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 obtained fiuoropropylene is taken as the most volatile component. from the pyrolysis. Thus, chlorotrifluoroethylene and TABLE I dichlorodifluoromethane boil within 2 C. of hexafluoro Azeotrope: Hexafluoropropylene 10 molpercent; chlorodifluoromethane propylene. The addition of the described aromatic hydro 90 mol percent carbons or halogenated aromatic and aliphatic hydro carbons was found to rearrange the Volatilities of the 25 Relative Wolatility Moles of Solventifivole of interfering fluorocarbons in Such a manner as to greatly Azeotrope Chloro- Carbon enhance the volatility of the tetrafluoroethylene, hexa form Tetra- Tollene fluoropropylene and perfluorocyclobutane, thereby pre chloride

venting the formation of the undesirable azeotropes. The preferred aromatic hydrocarbons and halogenated, ali 1.0 .0 ... O .52 1.29 1.82 phatic or aromatic hydrocarbons used in the present in 2.01 ------vention are hydrocarbons which are liquid under distill 2.27 1.52 2.75 lation conditions and include benzene, toluene, Xylene, 2.43 ethylbenzene and other alkyl-substituted benzenes, as well The relative volatilities of compounds which distill as chlorobenzene, p-dichlorobenzene, 2,4,6-trichloro 35 close to tetrafluoroethylene and form azeotropes with benzene, o-chlorotoluene, p-chlorotoluene, and o-chloro tetrafluoroethylene are shown in Table IV. Compounds ethylbenzene, methylene chloride, chloroform, methyl which form azeotropes with tetrafluoroethylene or boil chloride, carbon tetrachloride, dichloroethylene, tetra close to tetrafluoroethylene, generally do not constitute chloroethylene and trichloroethylene. The quantity of a significant problem when the pyrolysis is carried out the hydrocarbon to be employed can be varied over a 40 using chlorodifluoromethane. However, the fluorocar wide range. Even the addition of small quantities of the bons listed below are formed in substantially greater con hydrocarbon will significantly affect the volatility. In centrations when the pyrolysis is carried out with bromo order to readily facilitate the separation of the higher difluoromethane. Using bromodifluoromethane, the ex boiling fluorinated hydrocarbons obtained from the pyrol tractive distillation of the present invention is important ysis, after removal of starting material and tetrafluoro to the preparation of pure tetrafluoroethylene. The rela ethylene, it is preferred to employ from 1 to 25 times the tive volatilities shown in Table V were obtained at at weight of the hydrocarbons as compared to the weight of mospheric pressure using three parts of Solvent per part the higher boiling fluorocarbons. Thus, the relative of the fluorocarbon mixtures. This table further shows volatilities at various pressures of a mixture employing that normal hydrocarbons do not give rise to an increase three parts of toluene, one of the preferred hydrocarbons, 50 in relative volatilities. Tetrafluoroethylene is used as the and one part of the higher boiling fluorocarbons obtained most volatile component. from the pyrolysis of chlorodifluoromethane, is illus trated with respect to the major components of the TABLE IV mixture in Table I below. The mixture of hexafluoro propylene and perfluorocyclobutane is taken as the most 5 5 Solvents volatile component. Methyl- Methyl66% TABLE Components ele Toluene ene Chlo- ln-Hexane Chloride ride, 34% Toluene Relative Volatility 60 Component ------Relative Wolatilitics Atom. 15 p.s.i.g. 30 p.s.i.g. 45 p.s.i.g. pressure Trifluoroethylene.------3. Difluoromethane.--- 5. Chlorotrifluoroethylene------3. 2.78 2.25 2.05 Pentafluoroethane- 2. Dichlorodifluoromethane.----- 2.83 2.72 2.42 2.1 65 Trifluoromethane------. Chlorotetrafluoroethane.------7.8 6.06 4.5 3.75 Chlorodifluoromethane--- 13. Chlorohexafluoropropane---- 22.0 5.6 9.85 7.94 Chloropentafluoroethane.--- 3. Relative volatilities obtained with toluene at atmos Table shows the relative volatility of the major pyrol pheric pressure when used in a ratio of fluorocarbon ysis products with xylene and chlorobenzene indicating 70 mixture to toluene of 1:3 are shown in Table V. The their suitability in the process of the present invention, fluorocarbon mixture employed contained the components and also points out the reason why toluene is the pre formed in the pyrolysis of bromodifluoromethane. The ferred separating agent. The measurements were ob (relative volatilities shown were obtained with hexafluoro tained at atmospheric pressure using a mixture compris propylene and perfluorocyclobutane as the most volatile ing three parts of the hydrocarbon and one part of the 75 component. The table also includes the relative volatil 3,101,304 6 S preferred with a temperature of 150 to 165 C. at the ities of fluorocarbon compounds, the structures of which base and 14 C. at the head of the column. The over have not been definitely established, but which are formed head of the column, comprising essentially pure hexa in the pyrolysis. The unknowns are designated by their fluoropropylene and perfluorocyclobutane, G is passed retention time in the gas chromatography unit. to column 19 where the hexafluoropropylene H is sepa TABLE V 5 rated by distillation from the perfluorocyclobutane J. The column is maintained at a pressure of 45 p.s.i.g. Relative Wolatilities and at a temperature of -8° C. at the head of the col Component umn and --35°. C. at the base of the column. The resi Hexafluoro Perfluoro propylene as cyclobutane O due from column 8 containing the remaining higher M.W.C. as M.W.C. boiling fluorocarbons and the aromatic hydrocarbon K - is passed to column 20 where the aromatic hydrocarbon Bromotrifluoromethane F is separated from the remaining high boilers L. The Unknown 7.3------Operating conditions of the column will vary with the Unknown 14 Unknown 12.3 5 aromatic hydrocarbon employed. The conditions are Tetrafluoroethane.------Hexafluoropropane--- maintained such that the aromatic hydrocarbon F is re Unknown 15.8------covered from the base of the column and the remain Chlorotetrafluoroetha Unknown 21.3------ing fluorocarbons L are removed at the head of the Chlorobromodifluoronethane... Column. The aromatic hydrocarbon is dried if necessary Bromodifluoromethane------Unknown 37.0------20. and then recycled to column 18. Bromotetrafluoroethane FIG. 2 shows schematically the additions necessary Unknown 47.5- to include in the pyrolysis process of FIG. 1 an ex Unknown 54.9-l. tractive distillation of tetrafluoroethylene. As indicated Dibromotetrafluoroethane------Dibromodifluoromethane-- above, this becomes necessary when the pyrolysis is car The process of the present invention is further illus 25 ried out using bromodifluoromethane as the starting ma trated by the attached schematic drawings showing, in terial. When using the extractive distillation with bromo FIG. 1, the process of the present invention as employed diffuoromethane, the pyrolysis is conducted in the same in the pyrolysis of chlorodifluoromethane using an aro manner as with chlorodifluoromethane, but after the matic hydrocarbon as the extractive distillation agent, compressor 12, the distilling column 3 of FIG. 1 is op and in FIG. 2, the modifications necessary to include an 30 erated at somewhat different conditions, i.e., at a pres extractive distillation of tetrafluoroethylene with methyl Sure of 165 p.s.i.g. and a head temperature of -15° C. ene, chloride. and, thus, becomes column 21 of FIG. 2. This permits Chlorodifluoromethane A is charged into the pyrolysis more of the fluorocarbons boiling around tetrafluoro furnace 50 where the formation of the fluorocarbons oc ethylene and tetrafluoroethylene azeotrope to come over. curs. The reaction is generally carried out in a multiple 35 The distillate S is passed again through scrubber 14 where tube furnace employing platinum-lined Inconel tubes, al hydrobromic acid is removed. From there the distil though other materials of construction, such as silver late is passed into gas stripper 22 operated at 155 p.si.g. and carbon-lined steel or Inconel tubes, may be used. and a head temperature of -20° C. The overhead prod The chlorodifiuoromethane may be preheated if desired. uct T comprises those gases boiling below tetrafluoro The operating conditions for the furnace have been de 40 ethylene and its azeotropes, such as trifluoromethane and scribed hereinabove. From the furnace 10 the reac hexafluoroethane. The residue U is passed into extrac tion products are passed through a cooler a where the tive distillation column 23 where the tetrafluoroethylene gaseous products are cooled to below 150° C. The azeotrope is broken up. The extractive distillation agent, cooled gases are fed to the cooler and compressor 12 methylene chloride V, is pumped into the column near where the gas is cooled to below 50° C. and compressed 45 the top and above U. The column is operated at a to about 200 to 250 p.s.i.g. Tetrafluoroethylene and hy pressure of 140 p.s.i.g. and a head temperature of -19 drogen chloride are distilled off in column f3, main C. The Overhead B comprises polymerization grade tetra tained at a temperature of -25 C. at the head and fluoroethylene. The residue W is distilled in column 45' C. at the base of the column, and at a pressure 24, operated at a pressure of 60 psi.g. and a tem of 225 p.s.i.g. The tetrafluoroethylene B is separated 50 perature of -19 C., in which the extractive distillation from the hydrogen chloride by passing it through a agent is recovered and recycled and from which the re caustic scrubber 14. The residue from the distillation mainder of the fluorocarbons X comprising principally column containing the desired hexafluoropropylene and trifluoroethylene and difluoromethane, are obtained. perfluorocyclobutane C is passed to column 15 where The separation of hexafluoropropylene and perfluoro the unreacted starting material, chlorodifluoromethane, 55 Cyclobutane is slightly different in the pyrolysis with is separated from the residue of column 3 so far as it bromodifluoromethane, since it is the latter which forms does not form the azeotrope with hexafluoropropylene an azeotrope with the former, but will be apparent from and where additional chlorodifluoromethane A is charged the foregoing. to the system which is then fed to the pyrolysis furnace. The invention is further illustrated by the following The column is maintained at a temperature of 45° C. 60 Specific examples. at the base and --7 C. at the head of the column, and Example I at a pressure of 75 p.s.i.g. The residue from column 15 Into a 43' long, 1" in diameter, packed column is passed to the storage tank 6 where the higher boil (35-40 plates) operating at atmospheric pressure, was ing fluorocarbons may be stored prior to their separa charged continuously at a rate of 150 g/hr, a composi tion. The higher boiling fluorocarbons are passed from 65 tion comprising essentially as follows: the storage tank 16 into a heels column 17 where com Mol percent pounds boiling higher than chlorohexafluoropropane Hexafluoropropylene ------17.2 (B.P. 20 C.) M are removed to avoid contamination of the aromatic hydrocarbon. The overhead from the Perfluorocyclobutane ------25.4 heels column E passes to the extractive distillation col 70 Chlorodifluoromethane ------1.2 umn i8. The aromatic hydrocarbon F is charged to Dichlorodifluoromethane ------6.8 the column at a temperature of 0 to 50° C. higher than Tetrafluorochloroethane ------23.5 the head temperature of the column. The column can Hexafluorochloropropane ------8.3 be maintained over a wide pressure range from 0 to 200 Chlorotrifluoroethylene ------5.3 p.s.i.g., although pressures of about 50 to 75 p.s.i.g. are 75 Unknowns--minor components ------12.3 3, G1,304 8 This composition had been obtained by cracking chloro was boiled up the column at a rate of not less than 50 difluoromethane at a temperature of 600 C., and at a lb./hr. pressure of 2 p.si.g., for a contact time of approximately Toluene removed from the base of the column was 0.2 sec., and removing hydrogen chloride, tetrafluoro free from significant contamination with fluorocarbons ethylene and starting material so far as possible, from and was recycled to the extraction column for 8 hrs. the pyrolysate. The composition was fed to the column 5 without deterioration in the purity of the extraction col 25 in. up from the base of the column. Toluene at -9 umn overhead. C. was fed from the top of the column at a rate of 1260 Example III g/hr. maintaining a ratio of 3:1 of toluene to higher boiling fluorocarbons in the column which was refluxing Using the column described in Example i, crude tetra at a 5:1 reflux ratio. The temperature of the column 0. fluoroethylene, having the following composition, ranged from 72-74 C. at the base of the column to Mol percent -20° C. at the head. The pressure of the column was Tetrafluoroethylene ------95 atmospheric. Hexafluoropropylene and perfluorocyclo butane were taken from the head of the column at a rate Trifluoroethylene ------of 72 g./hr. Analysis of the product indicated that it 5 Difluoromethane ... - - - me a marm men one or -r a w- - - -an r------w 4 contained more than 99.99% of the perfluorocarbons. was fed into the column 25 in. up from the base at the The yield from the extractive distillation was 98%. rate of 200 g./hr. Methylene chloride at -65 C. was Equal results can be obtained with aromatic hydro fed from the top of the column at 600 g/hr., thus main carbons other than toluene, such as benzene, Xylene and taining a ratio of 1:3. Reflux was at an approximate ethyl benzene, under slightly modified conditions. 20 ratio of 1:1. The column was operated at atmospheric Using the same column and the same supply of high pressure. Temperature of the column ranged from boiling fluorocarbons, a batch distillation was conducted --30° C. at the base to -75° C. at the top of the column. without addition of toluene. A reflux ratio of 10:1 was Analysis of the overhead showed the product to be greater used. than 99.99% pure tetrafluoroethylene. The same results The purest cut of hexafluoropropylene that could be 25 are obtained when a chlorodifluoromethane pyrolysate obtained analyzed: containing 50 to 1000 p.p.m. of difluoromethane and 50 Mol percent to 100 p.p.m. of trifluoroethylene is employed, or a bromo Hexafluoropropylene ------81.5 difluoromethane pyrolysate containing 5 to 20 mol percent Dichlorodifluoromethane ------9.2 of difluoromethane and 0.1 to 0.2 mol percent of trifluoro Chlorotrifluoroethylene ------6.2 30 ethylene is employed. - Miscellaneous ------3.2 Example IV The purest cut of perfluorocyclobutane that could be Using the column of Example I, a mixture having the obtained analyzed: following composition, Mol percent 3 5 Mol percent Perfluorocyclobutane ------50.0 Perfluorocyclobutane ------0. Chlorotetrafluoroethane ------50.0 Hexafluoropropylene ------10 Example II Bromodifluoromethane ------80 into a 21' x 2' stainless steel column packed with %' 40 was fed into the column 25 in. up from the base at a rate Raschig rings (15-30 plates) maintained at 60 p.s.i.g. of 150 g./hr. Toluene was fed from the top of the was fed toluene at a temperature of 60° C. and at the column at a rate of 1000 g./hr. The reflux ratio was rate of 47 lbs./hr. A stream of high boiling fluorocar maintained at 5:1. Temperature of the column was 60 bons produced via the cracking of chlorodifluoromethane, to 65° C. at the base and -20° C. at the top. The as in Example I, was fed to the column 6' from the base, column was operated at atmospheric pressure. Hexa at a rate of 7 lb./hr. This stream contained 48.4% by fluoropropylene and perfiuorocyclobutane removed from weight of useful perfluorocarbons (11.3% hexafluoropro the top contained less than 0.1% of bromodifluorometh pylene and 37.1% perfluorocyclobutane by weight). ac. Total analysis of the high boiling fluorocarbons fed to It is to be understood that the relative volatility data, the column was: the pyrolysis processes described, and the examples repre Mol percent sent specific embodiments of the invention. Various Hexafluoropropylene ------7.50 modifications of the invention disclosed will be apparent Perfluorocyclobutane ------33.28 to those skilled in the art and are included herein. Thus, Chlorotrifluoroethylene ------4.18 it will be apparent that other saturated and unsaturated Dichlorodifluoromethane ------2.38 55 perfluorocarbons can be separated by the process of the Chlorodifluoromethane ------0.24 present invention from other fluorocarbons containing, in Chlorotetrafluoroethane ------24.30 addition to fluorine, either chlorine, bromine, iodine or Chlorohexafluoropropane ------13.83 hydrogen. In general, the fluorocarbons and perfluoro Unknowns ------14.32 carbons involved have up to 5 carbon atoms. Further 60 more, it will be apparent that the invention has been The column was operated at a reflux ratio of about principally described with respect to chlorinated and 2:1, base temperature was 110-120° C. For a period unhalogenated extractive distillation agents. Compounds, of 14 hrs., the overhead stream of useful perfluorocar such as bromobenzene and bromoform and methylene bons (hexafluoropropylene and perfluorocyclobutane) iodide, may, however, also be employed, although they was taken off at 3 lb./hr. Analysis indicated less than 65 are not preferred, since they have lower stabilities than the 100 p.p.m. of the other fluorocarbons in the overhead. chlorohydrocarbons. Recovery of useful perfluorocarbons was 87 weight The process of the present invention provides a simple, percent of that fed to the column. effective and economic method for the recovery of per Toluene from the base of the extraction column (47 fluorocarbons when formed in admixture with other fluoro lb./hr.), containing 4 lb./hr. of non-useful fluorocarbons 70 carbons. The perfluorocarbons which heretofore had to was fed to the toluene recovery column. Equipment used be recycled or vented off because of the extreme difficulty was a 20' X 3’ diameter, stainless steel column, packed or impossibility of separating them from by-products, can with %' Raschig rings, operating at 40 p.s.i.g. now be readily separated and recovered. The base temperature was maintained at the boiling The commercial utility of the perfluorocarbons obtained point of toluene at that pressure, 160° C., and toluene by the process of the present invention as monomers, 3,101,804 10 intermediates and solvents, has been well-established in 5. The process of claim 4 wherein the fluorocarbons the art, so that no further description thereof is deemed are formed by pyrolysis of chlorodifluoromethane. necessary. 6. The process of claim 4 wherein the fluorocarbons This application is a continuation-in-part of Serial No. are formed by pyrolysis of bromodifluoromethane. 755,030, filed August 14, 1958, now abandoned. 5 7. The process of claim 4 wherein the azeotropes sep I claim: arated are of the class consisting of tetrafluoroethylene 1. A process for separating volatile perfluorocarbons with methylene fluoride and tetrafluoroethylene with tri of one to five carbon atoms from close boiling and fluoromethane, azeotrope forming fluorocarbons of one to five carbon 8. The process of claim 7 wherein the extractive dis atoms containing, in addition to fluorine, elements selected 0. tillation compound is an alkyl benzene. from the class consisting of hydrogen, chlorine and 9. The process of claim 8 wherein the alkyl benzene is bromine, by extractive distillation in the presence of a toluene. compound, liquid at distillation conditions and selected 10. The process of claim 7 wherein the extractive dis from the class consisting of benzene, alkyl benzenes, tillation compound is a chlorinated aliphatic hydrocarbon. chlorinated aliphatic hydrocarbons, chlorinated benzenes 5 11. The process of claim 10 wherein the chlorinated and chlorinated alkylbenzenes. aliphatic hydrocarbon is methylene chloride. 2. A process for separating volatile perfluorocarbons 12. The process of claim 4 wherein the azeotropes sep of one to five carbon atoms from close boiling and azeo arated are of the class consisting of hexafluoropropylene trope forming fluorocarbons of one to five carbon atoms with chlorodifluoromethane and hexafluoropropylene with containing, in addition to fluorine, elements selected from 20 bromodifluoromethane. the class consisting of hydrogen, chlorine and bromine, 13. The process of claim 12 wherein the extractive by extractive distillation in the presence of an alkylben distillation compound is an alkyl benzene. Zene liquid at distillation conditions. 14. The process of claim 13 wherein the alkyl benzene 3. A process for separating perfluorocarbons having up is toluene. to 5 carbon atoms from fluorocarbons containing, in addi 25 15. The process of claim 4 wherein the azeotropes tion to fluorine, elements from the class consisting of separated are of the class consisting of perfluorocyclo hydrogen, chlorine and bromine, said fluorocarbons hav butane with chlorotetrafluoroethane and perfluorocyclo ing up to 5 carbon atoms, by extractive distillation in the butane with bromodifluoromethane. presence of a chlorinated aliphatic hydrocarbon, liquid at 16. The process of claim 15 wherein the extractive dis distillation conditions. 30 tillation compound is an alkylbenzene. 4. A process of separating perfluorocarbons of one to 17. The process of claim 16 wherein the alkylbenzene five carbon atoms, obtained by the pyrolysis of a com is toluene. pound selected from the class consisting of chlorodifluoro methane and bromodifluoromethane at a temperature of References Cited in the file of this patent 450 to 900 C., from the fluorocarbons of one to five 35 UNITED STATES PATENTS carbon atoms formed in said pyrolysis which comprises 2,339,160 Dunn et al. ------Jan. 11, 1944 distilling close boiling fractions and azeotrope forming 2,384,449 Benning et al. ------Sept. 11, 1945 fractions of said pyrolysate in the presence of a compound 2,551,573 Downing et al. ------May 8, 1951 Selected from the class consisting of benzene, alkyl ben Zenes, chlorinated aliphatic hydrocarbons, chlorinated 40 OTHER REFERENCES benzenes and chlorinated alkylbenzenes, liquid at distilla "Technique of Organic Chemistry,” volume IV, “Distil tion conditions, said compounds being present in greater lation,” by Weissberger, Interscience Publ., Inc., New than 1:1 weight ratios, and recovering pure perfluoro York, 1951 (pages 321, 338 and 339 relied upon). carbons.