US3262981A - Production of fluorinated olefins - Google Patents

Description

July 26, 1966 A. H. FAINBERG ETAL 3,262,981

PRODUCTION OF FLUORINATED OLEFINS Filed Oct. 5, 1964 INVENTORS ARNOLD H. FAINBERG DAVID S. FETTERMAN BY MURRAY HAUPTSCHEIN United States Patent 3,262,981 PRODUCTION OF FLUORINATED OLEFHNS Arnold H. Fainberg, King of Prussia, David S. Fetterman,

Maple Glen, and Murray Hauptschein, Glenside, Pa.,

assignors to Pennsalt Chemicals Corporation, Philadelphia, Pa., a corporation of Pennsylvania Filed Oct. 5, 1964, Ser. No. 401,499 3 Claims. (Cl. 260-6533) This is a continuation-in-part of application Serial No. 178,970, filed March 12, 1962, now US. Pat. No. 3,152,051.

This invention relates to improved methods for pyrolyzing fiuoroform to produce tetrafluoroethylene and/or hexafluorop-ropene.

In the pyrolysis of fiuoroform to produce the perfluorinated olefins tetrafluoroethyiene and/or hexafiuoropropene as disclosed in US. Patent No. 3,009,966 of Murray Hauptschein and Arnold Fainberg, one of the major by-products in the pyrolysis is hexafluoroethane C 1 While the hexafluoroethane is a relatively minor byproduct under conditions providing low fiuoroform conversions, it is produced in increasingly larger quantities as the pyrolysis is carried out at increasingly higher fluoroform conversions, and in view of the relatively high cost of the fiuoroform starting material, it is highly desirable to reduce the formation of this by-product substantially to zero. While it is possible to virtually eliminate the formation of C F by operating the pyrolysis at very low fiuoroform conversion rates operation at very low conversions is not desirable since the size of the pyrolysis reactor and associated distillation equipment increases as the rate of fiuoroform conversion decreases.

In accordance with the present invention it has now been found that the net production of hexafluoroethane can be reduced to zero by separating the hexafluoroethane from the pyrolysate and recycling the separated hexafluoroethane to the pyrolysis zone together with fluoroform feed-stock until the concentration of hexafluoroethane in the feed to the pyrolysis zone becomes equal to the concentration of hexafluoroethane in the product gases. Provided that the total one pass conversion of fiuoroform to products is maintained below about 65% it has been found that upon prolonged recycle of the C 1 a condition will be obtained in which the concentration by Weight of the hexafluoroethane in the product from the pyrolysis zone is equal to the concentration by weight of hexafiuoroethane in the feed to the pyrolysis, thus resulting in a net formation of C F of zero. In order to achieve this result it has been found necessary to maintain the total one pass conversion of fiuoroform below about 65 since above that conversion rate the net formation of C F cannot be suppressed by recycle Of the CZFB.

It is quite surprising that the net formation of hexafluoroethane in the pyrolysis of fiuoroform can be suppressed in the manner described above, since it appears that the mechanism for the formation of C 1 involves the production of elemental carbon, a solid, which tends to deposit out on the Walls of the reactor. Although the invention is not limited to any particular explanation for the reaction mechanism, it is believed that C F forms according to the following equation:

The observed fact of the suppression of C F by its recycle would appear to require an equilibrium between the C F in the reactor and carbon deposited on the reactor walls, which is quite unexpected.

It is quite surprising that the recycle of the C F does not result in its breakdown to CR, or other by-products. As will be seen, the C F recycle does not result in any ice corresponding increase in any of the normal by-products, or the formation of new by-products, even though the reaction is conducted under vigorous conditions of high temperature.

Finally, it is also surprising that the complete suppression of the C F is achieved at relatively low concentrations of C F in the product and feed gases. Even at relatively high conversions, up to about 65%, the complete suppression of C F occurs at a concentration of C F in the product and feed gases of not more than about 5%. The concentration of C 1 at low fiuoroform conversions necessary to provide complete suppression of net C 1 production is considerably lower than 5%.

Reference is now made to the sole figure of the drawing which shows in diagrammatic form a preferred embodiment of the invention.

Referring to the figure, the reference numeral 1 refers to the line for introducing fresh fiuoroform feed into the system. Into line 1 there is also introduced recycle streams 2 and 3 separated from the pyrolysis products which will be described in detail subsequently. The mixture of fresh fiuoroform feed and recycled material from lines 2 and 3 pass through valve 4 into a preheater 5 which may be gas fired as shown, then through line 6 t0 the pyrolysis reactor indicated generally by 7. The pyrolysis reactor comprises a tube or series of tubes 8 which may be heated by any convenient means such as by electrical resistance heater elements 9. The entire pyrolysis furnace is insulated by suitable thermal insulating material 10. The pyrolysis reactor 7 is maintained at pyrolysis temperatures (as described more in detail in US. Patent No. 3,009,966) ranging preferably from about 850 to 1300 C. and usually most desirably from about 900 to 1200 C. and at a contact time and pressure adjusted, depending upon the temperature used, to produce the desired rate of conversion of fiuoroform to products. The raw pyrolysate from the reactor is conducted by line 11 to a cooler 12 where the pyrolysate is cooled e.g. approximately to room temperature and is then conducted by line 13 to a vacuum compressor 14 which serves to draw a vacuum on the reactor and at the same time serves to compress the pyrolysate to pressures of about 5 to lbs/in. gage. The compressed pyrolysate is then fed by line 15 to a condenser 16 provided with a cooling coil 17. In the condenser 16, the compressed pyrolysate is cooled to a temperature of about 20 C. to +10 C. which is below the dew point of the hydrogen fluoride product from the reaction but above the dew point of the organic portion of the pyrolysate. As a result, the bulk of the hydrogen fluoride condenses from the mixture and is Withdrawn from the bottom of the condenser 16 by line 18 while the organic portion of the pyrolysate passes by line 19 to a compressor 20. The liquid condensate which is comprised almost entirely of hydrogen fluoride is introduced into column 21 where any organic portion is distilled overhead and is removed by line 22 and fed into line 19. The bottoms from column 21 are removed by line 23 and these bottoms comprising substantially pure hydrogen fluoride may be recovered for any desired use.

The organic portion of the pyrolysate leaving compressor 45 is compressed to about to 400 lbs/in. and is fed by line 24 into extractive distillation column 25. In extractive distillation column 25 the desired products tetrafluoroethylene and hexafluoropropene are separated from unreacted fiuoroform and to a large extent from the hexafiuoroethane obtained in the pyrolysate.

Where tetrafluoroethylene is a desired product, it has been found necessary to employ an extractive distillation procedure because of the formation of a fiuoroform tetrafluoroethylene azeotrope containing about 20% tetraindicated by the cross-hatching. As can be seen, the organic portion of the raw pyrolysate containing unreacted fluoroform, tetrafluoroethylene, perfluoropropylene, hexafiuoroethane and minor amounts of other constituents such as Q, olefins and C HF is introduced between sections B and C. The extractant is introduced between sections A and B by line 26. The extractant is preferably hexafluoropropylene and the operation of the column will be described with reference to that extractant. The short section A of the column above the point of introduction of the extractant prevents the hexafluoropropene extractant from passing out of the column as overhead. In section B the descending hexafluoropropene extractant in effect strips away C 1 from the CHF such that the descending liquid becomes progressively richer in C 1 and the ascending vapors richer in the CHF Unexpectedly, the hexafiuoroethane content of the pyrolysate is also separated from the C 1 in section B, rising up the column with the fluoroform rather than descending with the G l -C 1 mixture. Vapor liquid equilibria measurements for the quaternary system show that the relative volatility of C F with respect to C 1 is substantially in excess of unity, ranging eg, from 1.2 to 1.3 at temperatures of +5 to -5 respectively for the relatively low concentrations of C 1 that are normally encountered in section B of the column (usually from 0.1 to 10 mol percent of the quaternary mixture). This permits separation of some or virtually all of the C F from the C F in the extractive distillation column, the C F going overhead with the fluoroform.

In section C, the descending liquid mixture is progressively enriched in C 1 and depleted in CHF and C F The hexafluoropropene content of the feed stream 46 of course travels down the column with the circulating stream of C F extractant. The higher boiling C 1 and C 1 compounds comprising a minor fraction of the pyrolysate also travel down the column.

The extractive distillation column is provided with the usual reboiler coil 29 at the bottom of the column and with the usual condenser 30 at the top. The overhead from column 25 is taken oft by line 28 while the bottoms, consisting principally of hexafiuoropropene and tetrafluoroethylene, flow by line 27 through valve 31 to a second distillation column where the tetrafluoroethylene and other volatile products are separated from hexafluoropropene and other higher boiling components. The hexafluoropropene and other high boiling components leave the bottom of column 32 by line 33 and are conducted to a pump 34 and are then conducted by line 35 to lines 36 and 37 where the stream is divided into two portions.

The bulk of the stream from line 35 flows through line 36 through a heat exchanger 38 where the temperature is adjusted and then through line 26 to the extractive distillation column 25. A minor proportion of the steram conducted by line 37 to column 39 where the perfiuoropropene is distilled away from higher boiling materials such as Q, olefins and the like, the perfluoropropene being recovered as product from the top of the column by line 40. If desired, the higher boiling materials such as C, olefins may be recycled by line 41, valve 42 and line 2 and introduced with fresh fluoroform feed into the reactor Any minor amounts of even higher boiling materials are removed from the bottom of column 39 by line 43 controlled by valve 44.

As stated previously, the bulk of the fluoroform and in many cases the bulk of the C F contained in the raw pyrolysate is separated from the other products as overhead from column 25. A portion of the column overhead is refluxed back into the column by line 45 and valve 46 while another portion is recycled to the pyrolysis reactor by line 47, valve 48, and line 3.

In atypical operation, most of the fluoroform and hexafiuoroethane in the pyrolysis product is recovered as column overhead from extractive distillation column 25 and thus, the main source of fluoroform and C F recycle to the pyrolysis reactor in the preferred embodiment shown will be the overhead from the extractive distillation column 25.

In the practical operation of the extractive distillation column 25 however, there may be some minor amounts of fluoroform and C 1 which are contained in the bottom of column 25 and these will be introduced accordingly into the second distillation column 32, where the tetrafluoroethylene and other lower boiling products are separated from hexafluoropropene and other higher boiling products. Accordingly, the overhead from column 32 will often contain in addition to the tetrafluoroethylene, small amounts of hexafiuoroethane and fluoroform. This mixture is withdrawn from the top of column 32 by line 49 and introduced into a third distillation column 49a where the hexafiuoroethane and fluoroform are separated from the tetrafluoroethylene, the bulk of the tetrafluoroethylene being recovered from the bottom of the column by line 50, while the fluoroform and hexafiuoroethane are recovered from the top of the column through line 51. This overhead portion is recycled to the pyrolysis reactor by lines 51 and 52 and introduced into line 3 where it joins the bulk of the C F and CF H recovered as overhead from the extractive distillation column 25 and is introduced into the pyrolysis reactor together with fresh fluoroform feed from line 1.

As can be seen from the foregoing description of the embodiment shown in the drawing, essentially all of the hexafiuoroethane produced in the raw pyrolysate is recovered and recycled to the pyrolysis reactor. The particular techniques used in separating C F from the pyrolysis products does not form per se part of the present invention, and it is to be understood that the C F may be separated by methods other than by the particular ones shown.

As previously pointed out, it is essential in the present invention to operate the pyrolysis reactor under conditions providing not more than about a 65% one pass conversion of fluoroform products, i.e. to say not more than about 65 of the fluoroform in the mixture entermg the reactor through line 6 should be converted into other products during passage through the pyrolysis reactor. The factors of effecting conversion of the fluoroform are principally temperature and contact time. Higher temperatures and higher contact times strongly favor higher one pass conversions of fluoroform. As stated previously, the pyrolysis temperature will be preferably from about 850 C. to about 1300 C. and preferably 900 to 1200 C. while contact times are preferably in the range from about one second to about .001 second, with the shorter contact times being used at higher temperatures. Reaction pressures preferably of one atmosphere or less are used. Somewhat reduced pressures varying from about to 600 mm. Hg absolute are preferred because they appear to favor a more desirable product distribution giving higher total yields of tetrafluoroethylene and/or hexafluoropropene. As discussed in more detail in US. Patent 3,009,966 these reaction variables may be readily adjusted to obtain any desired rate of conversion of the fluoroform to products.

As stated previously, the rate of production of hexafiuoroethane tends to increase with increasing fluoroform,

5 6 conversions. This is shown by the following examples in TABLE H which no recycle of hexafluoroethane to the pyrolysis Zone 18 p y on. in Feed Lbs. of 02m Net formation produced per of C2Fa, lbs. of Examples 1 to 6 Example 100 lbS. of CzFo per 100 5 l Mole t P Wt. t lirolduct lbs. of product 6 Cell ercen 011 In In these examples fluorofor-m 1s fed through a platinum r g lined Inconel reactor tube maintained at a pressure of 7 25 +2.5 approximately 100 mm. Hg absolute by a vacuum pump. 1.3 3.0 +0.5 The reactor tube is heated over a length of about 20 inches to a temperature of 1250 C. as measured by a 10 thermocouple at the center of the tube. The feed rate of As can be seen from Ta le h no C2F6 Was fluoroform to the reactor was varied between 0.5 to 1.5 cycled the product contams 2.5 weight percent of C F pounds per hour. Over this range of fiuoroform feed representing a yield loss of 2.5% when tetrafiuoroethylene rates, the conversion of fluoroform to total product varied and/or hexafluoropropene are the desired products. between about 40% and 85%. When the feed contains 2.5% C F the C 1 in the prod- The product gases in these examples were analyzed by uct increases to 3.0 weight percent but the net formation passing samples through a bed of sodium fluoride mainof C 1 drops to +0.5 weight percent. After prolonged tained at 80 to 100 C. to obtain quantitative removal of recycle, the percent of C F in the product increases to hydrogen fluoride and were then analyzed by gas liquid 3.8% and when this C F is recycled both feed and prodpartition chromatography to obtain the mole percent comuct contains 3.8% with a net formation of C 1 of zero position of the product from which was calculated the percent. mole percent conversion of fluoroform and the percent Attempts to suppress the net formation of C 1 at total conversion of fluoroform to organic products. The yield fluoroform conversions of 70% and 80% respectively by of hexafluoropropene plus tetrafluoroethylene was also 5 recycling C F were unsuccessful. Although some supdetermined by calculation from this data. The results 2 pression occurred, the net formation could not be reduced of Examples 1 to 6 are shown in the following Table I. to z TABLE I Total percent Percent CHFI! Converted to Perc nt; Yi ld Example Cofngefisli on 1 Other of (31%: 013+ 0 z CF2=CF2 CF3=CF=CF2 OEFS roducts CF3CF=CF2 As can be seen from Table I, significant amounts of Example 10 hexafluoroethane are produced at conversions of with the conversion to C 1 increasing as the conversion 2 f w uslzlg a j if fi z increases. As the total fiuoroform conversion increases 20 h E 6 tlameier o B over above about 60%, the conversion to hexafiuoroethane o 1 S engt. O a emperature of and maintained at a reaction pressure of 150 mm. Hg with the increases markedly.

rate of flow ad usted to give a one pass fluoroform con- 7 to 9 version of 60%. During the run the reactor feed con- Examp es tamed about 3.3% by weight of C F the remainder being fl The following examples illustrate the effect of recyclmg uoroform A total of 1254 grams of fiuoioform and 43 grams of C 1 was fed to the react-or durlng the run C F to the reactor at a total one pass flu-oroform con- I (a total feed of 1297 grams). The results of this run version of about 65%. The same pyrolysis reactor was were as shown in Table III. employed as 1n the previous examples with a preactor temperature of about 1250 C. and a pressure of about TABLE III 100 mm. Hg absolute. The rate of feed to the reactor was maintained at a constant rate of 0.75 lb. per hour in C d W ht Percent CHF; these examples. To evaluate the net formation of C 1 ompoun rudh gt r ms i tigitii s i l d it was necessary to calculate a material balance across the product reactor for each set of feed and product analyses. For his purpose all analyses were converted to carbon per- 5 1 503 cent, i.e. the percent of the total carbon in the total prod- 8 3358 54%.; gg not contributed by each component. This was done by g 2 02 3-8 multiplying the mole percent of each component by the 1 0 1' CF 0.3 .03 5 number of ca1bon atoms 1n that component and then ex 5 g g g g 1.3 .10 g g pressing the thus weighted mole percents as carbon per- HF 223 cent by dividing the weighted mole percent of each com- Totals 1 271 100 0 ponent by the weighted mole percent of total products. Using this material balance method, the actual weights of C F entering and leaving the reactor per 100 pounds As can be seen from Table III, the recycle of C F of total feed were calculated. In Example 7, no C 1 resulted in the complete suppression of the net formation was introduced into the feed to the reactor; in Example of this by-product at a C 1 concentration in the feed and 8, 2.5 weight percent of C 1 was added while in Examproduct streams of 3.3% by weight. The material balple 9, 3.8 weight percent was added. The results are ance across the reactor confirmed this result within expeshown in Table II. rimental err-or. It is also to be noted that the recycle of the C F in this manner resulted in no increase in the formation of other by-products. In particular no formation of OR; was noted. Also there was no net formation of carbon as shown by no carbon deposition in the pyrolysis tube during the run.

Example 11 In another prolonged run at pyrolysis temperatures of about 900 C., giving one pass fluoroform conversions of 30%, recycle of C 1 suppressed the net formation of this compound at a concentration of C 1 in the feed and product gases of only about 0.2% by weight.

While the method of the invention will be useful in suppressing net C 1 formation at any rate of fluoroform conversion below about 65% it will be found most useful when the pyrolysis is operated to give fluor-oform conversion of from about 20 to 60%.

We claim:

1. In the production of olefins selected from the class consisting of tetrafiuor-oethylene and perfluoropr-opene by the pyrolysis of fiuoroform at temperatures of from 850 to 1300 C. and at contact times of from 0.001 to 1 second, under conditions producing C F as a by-product of the pyrolysis reaction, the improvement which comprises separating C 1 from the desired products of the pyrolysate and recycling the separated C 1 to the pyrolysis zone together with fiuoroform feed stock until the concentration of C 1 in the organic product from said pyrolysis zone becomes equal to a value, not more than about 5% by weight of the feed to said pyrolysis, which is substantially the same as the concentration of C F in the feed to said pyrolysis zone, while maintaining the total one pass conversion of the fluoroform to other products in the range of from about 20% to about thereby substantially eliminating the net production of C F in said pyrolysis.

2. A method in accordance with claim 1 in which the pressure in said pyrolysis zone is maintained at from about to 600 mm. Hg absolute.

3. A method in accordance with claim 1 in which the total one pass conversion of fiuoroform to other products is maintained in the range of from 20% to 60%.

References Cited by the Examiner UNITED STATES PATENTS 2,758,138 8/1958 Nelson 260-6533 2,902,521 9/1959 Cleaver et al. 260--653.3 3,009,966 11/1961 Hauptschein et a1. 260653.3

LEON ZITVER, Primary Examiner.

DANIEL D. HORWITZ, Examiner.

Claims (1)

1. IN THE PRODUCTION OF OLEFINS SELECTED FROM THE CLASS CONSISTING OF TETRAFLUOROETHYLENE AND PERFLUOROPROPENE BY THE PYROLYSIS OF FLUOROFORM AT TEMPERATURES OF FROM 850 TO 130*C. AND AT CONTACT TIMES OF FROM 0.001 TO 1 SECOND, UNDER CONDITIONS PRODUCING C2F6 AS A BY-PRODUCT OF THE PYROLYSIS REACTION, THE IMPROVEMENT WHICH CCOMPRISES SEPARATING C2F6 FROM THE DESIRED PRODUCTS OF THE PYROLYSATE AND RECYCLING THE SEPARATED C2F6 TO THE PYROLYSIS TOGETHER WITH FLUROFORM FEED STOCK UNTIL THE CONCENTRATION OF C2F6 IN THE ORGANIC PRODUCT FROM SAID PYROLYSIS ZONE BECOMES EQUAL TO A VALUE, NOT MORE THAN ABOUT 5% BY WEIGHT OF THE FEED TO SAID PYROLYSIS, WHICH IS SUBSTANTIALLY THE SAME AS THE CONCENTRATION OF C2F6 IN THE FEED TO SAID PYROLYSIS ZONE, WHILE MAINTAINING THE TOTAL ONE PASS CONVERSION OF THE FLUOROFORM TO OTHER PRODUCTS IN THE RANGE OF FROM ABOUT 20% TO ABOUT 65%, THEREBY SUBSTANTIALLY ELIMINATING THE NET PRODUCTION OF C2F6 IN SAID PYROLYSIS.

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