JPS627175B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for recovering a tetrafluoroethylene component, and more particularly, to recovering a tetrafluoroethylene component from a gas mixture discharged from the top of a distillation column when distilling a chlorodifluoromethane thermal decomposition product to obtain tetrafluoroethylene. This invention relates to a method for recovering fluorocyclobutane. Tetrafluoroethylene (hereinafter abbreviated as TFE) is generally produced by thermally decomposing chlorodifluoromethane and rectifying the reaction product. In this rectification process, carbon monoxide, carbon dioxide, trifluoromethane, difluoromethane, etc., which are lower boiling point substances than TFE in the reaction product, are removed, but several percent of the TFE produced is distilled. Because it is discharged from the top of the tower to the outside of the system, the production cost of TFE increases, and TFE using TFE as a raw material increases.
This has led to increases in manufacturing costs for polymers, TFE/HFP (hexafluoropropylene) copolymers, etc.
For this reason, various methods have been proposed to separate and recover the TFE component in the gas mixture discharged from the top of the tower. Typically, a liquid perhalogenated hydrocarbon is used to selectively absorb TFE in a gas mixture, and then the TFE is diffused from the perhalogenated hydrocarbon solution under lower pressure. There is a method to separate and recover the debris (see Japanese Patent Publication No. 10444/1983), but since TFE is absorbed once and then radiated again, a considerable amount of energy is required, making it difficult to achieve the desired goal of reducing TFE manufacturing costs. cannot be achieved. Furthermore, the recovery rate of TFE is not necessarily satisfactory, and furthermore, there is a drawback that separation from carbon dioxide, difluoromethane, etc. is not sufficient. In view of the above-mentioned shortcomings of the conventional technology, the present inventors conducted extensive research and found that the gas mixture discharged from the top of the tower
Passing through a heating zone between 450 and 650°C, the TFE in the gas mixture is converted into octafluorocyclobutane (hereinafter abbreviated as Freon C-318), a useful compound in itself, which has a higher boiling point than TFE. The present inventors have discovered that TFE can be easily separated from other components having a boiling point lower than that of TFE by utilizing the boiling point difference therebetween, and have completed the present invention. The gist of the present invention is to pass a TFE-containing gas mixture discharged from the top of a distillation column through a heating zone at 450 to 650°C when distilling a thermal decomposition product of chlorodifluoromethane to obtain TFE. , a method for recovering a TFE component, comprising converting TFE contained in the gas mixture into octafluorocyclobutane, and then separating octafluorocyclobutane from the reaction mixture. According to the method of the present invention, from TFE to Freon C-318
Not only can this contribute to reducing TFE manufacturing costs, but the C-318 that is separated has a boiling point that is sufficiently higher than other components in the reaction mixture. Because of this, it is easier to separate than TFE. When carrying out the method of the present invention, the gas mixture discharged from the top of the column is heated to a pressure of 0 to 30 kg/cm ² G, especially 1
~25Kg/ ^cm2G , temperature: 450~650℃, especially 500~600
It is preferable to pass through the heating zone under the conditions of ℃, space velocity: 10 to 5000 hr ^-1 (NTP). The pressure of the gas mixture passing through the heating zone is 0 Kg/cm ² G
At lower pressures, the yield of Freon C-318 is poor;
Not economical. On the other hand, a pressure higher than 30 Kg/cm ² G is higher than the pressure discharged from the top of the column, and it becomes necessary to pressurize the gas mixture again. In addition, equipment costs are high because the conversion reaction is a high-pressure reaction.
There are disadvantages such as the reaction becomes unstable, the production rate of toxic perfluoroisobutene increases, the efficiency decreases, and the cost required for perfluoroisobutene removal and detoxification increases. . Furthermore, if the temperature of the gas mixture is below 450°C, the production rate of Freon C-318 will be low, which is economically disadvantageous. On the other hand, a temperature of 650°C or higher is not preferable because more perfluoroisobutene is produced. Furthermore, it goes without saying that the space velocity, or flow rate, of the gas mixture needs to be appropriate for the above pressure and temperature; however, as the flow rate increases, Freon C-318
The conversion rate to decreases. On the other hand, when the flow rate is low, there is a tendency for more perfluoroisobutene to be produced. Generally, the space velocity can be appropriately selected within the range of 10 to 5000 hr ^-1 . The heating zone consists of a pyrolysis device consisting of a reaction tube made of a material that does not corrode under the above reaction conditions and does not inhibit the conversion reaction, such as quartz, nickel, or stainless steel, in the heating furnace. can do. This reaction can be carried out without using any particular catalyst. Freon C-318 in the reaction mixture where TFE in the gas mixture was converted to Freon C-318 by the above method.
318, the reaction mixture is heated below the boiling point of Freon C-318,
By cooling to a temperature above the boiling point of TFE,
Preferentially condenses and can be easily separated. The recovered Freon C-318 is known to be useful as an aerosol propellant (particularly useful as a food propellant), a gas for refrigerator thermostats, and a raw material for HFP production. Next, the method of the present invention will be explained using the drawings. The attached drawing shows a flow sheet showing one embodiment of the method of the present invention, in which raw material gas (chlorodifluoromethane) is thermally decomposed in a thermal decomposition device 1, and the resulting gas is deoxidized and dried in a cleaning tower 2. In distillation column 3, components with boiling points lower than TFE in the produced gas are discharged from the top of the column as a gas mixture, while components from the bottom of the column are discharged as a gas mixture.
Collect and separate TFE. The gas mixture discharged from the top of the column contains a large proportion of TFE. This gas mixture is sent to a pyrolysis device 5 installed in the heating furnace via a pressure control valve 4, and after converting TFE in the gas mixture into fluorocarbon C-318, it is passed through a cooling separation device 6 to fluorocarbon C-318. −318 is condensed and separated,
It is stored in the storage tank 7. Examples 1 to 6 The following composition is discharged from the top of the distillation column 3 in Figure 1: Carbon monoxide; 45.2% (by volume); carbon dioxide; 4.5
%, CHF ₃ ; 4.3%, CH ₂ F ₂ ; 0.8%, C ₂ F ₄ ; 41.1
%, unknown component; 4.1% gas mixture was passed through the pyrolysis apparatus shown in Fig. 1 (however, the pyrolysis tube was made of stainless steel with an inner diameter of 10 mm) under the conditions shown in Table 1, and a gas reaction with the following composition was carried out. A mixture was obtained. Table 1 shows the yield of Freon C-318 in the reaction mixture.

[Table] The results in Table 1 above indicate that in the temperature range of 450 to 650°C, most of the TFE in the raw material gas mixture can be converted to C-318 without being greatly affected by the reaction pressure or space velocity. I know what I can do.
Of course, if the TFE concentration in the reaction mixture is too high, it can also be recycled to the heating zone. In addition, Freon C- in the reaction mixture after the above reaction
Most of 318 could be recovered by passing the reaction mixture through cooling separator 6.

[Brief explanation of the drawing]

FIG. 1 shows a flow sheet illustrating one embodiment of the method of the present invention. 1, 5...Pyrolysis device, 2...Washing tower, 3...
Distillation column, 4...pressure control valve.

Claims (1)

[Claims] 1. A gas mixture containing tetrafluoroethylene discharged from the top of a distillation column when distilling a thermal decomposition product of chlorodifluoromethane to obtain tetrafluoroethylene is heated to a temperature of 450 to 650°C. A method for recovering a tetrafluoroethylene component, which comprises passing the gas mixture through a heating zone to convert the tetrafluoroethylene contained in the gas mixture into octafluorocyclobutane, and then separating octafluorocyclobutane from the reaction mixture. 2 The pressure of the gas mixture passing through the heating zone is reduced to 0.
30 Kg/cm ² G. The method according to item 1 above. 3. The method according to item 1, wherein the gas mixture passing through the heating zone has a space velocity of 10 to 5000 hr ^-1 . 4. The method according to item 1, wherein octafluorocyclobutane is separated from the reaction mixture by cooling the reaction mixture.