Title of Invention

"AN IMPROVED PROCESS FOR THE PURIFICATION OF 1,1,1.2-TETRAFLUOROETHANE"

Abstract

A process for the purification of 1,1,1,2-tetrafluoroethane: The present invention provides a novel continuous process for purification of 1,1,1,2 - tetrafluoroethane which is viable for commercialization. The process comprises passing a mixture of 1,1,1,2-tetrafluoroethane and 2-chloro-1,1-difluoroethylene through a tubular packed column with continuous circulation of potassium permanganate solution containing alkali in a counter current flow with gas liquid contact time of 4 - 14 min., to remove the 2-chloro - 1,1-difluoroethylene and recovering the pure 1,1,1,2-tetrafluoroethane by conventional method.

Full Text

The present invention relates to a process for the purification of 1,1,1,2 -tetrafluo-roethane (HFC-134a). More particularly this invention relates to the removal of 2-chloro-1,1-difluoroethylene (HCFC -1122). The 1,1,1,2-tetrafluoroethane (HFC-134a) is a refrigerant gas widely recommended as a substitute to replace chlorofluorocarbons (CFC's). 1,1,1,2-tetrafluoroethane is commercially produced by fluorination of 2-chloro-1,1,1-trifiuoroethane (HCFC-133a) with anhydrous hydrogenfluoride (AHF) in vapor phase in presence of a suitable catalyst. The product obtained is a mixture of 1,1,1,2-tetrafluoroethane, unreacted 2-chloro-1,1,1-trifiuoroethane, anhydrous hydrogenfluoride, hydrogenchloride (HC1), 2-chloro-1, 1-difluoroethylene (HCFC-1122) and others. 1,1,1,2-tetrafluoroethane forms an azeotrope with 2-chloro-1,1-difluoroethylene. The azeotrope containing 1,1,1,2- tetrafluoroethane and 2-chloro-1,1-difluoroethylene can be separated from all other components by conventional distillation methods. The 2-chloro-1,1-difluoroethylene is highly toxic and its presence in 1,1,1,2-tetrafluoroethane makes the latter unsuitable for use as refrigerant gas. For most uses of 1,1,1,2-tetrafluoroethane the concentration of 2-chloro-1,1-difluoroethylene must be in non-detectable range. Different methods of purification of 1,1,1,2-tetrafluoroethane to remove 2-chloro-1,1-difluoroethylene are known in the art. They can be classified into four categories. In the first category [CA.l, 124,264(1978); CA.l, 124,265(1978); JP.4-321632 (1992); EP.0507458 (1992); EP.0548742 (1992)], the 2-chloro-1,1-difluoroethylene was made to react with hydrogenfluoride either in an autoclave or in a catalytic reactor in vapor phase. But this reaction is not carried out using a mixture of 1,1,1,2-tetrafluoroethane and 2-chloro-1,1-difluoroethylene only. The feed mixtures contain one or more of the other components like 2-chloro-l,l,l-trifluoroethane, anhydrous hydrogenfluoride, hydrogenchloride, 1,1,1-trifluoroethane (HFC-143a) and 1,1,1,2,2-pentafluoroethane (HFC-125). In the second category, 1,1,1,2-tetrafluoroethane containing only 2-chloro-1,1-difluoroethylene or also containing 2-chloro-1,1,1-trifluoroethane, 1,1,1-trifluoroethane and 2-chloro-l, 1,1,2-tetrafluoroethane (HCFC-124) [USP.4, 906,796 (1990); EP 0526002 (1992)] were passed over molecular sieves to selectively adsorb 2-chloro-1,1-difluoroethylene. In the third category, the 2-chloro-1,1-difluoroethylene, present in 1,1,1,2-tetrafluoroethane is removed by destruction by passing over catalyst like Hopcolite described in Indian Pat. 171615. Canadian patents. 1, 124,264 (1978) and 1, 124, 265 (1978) discloses the removal of 2-chloro-1,1-difluoroethylene from 1,1,1,2-tetrafluoroethane using catalytic or potassium permanganate methods. A stream of 1,1,1,2-tetrafluoroethane, 2-chloro-1,1,1 -trifluoroethane, 2-chloro-1,1 -difluoroethylene (0.52%) having other minor components are passed over a bed of Chromia catalyst along with hydrogenfluoride and the product stream showed the presence of 2-chloro-1,1-difluoroethylene to the extent of 7 ppm. In another set of conditions the organic product containing four components having the product distribution as 16% of 1,1,1,2-tetrafluoroethane, 83% of 2-chloro-1,1,1-trifluoroethane, 0.8% of 1,1,1,2,2-penta fluoroethane and 1000 ppm of 2-chloro-1,1-difluoroethylene, stirred with 4% of alkaline potassium permanganate solution in an autoclave for 15 to 90 minutes. The lower organic layer is separated, analysed and the presence of 2-chloro-1,1-difluoroethylene is found to be in the range notdetectable to 10 ppm. This patent has not covered an exclusive method of purification of 1,1,1,2-tetrafluoroethane but for a mixture containing predominantly 2-chloro-l,l,l-trifluoroethane. The draw backs of this method are : the autoclave reaction is not feasible on commercial scale ; after distillation and separation of 1,1,1,2-tetrafluoroethane from the mixture, the content of 2-chloro-l,l-difluoroethylene may increase considerably and difficult to meet the specifications of international standards. In the fourth category a mixture containing 1,1,1,2-tetrafluoroethane and 2-chloro-l,l-difluoroethylene were subjected to extractive distillation to remove the impurity. [WO.95/24370 (1995)]. The methods described above suffer from drawbacks. The first category of methods give product mixture from which a 1,1,1,2-tetrafluoroethane should still be separated free from 2-chloro-l,l-difluoroethylene. In the adsorption techniques the 2-chloro-l,l-difluoroethylene should be disorbed and a method of disposal should be found. In the destructive method the efficiency of the removal of 2-chloro-l,l-difluoroethylene depends on the temperature of the catalyst bed for a given flow rate. In none of the methods the 2-chloro-l,l-difluoroethylene if present in concentration higher than 2000 ppm is removed completely. The present invention describes an improved process for removal of 2-chloro-1,1-difluoroethylene from 1,1,1,2-tetrafluoroethane by intimate contact with alkaline potassium permanganate solution. The main objective of the present invention is to provide an improved process for the purification of 1,1,1,2-tetrafluoroethane. The other objective of the present invention is to provide a novel continuous process which is viable for commercialisation. Yet another objective is to improve the contact time of the organic vapor to the alkaline potassium permanganate solution to 4 - 14 min. The another objective of the present invention is to eliminate the 2 - chloro-1,1-difluoroethylene to a nondetectable level from a maximum of as high as 5000 ppm. Accordingly the present invention provides an improved process for the purification of 1,1,1,2-tetrafluoroethane which comprises passing a mixture of 1,1,1,2-tetrafluoroethane and 2-chloro-1,1-difluoroethylene through a tubular packed column with continuous circulation of potassium permanganate solution containing alkali such as herein described in a counter current flow with gas liquid contact time of 4 - 14 min., to remove the 2-chloro - 1,1-difluoroethylene and recovering the pure 1,1,1,2-tetrafluoroethane by conventional method. In an embodiment of the present invention the alkali used is sodium hydroxide or potassium hydroxide. In another embodiment of the present invention the contact time of the organic vapour to alkaline potassium permanganate solution is 6 - 7 min. In the present process potassium permanganate selectively reacts with 2-chloro-1,1-difluoroethylene but not 1,1,1,2-tetrafluoroethane and the 2-chloro-1,1-difluoroethylene will be reduced to non-detectable range. The total removal of 2-chloro-1,1-difluoroethylene depends upon the factors like contact time and thorough mixing of the organic vapor with alkaline potassium permanganate solution. Thus the organic material was fed as liquid into a vaporizer and the resultant gas was allowed to pass through a packed tubular column in which the alkaline potassium permanganate solution was fed in counter current fashion. The superficial gas velocity of the organic vapor is adjusted to give a contact time in the range 4 to 14 min..depending on the concentration of 2 -chloro 1,1- difluoroethylene. The maximum quantity of 2 - chloro-l,l-difluoroethylene used in the present invention is 5000 ppm. The optimum concentration of the potassium permanganate solution is 1%. The use of higher concentrations of alkaline potassium permanganate offers no advantage. The alkaline solution may be prepared using sodium or potassium hydroxide. The present invention is described with reference to the following examples which are explained by way of illustration and should not therefore be construed to limit the scope of the invention. Example -1: l"dia stainless steel tubular reactor of 600 cm length packed with rasching rings of 8x8 mm size was used. The column is kept under constant circulation with an alkaline potassium permanganate solution containing 82.33g of sodium hydroxide, 124g of potassium permanganate in 12000g of demineralised water by feeding from top. The feed rate of alkaline potassium permanganate solution was maintained at 400 ml/min. The organic feed consisting of 1,1,1,2-tetrafluoroethane and 2-chloro-l,l-difluoroethylene taken in a pressure can and mounted on a balance was passed through a vaporizer maintained at 25-30 °C and then led into the tubular reactor in an upward flow. The flow rate of organic feed was maintained at the rate of 100g/h .The 1,1,1,2-tetrafluoroethane coming out of the reactor was passed through a drier and collected in a receiver placed in dry ice-acetone bath. The concentration of 2-chloro-l,l-difluoroethylene in the out coming gases is continuously monitored by online analysis by Gas Liquid Chromatography. The results are given in the Table-1 below: Table-1 (Table Removed) ExampIe-2: The experiment is carried out as described in example 1 but by using alkaline potassium permanganate solution containing 115.26 g. of potassium hydroxide, 124 g. of potassium permanganate in 12000 g. of demineralised water. The results are given in Table-2 below: Table-2 (Table Removed) The main advantage of the present invention is a continuous process viable to commercialisation. The other advantage of the invention is total removal of 2-chloro-l,l-difiuoroethylene from 1,1,1,2-tetrafiuoroethane makes the product suitable as per the specifications of international standard. We Claim:. 1. An improved process for the purification of 1,1,1,2-tetrafluoroethane which comprises passing a mixture of 1,1,1,2-tetrafluoroethane and 2- chloro-1,1-difluoroethylene through a tubular packed column with continuous circulation of potassium permanganate solution containing alkali such as herein described in a counter current flow with gas liquid contact time of 4 - 14 min., to remove the 2-chloro - 1,1- difluoroethylene and recovering the pure 1,1,1,2-tetrafluoroethane by conventional method. 2. A process as claimed in claim 1 wherein the alkali used is sodium hydroxide, potassium hydroxide. 3. A process as claimed in claims 1 - 2 wherein the contact time of the organic vapor to the alkaline potassium permanganate solution is 6 - 7 min. 4. A process for the purification of 1,1,1,2-tetrafluoroethane substantially as herein described with reference to the examples.

Documents:

471-del-2000-abstract.pdf

471-del-2000-claims .pdf

471-del-2000-correspondence-others.pdf

471-del-2000-correspondence-po.pdf

471-del-2000-description (complete).pdf

471-del-2000-form-1.pdf

471-del-2000-form-19.pdf

471-del-2000-form-2.pdf

471-del-2000-form-3.pdf