concentration of HCFC- 124 in the first mixture is greater than that in an azeotropic composition comprising PFC- C318/HCFC-124 formed under the distillation conditions, a HCFC- 124 product can be removed as a bottoms stream upon distillation of the first mixture with the PFC-C318 concentration in the bottoms stream reduced compared to the PFC- C318 concentration in the first mixture, while the azeotropic PFC-C318/HCFC- 124 composition is removed as an overhead stream from the distillation column. Obtaining PFC-C318 with the concentration of HCFC- 124 in it reduced compared to a first PFC-C318 HCFC-124 mixture (or obtaining HCFC- 124 with the concentration of PFC-C318 in it reduced compared to a first PFC-C318/HCFC- 124 mixture) in a single distillation would require starting with a composition higher in PFC-C318 (or HCFC- 124) concentration than the azeotrope composition formed at that same temperature and pressure, but some portion of the PFC-C318 (or HCFC- 124) would necessarily remain as the PFC-C318/HCFC-124 azeotrope. Where formation and distillation of the respective PFC-C318 azeotropes (e.g., PFC-C318/HCFC-124; PFC-C318/HFC-134; PFC-C318/HCFC- 124a; PFC-C318/HFC-134a; PFC-C318/HFC-152a) are employed for separating PFC-C318 from halogenated impurities, the resultant azeotropic PFC-C318- containing compositions are useful as feed streams to a thermal process for manufacturing TFE and HFP. Alternately, the PFC-C318 in the distillate product may be separated substantially-free from halogenated impurities by the extractive distillation process of the present invention. However, although use of said azeotropic compositions in a distillation is useful for partial purification of the PFC-C318 or halogenated impurity, it is difficult to obtain high-recovery efficiency of substantially-pure PFC-C318 product from the starting PFC-C318/halogenated impurity mixture by azeotropic distillation. Further, such azeotropic distillations most often do not provide for removal of other impurities that do not form azeotropic compositions with PFC-C318. A number of the other halogenated impurities found in PFC-C318 manufacturing streams have relative volatilities that approach 1.0 relative to PFC- C318. Such halogenated impurities include CFC- 12, HCC-20, HCFC-22, HFC- 32, CFC-114, CFC-114a, CFC-217ba, HFC-227ea, PFC-1318my, PFC-1318c and FC-31-10. CFC-114, CFC-114a, PFC-1318my and FC31-10 are particularly problematic because they frequently appear in concentrations ranging from several hundred to several thousand parts-per-millions-molar or higher in PFC-C318 process streams. Separating these impurities from the PFC-C318 would require tall and expensive distillation columns, and it would still be extremely difficult if not impossible to obtain substantially-pure PFC-C318 in high-recovery efficiency from such mixtures. The present inventors have found, unexpectedly, that PFC-C318 may be recovered substantially-free from these and other halogenated impurities by use of effective amounts of compounds that act in a non-ideal manner with at least one component of a PFC-C318/halogenated impurity first mixture. Such compounds, hereafter referred to as entraining-agents, increase or decrease the volatility of the PFC-C318 relative to at least one halogenated impurity in the first mixture under distillation conditions, thus allowing PFC-C318 that is substantially-free of halogenated impurity to be obtained from a first mixture comprising PFC-C318 and halogenated impurity. Thus, the present invention further comprises processes for separating PFC-C318 from at least one halogenated impurity, said processes comprising extractive distillation of a mixture comprising PFC-C318 and halogenated impurity in the presence of at least one entraining agent. The process further comprises a distillation in which the volatility of PFC-C318 or halogenated impurity is increased, one relative to the other, in the presence of an entraining agent. By entraining agent is meant any compound that, when added to a first mixture comprising PFC-C318 and halogenated impurity, interacts with at least one of PFC-C318 and halogenated impurity so as to change the volatility of one of these components relative to the other component in the mixture such that the PFC-C318 and halogenated impurity may be separated by distillation. By effective amount of entraining agent is meant an amount of at least one entraining agent that, in the presence of PFC-C318 and halogenated impurity, causes the volatility of the halogenated impurity to increase or decrease relative to PFC-C318 sufficiently to allow separation by distillation of the halogenated impurity from the PFC-C318. This definition includes the case when the effective 10 amount may vary depending on the pressure applied to the composition so long as the change in relative volatility continues to exist. By extractive distillation is meant a process in which an entraining agent is introduced at an upper feed point of a distillation column, whereas the mixture requiring separation is introduced at the same, or preferably a relatively lower feed point in the column than the point at which the entraining agent is introduced. The entraining agent passes down through trays or packing in the column and exits as a column bottoms stream with one or more components of the mixture to be separated. While in the presence of the entraining agent, at least one of the components to be separated becomes relatively either more or less volatile compared to at least one of the other components in the mixture, with that more volatile component exiting as a distillation column overhead stream. Entraining agents that are fed to a distillation column at a point equal to, or higher than, the mixture to be separated and that pass down through the column thus enabling a separation by distillation, are herein referred to as extractive distillation agents or extractants. Further defining the present extractive distillation process, the present inventors have discovered that any one of the halogenated impurities FC-31-10, FC-1318my, FC-1318c, HFC-134, HFC-134a, HCFC-124, HCFC-124a, CFC- 114, CFC- 114a, CFC-217ba, HCC-20 and optionally other halogenated compounds that may be present in a PFC-C318-containing stream may be separated from said PFC-C318 by the use of entraining agents in an extractive distillation process. Suitable entraining agents that may be used as extractants for the separation of PFC-C318 from these halogenated impurities include: ethers, ketones, alcohols, hydrocarbons, and hydrochlorocarbons. Suitable entraining agents that may be used as extractants in the present invention preferably have normal boiling points of from 30 °C to 120 °C. Suitable ethers include tetrahydrofuran (THF), 1 ,4-dioxane and dialkyl ethers such as methyl tertiarybutyl ether (MTBE). Suitable ketones include acetone and methylethylketone (MEK). Suitable alcohols include methanol and propanol. Suitable hydrocarbons include toluene and cyclohexane. Suitable hydrochlorocarbons include chloroform (CHC1 3 ). The preferred entraining agents for separating halogenated impurities from PFC-C318 by extractive distillation are THF, MEK and 1,4-dioxane. The relative volatility of said halogenated impurities compared to PFC-C318 in the absence of said extractants makes the halogenated impurity difficult to separate from the PFC-C318 by conventional distillation. In the presence of at least one said extractant, the volatility of PFC-C318 suprisingly changes compared to the aforementioned halogenated impurities. Thus, in the 11 presence of any one of the extractants, the PFC-C318 may be separated by extractive distillation from said halogenated impurities and obtained as a product substantially-free of said halogenated impurities. The present invention also provides a method of removing PFC-C318 from said halogenated impurities to recover said halogenated impurities as a product stream substantially-free of PFC- C318. With the exception of certain halogenated impurities such as FC-31- 10, the halogenated impurities unexpectedly become less volatile than PFC-C318 in the presence of the extractants of the presant invention. In the presence of the extractants, PFC-C318 substantially-free of halogenated impurities may thus be obtained as an extractive distillation column overhead stream, and with the halogenated impurities recovered together with the extractant as a distillation column bottom stream. Thus the present invention further comprises a process for separating PFC-C318 and at least one of said halogenated impurities, comprising: a.) contacting a first mixture comprising PFC-C318 and at least one halogenated impurity with an entraining agent to form a second mixture, and b.) distilling the second mixture and recovering a distillation column overhead stream comprising PFC-C318 and a distillation column bottom stream comprising entraining agent and at least one halogenated impurity. In contrast to the other aforementioned halogenated impurities, PFC- 31-10, which is normally higher boiling than PFC-C318, surprisingly becomes even more volatile than PFC-C318 in the presence of the present extractants such that the PFC-31-10 may be separated from the PFC-C318 as an extractive distillation column overhead stream, with PFC-C318 substantially-free of PFC- 31-10 obtained as an extractive distillation column bottom stream. Thus the present invention further comprises a process for separating PFC-C318 and PFC-31-10, comprising: a.) contacting a first mixture comprising PFC-C318 and PFC-31-10 with an entraining agent to form a second mixture, and b.) distilling the second mixture and recovering a distillation column overhead stream comprising PFC-31-10 and a distillation column bottom stream comprising entraining agent and PFC-C318. Figure 1 schematically illustrates a system that can be used to perform aspects of the present extractive distillation process. A first mixture comprising PFC-C318 and FC-31-10 is supplied via conduit 1 to distillation column 2. At least one extractive entraining agent, e.g. THF, is supplied via conduit 3 to distillation column 2_at a feed point higher in the column than the feed point of the mixture to be separated, e.g., PFC-C318 and FC-31-10. The overhead distillate 12 from the column is sent via conduit 4 to condenser 5_. At least part of the condensed distillate stream is returned to the column 2 as reflux 6. The remainder of the condensed distillate is recovered via conduit 7 as FC-31-10 product substantially-free of PFC-C318 and THF. A stream comprising PFC-C318 and THF that is substantially-free of FC-31 - 10 is removed from the column 2 bottoms via conduit _8 and may be recovered as product. Alternately, the column bottoms stream 8 may be fed to distillation column 9, which is operated so as to strip compounds from the entraining agent. The distillate from column 9 is fed via conduit 10 to condenser ϋ. From condenser H, some amount of condensed distillate is returned to the column 9 as reflux via conduit 12, while the remainder is recovered as product, e.g. as PFC-C318 substantially-free of FC-31-10 and extractive entraining agent, via conduit 13 ^ Extractive entraining agent, e.g. THF, with the concentration of non-THF compounds reduced compared to their concentrations in stream 8 is obtained as the distillation column bottoms 14. Stream 14 may optionally be returned to distillation column 2 as extractant feed, fed to the column at a feed point higher in the column than that feed point of the mixture to be separated, e.g. FC-31-10 and PFC-C318, or may be optionally mixed with stream 3. Figure 1 may also be used to schematically illustrate a system that can be used to perform another embodiment of the present inventive extractive distillation process. A first mixture comprising PFC-C318 and FC-1318my is supplied via conduit 1 to distillation column 2. At least one extractive entraining agent, e.g. THF, is supplied via conduit 3 to distillation column 2 at a feed point higher in the column than the feed point of the mixture to be separated, e.g., PFC- C318 and FC- 1318my . The overhead distillate from the column is sent via conduit 4 to condenser 5. At least part of the condensed distillate stream is returned to the column 2 as reflux 6. The remainder of the condensed distillate is recovered via conduit 7 as PFC-C318 product substantially-free of FC-1318my and THF . A stream comprising FC-1318my and THF that is substantially-free of PFC-C318 is removed from the column 2 bottoms via conduit 8 and may be recovered as product. Alternately, the column bottoms stream 8 may be fed to distillation column 9, which is operated so as to strip compounds other than the entraining agent from the entraining agent. The distillate from column 9 is fed via conduit 10 to condenser J . . From condenser IT, some amount of condensed distillate is returned to the column 9 as reflux via conduit 12, while the remainder is recovered as product, e.g. as FC-1318my substantially-free of PFC-C318 and extractive entraining agent, via conduit 13_. Extractive entraining agent, e.g. THF, with the concentration of non-THF compounds reduced compared to their 13 concentrations in stream 8, is obtained as the distillation column bottoms 14. Stream 14 may optionally be returned to distillation column 2 as extractant feed, fed to the column at a feed point higher in the column than that feed point of the mixture to be separated, e.g. PFC-C318 and FC-1318my, or may be optionally mixed with stream 3_. In conventional, azeotropic or extractive distillations, the overhead or distillate stream exiting the column may be condensed using conventional reflux condensers. At least a portion of this condensed stream can be returned to the top of the column as reflux, and the remainder recovered either as product or for other processing. The ratio of the condensed material that is returned to the top of the column as reflux to the material removed is commonly referred to as the reflux ratio. In those cases where an entraining agent is employed, the compounds and entraining agent exiting the column as bottoms stream can then be passed to a stripper or other distillation column for separation by using conventional distillation or other known methods, and, if desired, for recycle of the entraining agent to the first distillation column. The specific conditions that can be used for practicing the present processes depend upon a number of parameters, such as the diameter of the distillation column, feed points, number of separation stages in the column, among others. The operating pressure of the distillation system may range from 15 to 500 psia, normally 50 to 400 psia. Typically, an increase in the extractant agent feed rate relative to the feed rate of the mixture to be separated causes an increase in the purity of the product to be recovered with regard to those component(s) being removed. Increasing the reflux ratio normally results in a decreased extractant concentration in the distillate stream. But generally, the reflux ratio ranges between 1/1 to 200/1. The temperature of the condenser, that is located adjacent to the top of the column, is normally sufficient to substantially fully condense the distillate that is exiting from the top of the column, or is that temperature required to achieve the desired reflux ratio by partial condensation. Mixtures comprising C-318 suitable for purification by the presant invention can be obtained from any manufacturing process or source that produces or generates a PFC-C318-containing mixture. PFC-C318 may, for example, be produced by pyro lysis of HCFC-22. Alternately, the PFC-C318-containing mixture can be obtained from any manufacturing process that uses PFC-C318 and desires to recover said PFC-C318 from said process. If desired, conventional distillation can be used for reducing initial amounts of halogenated impurities. That is to say, conventional distillation can be used for removing relatively large or bulk quantities of halogenated impurities from the PFC-C318-containing 14 mixture that in turn may then be processed in accordance with the present inventive processes for recovering and purifying a PFC-C318. EXAMPLES The following examples are provided to illustrate aspects of the present inventive processes, and are not intended to limit the scope of the present invention. The following examples employ the NRTL interaction parameters identified earlier. In the following examples, each stage is based upon a 100% operational or performance efficiency. Differing column designs and operating conditions are employed using different extractive agents in order to maximize the performance of each distillation. In all examples, the total stages include condenser and reboiler, with the condenser counted as stage No. 1. In all examples, stream flows are expressed in pounds-per-hour (pph) or in moles-per- hour (mph); temperatures ("TEMP") are expressed in degrees Celsius (°C); concentrations are expressed in mole percent (mole%), weight percent (wt%), parts-per-million-molar (ppm-molar), and parts-per-million-by-weight (ppm-wt); heat flow rates ("DUTIES") removed from the condenser or put into the reboiler of the distillation columns are expressed in pcu/hour or pcu/hr; and pressures ("PRES") are expressed in pounds-per-square-inch-absolute (psia). Various values are also shown for the distillation column bottoms ("BTMS"), distillate ("DIST"), condenser ("CONDSR"), reflux ("REFLUX") and top ("TOP"). Values are also shown with regard to the stream fed to the column for separation ("FEED") and any extractant fed to the column ("EXTR" or "EXTR FEED"). In these Examples, by recovery efficiency ("RECOV. EFF.") is meant the percent of the C318 fed to the distillation that is recovered in the C318 product stream Comparative Examples 1.2.3 In Comparative Examples 1 through 3, a mixture comprising 980 pph PFC-C318 and 20 pph C 4 F 10 (PFC-31-10) is fed to a distillation column, then distilled using conventional distillation under conditions such that a PFC-C318 product stream is removed from the column as distillate and a C 4 F 10 product stream is removed as column bottoms. The specific conditions of and results from the distillations are shown in Table 1. TABLE 1 COMPARATIVE EXAMPLE NUMBER Extractant NONE NONE NONE 15 # STAGES 114 228 144 FEED STAGE 58 114 72 REFLUX TEMP (°C) 7.3 7.3 7.3 DIST TEMP (°C) 7.3 7.3 7.3 BTMS TEMP (°C) 10.7 10.7 13.5 FEED TEMP (°C) 10.5 10.5 10.5 TOP PRES (PSIA) 24.7 24.7 24.7 CONDSR PRES (PSIA) 24.7 24.7 24.7 BTMS PRES (PSIA) 27.7 27.7 24.7 DISTILLATE RATE (PPH) 800.0 800.0 975.0 REFLUX RATE (PPH) 13888.0 28551.0 160531.0 BOTTOMS RATE (PPH) 200.0 200.0 25.0 CONDENSER DUTY(PCU/HR) -385645. -770645. -4240779. REBOILER DUTY (PCU/HR) +385000. +770000. +4240000. FEED FLOWS PFC-C318 (PPH) 980. 980. 980. C4F10 (PPH) 20. 20. 20. COMPOSITION C 4 F 10 (PPM-WT) 20000. 20000. 20000. C F 10 (PPM-MOLAR) 16861. 16861. 16861. COLUMN DISTILLATE FLOWS PFC-C318 (PPH) 799.40 800.0 975.0 C F 10 (PPH) 0.60 0.00 0.0073 COMPOSITION I C 4 F 10 (PPM-WT) 754. 4.1 7.5 C 4 F 10 (PPM- MOLAR) 634. 3.4 6.3 PFC-C318 RECOV.EFF. (%) 81.57 81.63 99.49 COLUMN BOTTOMS FLOWS PFC-C318 (PPH) 180.6 180.0 5.0 C 4 F, [PPH) 19.40 20.00 19.99 In Comparative Example 1 , the PFC-C318 product stream from this conventional distillation contains 634 ppmm C 4 F 10 and the PFC-C318 recovery efficiency is only about 82%>. In Comparative Example 2, the number of column stages and the reboiler heat duty are both doubled compared to Comparative Example 1. In spite of this, the resulting PFC-C318 product still contains more than 3 ppmm C 4 F 10 , and the PFC-C318 recovery efficiency is only about 82%. In Comparative Example 3, the distillation is run with a reflux rate approximately ten times that of Comparative Example 1 , and the PFC-C318 bottoms takeoff rate is reduced to approximately one tenth that of Comparative 16 Example 1. Although the recovery efficiency increases to 99%>, the PFC-C318 product stream still contains 6.3 ppmm C 4 F 10 . These Comparative Examples demonstrate the difficulty of obtaining by conventional distillation a substantially-pure PFC-C318 product stream with high-recovery efficiency from a first stream comprising PFC-C318 and C 4 F 10 . Comparative Examples 4. 5 In Comparative Examples 4 and 5, a mixture comprising PFC-C318 and the impurities shown below in Table 2 is fed to a distillation column. The mixture is distilled using conventional distillation under conditions such that a product stream comprising PFC-C318 is removed from the column as distillate and a product stream comprising the impurities are removed as column bottoms. The specific conditions of and results from the distillations are shown in Table 2. TABLE 2 COMPARATIVE EXAMPLE NUMBER Extractant ONE NONE # STAGES 114 228 FEED STAGE 58 114 TOP TEMP (°C) 7.3 7.3 REFLUX TEMP (°C) 7.3 7.3 DIST TEMP (°C) 7.3 7.3 BTMS TEMP (°C) 10.5 10.5 FEED TEMP (°C) 10.5 10.5 TOP PRES (PSIA) 24.7 24.7 CONDSR PRES (PSIA) 24.7 24.7 BTMS PRES (PSIA) 27.7 27.7 DISTILLATE RATE (PPH) 800.0 800.0 REFLUX RATE (PPH) 20360. 41118. BOTTOMS RATE (PPH) 200. 200. CONDENSER DUTY (PCU/HR) -555645. -1100645. REBOILER DUTY (PCU/HR) +555000. +1100000. FEED FLOWS PFC-C318(PPH) 998.9 998.9 CFC-114 (PPH) 1.0 1.0 HFC-124a (PPH) 0.200E-01 0.200E-01 CFC-217ba (PPH) 0.10 0.10 PFC-1318my (PPH) 0.200E-01 0.200E-01 COMPOSITION [ TOT IMPURITIES (PPM-WT) 1140.0 1140.0 TOT IMPURITIES (PPM-M0LAR) 1317.2 1317.2 COLUMN DISTILLATE 17 FLOWS PFC-C318 (PPH) 799.9 800.0 CFC-114 (PPH) 0.345E- 01 0.440E-03 HCFC-124a (PPH) 0.200E- 01 0.200E-01 CFC-217ba (PPH) 0.552E- 02 0.117E-03 PFC-1318my(PPH) 0.139E- ■ 05 0.439E-10 COMPOSITION CFC-114 (PPM-MOLAR) 50.4 0.64 HCFC -124a (PPM-MOLAR) 36.6 36.6 CFC-217ba (PPM-MOLAR) 6.7 0.14 PFC-1318my(PPM-M0LAR) 0.0017 0.548E-07 TOT IMPURITIES (PPM-MOLAR) 93.8 37.4 PFC-C318 (WT %) 99.992503 99.997430 PFC-C318 (MOLE %) 99.990620 99.996257 PFC-C318 RECOV. EFF. ( ) 80.09 80.09 COLUMN BOTTOMS FLOWS PFC-C318 (PPH) 198.9 198.9 CFC-114 (PPH) 0.97 1.00 HCFC-124a (PPH) 0.105E- ■ 17 0.467E-33 CFC-217ba (PPH) 0.09 0.10 PFC-1318my (PPH) 0.200E- ■01 0.200E-01 Under the conditions of the distillation of Comparative Example 4, the PFC-C318 is recovered as distillate product with only 80% recovery efficiency and still containing 94 ppmm total impurities. In Comparative Example 5, the number of column stages and the reflux rate are both doubled compared to Comparative Example 4. In spite of this, the PFC-C318 is recovered still with only 80% PFC-C318 recovery efficiency and still containing 37 ppmm total impurities. These Comparative Examples show the difficulty in using conventional distillation to produce a substantially -pure PFC-C318 product stream with high-recovery efficiency from a first stream comprising PFC-C318 and a variety of impurities common to PFC-C318 process streams. Comparative Example 6.7.8.9 In Comparative Examples 6, 7, 8 and 9 a stream comprising 394 pph PFC-C318 and 606 pph HFC- 134 is fed to a distillation column These are distilled using the variety of conditions and with the results shown in Table 3 TABLE 3 COMPARATIVE EXAMPLE NUMBER Extractant NONE NONE NONE NONE # STAGES 62 62 62 62 Feed Stage 30 30 30 30 18 TOP TEMP (C) -10.1 -10.1 -10.1 -10.1 REFLUX TEMP (C) -10.2 -10.2 -10.2 -10.2 DIST TEMP (C) -10.2 -10.2 -10.2 -10.2 BTMS TEMP (C) -7.3 -7.3 -7.3 -7.3 FEED TEMP (C) -7.3 -7.3 -7.3 -7.3 TOP PRES (PSIA) 24.7 24.7 24.7 24.7 CONDSR PRES (PSIA) 24.7 24.7 24.7 24.7 BTMS PRES (PSIA) 27.7 27.7 27.7 27.7 DISTILLATE RATE (PPH) 100.0 900.0 100.0 900.0 REFLUX RATE (PPH) 2000. 2000. 20000. 20000. BOTTOMS RATE (PPH) 900. 100. 900. 100. CONDENSER DUTY (PCU/HR) -89975. -124251. -861187. -895463. REBOILER DUTY ( PCU/HR) +89896. +123544. +861108. +894755. FEED FLOWS PFC-C318 (PPH) 394.0 394.0 394.0 394.0 HFC-134 (PPH) 606.0 606.0 606.0 606.0 COMPOSITION PFC-C318 (WT %) 39.4 39.4 39.4 39.4 PFC-C318 (MOLE %) 24.9 24.9 24.9 24.9 COLUMN DISTILLATE FLOWS PFC-C318 (PPH) 39, .4 354.7 39.4 354.7 HFC-134 (PPH) 60, .6 545.3 60.6 545.3 COMPOSITION PFC-C318 (WT %) 39, .4 39.4 39.4 39.4 PFC-C318 (MOLE % ) 24 .9 24.9 24.9 24.9 COLUMN BOTTOMS FLOWS PFC-C318 (PPH) 354.6 39.3 354.6 39.3 HFC -134 (PPH) 545.4 60.7 545.4 60.7 COMPOSITION PFC-C318 ( T %) 39.4 39.3 39.4 39.3 PFC-C318 (MOLE %) 24.9 24.9 24.9 24.9 As may be seen in these Comparative Examples, there is essentially no separation of the PFC-C318 from the HFC- 134 as a result of this distillation, even over a wide range of distillate rates, bottoms rates and reflux rates. This is because the PFC-C318 and HFC-134 feed rates to this distillation column comprise the azeotrope composition formed by PFC-C318 and HFC- 134 at the column operating conditions. This Comparative Example shows the futility of separating an azeotropic composition comprising PFC-C318 and HFC- 134 by conventional distillation. Comparative Examples 10.11.12,13 19 In Comparative Examples 10, 11, 12 and 13 a mixture comprising PFC-C318 and HFC- 134 is fed to a distillation column. The conditions of and results from these distillations are shown in Table 4. TABLE 4 Comparative EXAMPLE NUMBER 10 11 12 13 Extractant NONE ONE NONE NONE #STAGES 32 32 32 32 FEED STAGE 12 12 12 12 TOP TEMP (°C) 1.4 -1.3 -10.0 -10.1 REFLUX TEMP (°C) -5.6 -7.7 -10.2 -10.2 DIST TEMP (°C) -5.6 -7.7 -10.2 -10.2 BTMS TEMP (°C) 10.5 10.5 10.5 10.5 FEED TEMP (°C) 3.7 3.7 3.7 3.7 TOP PRES (PSIA) 24.8 24.8 24.8 24.8 CONDSR PRES (PSIA) 24.7 24.7 24.7 24.7 BTMS PRES (PSIA) 27.7 27.7 27.7 27.7 DISTILLATE RATE (PPH) 338.8 226.5 86.7 82.5 REFLUX RATE (PPH) 150. 300. 400. 500. BOTTOMS RATE (PPH) 661. 774. 913. 917. CONDENSER DUTY (PCU/HR) -15741. -17960. -20505. -24961 REBOILER DUTY (PCU/HR) +16067. +18619. +21747. +26225 FEED FLOWS PFC-C318 (PPH) 950. 950. 950. 950. HFC-134 (PPH) 50. 50. 50. 50. COMPOSITION HFC-134 (PPM-WT) 50000. 50000. 50000. 50000. HFC-134 (PPM-MOLAR) 93532. 93532. 93532. 93532. DISTILLATE FLOWS PFC-C318 (PPH) 288.8 176.5 36.7 32.5 HFC-134 (PPH) 50.0 50.0 50.0 50.0 COMPOSITION PFC-C318 (WT %) 85.2 77.9 42.3 39.4 PFC-C318 (MOLE 74.7 64.3 27.2 24.9 BOTTOMS FLOWS PFC-C318 (PPH) 661.2 773.5 913.3 917.5 HFC-134 (PPH) 0.00003 0.00004 0.00005 0.00005 COMPOSITION HFC-134 (PPM-MOLAR) 0.1 0.1 0.1 0.1 PFC-C318 (WT %) 99.99999 99.99999 99.99999 99.99999 PFC-C318 (MOLE ) 99.99999 99.99999 99.99999 99.99999 PFC-C318 RECQV.EFF. ('- 69.6 81.4 96.1 96.6 20 In these examples, the distillation column is operated under conditions such that a PFC-C318/HFC-134 azeotrope composition is formed having a lower PFC-C318 concentration than that of the feed mixture. The low-boiling HFC- 134/PFC-C318 azeotrope goes overhead in the column and PFC-C318 that was fed in excess of that azeotrope composition is recovered in the column bottoms stream. As the reflux rate is increased between Comparative Examples 10, 11, 12 and 13, the efficiency of the distillation column in separating the excess PFC- C318 from the azeotropic composition increases, such that more of this excess may be recovered as an PFC-C318 substantially free of HFC- 134 as column bottoms. However, this increased efficiency requires significant increases in reflux rates and significant decreases in distillate product rates to obtain. Further, some of the PFC-C318 fed necessarily remains in the distillate as part of the PFC- C318/HFC-134 azeotrope composition, limiting the PFC-C318 recovery efficiency possible from such an azeotropic distillation. Nevertheless, these Comparative Examples show how low-boiling azeotropic compositions comprising PFC-C318 and HFC- 134 may be used to remove HFC- 134 from a first mixture comprising PFC-C318 and HFC- 134 such that a PFC-C318 product stream substantially-free of HFC- 134 may be obtained. Comparative Example 14. 15. 16. 17 In Comparative Examples 14, 15, 16 and 17, a mixture comprising PFC-C318 and HFC- 134 is fed to a distillation column. The conditions of and results from these distillations are shown in Table 5. TABLE 5 COMPARATIVE EXAMPLE NUMBER 14 15 16 17 Extractant NONE ONE NONE NONE # STAGES 62 62 62 62 FEED STAGE 30 30 30 30 TOP TEMP (°C) -8.3 -9.3 -10.1 -10.1 REFLUX TEMP (°C) -9.1 -9.9 -10.2 -10.2 DIST TEMP (°C) -9.1 -9.9 -10.2 -10.2 BTMS TEMP (°C) -4.3 -4.3 -4.3 -4.3 FEED TEMP (°C) -5.2 -5.2 -5.2 -5.2 TOP PRES (PSIA) 24.7 24.7 24.7 24.7 CONDSR PRES (PSIA) 24.7 24.7 24.7 24.7 BTMS PRES (PSIA) 27.7 27.7 27.7 27.7 DISTILLATE RATE(PPH) 389.2 206.9 126.9 126.9 21 REFLUX RATE (PPH) 500. 1000. 1500. 2000. BOTTOMS RATE (PPH) 611. 793. 873. 873. CONDENSER DUTY (PCU/HR) -44097. ■ 56438. -69704. -91127. REBOILER DUTY (PCU/HR) +43827. +56376. 69760. 91182. FEED FLOWS PFC-C318 (PPH) 50. 50. 50. 50. HFC-134 (PPH) 950. 950. 950. 950. COMPOSITION PFC-C318 (PPM-WT) 50000. 50000. 50000. 50000 PFC-C318 (PPM-MOLAR) 26144. 26144. 26144. 26144 COLUMN DISTILLATE FLOWS PFC-C318 (PPH) 50.0 50.0 50.0 50.0 HFC-134 (PPH) 339.2 156.9 76.9 76.9 COMPOSITION PFC-C318 (WT ) 12.8 24.2 39.4 39.4 PFC-C318 (MOLE %) 7.0 14.0 24.9 24.9 COLUMN BOTTOMS FLOWS PFC-C318 (PPH) 0.00012 0.00016 0.00017 0.00017 HFC-134 (PPH) 610.8 793.1 873.1 873.1 COMPOSITION PFC-C318 (PPM-MOLAR) 0.1 0.1 0.1 0.1 HFC-134 (WT %) 99.99998 99.99998 99.99998 99.99998 HFC-134 (MOLE %) 99.99999 99.99999 99.99999 99.99999 HFC-134 RECOV.EFF.(%) 64.3 83.5 91.9 91.9 In these examples, the distillation column is operated under distillate temperature conditions such that a PFC-C318/HFC-134 azeotrope composition is formed having a lower HFC- 134 concentration than that of the feed mixture. The low-boiling HFC-134/PFC-C318 azeotrope goes overhead in the column and HFC- 134 that was fed in excess of that azeotrope composition is recovered in the column bottoms stre,am. As the reflux rate is increased and the distillate rate decreased between Comparative Examples 14, 15, 16 and 17, the efficiency of the distillation column in separating the excess HFC- 134 increases, such that more of this excess may be recovered as an HFC-134 substantially-free of PFC-C318. However, this increased efficiency requires significant increases in reflux rates and decreases in distillate product rates increasing the energy expended for the distillation. Further, some of the HFC- 134 fed necessarily remains in the distillate as part of the PC-C318/HFC-134 azeotrope composition, limiting the HFC-134 recovery efficiency possible from such an azeotropic distillation. Nevertheless, these Comparative Examples show how the low-boiling azeotropic compositions comprising PFC-C318 and HFC- 134 may be used to 22 remove PFC-C318 from a first mixture comprising PFC-C318 and HFC- 134 such that a HFC- 134 product stream that is substantially-free of PFC-C318 may be obtained. Comparative Examples 18 and 19 In Comparative Examples 18 and 19, a feed stream comprising PFC- C318 and C 4 F 10 is fed to a distillation column operated under the conditions shown in Table 6. An extractant stream comprising perfluorohexane (C 6 F 14 ) is fed into the column as an extractant stream at a point above that of the PFC- C318/C 4 F 10 feed. The results of these distillations are shown in Table 6. TABLE 6 COMPARATIVE EXAMPLE NUMBER 18 19 Extractant CβΑ C 6 F 14 #STAGES 92 92 EXTR FEED STAGE 10 10 FEED STAGE 40 40 TOP TEMP (°C) 7.3 7.3 REFLUX TEMP (°C) 7.3 7.3 DIST TEMP (°C) 7.3 7.3 BTMS TEMP (°C) 74.8 75.4 EXTR FEED TEMP (°C) 10.0 10.0 FEED TEMP (°C) 10.5 10.5 TOP PRES (PSIA) 24.7 24.7 CONDSR PRES (PSIA) 24.7 24.7 BTMS PRES (PSIA) 27.7 27.7 DISTILLATE RATE (PPH) 25.0 250.0 REFLUX RATE (PPH) 3000. 3000. BOTTOMS RATE (PPH) 100975. 100750. EXTRACTANT RATE (PPH) 100000. 100000. EXTRACTANT ( LB- OLES/HR) 295.8 295.8 CONDENSER DUTY (PCU/HR) -79435. -85343. REBOILER DUTY (PCU/HR) +1730243. +1746685 FEED FLOWS PFC-C318 (PPH) 980. 980. C 4 F, 0 (PPH) 20. 20. EXTR. (PPH) 0. 0. COMPOSITION C 4 F 10 (PPM-WT) 20000. 20000. C 4 F 10 (PPM-MOLAR) 16861. 16861. EXTRACTION COLUMN DISTILLATE FLOWS PFC-C318 (PPH) 25.00 249.97 23 C 4 F 10 ( PPH ) 0.00 0.03 EXTR . ( PPH ) 0.00 0.00 COMPOSITION C 4 F 10 ( PPM- MOLAR ) 64. 104. PFC -C318 (WT %) 99.99235 99.98764 PFC-C318 (MOLE %) 99.99357 99.98961 EXTRACTION COLUMN BOTTOMS FLOWS PFC-C318 (PPH) 955. 730. C 4 F 10 (PPH) 20.00 19.97 EXTR. (PPH) 100000. 100000. COMPOSITION C 4 F 10 ( PPM- MOLAR ) 17293. 22471. PFC - C318 (WT %) 97.94891 97.33745 PFC-C318 (MOLE %) 98.27068 97.75294 PFC-C318 RECOV. EFF. (%) 97.45 74.49 In Comparative Examples 18 and 19, the feed stream composition is identical to that of Comparative Examples 1, 2 and 3. Even with an extremely high extractant feed rate, extremely tall columns and extremely high reflux rates, the C 4 F 10 concentration in the PFC-C318 product has been reduced down to only slightly less than 100 ppmm. Comparative Examples 18 and 19 show that when used as an extractant, C 6 F 14 offers no advantage over the conventional distillation shown in Comparative Examples 1,2 and 3 for this separation. C 6 F 14 is one example of the many compounds ineffective as an extractant for facilitating the separation of PFC-C318 and C 4 F 10 by extractive distillation. Examples 20 through 31 In each of Examples 20 through 31, a crude feed stream comprising HFC-C318 and C 4 F 10 is fed to a distillation column and operated under the conditions shown in Table 7. The concentrations of HFC-C318 and C 4 F 10 in the feed stream for each of Examples 20 through 31 are identical to those of Comparative Examples 1, 2, 3, 18 and 19. In each of Examples 20 through 31, a different compound is fed to the column as an extractive agent. The distillation columns in these Examples are operated to remove C 4 Fι 0 from the column as overhead distillate, while recovering a PFC-C318 product as column bottoms. The extractants for each Example and the results of the distillations are shown in Table 7. TABLE 7 EXAMPLE NUMBER 20 21 22 23 24 25 26 EXTRACTANT ACETONE MEK THF 1 ,4-DIOXANE CHC1 3 PROPANOL METHANOL #STAGES 72 72 72 72 72 72 72 24 EXTR FEED STAGE 15 15 15 5 10 10 10 FEED STAGE 30 30 35 20 20 20 20 TOP TEMP (°C) 37.7 40.5 40.5 40.6 40.3 40.5 37.7 REFLUX TEMP (°C) 37.6 40.3 40.3 40.3 40.1 40.3 37.5 DIST TEMP CO 37.6 40.3 0.3 40.3 40.1 40.3 37.5 BTMS TEMP (°C) 53.2 88.9 74.3 70.2 76.0 99.1 65.2 EXTR FEED TEMP (°C) 40.0 40.0 40.0 40.0 40.0 40.0 40.0 FEED TEMP CO 38.1 38.1 38.1 38.1 38.1 38.1 38.1 TOP PRES (PSIA) 64.7 64.7 64.7 64.7 64.7 64.7 64.7 CONDSR PRES (PSIA) 64.7 64.7 64.7 64.7 64.7 64.7 64.7 BTMS PRES (PSIA) 67.7 67.7 67.7 67.7 67.7 67.7 67.7 DISTILLATE RATE (PPH) 25.9 25.1 25.2 25.0 25.7 25.0 25.3 REFLUX RATE (PPH) 3000. 3000. 3000. 3000. 3000. 3000. 3000, BOTTOMS RATE (PPH) 5034. 9761. 10344. 13361. 19118. 22239. 15679, EXTRACTANT RATE (PPH) 4060. 8786. 9369. 12386. 18144. 21264. 14704, EXTRACTANT(LB-MOLEStHR) 69.9 121.9 129.9 140.6 152.0 353.8 458.9 CONDENSER DUTY(PCU/HR) -72400. - 61347. - 62079. - 61072. - 63803. - 61537. -70007. REBOILER DUTY(PCU.HR) +105352. +312053. +213459. +245962. +227529. +1004089. +316004 FEED FLOWS PFC-C318(PPH) 980. 980. 980. 980. 980. 980. 980. C 4 F,„ (PPH) 20. 20. 20. 20. 20. 20. 20. EXTR. (PPH) 0. 0. 0. 0. 0. 0. 0. COMPOSITION C 4 F 10 (PPM-WT) 20000. 20000. 20000. 20000. 20000. 20000. 20000. C 4 F 10 (PPM-MOLAR) 16861. 16861. 16861. 16861. 16861. 16861. 16861. EXTRACTION COLUMN DISTILLATE PFC-C318(PPH) 5.00 5.00 5.00 5.00 5.00 5.00 5.00 C 4 F 10 (PPH) 20.0 20.0 20.0 20.0 20.0 20.0 20.0 EXTR. (PPH) 0.95 0.07 0.16 0.05 0.71 0.05 0.30 EXTRACTION COLUMN BOTTOMS PFC-C318(PPH) 975. 975. 975. 975. 975. 975. 975. C 4 F 10 (PPH) 0.00116 0.00116 0.00116 0.00116 0.00116 0.00116 0.00116 EXTR. (PPH) 4059. 8786. 9369. 12386. 18143. 21264. 14704 COMPOSITION - (values exclude extractant. C 4 F 10 (PPM-MOLAR) 1.0 1.0 1.0 1.0 1.0 1.0 1.0 PFC-C318 (WT %) 99.99988 99.99988 99.99988 99.99988 99.99988 99.99988 99.99988 PFC-C318 (MOLE ' %) 99.99990 99.99990 99.99990 99.99990 99.99990 99.99990 99.99990 PFC-C318 RECOV.EFF. ( )99.49 99.49 99.49 99.49 99.49 99.49 99.49 TABLE 7 ( CONTINUED) EXAMPLE NUMBER - 27 28 29 30 31 EXTRACTANT MTBE CYANE TOLUENE DEE HEXANE NO. THEORETICAL STAGES 92 92 92 92 92 EXTR FEED STAGE 15 5 5 5 5 FEED STAGE 35 25 25 35 35 TOP TEMPERATURE (°C) 10.5 10.6 10.5 8.9 10.7 25 REFLUX TEMPERATURE (°C) 10.3 10.3 10.3 8.8 10.4 DIST TEMPERATURE (°C) 10.3 10.3 10.3 8.8 10.4 BTMS TEMPERATURE (°C) 63.2 52.1 51.4 47.5 85.1 EXTR FEED TEMP (°C) 10.0 10.0 10.0 10.0 10.0 FEED TEMP (°C) 10.5 10.5 10.5 10.5 10.5 TOP PRESSURE (PSIA) 24.7 24.7 24.7 24.7 24.7 CONDSR PRESSURE (PSIA) 24.7 24.7 24.7 24.7 24.7 BTMS PRESSURE (PSIA) 27.7 27.7 27.7 27.7 27.7 DISTILLATE RATE (PPH) 25.1 25.0 25.0 26.5 25.2 REFLUX RATE (PPH) 3000. 3000. 3000. 3000. 3000. BOTTOMS RATE (PPH) 20359. 19668. 24543. 29256. 60205. EXTRACTANT RATE (PPH) 19385. 18694. 23568. 28283. 59231. EXTRACTANT ( LB-MOLES/HR) 219.9 222.1 255.8 381.6 687.4 CONDENSER DUTY (PCU/HR) -69856. -69242. -68832. -81060. -70072. REBOILER DUTY (PCU/HR) +619018. +430629. +479421. +696617. +2617593. PFC-Cai Q IC F^ FEED PFC-C318 (PPH) 980. 980. 980. 980. 980 C 4 F 10 (PPH) 20. 20. 20. 20. 20 EXTR. (PPH) 0. 0. 0. 0. 0 COMPOSITION C 4 F 10 (PPM-WT) 20000. 20000. 20000. 20000. 20000 C 4 F 10 (PPM-MOLAR) 16861. 16861. 16861. 16861. 16861 EXTRACTION COLUMN DISTILLATE FLOWS PFC-C318 (PPH) 5.00 5.00 5.00 5.00 5.00 C 4 F 10 (PPH) 20.0 20.0 20.0 20.0 20.0 EXTR. (PPH) 0.15 0.05 0.00 1.48 0.16 EXTRACTION COLUMN BOTTOMS FLOWS PFC-C318 ( PPH) 975. 975. 975. 975. 975. C 4 F 10 (PPH) 0. 00116 0. 00116 0.00116 0.00116 0.00116 EXTR . ( PPH) 19384. 18693. 23568. 28281. 59230. COMPOSITION - (values exclude extractant ) C 4 F 10 ( PPM-MOLAR) 1 . 0 1 . 0 1.0 1.0 1.0 PFC-C318 (WT %) 99.99988 99.99988 99.99988 99.99988 99.99988 PFC -C318 (MOLE %) 99.99990 99.99990 99.99990 99.99990 99.99990 PFC-C318 RECOV . EFF . ( ) 99.49 99.49 99.49 99.49 99.49 By contrasting these Examples to Comparative Examples 1 ,2, 3 and to Comparative Examples 18 and 19, it may be seen that the extractive agents of Comparative Examples 20 through 33 significantly increase the effectiveness of the distillation for this separation, and are thus effective extractants for separating C 4 F 10 from PFC-C318. In Examples 20 through 31 , PFC-C318 having significantly reduced concentrations of C 4 F 10 versus the crude feed stream are recovered in the PFC-C318 product from the distillation column bottoms. Whereas concentrations of C 4 F 10 in the PFC-C318 product below 10 ppmm were extremely difficult or impossible to obtain in the Comparative Examples, 26 concentrations of C 4 F, 0 of 1 ppmm in the PFC-C318 product are obtained in each of the Examples in Table 7 with high-recovery efficiency of the PFC-C318 product. The eleven extractants are shown in the order from most effective to least effective for the PFC-C318/C 4 F 10 separation, with those more effective defined by those requiring lower molar flow rate of extractant required to produce PFC-C318 product containing 1 ppmm C 4 F 10 . On this basis, the ranking of extractants from "most" to "least" effective is: acetone, methylethylketone (MEK), tetrahydrofuran (THF), 1,4-dioxane, chloroform (CHC1 3 ), methanol, methyl tertiarybutyl ether (MTBE), cyclohexane (CYANE), toluene, diethyl ether (DEE) and n-hexane. All of these extractants reverse the normal relative volatility of PFC-C318 and C 4 F 10 compared to a conventional distillation as in Comparative Examples 1,2 and 3, such that in the presence of the extraction solvent the C 4 F 10 is recovered in the distillate stream, while the PFC-C318 is recovered as the extractive distillation column bottoms. The extractive agents indicated in each of Examples 20 through 33 exiting with the PFC-C318 from the column bottoms may optionally be separated from the PFC-C318 by distillation or other methods. While acetone is among the most effective extractants for the PFC- C318/C 4 F 10 separation, acetone forms a low-boiling azeotrope with PFC-C318. While MEK, THF and 1 ,4-dioxane are all just about equally effective in separating PFC-C318 and C 4 F 10 , MEK and 1,4-dioxane form two liquid phases with PFC-C318 in the extraction column which requires more extractant flow and adds to the difficulty of operating the column. The extractants most prefeπed for this separation are consequently THF, followed by MEK and 1,4-dioxane. Example 32 In this Example, a stream comprising PFC-C318 and C 4 F 10 is fed to an extractive distillation column, where THF is fed to the column as extractant at a point above that of the PFC-C318/C 4 F 10 feed. The bottoms stream from the extraction column, comprising PFC-C318 and THF extractant, is then fed to a stripping column. In the stripping column, a PFC-C318 substantially-free of both C 4 F 10 .and the THF extractant is obtained as distillate product, while the THF extractant is recovered as bottoms product, then recycled back to the extraction column as extractant feed. The operating conditions and results from these distillations are shown in Table 8. TABLE 8 EXTRACTION COLUMN STRIPPING COLUMN 27 # STAGES 72 # STAGES 42 EXTR FEED STAGE 15 FEED STAGE 35 FEED STAGE 35 TOP TEMP O 40.5 TOP TEMP CO 7.4 REFLUX TEMP (°C) 40.3 REFLUX TEMP CO 7.3 DIST TEMP CO 40.3 DIST TEMP C 7.3 BTMS TEMP CO 74.3 BTMS TEMP CO 85.6 EXTR FEED TEMP(°C) 40.0 FEED TEMP ("O 74.3 FEED TEMP (°C) 38.1 TOP PRES (PSIA) 64.7 TOP PRES (PSIA) 24.7 CONDSR PRES (PSIA) 64.7 CONDSR PRES (PSIA) 24.7 BTMS PRES (PSIA) 67.7 BTMS PRES (PSIA) 26.7 DISTILLATE RATE (PPH) 25.2 DISTILLATE RATE (PPH) 975.0 REFLUX RATE (PPH) 3000. REFLUX RATE (PPH) 4282. BOTTOMS RATE (PPH) 10344. BOTTOMS RATE (PPH) 9369. EXTRACTANT RATE (PPH) 9369. EXTRACTANT ( LB-MOLES/HR) 129.9 FEED TO EXTRACTION COLUMN FEED TO STRIPPING COLUMN FLOWS FLOWS PFC-C318 (PPH) 980. PFC-C318 (PPH) 975. C 4 F, 0 (PPH) 20. C 4 F,„ (PPH) 0.0117 EXTR. (PPH) 0. EXTR. (PPH) 9368. COMPOSITION C 4 F,o (PPM-WT) 20000. C„F,„ (PPM-MOLAR) 16861. EXTRACTION COLUMN DISTILLATE STRIPPING COLUMN DISTILLATE FLOWS FLOWS PFC-C318 (PPH) 5.00 PFC-C318 (PPH) 975. C 4 F 10 (PPH) 20.0 C 4 F 10 (PPH) 0.117E-02 EXTR. (PPH) 0.16 EXTR. (PPH) 0.176E-04 COMPOSITION PFC-C318 (MOLE%) 99.99989 C4F10 (PPM-MOLAR) 1.0 EXTR. (PPM-MOLAR) 0.05 PFC-C318 RECOV. EFF. (%, 99.49 EXTRACTION COLUMN BOTTOMS STRIPPING COLUMN BOTTOMS FLOWS FLOWS PFC-C318 (PPH) 975. PFC-C318 (PPH) 0.5 C 4 F 10 (PPH) 0.00117 C 4 F 10 ( PPH) 0.676E-08 EXTR. (PPH) 9368. EXTR . ( PPH) 9368. This example shows how the extractive distillation of the present invention may be used to produce a PFC-318 product substantially-free of both C 4 F i0 and the THF extractant, with high-recovery efficiency of the PFC-C318. Starting from a feed stream comprising 980 pph PFC-C318 and 20 pph C 4 F 10 , a PFC-C318 product is obtained comprised of 1.0 ppmm C 4 F, 0 and 0.05 ppmm THF, with 99.5 recovery efficiency of the PFC-C318. 28 Example 33 Table 9 shows both the extractive distillation and stripping steps for purifying a crude PFC-C318 feed containing halogenated impurities comprising CFC-114, HCFC-124a, CFC-217ba and PFC-1318my. The extractant used is THF. The PFC-C318 product is recovered as extraction column distillate substantially-free of both the halogenated impurities and THF. The halogenated impurities are recovered in the extraction column bottoms stream along with the THF, this bottoms stream then sent as feed to a stripping column. The stripping column removes the organic impurities from the THF, the organic impurities recovered as the stripping column distillate, with the THF recovered as stripping column bottoms then recycled back to the extraction column as extractant feed. TABLE 9 EXTRACTION COLUMN STRIPPING COLUMN #STAGES 72 #STAGES 42 EXTR FEED STAGE 40 FEED STAGE 25 FEED STAGE 55 TOP TEMP CO 7.4 TOP TEMPERATURE CO 11.3 REFLUX TEMP (°C) 7.3 REFLUX TEMPERATURE CO 8.6 DIST TEMP CO 7.3 DIST. TEMPERATURE CO 8.6 BTMS TEMP CO 86.3 BTMS TEMPERATURE CO 85.6 EXTR FEED TEMP CO 5.0 FEED TEMPERATURE CO 87.2 FEED TEMP CO 10.5 TOP PRES (PSIA) 24.7 TOP PRESSURE (PSIA) 24.7 CONDSR PRES (PSIA) 24.7 CONDSR PRESSURE (PSIA) 24.7 BTMS PRES (PSIA) 27.7 BTMS PRESSURE (PSIA) 26.7 DISTILLATE RATE (PPH) 993.9 DISTILLATE RATE (PPH) 6.2 REFLUX RATE (PPH) 5000. REFLUX RATE (PPH) 4274. BOTTOMS RATE (PPH) 7995. BOTTOMS RATE (PPH) 7989. EXTRACTANT RATE (PPH) 7989. EXTRACTANT (LB-MOLES/HR) 110.8 EXTRACTION COLUMN FEED STRIPPING ι COLUMN FEED FLOWS FLOWS PFC-C318 (PPH) 998.9 PFC-C318 (PPH) 5.0 CFC-114 (PPH) 1.0 CFC-114 (PPH) 1.0 HCFC-124a (PPH) 0, .200E-01 HCFC-124a (PPH) 0.220E-01 CFC-217ba (PPH) 0.10 CFC-217ba (PPH) 0.10 PFC-1318my (PPH) 0, .200E-01 PFC-1318 y (PPH) 0.200E-01 EXTR. (PPH) 0.0 EXTR. (PPH) 7989.0 COMPOSITION TOT IMP (PPM-WT) 1140.0 TOT IMP (PPM-MOLAR) 1317.2 29 EXTRACTION COLUMN DISTILLATE STRIPPING COLUMN DISTILLATE FLOWS FLOWS PFC-C318 (PPH) 993.9 PFC-C318 (PPH) 5.0 EXTR. (PPH) 0.678E-06 EXTR. (PPH) 0.10 COMPOSITION CFC-114 (PPH) 1.0 CFC-114 (PPM-MOLAR) 0.694E-02 HCFC-124a (PPH) 0. .200E-01 HCFC-124a (PPM-MOLAR) 0.202E-01 CFC-217ba (PPH) 0. .990E-01 CFC-217ba (PPM-MOLAR) 0.966E+00 PFC-1318my (PPH) 0. .200E-01 PFC-1318 y (PPM-MOLAR) 0.496E-02 EXTR. (PPM-MOLAR) 0.189E-02 TOT IMP (PPM-MOLAR) 0.100E+01 PFC-C318 (WT %) 99.999899 PFC-C318 (MOLE %) 99.999900 PFC-C318 RECOV. EFF. (%} 99.50 EXTRACTION COLUMN BOTTOMS STRIPPING COLUMN BOTTOMS FLOWS FLOWS PFC-C318 (PPH) 5.0 PFC-C318 (PPH) 0 .385E-17 CFC-114 (PPH) 1.0 CFC-114 (PPH) 0, .185E-03 HCFC-124a (PPH) 0.220E-01 HCFC-124a (PPH) 0 .200E-02 CFC-217ba (PPH) 0.10 CFC-217ba (PPH) 0 .190E-14 PFC-1318my (PPH) 0.200E-01 PFC-1318my (PPH) 0 .924E-15 EXTR. (PPH) 7989. EXTR. (PPH) 7989. Examples 34 - 40 Table 10 shows seven examples of different extraction solvents for removing PFC-C318 from HFC- 134 to make substantially-pure HFC- 134. The seven extractants are shown in the order from most effective to least effective for the PFC-C318/HFC-134 separation, with those more effective defined by those requiring lower mote flow rate of extractant required to produce 0.1 ppm PFC- C318 in the HFC- 134 bottoms product. On this basis, the order from "most" to "least" effective is: 1,4-dioxane, tetrahydrofuran (THF), methyl ethyl ketone (MEK), methyl tertiarybutyl ether (MTBE), methanol, toluene, and propanol. In the presence of the extraction solvent the PFC-C318 is recovered in the distillate stream as a PFC-C318 product substantially-free of both HFC- 134 and extractant, while the HFC- 134 is recovered as the extractive distillation column bottoms along with the extractant. The extractive agent indicated may then optionally be separated from the HFC- 134 by distillation or other methods. TABLE 10 EXAMPLE NUMBER 34 35 36 37 38 39 40 EXTRACTANT 1 ,4-Dioxane THF MEK MTBE METHANOL TOLUENE PROPANOL #STAGES 62 62 62 62 62 62 62 EXTR FEED STAGE 8 8 8 8 8 8 8 FEED STAGE 20 20 25 20 20 20 20 TOP TEMP CO 7.4 7.4 7.4 7.5 6.5 7.4 7.4 REFLUX TEMP CO 7.3 7.3 7.3 7.3 6.5 7.3 7.3 DIST TEMP O 7.3 7.3 7.3 7.3 6.5 7.3 7.3 30 BTMS TEMP CO 30.3 31.1 43.1 29.0 10.0 17.7 21.8 EXTR FEED TEMP CO 5.0 5.0 5.0 5.0 5.0 5.0 5.0 FEED TEMP CO -5.2 -5.2 -5.2 -5.2 -5.2 -5.2 -5.2 TOP PRES (PSIA) 24.7 24.7 24.7 24.7 24.7 24.7 24.7 CONDSR PRES (PSIA) 24.7 24.7 24.7 24.7 24.7 24.7 24.7 BTMS PRES (PSIA) 27.7 27.7 27.7 27.7 27.7 27.7 27.7 DISTILLATE RATE (PPH) 50.0 50.0 50.0 50.2 50.3 50.0 50.0 REFLUX RATE (PPH) 300. 300. 300. 300. 300. 300. 300. BOTTOMS RATE (PPH) 2245. 2156. 2382. 2780. 1748. 3540. 4776. EXTRACTANT RATE (PPH) 1295. 1206. 1432. 1830. 798. 2590. 3826. EXTRACTANT (LB-MOLES/HR) 14.7 16.7 19.9 20.8 24.9 28.1 63.7 FEED FLOWS PFC-C318 (PPH) 50. 50. 50. 50. 50. 50. 50. HFC-134 (PPH) 950. 950. 950. 950. 950. 950. 950. EXTR. (PPH) 0. 0. 0. 0. 0. 0. 0. COMPOSITION PFC-C318 (PPM-WT) 50000. 50000. 50000. 50000. 50000. 50000. 50000. PFC-C318 (PPM-MOLAR) 26144. 26144. 26144. 26144. 26144. 26144. 2614 . EXTRACTION COLUMN DISTILLATE PFC-C318 (PPH) 50.00 50.00 50.00 50.00 50.00 50.00 50.00 HFC-134 (PPH) 0.01 0.01 0.01 0.01 0.01 0.01 0.01 EXTR. (PPH) 0.124E -4 0.346E -1 0.293E -1 0. 143 0. 307 0.159E -3 0.544E-I EXTRACTION COLUMN BOTTOMS FLOWS PFC-C318 (PPH) I 0.00019 0.00019 0.00019 0.00019 0.00019 0.00019 0.00019 HFC-134 (PPH) I 950. 950. 950. 950. 950. 950. 950. EXTR. (PPH) I 1295. 1206. 1432. 1830. 798. 2590. 3826. COMPOSITION - (values exclude extractant) PFC-C318 (PPM-MOLAR) 0.1 0.1 0.1 0.1 0.1 0.1 0.1 HFC-134 (WT %) 99.99998 99.99998 99.99998 99.99998 99.99998 99.99998 99.99998 HFC- 134 (MOLE %) 99.99999 99.99999 99.99999 99.99999 99.99999 99.99999 99.99999 HFC-134 RECOV. EFF. (%) 100.00 100.00 100.00 100.00 100.00 100.00 100.00 Examples 41. 42 Table 11 shows both the operating conditions and results for Examples 41 and 42. These Examples show an extractive distillation with THF as the extractant in which PFC-C318 and HFC- 134, respectively, are recovered from a feed mixture comprising 60.6 wt% HFC-134 (75.1 mole %) and 39.4 wt% PFC- C318 (24.9 mole %). This feed composition comprises the azeotropic composition formed by HFC-134 and PFC-C318 at the temperatures of these distillation. As was shown in Comparative Examples 6, 7, 8 and 9, it is virtually impossible to separate said azeotropic composition of HFC- 134 and PFC-C318 by conventional distillation. Examples 41 and 42, however, show extractive distillation using THF as the extractant enables the separation. In Example 41 , the PFC-C318 product 31 recovered as distillation column distillate contains 0.1 ppmm total impurities (HFC- 134 plus THF), and the recovery of PFC-C318 product is more than 99% of the PFC-C318 in the feed to the column. In Example 42, the HFC- 134 product contains 0.1 ppmm PFC-C318, and the recovery of HFC-134 product is essentially 100% of the HFC- 134 in the feed to the column. TABLE 11 EXAMPLE NUMBER 41 42 EXTRACTANT THF THF #STAGES 62 62 EXTR FEED STAGE 40 15 FEED STAGE 50 30 TOP TEMP CC) 7.4 7.4 REFLUX TEMP CC) 7.3 7.3 DIST TEMP (°C) 7.3 7.3 BTMS TEMP CC) 49.4 31.4 EXTR FEED TEMP O 5.0 5.0 PFC-C318 FEED TEMP O -7.3 -7.3 TOP PRES (PSIA) 24.7 24.7 CONDSR PRES (PSIA) 24.7 24.7 BTMS PRES (PSIA) 27.7 27.7 DISTILLATE RATE (PPH) 392.0 394.2 REFLUX RATE (PPH) 1231. 600. BOTTOMS RATE (PPH) 2191. 1385. EXTRACTANT RATE (PPH) 1583. 779. EXTRACTANT ( LB-MOLES/HR) 22.0 10.8 CONDENSER DUTY (PCU/HR) -42621. -26149. REBOILER DUTY (PCU/HR) 83981. 43128. FEED FLOWS PFC-C318 (PPH) 394.0 394.0 HFC-134 (PPH) 606.0 606.0 COMPOSITION HFC-134 (PPM-WT) 606000.0 606000.0 HFC-134 (PPM-MOLAR) 750956.6 750956.6 EXTRACTION COLUMN DISTILLATE FLOWS PFC-C318 (PPH) 392.0 394.0 HFC-134 (PPH) 0.999E-05 0.01 EXTR. (PPH) 0.706E-05 0.177 COMPOSITION HFC-134 (PPM- MOLAR) 0.05 49.7 EXTR. (PPM-MOLAR) 0.05 1250 TOT IMP (PPM- MOLAR) 0.10 1300 PFC-C318 (WT % ) 99.999996 99.95 PFC-C318 (MOLE *) 99.999990 99.87 PFC-C318 RECOV. EFF. ( ) 99.49 100.00 32 EXTRACTION COLUMN BOTTOMS FLOWS PFC-C318 (PPH) 2.0 0.0 HFC-134 (PPH) 606.0 606.0 EXTR. (PPH) 1583. 779. COMPOSITION - (values exclude extractant) PFC-C318 (PPM-MOLAR) 1680 0.10 HFC-134 (WT %) 99.67 99.99998 HFC- 134 (MOLE %) 99.83 99.99999 HFC -134 RECOV. EFF. (%) 100.00 100.00 Examples 43, 44 In Examples 43 and 44, a stream comprising PFC-C318 and C 4 F 10 is fed to an extractive distillation column, where THF is fed to the column as extractant at a point above that of the PFC-C318/C 4 F 10 feed. The operating conditions and results from these distillations are shown in Table 12. TABLE 12 EXAMPLE NUMBER 43 44 Extractant THF THF #STAGES 72 72 EXTR FD STAGE 15 15 PFC-C318 FD STAGE 35 35 TOP TEMP (°o 40.5 40.5 REFLUX TEMP CC) 40.3 40.3 DIST TEMP (°C) 40.3 40.3 BTMS TEMP (°C) 74.3 82.7 EXTR FEED TEMP CO 40.0 40.0 FEED TEMP CC) 38.1 38.1 TOP PRES (PSIA) 64.7 64.7 CONDSR PRES (PSIA) 64.7 64.7 BTMS PRESSURE (PSIA) 67.7 67.7 DISTILLATE RATE (PPH) 25.2 25.2 REFLUX RATE (PPH) 3000. 3000. BOTTOMS RATE (PPH) 10344. 13742. EXTRACTANT RATE (PPH) 9369. 12767. EXTRACTANT ( LB-MOLES/HR) 129.9 177.1 CONDENSER DUTY (PCU/HR) -62079. -62089. REBOILER DUTY (PCU/HR) +213459. +316903. FEED FLOWS PFC-C318 (PPH) 980. 980. C 4 F 10 (PPH) 20. 20. EXTR. (PPH) 0. 0. COMPOSITION C 4 F 10 (PPM-WT) 20000. 20000. 33 C 4 F 10 ( PPM-MOLAR) 16861 . 16861 . EXTRACTION COLUMN DISTILLATE FLOWS PFC-C318 (PPH) 5. 00 5.00 C 4 F 10 ( PPH ) 20. 0 20 . 0 EXTR . ( PPH) 0 . 16 0 . 16 EXTRACTION COLUMN BOTTOMS FLOWS PFC-C318 (PPH) 975. 975. C 4 F 10 ( PPH ) 0 . 001 16 0 . 001 16 EXTR . ( PPH) 9369. 12767. COMPOSITION - (values exclude extracatant ) C 4 F 10 ( PPM- MOLAR ) 1 . 0 0 . 1 PFC-C318 (WT %) 99.99988 99.99999 PFC-C318 (MOLE %) 99.99990 99.99999 PFC-C318 RECOV . EFF . (%) 99.49 99.49 Example 43 is the same as the previous Example 22, and produces a PFC-C318 product as the extraction column bottoms stream comprising PFC- C318 containing 1.0 ppmm C 4 F 10 with a PFC-C318 recovery efficiency of 99.5%. In Example 44, the extractant flow to the column is increased compared to that of Example 43, and produces a PFC-C318 product as extraction column bottoms stream comprising PFC-C318 containing 0.1 ppmm C 4 F I0 with a 99.5% PFC- C318 recovery efficiency. EXAMPLE 45 This Example demonstrates the existence of azeotropic or azeotrope- like compositions between the binary pairs mixtures consisting essentially of PFC-C318 and HCFC- 124; PFC-C318 and HCFC- 124a; PFC-C318 and HFC- 134; PFC-C318 and HFC- 134a; and PFC-C318 and HFC- 152a. To determine the relative volatility of each binary pair, the PTx Method was used. In this procedure, for each binary pair, the total absolute pressure in a sample cell of known volume was measured at constant temperature for various known binary compositions. These measurements were then reduced to equilibrium vapor and liquid compositions using the NRTL equation. The vapor pressure measured versus the composition in the PTx cell for these binary systems are shown in Figures 3 through 7, respectively. The experimental data points are shown in each Figure as solid points and the solid line is drawn from data calculated using the NRTL equation. Referring now to Figure 2, Figure 2 illustrates graphically the formation of an azeotropic and azeotrope-like composition consisting essentially of PFC-C318 and HCFC-124 at 20°C, as indicated by a mixture of 26.9 mole % 34 PFC-C318 and 73.1 mole % HCFC- 124 having the highest pressure over the range of compositions at this temperature. Based upon these findings, it has been calculated that an azeotropic or azeotrope-like composition of 26.9 mole% PFC- C318 .and 73.1 mole% HCFC- 124 is formed at 0°C and 24 psia, and an azeotropic or azeotrope-like composition of 27.5 mole% PFC-C318 and 72.5 mole% HCFC- 124 is formed at 80°C and 234 psia. Accordingly, the present invention provides an azeotropic or azeotrope-like composition consisting essentially of from 26.9 to 27.5 mole% PFC-C318 and from 73.1 to 72.5 mole% HCFC-124, said composition having a boiling point of from 0°C at 24 psia to 80°C at 234 psia. Referring now to Figure 3, Figure 3 illustrates graphically the formation of an azeotropic and azeotrope-like composition consisting essentially of PFC-C318 and HCFC- 124a at 20°C, as indicated by a mixture of 22.4 mole % PFC-C318 and 67.6 mole % HCFC- 124a having the highest pressure over the range of compositions at this temperature. Based upon these findings, it has been calculated that azeotropic or azeotrope-like compositions of 33.1 mole % PFC- C318 and 66.9 mole % HCFC-124a is formed at 0°C and 24 psia, and an azeotropic or azeotrope-like composition of 33.9 mole % PFC-C318 and 66.1 mole % HCFC- 124a is formed at 80°C and 229 psia. Accordingly, the present invention provides an azeotropic or azeotrope-like composition consisting essentially of from 33.1 to 33.9 mole% PFC-C318 and from 66.9 to 66.1 mole% HCFC- 124a, said composition having a boiling point of from 0°C at 24 psia to 80 °C at 229 psia. Referring now to Figure 4, Figure 4 illustrates graphically the formation of an azeotropic and azeotrope-like composition consisting essentially of PFC-C318 and HFC- 134 at 0°C, as indicated by a mixture of 25.0 mole% PFC- C318 and 75.0 mole% HFC- 134 having the highest pressure over the range of compositions at this temperature. Based upon these findings, it has been calculated that an azeotropic or azeotrope-like composition of 24.6 mole% PFC- C318 and 75.4 mole% HFC- 134 is formed at -30°C and 10 psia, and an azeotropic or azeotrope-like composition of 23.2 mole% PFC-C318 and 76.8 mole% HFC- 134 is formed at 80°C and 326 psia. Accordingly, the present invention provides an azeotropic or azeotrope-like composition consisting essentially of from 24.6 to 23.2 mole% PFC-C318 and from 75.4 to 76.8 mole% HFC-134, said composition having a boiling point of from -30°C at 10 psia to 80°C at 326 psia. Referring now to Figure 5, Figure 5 illustrates graphically the formation of an azeotropic and azeotrope-like composition consisting essentially of PFC-C318 and HFC- 134a at 0°C, as indicated by a mixture of 7.4 mole% PFC- C318 and 92.6 mole % HFC- 134a having the highest pressure over the range of 35 compositions at this temperature. Based upon these findings, it has been calculated that an azeotropic or azeotrope-like compositions of 9.9 mole% PFC- C318 and 90.1 mole% HFC-134a is formed at -30°C and 12 psia, and an azeotropic or azeotrope-like composition of 0.6 mole% PFC-C318 and 99.4 mole% HFC- 134a is formed at 40°C and 147 psia. Accordingly, the present invention provides an azeotropic or azeotrope-like composition consisting essentially of from 9.9 to 0.6 mole% PFC-C318 and from 90.1 to 99.4 mole % HFC-134a, said composition having a boiling point of from -30°C at 12 psia to 40°C at about 147 psia. Referring now to Figure 6, Figure 6 illustrates graphically the formation of an azeotropic and azeotrope-like composition consisting essentially of PFC-C318 and HFC- 152a at 0°C, as indicated by a mixture of 23.1 mole% PFC-C318 and 76.9 mole% HFC- 152a having the highest pressure over the range of compositions at this temperature. Based upon these findings, it has been calculated that azeotropic or azeotrope-like compositions of 22.4 mole% PFC- C318 and 77.6 mole% HFC- 152a is formed at -20°C and 19 psia, and an azeotropic or azeotrope-like composition of 21.3 mole% PFC-C318 and 78.7 mole% HFC-152a is formed at 80°C and 234 psia. Accordingly, the present invention provides an azeotropic or azeotrope-like composition consisting essentially of from 23.1 to 21.3 mole% PFC-C318 and from 76.9 to 78.7 mole% HFC-152a, said composition having a boiling point of from -20°C at 19 psia to 80 °C at 349 psia. 36

Claims: WHAT IS CLAIMED IS: 1.) Perfluorocyclobutane (PFC-C318) containing less than 10 parts-per-million-molar of halogenated impurities. 2.) Perfluorocyclobutane (PFC-C318) containing less than 1 parts-per-million-molar of halogenated impurities. 3.) Perfluorocyclobutane (PFC-C318) containing less than 100 parts-per-billion-molar of halogenated impurities. 4.) An azeotropic or azeotrope-like composition consisting essentially of from 26.8 to 27.5 mole percent of perfluorocyclobutane (PFC-C318) and from 73.1 to 72.5 mole percent of 2-chloro- 1,1,1, 2-tetrafluoroethane (HCFC- 124), said composition having a boiling point of from 0°C at 24 psia to 80°C at 234 psia. 5.) An azeotropic or azeotrope-like composition consisting essentially of from 24.6 to 23.2 mole percent of perfluorocyclobutane (PFC-C318) and from 75.4 to 76.8 mole percent of 1,1,2,2-tetrafluoroethane (HFC-134), said composition having a boiling point of from -30°C at 10 psia to 80°C at 326 psia. 6.) An azeotropic or azeotrope-like composition consisting essentially of from 9.9 to 0.6 mole percent of perfluorocyclobutane (PFC-C318) and from 90.1 to 99.4 mole percent of 1 , 1 , 1 ,2-tetrafluoroethane (HFC- 134a), said composition having a boiling point of from -30°C at 12 psia to 40°C at 147 psia. 7.) An azeotropic or azeotrope-like composition consisting essentially of from 23.1 to 21.3 mole percent of perfluorocyclobutane (PFC-C318) and from 76.9 to 78.7 mole percent of 1,1-difluoroethane (HFC-152a), said composition having a boiling point of from -20°C at 19 psia to 80°C at 349 psia. 8.) A process for separating perfluorocyclobutane (PFC-C318) from a first mixture comprising perfluorocyclobutane (PFC-C318) and halogenated impurity, wherein the amount of perfluorocyclobutane (PFC-C318) in the first mixture is in excess of the amount of perfluorocyclobutane (PFC-C318) 37 in an azeotropic or azeotrope-like composition comprising perfluorocyclobutane (PFC-C318) and impurity, comprising: distilling the first mixture to form a second mixture comprising an azeotropic or azeotrope-like composition comprising perfluorocyclobutane (PFC- C318) and impurity, recovering the second mixture as a distillation column overhead stream, and recovering perfluorocyclobutane (PFC-C318) as a distillation column bottom stream. 9) A process for separating at least one halogenated impurity from a first mixture comprising perfluorocyclobutane (PFC-C318) and halogenated impurity wherein the amount of halogenated impurity in the first mixture is in excess of the amount of halogenated impurity in an azeotropic or azeotrope-like composition comprising perfluorocyclobutane (PFC-C318) and impurity, comprising: distilling the first mixture to form a second mixture comprising an azeotropic or azeotrope-like composition comprising perfluorocyclobutane (PFC- C318) and halogenated impurity, recovering the second mixture as a distillation column overhead stream, and recovering halogenated impurity as a distillation column bottom stream. 10.) A process for separating perfluorocyclobutane (PFC-C318) from halogenated impurity comprising distilling a mixture comprising perfluorocyclobutane (PFC-C318) and halogenated impurity in the presence of entraining agent. 11.) The process of claim 10 wherein the volatility of perfluorocyclobutane (PFC-C318) or impurity is increased, one relative to the other, in the presence of entraining agent. 12.) A process for separating perfluorocyclobutane (PFC-C318) and halogenated impurity, comprising: contacting a first mixture comprising perfluorocyclobutane (PFC-C318) and halogenated impurity with an entraining agent to form a second mixture, and 38 distilling the second mixture and recovering a distillation column overhead stream comprising perfluorocyclobutane (PFC-C318) and a distillation column bottom stream comprising entraining agent and impurity. 13.) A process for separating perfluorocyclobutane (PFC-C318) and halogenated impurity, comprising: contacting a first mixture comprising perfluorocyclobutane (PFC-C318) and halogenated impurity with an entraining agent to form a second mixture, and distilling the second mixture and recovering a distillation column overhead stream comprising halogenated impurity and a distillation column bottom stream comprising entraining agent and perfluorocyclobutane (PFC-C318). 14.) The process of Claims 12 or 13 wherein the first mixture comprises an azeotropic or azeotrope-like composition of perfluorocyclobutane (PFC-C318) and impurity . 15.) The process of Claims 8, 9, 10, 12, or 13 wherein the halogenated impurity comprises at least one of PFC-31-10 (C 4 F 10 ), PFC-41-12 (C S F 12 ), PFC-1318my (cis and trans-CF 3 CF=CFCF 3 ), PFC-1318c (CF 3 CF 2 CF=CF 2 ), PFC- 1216 (CF 3 CF=CF 2 ), PFC- 1114 (CF 2 =CF 2 ) perfluoroisobutene (CF 2 =C(CF 3 ) 2 ), CFC-114 (CF 2 C1CF 2 C1), CFC-114a (CFC1 2 CF 3 ), CFC-216ba (CF 3 CFC1CF 2 C1), CFC-217ba (CF 3 CC1FCF 3 ), CFC-1113 (CC1F=CF 2 ), HCFC-124 (CHFC1CF 3 ), HCFC-124a (CC1F 2 CHF 2 ), HFC-134 (CHF 2 CHF 2 ), HFC- 134a (CH 2 FCF 3 ), HFC- 152a (CH 3 CF 2 H), HFC- 125 (CF 3 CF 2 H), HFC-227ca (CF 3 CF 2 CHF 2 ), HFC-227ea (CF 3 CHFCF 3 ), HFC- 1225zc (CF 3 CH=CF 2 ), HFC-236ca (CHF 2 CF 2 CHF 2 ), HFC-236ea (CHF 2 CHFCF 3 ), HFC- 236fa (CF 3 CH 2 CF 3 ), HCC-30 (CH 2 C1 2 ), HCC-40 (CH 3 C1) and HCC-160 (CH 3 CH 2 C1). 16.) The process of Claims 10, 12, or 13 wherein the entraining agent is selected from the group consisting of ethers, ketones, alcohols, hydrocarbons, and hydrochlorocarbons. 17.) The process of Claim 16 wherein: the ethers are selected from the group consisting of methy tertiarybutyl ether, tetrahydrofuran, and 1,4-dioxane; the ketones are selected from the group consisting of propanone (acetone) and 2-butanone (methyl ethyl ketone); the alcohols are selected from the group consisting of methanol and propanol; the hydrocarbons are selected from 39 the group consisting of toluene and cyclohexane; and the hydrochlorocarbons are selected from the group consisting of chloroform. 18.) The process of Claims 8, 10, 12, or 13 wherein perfluorocyclobutane (PFC-C318) is recovered substantially-free of impurities. 19.) The process of Claims 8, 10, 12, or 13 wherein perfluorocyclobutane (PFC-C318) is recovered containing less than 10 parts-per- million-molar impurities. 40

Inventors: KAO CHIEN-PING CHAI, MAHLER BARRY ASHER, MILLER RALPH NEWTON, KAO, CHIEN-PING, CHAI, MAHLER, BARRY, ASHER, MILLER, RALPH, NEWTON
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1999
Main classification: C07C17
IPC codes: C07C17, C07C19, C07C23