DE1249853B - Process for the production of tetrafluoroethylene - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY

GERMAN

PATENT OFFICE

EDITORIAL

Int. Cl .:

C 07 c

German class: 12 ο - 19/02

Number: 1249 853

File number: O 9448 IV b / 12 ο

Filing date: May 29, 1963

Opened on September 14, 1967

In the known method disclosed in Japanese Patent 276,088 and the German Auslegeschrift 1,073,475 a gaseous mixture of chlorodifluoromethane, and water vapor is pyrolyzed at temperatures of 650-1400 ⁰ C, with a conversion of 60 to 70 mole percent of a 90 to 94molprozentigen yield of Tetrafluoroethylene can be achieved. However, it is stated that the proportion of water vapor in the starting mixture cannot be increased at will at a given temperature and contact time without a decrease in the yield of tetrafluoroethylene and an increased elimination of hydrogen fluoride occurring, so that if the amount of water vapor is over 70 mol percent, both a Decrease in the conversion of chlorodifluoromethane as well as the selective yield of tetrafluoroethylene is recorded.

The inventive method for the preparation of tetrafluoroethylene by pyrolysis of chlorodifluoromethane in the presence of steam is characterized in that chlorodifluoromethane in the presence of 70 to 90 mol percent, preferably 75 to 85 mol percent of water vapor under essentially adiabatic conditions in a reaction zone with a ratio of surface area to Volume of less than 130 m " ¹ at 700 to 85O ⁰ C, preferably 750 to 800 ° C, and a residence time of 0.002 to 0.5 seconds, preferably 0.005 to 0.2 seconds, the pyrolysis temperature by simultaneous mixing and Introducing the preheated chlorodifluoromethane and superheated steam is generated into the reaction zone.

It is already known to produce tetrafluoroethylene by pyrolysis of monochloridifluoromethane in the presence of increased amounts of water vapor, namely from 3 to 19 mol of water vapor per mole of monochlorodifluoromethane, but, as shown below, an optimum of conversion and yield in the pyrolysis of chlorodifluoromethane and a Reduction in the formation of hydrogen fluoride, which is extremely undesirable because of its highly corrosive effect, is achieved when the conversion of the chlorodifluoromethane is carried out in the presence of the stated amounts of water vapor in a reaction zone whose surface-to-volume ratio is less than 130 m ~ ^x .

That is to say, of essential importance for suppressing the hydrolytic elimination of hydrogen fluoride and the formation of high-boiling substances, due to the improved yields of tetrafluoroethylene high purity can be achieved, so carrying out the pyrolysis in a suitable Process for the production of tetrafluoroethylene

Applicant:

Daikin Kogyo Co., Ltd., Osaka (Japan)

Representative:

Dipl.-Ing. R. Beetz and Dipl.-Ing. K. Lamprecht,
Patent Attorneys, Munich 22, Steinsdorfstr. 10

Named as inventor:

Haruo Shingu,

Makoto Hisazumi, Kyoto (Japan)

Claimed priority:

Great Britain 9 April 1963 (14 182)

Reaction vessel with a surface area to volume ratio of less than 130 m ~ ^x . The elimination of hydrogen fluoride during pyrolysis becomes almost negligible, and the conversion of chlorodifluoromethane therefore increases without a decrease in the selective yield of tetrafluoroethylene. Such a ratio of surface to volume of the reaction zone is also advantageous for the construction of the reaction vessels, since suitable reaction vessels can be produced with a larger volume and less effort than usual.

The necessary for the pyrolysis temperature is inventively achieved rapidly and uniformly in the entire gaseous mixture, wherein the water vapor to a temperature of 750-1200 ° C, preferably between 800 and 1100 ⁰ C, and the chlorodifluoromethane at a temperature between 400 and 550 ° C , preferably between 430 and 500 ⁰ C, is to be preheated separately.

The advantageous effect of higher proportions of water vapor in the pyrolysis gas mixture is demonstrated by the following results (Table 1), which were obtained from a series of experiments in which a quartz glass flask with a surface area to volume ratio of 130 m " ^{1 was used} as the reaction vessel under the following conditions was used: space velocity of chlorodifluoromethane 4500

709 647/571

(V / V / h at NTP) pyrolysis temperature 800 ^° C., the conversion of the chlorodifluoromethane taking place up to about 75 mol percent.

Table 1

were examined for the ratio of surface area to volume of the reaction zone.

Table 2

water
amount of steam
[Mol
percent] Selective
yield
to Tetra
fluoroethylene
[Mole percent] Telomeres
containing
high-boiling
Fabrics
[Mole percent] Spin-off
of fluorine
hydrogen*
[Mole percent] 50
70
75
85 79
87
93
95 18.6
11.1
5.3
3.3 2.4
1.9
1.7
1.7

relationship Spin-off Selective from surface of fluorine Yield to to volume hydrogen Tetrafluoroethylene [m- ¹ ] [Mole percent] [Mole percent] 700 9.0 65 300 3.6 78 200 1.5 84 130 0.7 91 100 0.5 94 80 0.3 97

Spin-off
of hydrogen fluoride
(Mole percent) =

Total moles of cleavage

of hydrogen fluoride

Total moles converted

Chlorodifluoromethane ■ 2

From Table 1 it can be seen that the pyrolysis in the process according to the invention because of the presence of water vapor in amounts over 70 mol percent more favorable at higher reaction temperatures can be carried out than in the previously known methods.

It is not advisable because of the decomposition of chlorodifluoromethane in the preheating stage, to raise the temperature above 550 ° C, and a temperature below 400 ⁰ C is unsuitable because the necessary superheating temperature then the steam for pyrolysis under adiabatic conditions too high.

In order to avoid overheating of the steam to excessively high temperatures, it is advisable to to supplement the heat required for pyrolysis by external heating of the reaction zone. That The extent to which the additional external heating occurs will vary with the reaction conditions and also with the properties of the reaction vessel. However, the extra warmth usually poses represents only a small proportion of the total heat input necessary for the production of the inventive Pyrolysis required temperature of the reaction zone is necessary.

When carrying out the process according to the invention under the conditions given above the pyrolysed gas mixture, which is the reaction vessel, quickly in a cooler or water scrubber cooled, washed and separated from condensable product, dried and fractionally distilled. There is a conversion of chlorodifmomethane of about 75 to 85 mole percent and one selective yield of tetrafluoroethylene of about 90 to 95 mol percent with little elimination of Hydrogen fluoride.

In general, it is observed that the elimination of hydrogen fluoride increases and the selective Tetrafluoroethylene yield decreases as the surface area to volume ratio of the reaction zone increases.

In the following Table 2 are represented by the pyrolysis of chlorodifluoromethane in the presence of 85 mole percent steam at a pyrolysis temperature of 75O ⁰ C and a ChlordifiuormethanumWandlung of about 70 mole percent results obtained, said reaction vessels made of quartz with various diameters comparatively with respect Due to the improvement, which is achieved by the present invention, in construction practice commercially available RcuklionsgcTüßc (Gc-

ao vessel types z. B. a multi-tube reactor, a single drum reactor and a drum reactor with tubular! Heat exchanger) can be used. Quartz can be used as the material for the construction of the reaction vessels as well Ordinary refractories are used in place of the commonly used precious metals.

example 1

To a temperature of 1150 ⁰ C superheated steam and chlorodifluoromethane, which was preheated to a temperature of 500 ⁰ C, were mixed at the same time and in a molar ratio of 80: 20 in a tubular reactor made of quartz having a ratio of surface area to volume of 130 irr ^"1 fed in. the mixture was allowed in the reaction zone, which was maintained by external heating with hot air of 840 ⁰ C at approximately adiabatic conditions. the temperature of the gaseous mixture in the reaction zone was thus kept constant at an average value of 800 ⁰ C. The gaseous mixture in the reaction zone was removed therefrom through a very narrow quartz tube and passed into a water scrubber, where the gaseous mixture was rapidly cooled and the water vapor and hydrogen fluoride formed were condensed and separated from the effluent gas of chlorodifluoromethane of about 4500 (V / V / h at NTP) a conversion of the chlorodifluoromethane of 75 mole percent was achieved, and a selective yield of tetrafluoroethylene of 94 mole percent was detected by gas chromatographic analysis. The hydrogen fluoride formed was determined by titration in the water condensate and was 1.8 mol percent, based on half the moles of chlorodifluoromethane converted.

Example 2

Chlorodifluoromethane was pyrolyzed at a temperature of 750 ° C in the reaction zone in the same manner as described in Example 1, except that the reaction zone, which had a platinum-lined reactor of the single drum type, had a surface area to volume ratio of 80 m- ¹ showed. The steam, which was superheated to a temperature of 1050 ° C and the chlorodifluoromethane heated to 500 ° C, were in a molar ratio of 85:15 with a space velocity of the chlorine

difluoromethane of 2600 (V / V / h at N.T.P.) mixed. In this case the conversion of chlorodifluoromethane was about 70 mole percent, the selective yield of tetrafluoroethylene 98 mole percent, and the hydrogen fluoride formed was 0.25 mol% (by analysis) of that obtained in Example 1.

Claims (2)

Patent claims: 1. A process for the preparation of tetrafluoroethylene by pyrolysis of chlorodifluoromethane in the presence of steam, characterized in that chlorodifluoromethane in the presence of 70 to 90 mol percent, preferably 75 to 85 mol percent of water vapor under essentially adiabatic conditions in a reaction zone with a ratio of surface area to Volume of less than 130 ην ¹ at 700 to 850 ⁰ C, preferably 750 to 800 ⁰ C, and a residence time of 0.002 to 0.5 seconds, preferably 0.005 to 0.2 seconds, the pyrolysis temperature by simultaneous mixing and introduction the preheated chlorodifluoromethane and superheated steam is generated in the reaction zone. 2. The method according to claim 1, characterized in that the water vapor to a temperature between 750 and 1200 ° C, preferably between 800 and 1100 ° C, and the chlorodifluoromethane to a temperature between 400 and 550 ° C, preferably between 430 and 500 ⁰ C, is preheated. Considered publications:
German interpretative document No. 1 073 475. 709 647/571 9. 67 © Bundesdruckerei Berlin