Field of the invention
The present invention provides an improved process for the preparation of 2,3,3,3-tetrafluoropropene by recycling the reaction by-products and attaining an equilibrium between feed and outlet composition.

Background of the invention
Fluoro olefins play an important role as refrigerants. In recent years a fluoro olefin viz. 2,3,3,3-tetrafluoropropene (HFO-1234yf) has attracted attention as a new refrigerant to replace another fluorinated refrigerants namely 1,1,1,2-tetrafluoroethane (HFC-134a).
US Patent No. 2,931,840 describes a process for the preparation of HFO-1234yf by heating and decomposing a mixture of methyl chloride and chlorodifluoromethane or tetrafluoroethylene at a temperature from 700 to 950°C by a common heating means such as an electric heater in a reactor.
It has been observed that the said process results in low yield due to formation of several by-products. The said patent has not disclosed any method for recycling of by-products.
Japan Publication No. 2016-027004 describes a process for the preparation of HFO-1234yf by feeding heat source selected from steam, nitrogen or carbon dioxide into a reactor containing a mixture of methyl chloride and tetrafluoroethylene at a temperature from 400 to 870°C. However the said Japanese patent application discloses recycling of vinylidenefluoride (VDF), hexafluoropropene (HFP), and chlorotrifluoroethylene (CTFE), which are used as raw materials for preparing HFO-1234yf, but it discloses nothing about equilibrium between feed and outlet composition.
Surprisingly, the present inventors observed that recycling of the composition comprising vinylidenefluoride (VDF), hexafluoropropene (HFP), octafluorocyclobutane (OFCB), 1-chloro-1,1,2,2,3,3-hexafluoropropane (R-226cb), chlorotetrafluoroethane, difluoromethane and trifluoromethane back into the reactor reduces further formation of these compounds by attaining an equilibrium, thereby increasing the yield of 2,3,3,3-tetrafluoropropene.

Summary of the invention
The present invention provides an improved process for the preparation of 2,3,3,3-tetrafluoropropene comprising:
a) providing the heat source in the reactor;
b) providing preheated methyl chloride, tetrafluoroethylene and initiator, either pre-mixed or mixed separately, in the reactor to obtain the first reaction mixture;
c) heating the first reaction mixture using heat source to obtain 2,3,3,3-tetrafluoropropene and second reaction mixture components;
d) recycling the second reaction mixture components into the reactor to attain an equilibrium; and
e) isolating 2,3,3,3-tetrafluoropropene.

Detailed description of the invention
The present invention provides an improved process for the preparation of 2,3,3,3-tetrafluoropropene comprising:
a) providing the heat source in the reactor;
b) providing preheated methyl chloride, tetrafluoroethylene and initiator, either pre-mixed or mixed separately, in the reactor to obtain the first reaction mixture;
c) heating the first reaction mixture using heat source to obtain 2,3,3,3-tetrafluoropropene and second reaction mixture components;
d) recycling the second reaction mixture components into the reactor to attain an equilibrium; and
e) isolating 2,3,3,3-tetrafluoropropene.

Methyl chloride, tetrafluoroethylene and initiator used in the step b) are preheated at 250°C to 400°C using the electric furnace and are continuously introduced into the reactor either premixed or separately and first reaction mixture is obtained.
The heat source provided in the reactor is electric furnace.
The second reaction mixture components obtained in step c) comprises vinylidenefluoride, chlorotetrafluoroethane, hexafluoropropene, octafluorocyclobutane, chlorodifluoromethane, 1-chloro-2,2-difluoroethene, 1,1,2,2,3,3-hexafluoropropane, 1-chloro-1,1,2,2,3,3-hexafluoropropane, difluoromethane and trifluoromethane.

The present invention is carried out in continuous reaction mode wherein reaction by-products are recycled back into the reactor.

The reaction by-products comprise second reaction mixture components.

Initially as the concentration of second reaction mixture components in reactor inlet increased, the concentration of second reaction mixture components in reactor outlet also increased however after a certain level further increase in second reaction mixture components in reactor outlet stopped with the increase in second reaction mixture components in reactor inlet, thereby attaining an equilibrium between feed and outlet of the reactor.

The recycling of the second reaction mixture back into the step b) reactor also resulted in controlling the heat generated during reaction, thereby controlling reaction exothermicity which reduced the further formation of second reaction mixture components and resulted in increasing the yield of 2,3,3,3-tetrafluoropropene.

The initiator used in the step b) is selected from the group consisting of carbon tetrachloride, hexachloroethane, trichloroacetylchloride, chloroform, phosgene, thionyl chloride, sulfonyl chloride, trichloromethylbenzene, organic hypochlorites and inorganic hypochlorites or mixture thereof.
The concentration of initiator with respect to methyl chloride is selected from 0.1 to 8 %.
The mole ratio of methyl chloride to tetrafluoroethyelene is selected from 0.1-3:1.
The present invention is carried out at a reaction temperature selected from 300oC to 750oC.

The process for the preparation of 2,3,3,3-tetrafluoropropene is optionally carried out in the presence of an inert gas selected from argon and nitrogen.

The isolation of 2,3,3,3-tetrafluoropropene is carried out using several techniques known in the prior art such as distillation, adsorption, absorption and a like or combination thereof.
The residence time in the reactor is between 0.1 to 3.5 second.

Examples
Example 1: Preparation of 2,3,3,3-tetrafluoropropene by attaining an equilibrium by recycling of vinylidenefluoride at 6800C.
A mixture of methyl chloride and tetrafluoroethylene in the mole ratio of 0.88 together with carbon tertrachloride was preheated and then superheated to 350oC and was fed to the reactor which was maintained at 620oC by electrical heater. The reaction was exothermic and the temperature increased to 6800C. The reaction mixture thus obtained consisted of HFO1234yf, VDF and other by-products. The VDF was recycled back into the reactor. The reactor outlet samples were analyzed initially and after recycling of VDF using a gas chromatograph equipped with thermal conductivity detector. The results are shown in Table-1.
Table-1
Reactor Inlet
Sample Number 1 2 3 4
Reaction temperature (0C) 680 680 680 680
Pressure (Kg/cm2) 1 1 1 1
Mole Ratio (R40:TFE) 0.8 0.8 0.8 0.8
VDF Mole % 0.0% 14.1% 28.1% 27.9%
Reactor outlet (analysis in mole %)
Methane 0.05% 0.08% 0.07% 0.08%
Tetrafluoroethyelene 2.58% 3.38% 3.70% 3.51%
Trifluoromethane 3.62% 2.29% 1.98% 1.61%
Vinylidenefluoride 12.96% 15.69% 18.27% 16.30%
Difluoromethane 1.31% 1.58% 1.49% 1.40%
Hexafluoropropene 5.71% 6.01% 6.62% 6.80%
Trifluoroethene 1.01% 0.91% 0.85% 0.72%
Octafluorocyclobutane 4.17% 4.63% 4.80% 4.93%
2,3,3,3-Tetrafluoropropene 18.80% 18.98% 18.04% 16.61%
Tetrafluoroethane 1.15% 1.25% 1.24% 1.16%
Chlorodifluoromethane 5.30% 4.64% 4.45% 4.08%
Methyl chloride 2.04% 5.17% 5.93% 7.15%
Chlorotetrafluoroethane 3.39% 1.59% 1.36% 1.20%
1,1,2,2,3,3-Hexafluoropropane 2.18% 2.56% 2.47% 2.58%
1-Chloro-2,2-difluoro ethylene 7.47% 6.27% 5.90% 5.64%
1-Chloro-1,1,2,2,3,3-hexafluoropropane 1.78% 1.61% 1.48% 1.43%
1,1-Dichloro-2,2-difluoro ethane 3.62% 1.97% 1.79% 1.73%
1-Chloro-2,2,3,3-tetrafluoro propane 0.93% 1.97% 2.02% 2.54%
Based on TFE conversion, HFO-1234yf selectivity 19.72% 24.56% 28.99% 27.09%

Example 2: Preparation of 2,3,3,3-tetrafluoropropene by attaining an equilibrium by recycling of vinylidenefluoride at 7080C.
A mixture of methyl chloride and tetrafluoroethylene in the mole ratio of 0.88 together with carbon tertrachloride was preheated and then superheated to 350oC and was fed to the reactor which was maintained at 620oC by electrical heater. The reaction was exothermic and the temperature increased to 7080C. The reaction mixture thus obtained consisted of HFO1234yf, VDF and other by-products. The VDF was recycled back into the reactor. The reactor outlet samples were analyzed initially and after recycling of VDF using a gas chromatograph equipped with thermal conductivity detector. The results are shown in Table-2.
Table-2
Reactor Inlet
Sample No. 1 2 3 4
Reactor Skin Temperature (0C) 620 620 620 620
Temperature Process (0C) 708 708 708 708
Pressure Kg/cm2 1 1 1 1
Mole Ratio (R40:TFE) 0.9 0.9 0.9 0.9
VDF Mole % 0.0% 21.7% 15.7% 20.0%
Reactor outlet (analysis in mole %)
Methane 0.21% 0.11% 0.13% 0.08%
Tetrafluoroethyelene 4.16% 3.26% 3.12% 3.85%
Trifluoromethane 2.70% 2.30% 1.87% 1.91%
Vinylidenefluoride 15.85% 20.66% 17.88% 17.76%
Difluoromethane 1.35% 1.63% 1.55% 1.41%
Hexafluoropropene 4.47% 5.02% 5.64% 7.46%
Trifluoroethene 0.98% 0.79% 0.65% 0.76%
Octafluorocyclobutane 4.11% 2.93% 3.22% 3.96%
2,3,3,3-Tetrafluoropropene 19.88% 19.57% 18.30% 18.31%
Tetrafluoroethane 1.05% 1.05% 1.00% 0.95%
Trifluoropropene 0.25% 0.28% 0.25% 0.22%
Chlorodufluoromethnae 5.01% 4.43% 4.17% 1.10%
Methyl chloride 6.52% 7.06% 9.09% 7.54%
Chlorotetrafluoroethane 2.00% 1.38% 1.73% 1.95%
1,1,2,2,3,3-Hexafluoropropane 2.12% 1.82% 1.94% 1.91%
1-Chloro-2,2-difluoroethylene 6.98% 6.06% 5.65% 5.57%
1-Chloro-1,1,2,2,3,3-hexafluoropropane 0.91% 1.14% 1.19% 1.15%
1,1-Dichloro-2,2-difluoroethene 2.60% 2.60% 2.37% 2.18%
Based on TFE conversion, R1234yf selectivity 22.30% 28.84% 25.43% 26.71%

From the Tables-1 and Table-2, it is evident that initially as the concentration of VDF in reactor inlet increases, the concentration of VDF in reactor outlet also increases but after a certain level, further increase in VDF concentration in reactor outlet stops with the increase in VDF concentration in reactor inlet. Thus an equilibrium was said to be attained.

Example 3: Preparation of 2,3,3,3-tetrafluoropropene by attaining an equilibrium by recycling OFCB, R-124 or R-226cb.
A mixture of methyl chloride and tetrafluoroethylene in the mole ratio of 0.88 was preheated and then superheated to 350oC and was fed to the reactor which was maintained at 645oC by electrical heater. The reaction was exothermic and the temperature increased to 6800C. The reaction mixture components thus obtained consisted of HFO-1234yf, octafluorocyclobutane (51.6 mole%), chlorotetrafluoroethane (33.4 mole%), 1-chloro-1,1,2,2,3,3-hexafluoropropane (9.8 mole%) and other by-products. The octafluorocyclobutane (51.6 mole%), chlorotetrafluoroethane (33.4 mole%), 1-chloro-1,1,2,2,3,3-hexafluoropropane (9.8 mole%) were recycled back into the reactor one by one. The reactor outlet samples were analyzed initially and after recycling of said reaction components using a gas chromatograph equipped with thermal conductivity detector. The results are shown in Figure 1,2 and 3.
The above experimental data clearly indicates that the presence of second reaction mixture components in reactor inlet results in an equilibrium which enhances the selectivity for the formation of 2,3,3,3-tetrafluoropropene.
However, this equilibrium is not limited to the compounds in the examples, it can be applied to most or all of the components of second reaction mixture.

WE CLAIM:
1. A process for the preparation of 2,3,3,3-tetrafluoropropene comprising:
a) providing the heat source in the reactor;
b) providing preheated methyl chloride, tetrafluoroethylene and initiator, either pre-mixed or mixed separately, in the reactor to obtain the first reaction mixture;
c) heating the first reaction mixture using heat source to obtain 2,3,3,3-tetrafluoropropene and second reaction mixture components;
d) recycling the second reaction mixture components into the reactor to attain an equilibrium; and
e) isolating 2,3,3,3-tetrafluoropropene.
2. The process as claimed in claim 1, wherein in the step b) methyl chloride, tetrafluoroethylene and initiator are preheated at a temperature in the range of 250°C to 400°C using the electric furnace.
3. The process as claimed in claim 1, wherein the heat source provided in the reactor is electric furnace.
4. The process as claimed in claim 1, wherein the initiator used in the step b) is selected from 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, wherein in the step b) the concentration of initiator with respect to methyl chloride is selected from 0.1 to 8 %.
6. The process as claimed in claim 1, wherein in the step b) the mole ratio of methyl chloride to tetrafluoroethyelene is selected from 0.1 to 3:1.
7. The process as claimed in claim 1, wherein in the step c) the second reaction mixture component comprising of vinylidenefluoride, chlorotetrafluoroethane, hexafluoropropene, octafluorocyclobutane, chlorodifluoromethane, 1-chloro-2,2-difluoroethene, 1,1,2,2,3,3-hexafluoropropane, 1-chloro-1,1,2,2,3,3-hexafluoropropane, difluoromethane and trifluoromethane.
8. The process as claimed in claim 1, wherein the reaction is carried out at temperature in the range of 300oC to 750oC.
9. The process as claimed in claim 1 is carried out in continuous reaction mode.
10. The process as claimed in claim 1, wherein the reaction is optionally carried out in the presence of an inert gas selected from argon and nitrogen.
Dated this 7th day of July 2017.