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Al-46% ATTORNEY Aug. 20, 1963 H. A. WIIST 3,101,304 DISTILLATION PROCESS FOR FLUOROCARBONS Filed April 28, 1960 INVENTOR ... al-46%HERBERT A. WIST ATTORNEY 3, 2,304 United States Patent Office Paterated Aug. 20, 1963 2 17 to 53 inclusive by extractive distillation. Compounds 3,105,304 causing the separation of the perfluorocarbons from the DSTELLATION PRO-CESS FOR FLUORO CARBONS close boiling or azeotrope forming fluorocarbons are aro Herbert Albert Wist, Wilmington, Dei. assignor to matic hydrocarbons, halogenated aromatic hydrocarbons E. I. du Pont de Nemours and Company, Wilmington, and halogenated aliphatic hydrocarbons in which the halo Del, a corporation of Delaware gen has an atomic number of 17 to 53, and which are Filed Apr. 28, 1960, Ser. No. 25,362 liquid at distillation conditions. The process of the pres 17 Clains. (C. 202-39.5) ent invention is based on the discovery that the volatility The process of the present invention relates to the sepa of fluorocarbons boiling close to the volatile perfuoro ration of fluorocarbons, and, more particularly, to the ex 10 carbons, which generally have from 1 to 5 carbon atoms, tractive distillation of fluorocarbons. or form. azeotropes with the fluorocarbons, is so changed Fluorocarbons, i.e., compounds of carbon and fluorine by the addition of the extractive distillation agents of the which may also contain other elements, such as hydro present invention, that the separation of these groups of . gen and chlorine, and particularly, perfluorocarbons, i.e., compounds becomes a matter of simple distillation. The compounds consisting solely of carbon and fluorine, are 15 close boiling or azeotrope forming fluorocarbons generally highly useful as refrigerants and solvents. The unsatu also have from 1 to 5 carbon atoms. Examples of azeo rated fluorocarbons are extremely useful monomers which tropes of perfluorocarbons and fluorocarbons separated on polymerization give rise to high molecular weight by the extractive distillation of the present invention are resins having outstanding corrosion- and heat-resistance. hexafluoropropylene and chlorodifiuoromethane, per The most useful of these completely fluorinated hydro 20 fluorocyclobutane and chlorotetrafluoroethane, hexafluoro carbons are tetrafluoroethylene and hexafluoropropylene propylene and bromodifluoromethane, perfluorocyclo as Inonomers, and perfiuorocyclobutane as a solvent and butane and bromodifluoromethane, tetrafluoroethylene and refrigerant. One of the preferred methods of preparing difluoromethane, tetrafluoroethylene and trifluoromethane. tetrafluoroethylene is by pyrolysis of chlorodifluorometh Most of the close boiling fuorocarbon and perfluoro ane attemperatures above 700° C., as disclosed in greater 25 carbon compounds will be apparent from any table list detail in U.S. 2,551,573, issued to Downing et al. on May ing the physical properties of fluorocarbon and perfluoro 8, 1951. The Downing et al. method results in the pro carbon compounds; examples are trifluoroethylene and duction of a large number of fluorocarbons in addition to tetrafluoroethylene, dichlorodifluoromethane and hexa tetrafluoroethylene. These compounds include hexafluoro fluoropropylene. ". propylene, perfluorocyclobutane, trifluoromethane, di 30 The process of the present invention is of particular fluoromethane, chlorotrifluoromethane, perfluoroisobutyl utility in combination with processes for the preparation ene, dichlorodifluoromethane, chlorotrifluoroethylene, di of such fluorocarbons as tetrafluoroethylene, hexafluoro chlorotetrafluoroethane, chlorotetrafluoroethane, chloro propylene and perfluorocyclobutane. These compounds hexafluoropropane, a homologous series of compounds can be prepared by the pyrolysis of chlorodifluorometh CHF2(CF2)CF2C (in up to 13), and a number of other 35 ane, bromodifluoromethane or by inorganic and organic compounds which have only to date been identified by fluoride arc processes known in the art. their location on a gas chromatogram. Of these com The pyrolysis of chlorodifluoromethane gives rise, in pounds, hexafluoropropylene and perfluorocyclobutane addition to tetrafluoroethylene the major product, to a are highly useful perfluorocarbons. Heretofore, it has number of fluorocarbons higher boiling than tetrafluoro been necessary to discard these two highly useful per 40 ethylene. These higher boiling fluorocarbons include fluorocarbons, since it was impossible to separate these hexafluoropropylene, perfluorocyclobutane, perfluoroiso compounds from the other higher boiling fluorocarbons butylene, dichlorotetrafluoroethane, chlorotetrafluoro by either distillation or absorption, in view of azeotropes ethane, chlorohexafluoropropane, a homologous series of formed between the desired perfluorocarbons and the re compounds CHF (CF)CF2Cl (n up to 13), and a maining fluorocarbons, their exceedingly close boiling 45 number of other compounds which have only to date been points and their inertness to absorption. In another meth identified by their location on a gas chromatogram. The od, tetrafluoroethylene is produced by pyrolysis of bro higher boiling fluorocarbons also contain some of the modifluoromethane. Similar to the pyrolysis of the chlo starting material, chlorodifluoromethane, which, although rine analog, the pyrolysis of the bromodifluoromethane having a lower boiling point, can only be separated in produces a large number of fluorocarbons in addition to 50 part from the higher boiling fluorocarbons due to the the more valuable perfluorocarbons, tetrafluoroethylene, fact that it forms an azeotrope with hexafluoropropylene. hexafluoropropylene and octafluorocyclobutane. As in The composition of these higher boiling fluorocarbons the case of the chlorine analog, the pyrolysate contains will vary with the pyrolysis conditions, but will, in gen a number of compounds which form azeotropes with the eral, contain about 50% of the useful perfluorocarbons, desired perfluorocarbons, or boil so close to the perfluoro 55 hexafluoropropylene and perfluorocyclobutane. carbons that separation by distillation is impossible. The The pyrolysis of chlorodifluoromethane is carried out azeotropes formed in the bromodifluoromethane pyrolysis attemperatures of 4450° C. to 900° C., and, generally, at are, however, formed with different fluorocarbons than atmospheric pressure, although lower or higher pressures those formed with the chlorine analog. may be employed in the pyrolysis. The contact time of It is, therefore, one of the objects of the present inven 60 the pyrolysis feed in the furnace will vary from 0.05 to tion to separate perfluorocarbons from fluorocarbons by a 5 seconds. The pyrolysis conditions will affect the con simple, econcmic and novel method. It is another object version of the chlorodifluoromethane and also the compo to prepare perfluorocarbons. A further object is to purify sition of the pyrolysate. In general, with increasing perfluorocarbons. Still another object is to separate tetra pyrolysis temperatures and contact times, a higher con fluoroethylene, hexafluoropropylene and octafluorocyclo version to the products mentioned is obtained. Increases butane from close boiling, or azeotrope forming fluoro in pyrolysis temperature and contact times will also lead carbons. Other objects will become apparent hereinafter. to the formation of increased proportions of the higher In accordance with the present invention, volatile per boiling fluorocarbons and to a decrease in the propor fluorocarbons are separated from volatile non-perfluori tions of tetrafluoroethylene. Thus, at 35% conversion, nated fluorocarbons, which, in addition to fluorine, con 70 a yield of ~95% of tetrafluoroethylene is obtained and tain at least one element selected from the class consisting a yield of 5 to 7% of higher boiling fluorocarbons of of hydrogen and halogens having atomic numbers from which 50% is hexafluoropropylene and perfluorocyclo 3,101,304 A. butane. At a 90% conversion of the starting material, higher boiling fluorocarbons. The mixture of hexa a 30% yield of tetrafluoroethylene is obtained and a fluoropropylene and perfluorocyclobutane is taken as the 70% yield of higher boiling fluorocarbons of which 50% most volatile component. is hexafluoropropylene and perfluorocyclobutane. The pyrolysis of bromodifluoromethane is carried out under TABLE substantially similar conditions and gives rise to Sub stantially similar results. Relative Volatility The separation of the higher boiling fluorinated hydro Component carbons by straight distillation was found to be impos Xylene Chloro sible, since the various fluorocarbons formed azeotropes 10 benzene which prevented their separation. Thus, it was found Chlorotrifluoroethylene.--- 2, 32 1.99 that difluorochloromethane and hexafluoropropylene Dichlorodifluoronethane- 2.46 1.98 Chlorotetrafluoroethane-- 5.45 3.48 formed an azeotrope, and that perfluorocyclobutane and Chlorohexafluoropropane- 13.2 8.6 chlorotetrafluoroethane formed an azeotrope. Since the relative volatility of these compounds is one or close to one, their substantial separation can not be achieved Relative volatilities of the azeotrope formed between directly by distillation, the only separation possible being hexafluoropropylene and chlorodifluoromethane and the as to excess quantities present beyond the quantities in effect of various extractive distillation agents at increas the azeotrope composition. A further difficulty is the ing concentrations is shown in Table III, in which hexa close boiling points of some of the fluorocarbons
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