Field of invention
The present invention provides a process to prepare hexafluoropropene (HFP).
Background of the invention
The HFP is finding increasing use as a co-monomer for the industrial preparation of polymers based on tetrafluoroethylene (TFE). HFP is commercially prepared by pyrolyzing TFE.
The U.S Patent No. 3,306,940 describes a process to prepare TFE and HFP simultaneously by pyrolyzing chlorodifluoromethane to a mixture of TFE and HFP. The U.S Patent No. 3,459,818 describes a process to prepare TFE and HFP by pyrolyzing a mixture of chlorodifluoromethane and TFE formed by partial pyrolysis of chlorodifluoromethane to TFE, followed by removal of HCl.
The U.S Patent No. 5,334,783 describes a process to prepare HFP by thermal cleavage of a mixture consisting of chlorotetrafluoroethane and perfluorocyclobutane and TFE. The process describes specific mole(s) of each components at specific temperature and pressure.
Problem
The present inventors observed that the prior art results in lower selectivity of HFP, that is, less than almost 10% conversion of reactants to HFP and more amount of unwanted products such as chlorotrifluoroethylene, chlorohexafluoropropane and heavies, thereby reducing yield of process. Also, TFE is highly reactive and explosive raw material thus, leading to safety concerns. Further, reaction conditions like involvement of high temperature (>1000deg. C) makes the reaction industrially non-feasible and difficult to handle.
Solution to problem
The present inventors observed that upon using feed of chlorotetrafluoroethane and octafluoro cyclobutane to prepare HFP led to the high conversion of reactants to HFP and much higher selectivity of 30% of HFP is obtained. Additionally, the present invention avoids the use explosive reactants like TFE and high temperature. 2
The diluent will help in uniform heat distribution and reduce partial pressure leading to less unwanted products and thus improving yield.
Summary of the invention
The present invention provides a process to prepare hexafluoropropylene comprising;
a) supplying chlorotetrafluoroethane and octafluoro cyclobutane to a reactor, as preliminary mixed or separately, to a reaction vessel;
b) optionally, supplying a diluent to the reaction vessel to obtain a first mixture;
c) removing hydrogen chloride from the first mixture to obtain a second mixture; and
d) subjecting the second mixture to compression and distillation to obtain hexafluoropropylene.
Detailed Description of the Invention
The present invention provides a process to prepare hexafluoropropene comprising;
a) supplying chlorotetrafluoroethane and octafluoro cyclobutane to a reactor, as preliminary mixed or separately, to a reaction vessel,
b) optionally, supplying a diluent to the reaction vessel to obtain a first mixture,
c) removing hydrogen chloride from the first mixture to obtain a second mixture, and
d) subjecting the second mixture to compression and distillation to obtain hexafluoropropylene.
The process of the present invention may be continuous system process or a batch system process. In the continuous process, the supply of chlorotetrafluoroethane and octafluoro cyclobutane to the reactor and the withdrawal of the reaction mixture containing hexafluoropropene from the reactor are continuously conducted, respectively. In the batch system production, either one of the supply of chlorotetrafluoroethane and octafluoro cyclobutane in step a) may precede the other, or they may be conducted simultaneously. 3
From the viewpoint of the production efficiency, the process of the present invention is preferably a continuous system process.
It is considered that raw materials, chlorotetrafluoroethane and octafluoro cyclobutane undergo a thermal decomposition to form hexafluoropropene. The diluent may be steam, which may be supplied into the raw materials.
The ratio of the supply amount of chlorotetrafluoroethane and octafluoro cyclobutane to be supplied to the reactor is chlorotetrafluoroethane (20 to 50 wt%) and octafluoro cyclobutane (20 to 55 wt%).
The diluent, for example, steam may be added to the mixture in 20 mol% to 95 mol%.
In the process of the present invention, the reaction mixture withdrawn from the outlet of the reactor contains unreacted raw material, the reaction product, byproducts, etc. Thus, byproduct such as hydrogen chloride is removed and the desired product hexafluoropropene is separated from outlet. The rest of the outlet materials such as unreacted raw material along with fresh chlorotetrafluoroethane and octafluoro cyclobutane are supplied to the reactor.
The temperature of diluent and mixture of chlorotetrafluoroethane and octafluoro cyclobutane to be supplied to the reactor are preferably made to be from 700-880°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 terms “furnace”, “reaction vessel” and “reactor” are interchangeable.
The following examples are given by way of illustration and therefore should not be construed to limit the scope of the present invention.
Example 1
Process for the preparation of hexafluoropropene. 4
In the experiment one reaction furnace was employed which consists of High nickel alloy tube of length 470mm and 1 inch in diameter and having wall thickness of 2.6 mm. The electrical resistance furnace of 1500 watt rated capacity was employed to heat this tube. The effective heating zone of this tube was 175 mm. The feed of chlorotetrafluoroethane and octafluoro cyclobutane blend was fed with flow of 21 LPH (liters per hour) from the cylinders. These gases were entered into the tube which was at temperature 820oC. Inconel tubing outlet was immersed in water bath. The product gases were cooled and passed through caustic trap (300 ml 20% caustic) followed by sulphuric acid trap (100 ml sulphuric acid). After the sulphuric trap the sample of the gases were taken in gas sampling tube and checked in the GC wt % analysis.
Feed Composition:
Results are follows:
Temperature (°C)
820
RT (sec)
1.8
TFE (%)
12.77
HFP (%)
29.94
Chlorotrifluoroethylene (%)
1.46
Chlorohexafluoropropane (%)
0.25
Heavies (%)
24.77
Perfloroisobutene%
0
Feed Composition
%
Perfluoroisobutene
0.68
Chlorodifluoromethane
0.12
Chlorotetrafluoroethane
31.6
Octafluorocyclobutane
53.69
Heavies
13.59
5
Chlorodifluoromethane%
1.18
Octafluorocyclobutane%
9.32
Chlorotrifluoroethane%
4.87
Conversion: 90%
Yield: 80%
Example 2
Process for the preparation of hexafluoropropene.
In the pyrolysis experiment three furnaces were employed, which consists of two high nickel alloy tube and one SS tube joined in “T” shape as shown in diagram. From one end (inlet-1) steam was passed and entered into SS tube which was heated at temperature 700oC (using furnace-1) and from second end (inlet-2) feed gases of chlorotetrafluoroethane and octafluoro cyclobutane with flow 18 LPH were passed through a high nickel alloy tube which was pre-heated to 600 oC (using furnace-2). These both steams were combined at T joint and then entered into reaction furnace-3 at temperature 800 oC in the high nickel alloy tube. These outlet gases were passed through the high nickel alloy tube immersed in water bath. These product gases were passed through caustic trap (300 ml 20% caustic) followed by sulphuric acid trap (100 ml sulphuric acid). After the sulphuric acid trap, the sample of the gases was collected in gas sampling tube and checked in GC analysis.
Feed Composition:
Feed Composition
%
Perfluoroisobutene
0.68
Chlorodifluoromethane
0.12
Chlorotetrafluoroethane
31.6
Octafluorocyclobutane
53.69
Heavies
13.59
6
Results are as follows:
Temperature (°C)
800
RT (sec)
1.34
TFE (%)
10.12
HFP (%)
31.1
Chlorotrifluoroethylene (%)
2.67
Chlorohexafluoropropane (%)
0
Heavies (%)
20.27
Perfloroisobutene%
2.45
Chlorodifluoromethane%
0.73
Octafluorocyclobutane%
0
Chlorotrifluoroethane%
8.04
Conversion: 80%
Yield: 80%

We claim:
1. A process to prepare hexafluoropropene comprising;
a) supplying chlorotetrafluoroethane and octafluoro cyclobutane, as preliminary mixed or separately, to a reaction vessel to obtain a reaction mixture;
b) optionally, supplying a diluent to the reaction vessel to obtain a first mixture;
c) removing hydrogen chloride from the first mixture to obtain a second mixture; and
d) subjecting the second mixture to compression and distillation to obtain hexafluoropropene.
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 for continuous system process, the supply of chlorotetrafluoroethane and octafluoro cyclobutane to the reaction vessel and the withdrawal of the reaction mixture containing hexafluoropropene from the reaction vessel are continuously conducted.
4. The process as claimed in claim 1, wherein the diluent is steam.
5. The process as claimed in claim 1, wherein the diluent is supplied in the range of 20 mol% to 95 mol% with respect to the reaction mixture.
6. The process as claimed in claim 1, wherein chlorotetrafluoroethane and octafluoro cyclobutane are supplied to the reaction vessel in the range of chlorotetrafluoroethane (20 to 50 wt%) and octafluoro cyclobutane (20 to 55 wt%). 8
7. The process as claimed in claim 1, wherein the first mixture comprising diluent and mixture of chlorotetrafluoroethane and octafluoro cyclobutane has a temperature in range of 700°C to 880°C.