FIELD OF THE INVENTION
The present invention provides an improved process for purification of halogenated compounds facilitating an efficient removal of sulphur oxide impurities.
BACKGROUND OF THE INVENTION
Halogenated compounds such as chlorofluoroalkanes, fluoroalkanes, fluoroalkenes, fluorinated anhydrides etc are valuable reagents for manufacture of various products in the pharmaceutical and agrochemical industry. They also find applications as refrigerants, blowing agents, solvents, polymers and fine chemicals.
Most of the methods of preparation of halogenated compounds involves use of sulphuric acid as a reagent or a dehydrating agent.
WO 2014/195929 discloses a process for preparation of trifluoroacetic anhydride by reaction of trifluoroacetic acid with sulphuric acid, oleum and/or disulphuric acid. The use of sulphuric acid, oleum and/or disulphuric acid results in the formation of sulphur oxide impurities that are very difficult to remove below certain level (50ppm). Fluorinated anhydrides are used as important reagents for preparation of various pharma and agrochemicals. The presence of oxides of sulphur in fluorinated anhydrides cause poisoning of the catalyst and makes them unfit for use in any catalytic reaction. Most of the commercially available fluorinated anhydrides have sulphur oxide impurities in level of 50ppm to lOOOppm making them less desirable for industrial purposes.
The presence of sulphur oxide impurities in halogenated compounds impart obnoxious odour to them making them unfit for various commercial applications such as refrigeration.
So, there is an urgent need to evolve an efficient method to remove sulphur oxide impurities from halogenated compounds.

The inventors have evolved a simple and cost-effective method of removing sulphur oxide impurities from halogenated compounds by purging inert gas under specific conditions.
OBJECTS OF THE INVENTION
The object of the present invention is to provide a simple, cost effective and industrially viable process for purification of halogenated compounds by efficiently removing sulphur oxide impurities to a level of below 50ppm.
SUMMARY OF THE INVENTION
First aspect of the present invention provides halogenated compounds having less than 20ppm of oxides of sulphur.
Second aspect of the present invention provides a process for purification of crude halogenated compounds having oxides of sulphur in the range of 50ppm to 2000ppm or more, comprising the steps of,
a) heating crude halogenated compound in a reactor to obtain a mixture comprising vapours of halogenated compound and oxides of sulphur;
b) condensing the mixture of step a) and collecting it in condensate collection pot;
c) purging inert gas in the condensate collection pot; and
d) isolating pure halogenated compound.

BRIEF DESCRIPTION OF DRAWING
Figure 1 describes the reactor setup used in the purification process of the present invention.
As referred in figure 1, crude halogenated compound feed 'SI' is fed to the reactor ' 1'. The reactor '1' is connected with a column '2'. The product stream 'S2' enriched with sulphur oxide impurities leaves from the top of the column '2' to enter into a heat exchanger (condenser) '3', the condensed stream from '3' enters a first condensate collection pot '4', to be purged with an inert gas 'S5' having desired flow rate. The product condensate 'S4' with reduced level of sulphur oxides is recycled back to the top of the column '2'. The stream 'S6' comprising an inert gas, sulphur oxides and small amount of product is further passed to another heat exchanger (condenser) '5'. The product condensate 'S8' from heat exchanger '5' is collected in another condensate collection pot '6'. The carryover condensate 'S9' is recycled back to the column '2' via another heat exchanger '7' to heat the condensed stream before entering into the bottom of the condenser '2' and through it, finally to the reactor ' 1'. The stream 'S10' enriched with oxides of sulphur is taken to the scrubbing media for final removal. The pure halogenated compound having sulphur oxide impurities below desirable limit is collected from "S11".
DETAILED DESCRIPTION OF INVENTION
As used herein, the halogenated compounds referred in the present invention include chlorofluoroalkanes, fluoroalkanes, fluoroalkenes, and fluorinated anhydrides such as trifluoroacetic anhydride, chloro-difluoroacetic anhydride or the like.
As used herein, the "crude halogenated compound" of the present invention contains 50 ppm to 3000ppm or more of sulphur oxides impurities. The crude halogenated

compound has been generated by a process comprising contact with sulphuric acid, disulphuric acid, oleum or the like.
As used herein, the "inert gas" referred in the present invention means nitrogen, helium, argon or the like.
As used herein, LPM referred to liter per minute
As used herein, the oxides of sulphur refers to mixture of majorly sulphur dioxide along with sulphur trioxide.
In an embodiment of the present invention, the heat exchanger '3' is kept at a temperature range to generate higher vapour pressure of sulphur oxides in order to facilitate higher sulphur oxides carryover along with inert gas and minimal carryover of the desired product in the first condensate collection pot '4'. Preferably, the temperature of the heat exchanger '3' is kept at a temperature of 10-15°C lower than the boiling point of the halogenated compound.
In another embodiment of the present invention, the heat exchanger '5' is kept at a temperature range to generate higher vapour pressure of sulphur oxides in order to facilitate higher sulphur oxide carryover along with inert gas and minimal carryover of the desired product in the first condensate collection pot '6'. Preferably, the temperature of the heat exchanger '5' is kept at a temperature of 20-25°C lower than the boiling point of the halogenated compound.
In another embodiment of the present invention, the nitrogen is purged at a flow rate to generate higher vapour pressure of sulphur oxides to facilitate their removal along with the inert gas.
In another embodiment of the present invention, the nitrogen is purged at 1 to 20 liter per minute, preferably, the nitrogen is purged at a rate of 5 to 15 liter per minute. The specific nitrogen flow rate and specific condenser temperature helps in the removal of

oxides of sulphur from the top of condenser while retaining pure halogenated compound.
The content of sulphur oxides in the product is analyzed through different techniques by converting them into corresponding sulphates through oxidation by hydrogen peroxide then total sulphate content analyzed using UV spectrophotometer. Thus, the sulfate content represents the amount of sulphur dioxide and sulphur trioxide gases present in the halogenated compound.
In an embodiment, the sulphate content in the pure halogenated compound is less than 20 ppm. In another embodiment, the sulphate content in the pure halogenated compound is less than 10 ppm.
In an embodiment, the sulfate content in the pure fluorinated anhydrides is less than 20 ppm.
Preferably, the fluorinated anhydrides obtained by the purification process of the present invention have sulfate content less than 10 ppm. The oxides of sulphur being measured using UV spectrophotometer method at 525nm wavelength.
In an embodiment, the present invention provides halogenated compound having less than lOppm of oxides of sulphur.
The crude product to be purified (SI) goes to the reactor (1) and heated to the desired temperature to vaporize it. The product vapours enriched with sulphur oxide impurities (S2) reaches to the column (2) top and get condensed (S3) while passing through first heat exchanger (3) and collected in the first condensate collection pot (4). A stream of inert gas (S5) is purged in the collected first condensate with a desired flow rate of an inert gas. The inert gas enriched with sulphur oxides and some carryover product (S6) comes out from '4' and goes to second heat exchanger (5) resulting the carryover product condensate (S8) in the second condensate collection pot (6). The inert gas, enriched with sulphur oxide (S10) goes out to the appropriate scrubber. The carryover

product condensate stream (S9) goes back to the column (component 2) bottom for recycling.
If feed stream (SI) containing sulphur oxide impurities is of good quality with respect to the organic impurities, the sulphur oxide free product can be collected from the bottom line (SI 1) of reactor (1) after complete removal of sulphur oxide impurities from the vent line (S10).
If the feed stream (SI) is crude product containing organic impurities, the sulphur oxide free product can be collected (S7) from the first condensate collection pot (component 4) after proper distillation of organic impurities and removal of sulphur oxides.
The process of the present invention is carried out in a continuous mode or a batch mode. In continuous mode, the feed of crude halogenated compound and an inert gas is continuously supplied to reactor, and pure halogenated compound is continuously separated from reflux divider and unwanted oxides of sulphur are separated continuously from the top of condenser.
In an embodiment, the process of present invention involves use of packed column. The packed column provides an elevation, which further helps in settling down heavies such as halogenated compound back into reactor.
In an embodiment, the halogenated compounds obtained by the purification process of the present invention have yield greater than 90%. In a preferred embodiment, the yield is greater than 95%.
In another embodiment, the fluorinated anhydrides obtained by the purification process of the present invention have yield greater than 90%. In a preferred embodiment, the yield is greater than 95%.
The reduction of sulphate content is monitored by UV spectrophotomer method.

Unless stated to the contrary, any of the words "comprising", "comprises" and includes mean "including without limitation" and shall not be constructed to limit any general statement that it follows to the specific or similar items or matters immediately following it.
Embodiments of the invention are not mutually exclusive, but may be implemented in various combinations. The described embodiments of the invention and the disclosed examples are given for the purpose of illustration rather than limitation of the invention as set forth in the appended claims.
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: Purification of crude chloro-difluoroacetic anhydride.
Charged crude chloro-difluoroacetic anhydride (lOOOg) having sulfate content of 1 OOOppm in a reactor as described in Figure 1. The reaction mass was heated to obtain vapours of chloro-difluoroacetic anhydride along with sulphur oxide impurities. The vapours were condensed in condenser and were sent back to condensate pot. In condensate pot, the flow of nitrogen gas was maintained at (10-15 LPM). The condenser temperature was maintained at 10 to 15°C. The vapours of sulphur oxides were passed through condenser and collected from the top and pure chloro-difluoroacetic anhydride was collected.
Yield: 97.6%, Purity (by GC): 99.8%; Sulphate content: NMT lOppm
Example 2: Purification of crude trifluoroacetic anhydride.
Charged crude trifluoroacetic anhydride (1039kg) having sulfate content of 686ppm in a reactor fitted as described in Figure 1. The reaction mass was heated to obtain vapours

of trifluoroacetic anhydride along with sulphur oxide impurities. The vapours were condensed in condenser and were sent back to condensate collection pot. In condensate collection pot, the flow of nitrogen gas was maintained at 5-10 LPM. The vapours of sulphur oxides were passed through condenser and collected from the top and pure trifluoroacetic anhydride was collected.

WE CLAIM:
1. A process for purification of crude halogenated compounds having oxides of sulphur
in the range of 50ppm to 2000ppm or more, comprising the steps of,
a) heating crude halogenated compound in a reactor to obtain a mixture comprising vapours of halogenated compound and oxides of sulphur;
b) condensing the mixture of step a) and collecting it in condensate collection pot;
c) purging inert gas in the condensate collection pot; and
d) isolating pure halogenated compound.
2. The process as claimed in claim 1, wherein the halogenated compounds are selected
from a group consisting of chlorofluoroalkanes, fluoroalkanes, fluoroalkenes; and
fluorinated anhydrides.
3. The process as claimed in claim 2, wherein the fluorinated anhydrides are selected
from a group consisting of trifluoroacetic anhydride and chloro-difluoroacetic
anhydride.
4. The process as claimed in claim 1, wherein the "inert gas" is selected from a group consisting of nitrogen, helium and argon.
5. The process as claimed in claim 1, wherein the oxides of sulphur is sulphur dioxide, sulphur trioxide or a mixture thereof.
6. The process as claimed in claim 1, wherein the inert gas is purged at 20 to 70 grams per hour.
7. The process as claimed in claim 6, wherein the inert gas is purged at 30 to 40 grams per hour.
8. The process as claimed in claim 1, wherein the sulphate content in the pure
halogenated compound is less than 20 ppm.

9. The process as claimed in claim 1, wherein the sulphate content in the pure halogenated compound is less than 10 ppm.