US3367983A - High temperature, short contacttime pyrolysis of dichlorodifluoromethane - Google Patents

Description

United States Patent ()fiFice 3,307,983 Patented Feb. 6, 1968 3,367,983 Hll-GH TEMPERATURE, SHORT CONTACT- TiME PYRGLYSHS F DICHLORODIFLUO- RQMETHANE John Richard Soulen, Narberth, and William Ford Schwartz, ing of Prussia, Pen, assignors to Pennsalt Chemicals Corporation, Philadelphia, Pa, a corporation of Pennsylvania No Drawing. Filed Oct. 1%, 1965, Ser. No. 497,535 6 Claims. (Cl. 260-453) ABSTRACT OF THE DISCLOURE Dichlorodifiuoromethane (CCl CF is pyrolyzed at a temperature within the range of about 1100 C. to about 2000" C. for a period of from 0.0002 to 0.05 second to produce, as a major product, 1,2-dichlorotetrafiuoroethane (CClF CClF This invention relates to the preparation of other organic fluorochloro compounds by the pyrolysis of dichlorodifluoromethane. More particularly, this invention concerns pyrolyzing dichlorodifluoromethane (CCl F to produce as the major products, primarily, 1,2-dichlorotetrafluoroethane (CClF CCIF and secondarily, chlorotrifiuoromethane (CClF By pyrolysis is meant the transformation of a compound into another compound or other compounds through the agency of heat alone, and therefore the term includes not only the rearrangement of a compound but also the making of more complex compounds.

A. B. Trenwith and R. H. Watson, J. Chem. Soc, 1957, pp. 2368-2372, describe the pyrolysis of CCl F at a temperature ranging from 700 C. to 900 C., whereby the principal products were CClF and chlorine, and smaller amounts of CCl F and CCl were obtained as minor products. Dichlorotetraliuoroethane was detected in merely a trace amount in the reaction products only from the pyrolysis at 750 C. F. B. Downing, A. F. Benmng and R. C. McHarness, US. Patent No. 2,551,573, pyrolyzed CCl F at 950 to 970 C. and obtained as the reaction products CF, and CF CI; however, no CCIF CCIP was produced by this method.

It has now been discovered that high temperature pyrolysis of CCl F i.e., in the range of about 1100 C. to about 2000 C., unexpectedly produces CClF CClF in relatively high yield as a principal reaction product, CClF as a major product of secondary importance, and as minor products, CCl F, CCl FCCl F and CCl FCC1F However, the time of the pyrolysis reaction embodied herein is very short and should not exceed about 0.05 second. Thus, a principal product of the pyrolysis of CCl F embodied in this invention is a haloethane although the principal products of the pyrolyses of CCl F using prior art processes are halomethanes. The most desired product of the process of this invention, CClF CClF is a valuable refrigerant used mainly in systems having centrifugal rotary compressors. The product CClF is valuable as a refrigerant for ultralow temperatures. The minor products are also useful as well known refrigerants. Moreover, CClF and CCl F can be pyrolyzed at high temperatures to produce additional CCIF CCIF as de scribed in our copending applications, Ser. No. 499,084, filed Oct. 20, 1965 and Ser. No. 495,348, filed Oct. 12, 1965, respectively.

As stated above, the pyrolysis of CCI F is carried out according to this invention at a temperature within the range of about 1100 C. to about 2000 C. The preferred temperature range with regard to obtaining the highest conversion of the CCI F and highest yields of CClF CClF therefrom is from about 1400 C. to about 1800 C.

As earlier stated, in combination with the high pyrolysis temperatures used in the practice of this invention, very short contact times of the CCl F at such temperatures are employed, that is, pyrolysis times on the order of about 0.0002 to 0.05 second, preferably in the range of about 0.0006 to about 0.01 second. At contact times of the usual order of magnitude used in pyrolyses of this type, i.e., on the order of about 10 seconds and higher, the process of this invention is inoperative because of considerable degradation of the reactant and reaction products. As used herein contact time is defined as follows:

Contact time (seconds):

heated reactor volume volume of gas per second (calculated at rcact'on temperature and pressure) fed to reactor.

The short contact times indicated above for the pyrolysis of this invention correspond to very high space velocities ranging from about 500 to about 120,000 per hour which permit a high rate of feed and reduce reactor volume needed. Space velocity is defined as volumes of reactant (measured at standard temperature and pressure (STP), i.e., 0 C. and 760 mm. Hg) per volume of heated reactor per hour. This is in sharp contrast to the much lower space velocities previously employed in the pyrolyses of CCl F on the order of about 6 to per hour. For example, A. B. Trenwith and R. H. Watson, Journal of Chemical Society, ibid, used extended reaction times of about to seconds for pyrolyzing CCI F and in the above-mentioned US. Patent No. 2,551,573, pyrolysis times on the order of about 11 seconds were used (Example XXIII).

The reaction pressure in the present process is not critical and may be atmospheric, sub-atmospheric or superatmospheric. Super-atmospheric pressures may range, e.g., up to about 10 atmospheres. However, atmospheric and sub-atmospheric pressure operation will generally be found most convenient. As a practical matter, pressures lower than about one mm. Hg absolute are not recommended. Preferred operating pressures will generally range from about 10 mm. Hg to atmospheric pressure.

The pyrolysis is conveniently carried out by continuously passing a stream of the CCl F feed through an elongated tube preferably having a high ratio of wall area to cross-sectional area so that heat may be rapidly and continuously transferred from the heated reactor wall to the gaseous reactant. The reactor should be constructed of a material resistant to attack by the reactant and reaction products at the high operating temperatures. Materials of this type include, for example, inert graphite, boron nitride and like inert materials. The reactor can be heated to the desired pyrolysis temperatures in any suitable manner such as by electrical induction heating or by placing the reactor in an electrically heated furnace.

The products of the pyrolysis reaction passing from the reactor are cooled and usually will be scrubbed in caustic solution or other alkaline solution to remove acidic, inorganic by-products (e.g., chlorine and fluorine). The organic products are separated from the reaction mixture in the conventional manner by fractional distillation. The unreacted CCI F can, of course, be recovered for re cycling purposes.

Examples 1-6 In the experiments herein described, specific embodiments of the invention are set forth to illustrate and clarify the invention.

Gaseous CCI F is passed continuously at a measured rate through a A; inch ID. x /2 OD. x 13" long, inert graphite tube reactor centered within a 2" diameter Vycor high-silica glass tube, inches long. The reactor is inductively heated with a 3% long load coil of 12 turns of A inch copper tubing about the Vycor tube, the power for said coil supplied by a high frequency generator with a maximum output of 7.5 kilowatts operating at 450 kilocycles. The effective reaction zone in the tube is thus 3% inches. The temperature of the reactor is measured with an optical pyrometer focused on the center of the heated portion of the tube. Examination of the inert graphite reactor after repeated runs therein reveals that its inner surface is unaffected by the passage of the hot gases therethrough.

The product mixture passes from the reactor and is condensed in a trap cooled with liquid nitrogen. The condenser is vented to a mechanical vacuum pump which maintains the subatmospheric reaction pressure employed in these examples. After completion of the run, the reaction products are warmed to room temperature and transferred to an evacuated stainless steel cylinder. The reaction products are then passed through a series of scrubbers containing aqueous solutions of sodium hydroxide to remove the inorganic by-products. The organic reaction products are analyzed using gas-liquid chromatographic and infrared analyses techniques.

The data from six runs are summarized in Table I. In addition to the components listed in the Product" column of Table I, the reaction products contain unreacted CCI F and varying minor amounts of CF CCL CFQZCFZ CCIF CCIF, and CCl FCF an isomer of the desired reaction. The principal reaction products are CC1F CClF (20 weight percent), CClF (63%) and C1 (17%).

It is to be understood that the foregoing iilustrative examples should not be construed as limitative of the scope of the invention which is defined by the appended claims.

We claim:

1. The method which comprises pyrolyzing dichlorodifluoromethane at a temperature of from about 1100 C. to about 2000 C., wherein the pyrolysis time is from about 0.0002 to about 0.05 second, a major product of the pyrolysis being 1,2-dichlorotetrafiuoroethane.

2. The method of claim 1 wherein the yrolysis temperature is from 1400 C. to about 1800 C.

3. The method of claim 1 wherein the pyrolysis time is from about 0.0006 to about 0.01 second.

4. The method which comprises pyrolyzing dichlorodifluoromethane at a temperature of from about 1400 to 1800 C. wherein the pyrolysis time is from about 0.0006 to 0.01 second, a major product of the pyrolysis being 1,2-dichlorotetrafiuoroethane.

5. The method of pyrolyzing dichlorodifiuoromethane which comprises passing dichlorodifiuoromethane through a tube heated to a temperature of from about 1100 C. to about 2000 C., wherein the contact time is within the range of about 0.0002 to 0.05 second, a major product of the pyrolysis being 1,2-dichlorotetrafluoroethane.

6. The method of pyrolyzing dichlorodifiuorornethane which comprises passing dichlorodifluoromethane through a tube heated to a temperature of from about 1400 C. to about 1800" C., wherein the contact time is within the range of about 0.0006 to 0.01 second, a major product of product, CCCIF CIF the pyrolysis being l,2-dichlorotetrafiuoroethane.

TABLE I Pyrolysis Conditions t Weight Percent in Recovered Converted Product of- Yemen Example Feed rate Pressure, Temp, Contact conversion 01 GCI F mm. Hg 0 0. time, 01' CCIQFQ CCIF1CCIF2 GClFs GCI3F CClQFCCIQF CCIZFCCiFi gins/min. abs. seconds 1. 087 29 1,130 0. 0013 4. 7 71. 7 8. 7 4. 9 Nil 3. 4 0. 645 47 1, 330 0. 0032 25. 0 35. 5 42. 3 7. 2 2. 1 8. 7 0. 485 31 1, 500 0. 0025 37. 0 55. 1 32. 7 5. 4 0. 4 5. 5 0.685 30 1,650 0.0016 61.6 70. 1 14.3 3. 2 1.0 7.4 0. 431 29 1, 750 0. 0023 70. 8 76. 5 14. 1 1. 7 Nil 3. 3 0. 332 1, 840 0. 0050 42. 9 47. 2 19. 1 12. 0 Nil 21. 0

Example 7 50 References Cited UNITED STATES PATENTS 2,551,573 5/1951 Downing et al 260653 3,009,966 11/1961 Hauptschein et a1. 260-6535 3,188,356 6/1965 Hauptschein et al. 260653.5

DANIEL D. HORWITZ, Primary Examiner.