US3652691A - Pyrolysis of octafluorocyclobutane - Google Patents

Abstract

OCTAFLUOROCYCLOBUTANE IS CONVERTED PRINCIPALLY TO TETRAFLUROETHYLENE BY PYROLYSIS AT A GIVEN CONVERSION LEVEL IN A PROCESS COMPRISING HEATING SAID OCTTAFLUOROCYCLOBUTANE TO A TEMPERATURE SUFFLICIENT TO SELECTIVELY PRODUCE, IN THE CONVERTED PRODUCTS, A HIGH YIELD OF TETRAFLUOROETHYLENE BASED ON WEIGHT PERCENT OF THE TOTAL TETRAFLUOROETHYLENE AND HEXAFLUOROPROPENE PRODUCED.

Description

March 28, 1972 Filed Nov. 5, 1969 CONVERSION /0 0: b O O s. BJORNSON ETAL 3,652,691

PYROLYSIS 'OF OGTAFLUOROCYLOBUTANE 2 Sheets-Sheet 2 PYROLYSI 5 OF OCTAFLUOROCYCLOBUTANE O IN I/8" O. D. 304 5.5. TUBE O IN 1/4" 0. D. INCONEL TUBE A IN |/4" o. D. 304 5.5. TUBE o AV/ 0 LESS THAN 90 SELECTIVITY To TFE 2! AT LEAST 90 SELECTIVITY To TFE f/ I200 1300 I400 I500 I600 REACTTON TEMP F F/G. Z

INVENTORS GEIR BJORNSON H. M. FOX

A T TORNEYS United States Patent 3,652,691 PYROLYIS 0F OCTAFLUQROCYCLOBUTANE Geir Bjornson and Homer M. Fox, Bartlesville, Okla assiguors to Phillips Petroleum Company Filed Nov. 3, 1969, Ser. No. 873,433 Int. Cl. C07c 17/24, 21/18 US. Cl. 260--653.3 11 Claims ABSTRACT OF THE DISCLOSURE This invention relates to the pyrolysis of octafluorocyclobutane to produce tetrafluoroethylene and hexafluoropropene with high selectivity to the production of tetrafiuoroethylene.

For convenience, tetrafiuoroethylene will sometimes be referred to hereinafter as TFE. Similarly, hexafluoropropene will sometimes be referred to hereinafter as HFP. Tetrafluoroethylene is unsaturated fluorocarbon having valuable utilityin various applications. One particularly valuable utility is in the form of its various polymers, several of which have achieved commercial success. For example, the polymer or resin Teflon is widely used as a coating material in many applications where a coating material having release properties is desired, e.g., in the coating of cooking utensils. Tetrafiuoroethylene would find even wider application if more efiicient and economical methods of producing same were available.

It is known that octafluorocyclobutane can be pyrolyzed to tetrafiuoroethylene and hexafluoropropene. However, in the methods of the prior art there is produced a considerable amount of hexafluoropropene, i.e., much more than is desirable when TFE is the desired product. Thus, even though hexafiuoropropene is a valuable fluorocarbon, its production in considerable amounts along with tetrafluoroethylene adversely affects the economics of prior art processes when tetrafiuoroethylene is the desired product. It would be desirable to have a pyrolysis process for producing tetrafiuoroethylene from octafluorocyclobutane in which the production of hexafiuoropropene can be essentially eliminated, or at least minimized.

The present invention provides such a process. We have now discovered the conversion and temperature relationship in the pyrolysis of octafiuorocyclobutane which must apply in order to achieve high selectivity to the production of tetrafluoroethylene with substantially complete elimination of hexafiuoropropene production.

An obejct of this invention is to provide an improved process for the pyrolysis of octafiuorocyclobutane to tetrafiuoroethylene. Another object of this invention is to provide a more economical process for the production of tetrafluoroethylene. Another object of this invention is to provide an improved process for the pyrolysis of octafiuorocyclobutane in which tetrafiuoroethylene is selectively produced in high yields. Another object of this invention is to provide an improved process for the pyrolysis of octafluorocyclobutane to tetrafluoroethylene in which the production of hexafiuoropropene can be substantially completely eliminated. Another object of this invention is to provide an improved process for the production of tetrafluoroethylene by the pyrolysis of octafiuorocyclobutane in which the production of by-products other than hexafluoropropene is essentially complete- 1y eliminated. Another object of this invention is to provide an improved process for the pyrolysis of octafiuorocyclobutane to tetrafiuoroethylene in which process operating difliculties such as plugging of the reactor tubes can be eliminated. Other aspects, objects and advantages of the invention will be apparent to those skilled in the art in view of this disclosure.

Thus, according to the invention, there is provided in a process for the conversion of octafluorocyclobutane by pyrolysis, wherein said octafiuorocyclobutane is pyrolyzed to convert same into products including tetrafiuoroethylene and hexafluoropropylene, the improvement comprising: pyrolyzing said octafluorocyclobutane at a given conversion level by heating same to a temperature sufficient, relative to said conversion level, to selective produce, in the converted products resulting from said pyrolysis, at least about 90 weight percent tetrafluoroethylene based on weight percent of the total tetrafluoroethylene and hexafluoropropylene produced.

A number of advantages are obtained or realized in the practice of this invention. An outstanding advantage is that tetrafluoroethylene can be produced in essentially pure form. Another advantage of the invention process is the essentially complete elimination of the production of so-called unrecoverables, i.e., unwanted side products. Another advantage is that the invention process can be operated at relatively high conversions of octafluorocyclobutane and still maintain a high selectivity for the production of tetrafluoroethylene. Another advantage of the invention process is that one can employ economical materials of construction. Another advantage of the invention process is that operating difficulties, e.g., the plugging of the reaction tube either by coke or by solidification of by-products, can be eliminated. Another advantage of the invention process is that it can be easily controlled by simple analysis of the etfluent product for (a) the extent of conversion of octafluorocyclobutane or (b) the selectivity to the production of tetrafiuoroethylene.

In the practice of the invention the operating variables of percent conversion, temperature, and contact time, are all interrelated and any change in the level or magnitude of any one of said variables must be considered in selecting the level or magnitude of the others. However, we have discovered that, when pyrolyzing octafluorocyclobutane for high selectivity to the production of tetrafiuoroethylene there is a definite relationship between the conversion level and the temperature level which must be observed. For example, at a given conversion level there is a reaction temperature which must not be exceeded. Stated conversely, when operating at a given reaction temperature, there is a maximum conversion level which must not be exceeded. This relationship can be expressed by the formula conversionO.l42t 166 In the above formula, conversion of octafiuorocyclobutane is expressed in percent, and t is the reaction temperature in degrees F. The above formula is considered further hereinafter in connection with the examples and the drawings.

Generally speaking, the temperature employed in the practice of the invention will be within the range of from about 1200 to about 1600 F. The conversion level can be maintained within the range of 0.1 to 60 percent, in most instances preferably within the range of 0.5 to 50 percent. A conversion level of from about 8 to about 37 percent has been found to be practical. The pressures employed will be substantially atmospheric, e.g., 0.5 to 1.5 atmospheres, it usually being preferred to operate at about one atmosphere. It is within the scope of the invention to obtain elfective pressures of less than one atmosphere by employing vacuum or by diluting the octafluorocyclobutane feedstock, e.g., with an inert gas such as nitrogen, so as to lower the partial pressure of said octafluorocyclobutane. The contact time can vary over a relatively wide range of values within the range of 0.001 to 2 seconds, or longer, preferably 0.01 to 1.5 seconds. Usually the contact time will be within the range of 0.05 to 1 second. At any given conversion level and at any given reaction temperature level, there will be a contact time which should be employed. However, said contact time is not controlling in that it is fixed, depending upon the conversion level and temperature.

Practically speaking, it is usually preferred to employ a tubular furnace, fix the temperature at a desired level, and then control the degree of conversion by varying the rate of flow of the reactant feedstock stream to said furnace. Such control can be performed manually on the basis of periodic analyses of the furnace effluent stream. One method of control which is sometimes preferred is to monitor the product stream by measuring the concentration (and thus the conversion) of the octafiuorocyclobutane in the effluent stream by such methods as infrared spectrometry, gas chromatography, mass spectrometry, and the like. One can then control a valve, either manually or automatically and continuously, regulating the flow of reactant feedstock to the furnace in order to maintain the conversion level measured in the product stream at the predetermined value. In a like manner, the feed rate can be fixed, and the temperature can be controlled responsive to the effluent analysis. As will be understood by those skilled in the art, other methods of control can be employed to control the level of conversion at any given temperature, and thus control the selectivity to the production of tetrafiuoroethylene in accordance with the invention.

The term pyrolysis, as used herein and in the claims, unless otherwise specified, is employed to describe the conversion of one chemical species to another by the action of heat. Thus, the process of the present invention comprises a pyrolytic reaction which depends upon the transfer of heat to the reactant gas, and is accomplished by the action or agency of heat.

In the construction of the furnaces used in the practice of the invention any suitable materials can be employed. By suitable materials is meant any material which will withstand the necessary temperatures, pressures, and the chemical action of the reactants, or reaction products or intermediates. In the prior art the noble metals, and particularly silver, have been preferred as materials of construction, or as a lining of the surface exposed to the reaction conditions. However, it is a feature of the present invention that the use of the noble metals is not required. We have found that the various stainless steels, and particularly stainless steel Type 304, are preferred materials of construction for the reaction tubes in the furnace. Inconel is another preferred material for the construction of the furnace reaction tubes. Carbon steel tubes can also be employed but are less preferred than those previously mentioned. In general, copper tubes are not satisfactory because of a tendency toward coke formation. The furnace tubes are preferably constructed to have a large surface to volume ratio, e.g., as small diameter as practical, so as to promote effective and uniform heat transfer at the generally preferred short contact times.

The pyrolysis furnace can be heated by any convenient or suitable means. Electrical resistance heaters have been found very satisfactory. However, other heating means such as a molten metal bath heated by combustion of gas, oil, etc. can be employed to heat the pyrolysis furnace. It is also within the scope of the invention to carry out the pyrolysis in a plasma reactor, using a gas such as argon.

The term unrecoverables is sometimes employed herein to refer to unwanted by-products. Included as such by products are perfiuoroisobutylene, cis and trans perfluorobutene-2, perfluoromethane, perfiuoroethane, perfiuoropropane, and polytetrafiuoroethylene.

FIG. 1 is a block flow diagram illustrating one present preferred embodiment of the invention.

FIG. 2 is a plot of portions of the data obtained in the illustrative examples given hereinafter, and illustrates the relationship between conversion level and reaction temperature.

Referring now to the drawings, the invention will be more fully explained. In FIG. 1 there is illustrated a pyrolysis furnace comprising a heat sink 10 and a reaction tube 12 mounted within said heat sink. Said heat sink can comprise any suitable type of heat sink. For example, one convenient form comprises a copper cylinder having a hole or holes drilled therein of a sufficient diameter to accommodate the outside diameter of the reaction tube or tubes extending through the heat sink. If desired, said heat sink can comprise a molten metal bath with the reaction tube or tubes disposed therein. Although not shown in the drawing, it will be understood that said heat sink is provided with suitable heating means, such as electrical heaters, and is also provided with suitable insulation.

It will be noted that the downstream end of reaction tube 12 is enlarged. This enlargement makes possible an expansion of volume of the reaction mixture in said reaction tube 12. Said reaction tube enlargement and expansion of the reaction mixture provide an additional important feature of the invention. We have found that a small amount, usually less than than about 0.02 weight percent, of polytetrafluoroethylene forms in the pyrolysis of octafluorocyclobutane. This polymer melts and forms a constriction in the reaction tube if the effluent is not expanded in the hot zone of the reaction, or at least prior to cooling of the reaction tube efliuent. We have found that an expansion of the reaction tube efliuent, preferably located immediately prior to the outlet of the reaction tube, will keep the polymer in a finely divided solid particle form. It is also within the scope of the invention to locate said expansion immediately downstream from the furnace. In the absence of said expansion of the reaction tube efiiuent, melted polymer will solidify and collect at the end of the reaction tube. The polymer in solid finely divided form can be removed from the reaction effluent stream by hot filtration, cyclones, or other suitable means. The amount of expansion on the effluent from the reaction tube will usually be Within the range of from 2 to 8, preferably 4 to 6 times the normal volume of said efiluent.

In the operation of the apparatus illustrated in FIG. 1 a gaseous stream of octafiuorocyclobutane is passed via conduit 14 at the desired flow rate, through reaction tube 12 which is heated to the desired temperature, through expansion section 16, and then via conduit 18 into cyclone 20. Said cyclone 20 removes the finely divided solid particles of polytetrafiuoroethylene. The remaining efiluent is withdrawn via conduit 22, passed through heat exchanger 24 where it is heat exchanged with liquid octafluorocyclobutane feedstock from conduit 26, with the vaporized feedstock being passed into conduit 14 for passage to furnace 10. If desired, the reaction effluent can be passed from heat exchanger 24 through auxiliary cooler 28 for further cooling. The now cooled reaction eflluent is passed via conduit 30 into one of the mo] sieve adsorbers 32 or 34 by means of the manifold arrangement shown. Said mol sieve adsorbers contains a fixed bed of Linde 5A molecular sieve adsorbent which preferentially adsorbs perfiuoroolefins and does not adsorb saturated materials such as the perfluorocyclobutane feedstock. Said mol sieve adsorbers are operated in an on-off cycle, i.e., one is on stream while one is being regenerated, by means of the manifold arrangement shown. The adsorbed perfiuoroolefins can be desorbed by heating the bed to about 300 F. by means of heating coils (not shown) embedded in said beds. The desorbed perfiuoroolefins are passed from Table 1 below.

ditions and results of analysis on tography analysis unit and analysed in conventional manner.

One series of runs was carried out employing a /s-inch OD stainless steel Type 304 tube having an ID of 0.069

the product efiluent stream are given in Another series of runs was carried out employing a it-inch OD Inconel reaction tube having an ID of 0.1835 inch. The operating conditions and results of analysis and 34, the un- 10 of the product stream are given in Table II below.

Another series of runs was carried out employing a A-inch OD stainless steel Type 304 tube having an ID of 0.1980 inch. Operating conditions and results of analysis of the product stream are given in Table HI below.

TABLE I [The pyrolysis of perfluorocyclobutane in a 14-inch O.D. stainless steel-304 tube] the on stream adsorber by means of the manifold arrangement shown into conduit 36, compressed in compressor Returning now to said adsorbers 32 adsorbed perfluorocyclobutane unreacted feedstock, and

38, and then passed via conduit 40 into distillation column or zone 42. In said column or zone 42 the indicated separation is effected with tetrafluoroethylene being withdrawn 5 inch. The operating con via conduit 44, hexafluoropropene being withdrawn via conduit 46, and perfluoroisobutylene being withdrawn via conduit 48. Any higher boiling materials can be withdrawn via conduit 50.

any saturated by-products such as perfluoromethane, perfluoroethane, and perfluoropropane, which may be present, are withdrawn from the adsorber being regenerated Runnumber Run condltions:

Total TFE plus HER...

at the middle of the reaction section and about 1% inches 0 Control run.

Referring to the above Table I, it will be noted that in all of the runs made in accordance with the invention so as to obtain at least 90% selectivity to TFE, i.e., Runs also under isothermal conditions. In the runs described 65 1 4 7 0 1 and 13, the production f unwanted hereinafter octafluorocyclobutane was passed at the de- FIG. 2 is a plot of a portion of the data from all the runs in Tables I, II, and III. It will be noted that all the 13, the production of said unwanted by-products was less than 0.06%. In nuns 1, 2, and 3, the by-product production was 0. Thus, the invention provides an improved efiifour to malntam the Small amount of polytetrafluo' 70 cient process for the production of essentially pure roethylene by-product in finely divided solid particle form.

by-products was less than 0.1%. In Runs 1, 2, 3, 6, 10, and

tetrafluoroethylene in good yields at a high selectivity of at least 90%.

section of the reactor tube was about inches in length. Thermocouples were embedded in the copper heat sink isothermal. Since there was a close fit between the reaction tubes and the heat sink, the reaction tubes were sired flow rate through the reaction tube which was heated to the desired temperature, in accordance with the invention. Efiiuent from the reaction tube was expanded about Said efl'luent was then passed through a water cooled copper tube, then through a Water bubbler, and then through a tube containing Drierite to remove the water. The dried and polymer free efiluent was then passed to a gas chroma- 75 points to the right of the line A are for runs wherein the 55 9 a 00 8 97 6 a 0755 99 mm. a 5 "m5 .7 3 5 a 5 me t 5 a 1 m 9 b L105 001 9 b :105 81 u u t t 1 e we m m H a M a our... .2 W C 1 03 9 0 MW 0 .m 1 1 ow 1 035 606 227. 8 7 D 3 0089 O0 0 D 3 0041 19 flnm wwwu. WW 9 M M W2 0 fi mz &L 10 &2 0 &0 9 9 h 1. m 1 01 9 5 3 m m mmwmmmmn a m u we m n sss 50 580. 409 7 2 9 47 .0 W7 9 42 .6 02m 10 1 0 4 0 9 9 n 1. 3 9 I n 1,102 9 4 5 1.... 1 I e I 8 005 9716 80 15 6 n 1 00 1 84 2 n 1 0093 19 0mm amen ma 8 m m. m ml t m m 0. 1 .i 1 9 1 1 9 1 u OO 0 0 mmw 1 1 BM 1 085 Am 043 1 5% O B% 3 y n u n v. u h h 4 4 0 .000M417 36 C G v01 ..3 81 9 0 n... O 1.. 1 "0000.410. m L DIW m "I p I n u M &n u man awn hmwwnnn 1 m H 68 mm 2m m m mms wm I 4 600 0000106 27 9 e d 6 8 B 02 .....2 9 9 D n mpn m D. m 3 M e w n D. m nwoc un 000 .m me w: E s hmmmw: 069 M7" 0 "n% 0 s D F n .q. D .0 21 n n s1 0. w. a mm u T m .w. r mdeoo 101 .a0 ..0".. 9 w o b emW 1 1 o m emmmy I a r 1 0. I w. m mhown w v. m m hown a 0 8 mmw www nammow 0 w m mmmmmrr T .n. w n .wrammmr 3 37.O"00O0118. 2 0 h n fiCtmW mTH m h n MnWm fiTH 1 n20 00 000 0 1 9 W. u wmymam a W n mwmm 008 8 R ROOP& R R0CP& 51 62903 7 96 5 new wmmmwamo 9 m. M m M 1 BOQHQQQQMQI 9 nd nn e d o e2 n mmmmmmmnm w 46 0 m mmm w u smm wm mm r t rm 9 11M m mnmm ma mtmmmmm O n 0 .6 3 00 d w w fl nompb m 00 NM 00 640 n elnce aW Sm 2. 70. && 1% 0 df 1 a. te v10 50". 9 0 m uudye 1 d "m 1 em m mmw oe $0 mi mam bnsmesahn. 69 mum" 0 Ot. u I d t 0.20. no 35 mw u yef euM .mmy b 1 0 0 0 "a 9 a n2md mo u W a csm I m m w n wmtOchYla 1.1.00 d n ie n aHaW a W moohr a 0 .T .m ohue so. S .ficesc kfi T f 0 g \l u f .I 1H4 u t np C .C t. 0 ts S E .1 In 5.. m 1 L M30 00; "um" F mm: we mw mm W m wk m .c. 0 "0e.. U W n re .r. .1 d U e "at": M fia mgm nw s A ame; aha r 3h fl mfwm tn Wwm h X Wu h m 1 1 h h u m Inm m Em M mn D ,.m ew; 0 o efirm r c .mierP n. .m S6 3 6 ynmmrhhupo mm mn m im d nwtarsmhm pwwmmw w t .i u 4 du "n r 3 SO .f C S m 0 .ed 6 :1 g a 6 r T I bl d D 4 n a co dr sF 0r F n m e ecc w .1 Y S t mmcwrkwm nnmnw m I 0 m mmm m m. m w. m wm 0 V3500 p. S4 t 1 y. b .1 n p o n ti 0 s C vl a 01 qw d fioo h ta eh ..n; v 0 .i t o i m as c... .mqrm mramwm mv w mm mm mm m w m dah tale C r GYOGfiF t S nC .lXmflUHteEt moo(Too0HoTP TH mm mm mo n w ou w .e C 1 6C C oe CCLU S 08 RosE & w .wmm ma fl mm wm mmb e e C CO m m h h no n n aibioSt t itwEiPcim from eah end thereof. Temperature measurements by means of said thermocouples showed the heat sink to be 7 selectivity to TFE was at least 90 weight percent. All points on or to the left of said line A are from runs wherein the selectivity to TFE was less than 90 weight percent. Calculating from the slope of said line A shows that the relationship between conversion and temperature can be expressed by the formula conversion0.l42t166 where conversion of octafluorocyclobutane is expressed in percent and the temperature is in degrees F. Thus, a person skilled in the art, in possession of this disclosure, can readily determine the operating conditions to be used in pyrolyzing octafluorocyclobutane to tetrafiuoroethylene with at least 90 weight percent selectivity to said tetrafluoroethylene. For example, when operating at a 40% conversion level, a temperature above 1450 P. will be necessary. Conversely, if one sets the temperature, one can then determine the conversion level at which the run should be made.

While certain embodiments of the invention have been described for illustrative purposes, the invention is not limited thereto. Various other modifications of the invention will be apparent to those skilled in the art in view of this disclosure. Such modifications are within the spirit and scope of the disclosure.

We claim:

1. In a process for the conversion of octafluorocyclobutane by pyrolysis, wherein said octafluorocyclobutane is pyrolyzed to convert same into products including tetrafiuoroethylene and hexafluoropropylene, the improvement comprising:

pyrolyzing said octafluorocyclobutane at a temperature within the range of from about 1200 to about 1600 F. and at a conversion level determined in accordance with the formula Percent conversion0.142t- 166 wherein t is said temperature; and selectively producing, in the converted products from said pyrolysis, tetrafluoroethylene in an amount of at least 90 Weight percent based on the weight percent of the total tetrafluoroethylene and hexafiuoropropylene produced.

2. A process according to claim 1 wherein the conversion of said octafluorocyclobutane is within the range of from 0.5 to 50 percent.

3. A process according to claim 2 wherein said conversion is within the range of from about 8 to about 37 percent.

4. A process according to claim 1 wherein said pyrolysis is carried out by continuously passing said octafluorocyclobutane through a tubular pyrolysis zone under essentially isothermal temperature conditions throughout the length of said zone.

5. In a process for the conversion of octafluorocyclobutane by pyrolysis, wherein said octafiuorocyclobutane is pyrolyzed by continuously passing same through a tubular pyrolysis zone to convert same into products including tetrafiuoroethylene and hexafiuoropropylene, the improvements comprising:

expanding the efiluent from said pyrolysis zone immediately prior to the outlet of said tubular pyrolysis zone;

passing said expanded effluent through a separation zone to remove solid-form reaction products therefrom; and

recovering tetrafluoroethylene product from the resulting essentially solids-free effluent.

6. A process according to claim 5 wherein said efiluent is expanded an amount within the range of from 2 to 8 times its normal volume.

7. A process according to claim 5 wherein the effluent from said pyrolysis zone is expanded immediately downstream from the outlet of said tubular pyrolysis zone.

8. A process according to claim 7 wherein said effluent is expanded an amount within the range of from 2 to 8 times its normal volume.

9. A process according to claim 5 wherein said octafiuorocyclobutane is pyrolyzed at a temperature within the range of from about 1200 to about 1600" F. and at a conversion level determined in accordance with the formula Percent conversion0.l42t 166 wherein t is said temperature; and

there is selectively produced, in the converted products from said pyrolysis, tetrafluoroethylene in an amount of at least 90 weight percent based on the weight percent of the total tetrafluoroethylene and hexafluoropropylene produced. \10. A process according to claim 9 wherein said conversion is within the range of from 0.5 to percent.

11. A process according to claim 10 wherein said conversion is within the range of from about 8 to 37 percent.

References Cited UNITED STATES PATENTS 3,306,940 2/1967 Halliwell 260653.3

DANIEL D. HORWITZ, Primary Examiner US. Cl. X.R. 260653, 899

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