US3183277A - Process for the manufacture of fluorethylenes - Google Patents

Description

y 11, 1965 o. SCHERER ETAL 3,183,277

PROCESS FOR THE MANUFACTURE OF FLUCRETHYLENES Filed March 10, 1961 a I b E n E INVENTORS Otto Scherer Alfons Sieinmez Heinrich Kit/Zn WELZer Wei-zed Kargzeinz 9I'Gif6n M 1 *XZW ATTOR N United States Patent ice 3,183,277 PRGCESS FOR THE MANUFAQTURE 0F FLUORETHYLENES Otto Scherer, Frankfurt am Main, Alfons Steinmetz, Kellsheim, Taunus, and Heinrich Kiihn, Walter Wetzel, and Karlheinz Grafen, Frankfurt am Main, Germany, assignors to Farbwerke Hoechst Aktiengesellschaft vormals Meister Lucius & Briining, Frankfurt am Main, Germany, a corporation of Germany Filed Mar. 10, 1961, Ser. No. 94,864 Claims priority, application Germany, Feb. 6, 1958,

16 Claims. oi. 260-6535) This application is a continuation-in-part of US. application Serial No. 790,937 filed February 3, 1959 (now abandoned) and relates to a process for the manufacture of fluorethylenes.

It is known to prepare tetrafiuorethylene by heating difluoromonochloromethane at a high temperature for a short time. For example, by passing difluorochloromethane at the rate of 180 grams per hour at 650 C. to 800 C. through a tube having a diameter of 8 mm. and a length of 700 mm., and which is open at both ends, tetrafiuorethylene is obtained at a conversion of 25 to 30 percent by weight calculated on the starting material used and in a yield of 90 percent by weight calculated on the starting material that undergoes reaction. The tube used in the aforesaid pyrolysis process is made of graphite, silver or platinum.

Experiments have been carried out in an attempt to increase the output (grams per hour) of tetrafluorethylone by increasing the dimension of the tube so as to increase the quantity of the gaseous starting material passed through the tube in unit time, and it has been found that increasing the diameter of the tube decreases the yield.

For example, when the gaseous starting material was passed through a tube having an internal diameter of 24 mm., and of which a length of 500 mm. was heated, at the rate of 865 grams per hour at a temperature of about 700 C., 28 percent by weight of the difluorochloromethane used underwent reaction and the yield of tetrafluorethylene was 84% calculated on the quantity of difluorochloromethane that underwent reaction. A further increase of the diameter of the tube caused a further decrease in the yield. For example, when the gaseous starting material was passed through a tube having an internal diameter of 50 mm, and of which a length of 1.0 meter was heated, at the rate of 6,900 to 7,200 grams per hour at a temperature Within the range of 750 C. to 800 C., 21 percent by weight of the difiuoromonochloromethane used underwent reaction and the yield of tetrafluorethylene was only 55 to 60 percent by weight calculated on the difluoromonochloromethane that underwent reaction.

The decrease in yield that occurs when the internal diameter of the tube exceeds a certain value, which is 7 within the range of about 20 to 30 mm., can be explained by the fact that, due to the altered conditions of flow, temperature distribution is not uniform within the reaction gas. The favorable conditions necessary for heating all the molecules of the reaction gas as uniformly as possible cannot be ensured in a tube of large diameter, because the surface of the tube available for heating be comes too small relative to the volume of the tube, which increases with the diameter, and therefore, also, relative to the quantity of gas to be reacted. The unsatisfactory conduction of heat is increased when the smooth tube is packed with fillers which divert the mixture in a direction perpendicular to the direction of flow, i.e. complicate dilfusion to the heated wall. For example, apparatus for the manufacture of chloride-containing fluor- 3,183,277 Patented May 11, 1955 ethylenes has been described which comprises a tube having an internal diameter of 18 mm., heated over a length of cm. The tube is packed, preferably with glass balls, and is provided with a central shaft into which thermo-elements are placed. It has been stated that 0.4 mol of substance can be passed per hour through that apparatus. A comparison of the last-mentioned apparatus with the above-mentioned smooth tube which has an internal diameter of 8 mm. and through which about 2 mols of substance can be passed per hour clearly shows that the yield which may be obtained with the tube having an internal diameter of 18 mm. is considerably decreased in spite of the larger diameter and that the apparatus is consequently completely unsuitable for use in processes in which larger quantities, for example, quantities that are a hundred times the above-mentioned quantity, are to be passed through per unit of time.

It has now been found that, in the pyrolysis of compounds of the formula in which R represents a hydrogen atom or a methyl group which may be halogenated and contains at least one hydrogen atom and of which the halogen advantageously has an atomic weight of at most 80, and preferably of at most 36, larger quantities of the starting materials undergo reaction in unit time with the splitting off of hydrogen halide having a higher molecular weight than hydrogen fluoride than is the case With reaction tubes of 8 mm. internal diameter, and equally good yields of fiuorethylenes, for example, tetrafiuorethylene, are obtained by carrying out the reaction in a reaction zone which has a slit-shaped cross-sectional area. The slitshaped cross-sectional area may be rectangular, elliptical or of another form; it may be partly or completely limited by straight lines, circles or other curved or bent lines and may in particular he recurrent in itself, that is to say endless, for example, have an annular form, all this provided that, according to the term slit, one dimension of the cross-sectional area is visibly greater than the other. The proportion of the length to the width of the slit shall at least be 2:1 and is advantageously within the range of 10:1 to 1000:1. Greater proportions, for example, proportions of 10,000:l or 100,00021, are still suitable but the preparation of such slits is more difficult. In the light of what has been said the endless slit may be defined geometrically (see the accompanying drawings) as follows: The cross-sectional area of the reaction chamber is bounded by two lines recurrent in themselves, which do not intersecteach other and the distance between which is advantageously not more than 18 mm. and not less than 1.5 mm. The radius of curvature of the boundary lines may be infinitely great in parts of the lines, and the boundary lines may partly be parallel. By cutting out sections of the endless slits of this type which are recurrent in themselves finite slits of any type may be made.

It is suitable to heat only one wall of such a slit--in endless slits, for example, the outer wall--and to cool the other wall, advantageously by the incoming gaseous starting material which outside the reaction zone flows along said wall and is there advantageously conducted in counter-current to the gaseous material flowing through the reaction zone. The cooling agent, in particular the said reaction gases, may be preheated to a certain temperature in an additional heating device.

The width of the slit is to be within the range of 1.5 to 18.0 mm. and advantageously Within the range of 1.5 to 6 mm. Curved slits may have any radius of curvature. It is, however, advantageous that in endless slits the smallest internal diameter is larger than 15 mm. and preferably greater than 20 mm., in particular greater than 25 mm. The smallest external diameter of endless slits is advantageously larger than 28 mm. and preferably greater than 32 mm., in particular greater than 37 mm.

According to the dimensions of the slit and the quantity of substance to be passed through in unit time the reaction chamber should have a length that is such that the time of sojourn in the heated reaction chamber is within the range of 0.01 to 2 seconds and preferably within the range of 0.1 to 0.8 second. The splitting off of hydrogen halide having a higher molecular weight than hydrogen fluoride can be brought about at temperatures within the range of 500 to 1100 C., advantageously 650 to 800 C. Various forms of apparatus suitable for carrying out the process of this invention are shown diagrammatically by way of example in the accompanying drawings:

In the apparatus shown in FIGURES 1 and 2 an inlet tube a is arranged concentrically within an enclosing tube b, which latter tube is surrounded by a jacket for heating the tube b. As shown in FIGURE 1, the gas used as starting material is passed into the tube a and is deflected at the inner end of this tube so as to flow in the opposite direction through the reaction zone d between the inner tube a and the outer tube b. Reaction zone a, which has a slit-shaped cross-sectional area, is bounded by the external surface of the tube a and the internal surface of the tube 17. The stream of gas flowing through the reaction zone is heated on the outside by the wall of the tube b and is cooled on the inside by the wall of the inlet tube a through which the gas is introduced, the tube b being heated by a heating element and the inner wall of the tube a being cooled by the incoming gas.

The cross-sectional area of the reaction zone d may have various forms, for example, it may be circular, elliptical or rectangular (see FIGURE 3), so that the reaction zone surrounds the inlet tube a like a ring, that is to say, the slit-shaped cross-sectional area of the zone is closed upon itself and endless. Alternatively, the reaction zone may be arranged only on one side of the gas inlet tube. This form of apparatus is shown in FIGURES 4 and 5, and is especially suitable for carrying out the process of the invention on a large industrial scale, because of the space.

As compared with carrying out the pyrolysis in a simple reaction tube, carrying it out in a reaction zone having a slit-shaped cross-sectional area has, for example, the advantage that a more uniform distribution of the temperature in the reaction zone is achieved because of the countercurrent method of cooling. Substantially higher area outputs are attainable than with simple reaction tubes. By area output is meant the quantity in grams of the desired gaseous reaction product, for example, tetrafluorethylene obtained from difluoromonochloromethaue, obtainable per square centimeter of available heating surface per hour.

The considerably higher area outputs of tetrafiuorethylene obtained by the pyrolysis of difluoromonochloromethane at temperatures within the range of 650 C. to 900 C., and preferably 700 C. to 850 C., is illustrated by the fact that about 0.09 gram of tetrafluorethylene is obtained per hour per square centimeter of surface of a known reaction tube having a diameter of 8 mm., whereas about 0.82 gram of tetrafluorethylene is obtained per hour per square centimeter of surface of reaction tube in the process of the invention. In the latter case the area output is therefore about 900 percent greater. Moreover, the rate of output of the desired product in the process of the invention can be increased to any desired extent by enlarging the dimensions of the apparatus without reducing the quantity of starting material that undergoes conversion or the yield obtained.

In constructing the apparatus used in the process of the invention care must be taken that the dimensions of the gas inlet zone and of the reaction zone, and especially the width of the slit-shaped reaction zone, are suited to the quantity of gas to be passed through in unit time and to the desired reaction temperature. For example, the conditions of heat transfer should be as favourable as possible, the mean residence time of the reaction gas in the reaction zone and the pressure drop should not be too great, and the preheating of the gas in the gas inlet tube should not be excessive. The width of the slit-shaped reaction zone is advantageously so chosen that the heat transfer coefficient, which determines the heat transfer from the heated wall of the reaction chamber to the interior thereof, is within the range of 70 to 120 kilogramcalories per square meter, per hour per degree centigrade, including the radiant heat. The quantity of gas passed through the apparatus in unit time or the velocity of flow in the reaction zone is so chosen that the temperature required for the pyrolysis is attained and that the residence time of the gases in the reaction zone is adequate, and that the difierence between the temperature of the heated Wall of the reaction chamber and the temperature of the reaction gas therein does not exceed 30 to 40 C. If the diameter of the gas inlet tube is relatively large a heatable displacement body may additionally be incorporated in the tube. In apparatus of large dimensions it is particularly suitable to use double slits. In this case two reaction zones having slit-shaped cross-sectional areas are arranged in such a way that their cooled surfaces are opposite each other and cooled by the same stream of gas. For example, two reaction zones having endless, slit-shaped cross-sectional areas may be arranged concentrically in such a manner that they are separated by a common inlet zone and that their surfaces which are opposite each other are cooled by the same stream of gas while the external surface of the outer reaction zone and the internal surface of the inner reaction zone are heated. In this manner it is also possible to combine more than two reaction zones of slit-shaped cross-sectional area. The process of the invention can be used not only for the manufacture of tetrafluorethylene from difiuoromonochloromethane, but also, for example, for the manufacture of 1,1-difluorethylone from l-chloro-l,1-difluorethane at a temperature within the range of 650 C. to 900 C., and preferably 700 C. to 850 C.; or for the manufacture of 1,1-difiuoro 2,2 dichlorethylene from 1,1-difluoro-1,2,2-trichlorethane at a temperature within the range of 200 C. to 550 C. or 600 C., and preferably of 450 C. to 580 C.; or for the manufacture of tetrafluorethylene from l,l,2,2-tetra-fiuorochloroethane at a temperature within the range of 600 C. to 1000 C., and preferably 700 C. to 800 C.; or for the manufacture of trifiuorochlorethylene from 1,1,Z-trifluoro-1,2-dichlorethane.

The manufacture of 1,1-difluorethylene from l-chloro- 1,1-difluorethane by the process of the invention can be carried out at a temperature within the range of 850 C. to 900 C., with a conversion of starting material of 98% and a yield of final product amounting to 98%, calculated on the quantity of l-chloro-1,1-difluorethane that undergoes conversion.

The extent of the improvement in carrying out the last mentioned reaction is apparent from the fact that the literature shows that the thermal splitting of 1-chloro-l,l-difluorethane as hitherto carried out is always accompanied by the splitting off of hydrogen fluoride, in addition to hydrogen chloride, with the formation of large quantities of l-chloro-l-fiuorethylene as by-product (see US. Patents Nos. 2,627,529; 2,551,573; 2,628,989 and 2,774,799). The statements in the literature show that the conversions of starting material were within the range of 20 to by weight, and the yields of 1,1-difluorethylene were at best 64% by weight. This was due to the formation of l-chloro-l-fiuorethylene as an undesired by-product (see US. Patent No. 2,774,799). It is known from the literature that in the latter reaction the formation of l-chlorol-fiuorethylene as an undesired by-product can be avoided by using a catalyst, for example, a copper catalyst in the form of the metal or as a copper salt or as a mixture of a copper salt with another metal salt. When the reaction is carried out in the presence of such a catalyst the yield of 1,1-difiuorethylene at a reaction temperature within the range of 650 C. to 700 C. amounts, according to the statements made in the literature, to 96% by weight, while 94% by weight of the starting material undergoes conversion. In this case the area output was 0.17 gram of the desired final product per hour per square centimeter of the surface of the reaction chamber. Accordingly, as compared with this, the area output of the aforesaid compound is 750% greater when made by the process of the invention under the conditions described above. Accordingly, in addition to the higher area output, the process of the invention has the further advantage that, in the manufacture of 1,1-difiuorethylene from 1-chloro-1,1-difiuorethane, the excellent yields of 97 to 98% by weight can also be obtained in the absence of a catalyst and without the undesired splitting oil of hydrogen fluoride.

An important advantage of the process of the invention is that very good conversions of starting material and yields are obtained without the use of a catalyst. However, the process of the invention may be carried out in the presence of a catalytically active non-metal, metal, metal alloy or compound, for example, carbon, copper, a copper-nickel alloy, platinum, platinum-iridium, platinum-rhodium or single or mixed sintered metal oxides, such as aluminum oxide, beryllium oxide or magnesium oxide or spinels. The catalyst may be disposed in the reaction zone in various ways. For example, the walls of the reaction zone may consist of the catalytic material or of silver or they may be lined therewith. Metals or metal .alloys may be present in the reaction zone, for example, in the form of wire or wire netting. Particularly good yields of fluor-ethylenes can be obtained by carrying out the reaction in the presence of platinum or a platinum alloy.

The following examples serve to illustrate the invention but they are not intended to limit it thereto.

Example 1 3,500 grams (40.4 mols) of difluoromonochloromethane were passed in the course of 30 minutes at a temperature of 760 C. and a rate of about 7.0 kilograms per hour through a reaction apparatus analogous to that shown in FIGURES 1 and 2. The gas inlet tube a which was made of stainless steel and lined with platinum had a length of about 940 mm. and a diameter of 44 mm. The outer tube b had a diameter of 50 mm., so that the slit-shaped annular reaction zone had a width of 3 mm. The mean residence time of the gas in the reaction zone was 0.2 second. The products leaving the reaction apparatus were cooled in a cooling apparatus, after the hydrogen chloride that had been split off had been washed out with water, and were condensed in a cooling trap.

Distillation of the condensate gave 547 grams (5.45 mols) of tetrafluorethylene, 2.491 grams (28.8 mols) of difiuoromonochloromethane and 40 grams of a fraction having a higher boiling point.

409 grams (11.2 mols) of hydrogen chloride were found by titration. Accordingly 28.8% of the difluoromonochloromethane used as starting material had undergone conversion and the yield of tetrafiuorethylene was 94% calculated on the difluoromonochloromethane that had undergone conversion.

If desired, the gas inlet tube may have a different external diameter within the range of, for example, 14 to 47 mm.

Example 2 1.997 grams (19.7 mols) of 1-chloro-1,1-difluorethane were passed in the course of 38 minutes at 850 C. and a rate of 3.10 kilograms per hour through the reaction apparatus described in Example 1 (see FIG- URES 1 and 2 of the accompanying drawings), but it was lined with platinum or a platinum alloy. The mean residence time of the reaction gas in the reaction zone was 0.6 second. On distilling the condensate 1.189 grams (18.6 mols) of 1,1-difluorethylene and 59 grams (0.59

6 mol) of unreacted 1-chloro-1,1-difluorethane were obtained. The washing water contained 699 grams (19.15 mols) of hydrogen chloride determined by titration. Accordingly, 97 percent of the 1-chloro-1,1-difluorethane used as starting material had undergone conversion and the yield of 1,1-difluorethylene was 97.5 percent calculated on the l-chloro-l,l-difluorethane that underwent conversion.

Example 3 288 grams (1.7 mols) of 1,1-difluoro-1,2,2-trich1orethane were passed at 550 C. in the course of 1 hour through the reaction apparatus described in Example 1 (see FIGURES 1 and 2 of the accompanying drawings). From the quantity of hydrogen chloride that had been split off, which was determined by titration, it was found that 92 percent of the starting material had undergone conversion. The yield was percent calculated on the 1,1-difluoro-1,2,2-trichlorethane that underwent reaction.

We claim:

1. In the process for preparing fluorethylenes by pyrolytic dehydrochlorination of a compound of the formula RCF Cl, in which R is a member selected from the group consisting of hydrogen, methyl, and methyl substituted by from one to two halogen atoms having an atomic Weight of at most 36, the improvement which comprises pyrolyzing said compound by passing it in the gaseous state through a reaction zone having a slit-shaped crosssection defined by a single self-recurrent line, the lengthto-width ratio of said slit-shaped cross-section being at least 2: 1, and the width of said section being between 1.5 mm. and 18 mm.

2. A process as in claim 1 wherein said reaction zone is bounded by at least two opposing walls, one of which is heated and the other of which is cooled.

3. The process as in claim 2 wherein said cooled wall is cooled by a stream of said compound to be pyrolyzed before said stream enters said reaction zone.

4. The process as in claim 2 wherein the coeflicient of heat transfer at said heated wall is from 70 to kilocalories per square meter per hour per degree centigrade.

5. The process as in claim 1 wherein said compound is pyrolyzed in the presence of a catalyst selected from the group consisting of platinum and platinum alloys.

6. The process as in claim 5 wherein said catalyst is present at the walls of said reaction zone.

7. A process as in claim 1 wherein difiuoromonochloromethane is pyrolyzed at a temperature of from 650 C. to 900 C. to produce tetrafluorethylene.

8. The process as in claim 1 wherein 1,1-difluoro-1- chloroethane is pyrolyzed at a temperature of from 650 to 900 C. to form 1,1-difluoroethylene.

9. In the process for preparing fluorethylenes by pyrolytic dehydrochlorination of a compound of the formula RCF Cl, in which R is a member selected from the group consisting of hydrogen, methyl, and methyl substituted by from one to two halogen atoms having an atomic Weight of at most 36, the improvement which comprises pyrolyzing said compound by passing it in the gaseous state through a reaction zone having a cross-section in the form of an endless slit defined by a first self-recurrent line separated by at least 15 mm. from a central point enclosed by said first line, and by a second self-recurrent line surrounding said first line and said central point, said second line being separated by at least 28 mm. from said central point and by from 1.5 mm. to 18 mm. from said first line.

10. The process as in claim 9 wherein said reaction zone is bounded by at least two opposing walls, one of which is heated and the other of which is cooled.

11. The process as in claim 10 wherein said cooled wall is cooled by a stream of said compound to be pyrolyzed before said stream enters said reaction zone.

12. The process as in claim 10 wherein the coefficient of heat transfer at said heated wall is from 70 to 120 7 kilocalories per square meter per hour per degree centigrade.

13. The process as in claim 9 wherein said compound is pyrolyzed in the presence of a catalyst selected from the group consisting of platinum and platinum alloys.

14. The process as in claim 13 wherein said catalyst is present at the walls of said reaction zone.

15. A process as in claim 9 wherein difluoromonoehloromethane is pyrolyzed at a temperature of from 650 C. to 900 C. to produce tetrafluorethylene.

16. The process as in claim 9 wherein 1,1-difluoro-lchloroethane is pyrolyzed at a temperature of from 650 C. to 900 C. to form 1,1-difiuoroethylene.

8 References Cited by the Examiner UNITED STATES PATENTS 2,551,573 5/51 Downing et al. 260-653.5

2,566,807 9/51 Padburg et al. 260-6 53.5

2,763,532 9/56 McKinnis 23-277 FOREIGN PATENTS 1,216,649 4/ 60 France.

10 LEON ZITVER, Primary Examiner.

ALPHONSO D. SULLIVAN, Examiner.

Claims (1)

1. IN THE PROCESS FOR PREPARING FLUORETHYLENES BY PYROLYTIC DEHYDROCHLORINATION OF A COMPOUND OF THE FORMULA RCF2CL, IN WHICH R IS A MEMBER SELECTED FROM THE GROUP CONSISTING OF HYDROGEN, METHYL, AND MEHYL SUBSTITUTED BY FROM ONE TO TWO HALOGEN ATOMS HAVING AN ATOMIC WEIGHT OF AT MOST 36, THE IMPROVEMENT WHICH COMPRISES PYROLYZING SAID COMOUND BY PASSING IT IN THE GASEOUS STATE THROUGH A REACTION ZONE HAVING A SLIT-SHAPED CROSSSECTION DEFINED BY A SINGLE SELF-RECURRENT LINE, THE LENGTHTO-WIDTH RATIO OF SAID SLIT-SHAPED CROSS-SECTION BEING AT LEAST 2:1, AND THE WIDTH OF SAID SECTION BEING BETWEEN 1.5 MM. AND 18 MM.

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