Field of the Invention
The present invention provides a process for the preparation of tetrafluoro ethylene.
Background of the invention
The US Patent No. 4,849,554 describes a process for the preparation of tetrafluoroethylene and hexafluoropropylene by pyrolysis of chlorodifluoromethane using gaseous diluents.
The US Patent No. 6,924,402 describes a process for the simultaneous preparation of tetrafluoroethylene and hexafluoropropylene by the pyrolysis of chlorodifluoromethane mixed in the molar ratio of super-heated steam/pre-heated chlorodifluoromethane.
The present inventors observed that the prior art results in lower selectivity of tetrafluoro ethylene (TFE), that is, less than almost 25% conversion of reactants to TFE and more amount of unwanted products such as chlorotrifluoroethylene, chlorohexafluoropropane and heavies, thereby reducing yield of process.
The present inventors observed that upon supplying steam as a diluent, in the range of 20 mol% to 60 mol% with respect to mol% of chlorodifluoromethane, to the feed of chlorodifluoromethane, high selectivity of TFE is achieved.

Summary of the invention
The present invention provides a process for the preparation of tetrafluoro ethylene comprising;
a) supplying chlorodifluoromethane to a reactor and preheating,
b) supplying steam to step a) in the range of 20 mol % to 60 mol % with respect to mol % of chlorodifluoromethane, to obtain a first reaction mixture,
c) providing heat to the first reaction mixture to obtain a second reaction mixture,
d) removing hydrogen chloride and steam from the second reaction mixture to obtain a third reaction mixture, and
e) subjecting the third reaction mixture to compression and distillation to obtain tetrafluoro ethylene.
Brief description of drawings
Figure 1 shows arrangement of furnaces which are employed for pyrolysis experiment of the present invention.
Detailed description of the invention
The present invention provides a process for the preparation of tetrafluoro ethylene comprising;
a) supplying chlorodifluoromethane to a reactor and preheating,
b) supplying steam to step a) in the range of 20 mol % to 60 mol % with respect to mol % of chlorodifluoromethane, to obtain a first reaction mixture,
c) providing heat to the first reaction mixture to obtain a second reaction mixture,
d) removing hydrogen chloride and steam from the second reaction mixture to obtain a third reaction mixture, and
e) subjecting the third reaction mixture to compression and distillation to obtain tetrafluoro ethylene.
The process of the present invention may be continuous system process or a batch system process. In the continuous process, the supply of chlorodifluoromethane to the reactor and the withdrawal of the reaction mixture containing TFE from the reactor are continuously conducted, respectively.
From the viewpoint of the production efficiency, the process of the present invention is preferably a continuous system process.
The diluent, for example, steam may be added to the mixture in 20 mol% to 60 mol% with respect to mol % of chlorodifluoromethane.
In the process of the present invention, the reaction mixture withdrawn from the outlet of the reactor contains unreacted raw material components, the reaction product, by products, etc. Thus, by product such as hydrogen chloride is removed and the desired product TFE is separated from outlet. The rest of the outlet materials such as unreacted raw material along with fresh chlorodifluoromethane are supplied to the reactor.
The temperature of diluent and chlorodifluoromethane to be supplied to the reactor are preferably made to be from 600-850°C.
While the present invention has been described in terms of its specific embodiments, certain modifications and equivalents will be apparent to those skilled in the art and are intended to be included within the scope of the present invention.

The following examples are given by way of illustration and therefore should not be construed to limit the scope of the present invention.
Examples
Example 1
In the pyrolysis experiment three furnaces were employed which consists of two high Ni-Cr tube and one SS tube joined in “T” shape as shown in figure 1. From one end (inlet-1) 60 mol% steam was passed and entered into SS tube which was heated at temperature 800oC (using furnace-1) and from second end (inlet-2) feed gas R22 (chlorodifluoromethane) with a flow rate at 60 litre per hour were passed through a high Ni-Cr tube which was pre-heated to 600oC(using furnace-2). These both steams were combined at T joint and then entered into reaction furnace-3 at temperature 720oC in the Ni-Cr tube. These outlet gases were passed through the Ni-Cr tube immersed in water bath. These product gases were passed through caustic trap (300 ml 20% caustic) followed by concentrated sulphuric acid trap (100 ml sulphuric acid). After the sulphuric acid trap, the sample of the gases were collected in gas sampling tube and checked in GC analysis.
Feed composition:
R22 (chlorodifluoromethane) = 100%
Results are as follows:
Temperature (°C) 720
RT (sec) 1.34
TFE (%) 30.09
Hexafluoropropylene (%) 0.7
chlorotrifluoroethylene (%) 0.33
Chlorohexafluoropropane (%) 0.64
Heavies (%) 0.63
Perfloroisobutene(%) 0
Chlorodifluoromethane(%) 63
Octafluorocyclobutane(%) 1.34
Chlorotrifluoroethane(%) 0.79

Conversion: 37%
Yield: 95%
Example -2
In continuation to example -1 both R22 and steam were combined at T joint and then entered into reaction furnace-3 at temperature 760°C in the Ni-Cr tube.
Feed composition:
R22 = 100%
Results are as follows:
Temperature (°C) 760
RT (sec) 1.22
TFE (%) 43
Hexafluoropropylene (%) 3.4
chlorotrifluoroethylene (%) 1.4
Chlorohexafluoropropane (%) 1.88
Heavies (%) 2.79
Perfloroisobutene(%) 0
Chlorodifluoromethane(%) 31
Octafluorocyclobutane(%) 5.5
Chlorotrifluoroethane(%) 6.5

Conversion: 69%
Yield: 90%

CLAIMS:
We claim:
1. A process for the preparation of tetrafluoro ethylene comprising:
a) supplying chlorodifluoromethane to a reactor and preheating,
b) supplying steam to step a) in the range of 20 mol % to 60 mol % with respect to mol % of chlorodifluoromethane, to obtain a first reaction mixture,
c) providing heat to the first reaction mixture to obtain a second reaction mixture,
d) removing hydrogen chloride and steam from the second reaction mixture to obtain a third reaction mixture, and
e) subjecting the third reaction mixture to compression and distillation to obtain tetrafluoro ethylene.
2. The process as claimed in claim 1, wherein the process is continuous system process or a batch system process.
3. The process as claimed in claim 2, wherein in the continuous system process, the supply of chlorodifluoromethane to the reactor and the withdrawal of the reaction mixture containing tetrafluoro ethylene from the reactor are continuously conducted.
4. The process as claimed in claim 2, wherein the process is a continuous system process.
5. The process as claimed in claim 4, wherein chlorodifluoromethane which is unreacted is supplied back to the reactor.
6. The process as claimed in claim 1, wherein steam and chlorodifluoromethane before supplying to the reactor is maintained at a temperature in the range of 600-850°C.