FIELD OF THE INVENTION

The present invention relates to a process for purifying olefin feed.

BACKGROUND OF THE INVENTION

Refrigerant is the substance which is used as working fluid in a thermodynamic cycle, undergoes a phase change from liquid to vapour and produces cooling. They absorb heat from one area, such as an air conditioned space, and reject it into another, such as outdoors, usually through evaporation and condensation, respectively. These phase changes occur both in absorption and mechanical vapour compression refrigeration systems, but they do not occur in systems operating on a gas cycle using a fluid such as air.
Tetrafluoropropenes, having zero ozone depletion and low global warming potential, have been identified as favorable refrigerant over other refrigerants. One tetrafluoropropene having valuable properties is 2,3,3,3-tetrafluoropropene (R-1234yf).
Production of tetrafluoropropene compounds often results in the formation of other organofluorine compounds, organochlorines, and chlorofluorocarbons (collectively referred to herein as ‘coproducts of organofluorine production’ or simply ‘coproducts’), as both intermediate products and coproducts that appear in the final reaction mixture.
In prior art several methods have been disclosed for preparing R-1234yf, wherein by-products have been removed by using several techniques such as distillation, azeotropic distillation, adsorption, absorption, membrane separation and a like or combination thereof to achieve high yield.
U.S. Pat. No. 2,931,840 describes a reaction involving the pyrolysis of chloromethane with tetrafluoroethylene, or alternatively chloromethane with chlorodifluoromethane. It is very difficult to remove unreacted chloromethane and other side products from the reaction product stream. It does not mention the process for removing unreacted chloromethane and the side products from the reaction mixture.
The pyrolysis reaction results in the formation of carbon monoxide which impedes pyrolysis resulting in reduction of efficacy of reaction and hence yield of final 1234yf. The inventors of present patent application realized this deficiency and have evolved an efficient process for purification and preparation of 1234yf.

SUMMARY OF THE INVENTION
A first aspect of the present invention provides a process for preparing 1234yf, the process comprising the steps of:
a) combining tetrafluoroethylene and chloromethane at a temperature to establish pyrolysis reaction to obtain a mixture 1;
b) distilling the mixture 1 to vent out carbon monoxide to obtain mixture 2;
c) distilling mixture 2 to vent out mixture 3 and obtain mixture 4;
d) recycling the mixture 3;
e) distilling mixture 4 to obtain 1234yf.

BRIEF DESCRIPTION OF DRAWINGS:
Figure 1 illustrates the process for preparation of 1234yf.

DETAILED DESCRIPTION
As used herein, the “mixture 1” comprises 1234yf, unreacted tetrafluoroethylene, chloromethane, chlorodifluoromethane and carbon monoxide.
As used herein, the “mixture 2” comprises 1234yf, unreacted tetrafluoroethylene, chloromethane, and chlorodifluoromethane.
As used herein, the “mixture 3” comprises unreacted tetrafluoroethylene, chloromethane, and chlorodifluoromethane.
As used herein, the “mixture 4” comprises 1234yf.
In an embodiment of the present invention, the reaction of tetrafluoroethylene and methylchloride, is carried out in the presence of an initiator..
The initiator is selected from a group consisting of carbon tetrachloride, hexachloroethane, trichloroacetylchloride, chloroform, phosgene, thionyl chloride, sulfonyl chloride, trichloromethylbenzene, organic hypochlorites and inorganic hypochlorites or mixture thereof.
In an embodiment of the present invention, the mixture 2 is substantially free from carbon monoxide.
In another embodiment of the present invention, the mixture 3 comprising unreacted tetrafluoroethylene, chloromethane, and chlorodifluoromethane is recycled back to the pyrolysis reactor.
In another embodiment of the present invention, the mixture 4 is passed through three successive distillation columns to obtain pure 1234yf.
In another embodiment of the present invention chlorodifluoromethane is charged to the reactor maintained at a temperature of about 700°C to about 800°C. The stream of gases mainly comprising tetrafluoroethylene, along with R-40 and an initiator such as carbon tetrachloride is fed to another reactor “1” maintained at a temperature of about 600°C to about 700°C. The stream passed through quencher (heat quencher), alkali scrubber, sulfuric acid and alumina to obtain a stream of gases free of acid and moisture to obtain feed “2” comprising carbon monoxide, tetrafluoroethylene, HFP, OFCB, R-1225zc, R-134a, R-134, R-22, CTFE, R-236ca, R-236ca, R-1122, heavies and is fed to the distillation column ‘3’. ‘4’, comprising carbon monoxide is vented out from the distillation column and heavier boiling fraction “5” comprising mixture 2 is fed to another column “6” to distill and recycle “7” comprising tetrafluoroethylene, the heavies “8” comprising 1234yf are passed through successive distillation columns to obtain 1234yf of high purity.
In another embodiment of the present invention, the reaction of tetrafluoroethylene/chlorodifluoromethane with chloromethane can be done in a batch reactor or continuous reactor.
In a preferred embodiment of the present invention, the process of purification is performed in a continuous reactor.
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
Comparative example 1: Tetrafluoroethene and chloromethane was reacted in the presence of carbon monoxide at a reaction temperature of 650°C - 700°C.
Table 1 shows the composition of the furnace (reactor) inlet and outlet data when the pyrolysis is carried out in presence of carbon monoxide.
Table 1
Reactor inlet Reactor Outlet

CO 8.237% 10.969%
CH4 0.097% 0.088%
R-116 0.031% 0.000%
TFE 19.260% 3.177%
R23 12.705% 14.783%
VDF 7.683% 10.141%
R-32 0.736% 1.037%
HFP 0.009% 0.652%
TriFethene 0.000% 0.239%
OFCB 1.513% 2.448%
2,3,3,3-tetrafluoropropene 0.000% 4.917%
C1 20.39% 11.40%
Heavies 29.34% 36.45%

TFE Conversion 84%
2,3,3,3-tetrafluoropropene Selectivity 31%

Comparative example 2: Tetrafluoroethene and chloromethane was reacted in the presence of carbon monoxide at a reaction temperature of 650°C - 700°C.
Table 2 also shows the composition of the furnace (reactor) inlet and outlet data when the pyrolysis is carried out in presence of carbon monoxide.

Table 2
Reactor inlet Reactor Outlet
CO 9.018% 10.907%
CH4 0.121% 0.081%
R-116 0.038% 0.000%
TFE 16.915% 3.086%
R23 14.028% 15.928%
VDF 8.524% 10.315%
R-32 0.471% 0.813%
HFP 0.102% 0.562%
TriFethene 0.000% 0.195%
OFCB 1.202% 1.811%
2,3,3,3-tetrafluoropropene 0.000% 4.718%
C1 20.21% 12.78%
Heavies 27.92% 33.65%

TFE Conversion 82%
2,3,3,3-tetrafluoropropene Selectivity 34%

The comparative examples clearly indicate that when pyrolysis is carried out in presence of carbon monoxide, the formation of 2,3,3,3 tetrafluoropropene is less than 5 %, conversion of TFE is less than 85 % and selectivity towards 2,3,3,3-tetrafluoropropene is below 35 %.
Example 1: Tetrafluoroethene and chloromethane was reacted in the absence of carbon monoxide at a reaction temperature of 650°C - 700°C.
Table 3 shows the composition of the furnace (reactor) inlet and outlet data when the pyrolysis is carried out in absence of carbon monoxide.
Reactor inlet Reactor Outlet
CO 0.130% 0.190%
CH4 0.014% 0.063%
R-116 0.025% 0.000%
TFE 23.271% 1.813%
R23 5.909% 9.751%
VDF 5.256% 10.859%
R-32 1.724% 2.984%
HFP 0.056% 1.548%
TriFethene 0.000% 0.623%
OFCB 0.781% 2.477%
2,3,3,3-tetrafluoropropene 0.000% 9.090%
C1 26.22% 5.17%
Heavies 34.91% 40.21%

TFE Conversion 92%
2,3,3,3-tetrafluoropropene Selectivity 42%

Example 2: Tetrafluoroethene and chloromethane was reacted in the absence of carbon monoxide at a reaction temperature of 650°C - 700°C.
Table 3 shows the composition of the furnace (reactor) inlet and outlet data when the pyrolysis is carried out in absence of carbon monoxide.
Reactor inlet Reactor Outlet
CO 0.945% 0.451%
CH4 0.157% 0.048%
R-116 0.000% 0.000%
TFE 21.408% 2.154%
R23 3.691% 6.676%
VDF 3.159% 8.526%
R-32 4.024% 3.889%
HFP 0.723% 2.215%
TriFethene 2.428% 1.459%
OFCB 3.394% 4.658%
2,3,3,3-tetrafluoropropene 0.000% 10.210%
C1 24.77% 4.92%
Heavies 32.69% 40.71%

TFE Conversion 90%
2,3,3,3-tetrafluoropropene Selectivity 53%

The experimental data obtained in examples 1 and 2 clearly indicate the advantage of CO removal on the conversion of TFE and on selectivity of 2,3,3,3-tetrafluoropropene.

We claim

1. A process for preparation of 1234yf, comprising the step of:
a) combining tetrafluoroethylene/chlorodifluoromethane and chloromethane at a temperature to establish pyrolysis reaction to obtain a mixture comprising 1234yf;
b) distilling carbon monoxide from the mixture comprising 1234yf.

2. A process for preparation of 1234yf, the
comprising the steps of:
a) combining tetrafluoroethylene/chlorodifluoromethane and chloromethane at a temperature to establish pyrolysis reaction to obtain a mixture 1;
b) distilling the mixture 1 to vent out carbon monoxide to obtain mixture 2;
c) distilling mixture 2 to vent out mixture 3 and obtain mixture 4;
d) recycling the mixture 3;
e) distilling mixture 4 to obtain 1234yf.

3. The process as claimed in claim 1 and claim 2 wherein, pyrolysis reaction is carried out in presence of an initiator.

4. The process as claimed in claim 3 wherein, the initiator is selected from a group consisting of carbon tetrachloride, hexachloroethane, trichloroacetylchloride, chloroform, phosgene, thionyl chloride, sulfonyl chloride, trichloromethylbenzene, organic hypochlorites and inorganic hypochlorites or mixture thereof.

5. The process as claimed in claim 1 and claim 2 wherein, pyrolysis is carried out at a temperature of 600°C to 700°C.

6. The process as claimed in claim 1 and claim 2 wherein, the mixture 1 comprises 1234yf, unreacted tetrafluoroethylene, chloromethane, chlorodifluoromethane and carbon monoxide.

7. The process as claimed in claim 1 wherein, the mixture 2 comprises 1234yf, unreacted tetrafluoroethylene, chloromethane, and chlorodifluoromethane.

8. The process as claimed in claim 1 wherein, the mixture 3 unreacted tetrafluoroethylene, chloromethane, and chlorodifluoromethane.

9. The process as claimed in claim 1 wherein, the mixture 4 comprises 1234yf.