Distillation: processes – separatory – Adding material to distilland except water or steam per se – Organic compound
Patent
1996-09-19
1999-07-27
Bhat, Nina
Distillation: processes, separatory
Adding material to distilland except water or steam per se
Organic compound
203 99, 570178, 570179, B01D 334
Patent
active
059284790
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a process for the purification of pentafluoroethane (HFA 125) and particularly to a process for removing the impurity chloropentafluoroethane (CFC 115) from pentafluoroethane.
Recently, processes have been proposed for the production of pentafluoroethane which has been proposed, together with other hydrofluoroalkanes such as 1,1,1,2-tetrafluoroethane, difluoromethane and 1,1,1-trifluoroethane, and blends thereof, as a suitable replacement for chlorofluorocarbons and hydrochlorofluorocarbons due to the zero ozone depleting potential of hydrofluoroalkanes.
Thus it has been proposed in Japanese Patent Application Nos Kokai 3099026, 1258632 and 4029941 to produce pentafluoroethane by contacting pentafluorochloroethane with hydrogen in the presence of a hydrogenation catalyst. Suitable conditions of temperature and pressure and suitable catalysts are described in the aforementioned Japanese patent applications.
It has also been proposed to manufacture HFA 125 by the fluorination of perchloroethylene, chlorotetrafluoroethane and/or dichlorotrifluoroethane by reacting perchloroethylene, chlorotetrafluoroethane and/or dichlorotrifluoroethane with hydrogen fluoride in the vapour phase at a temperature in the range of about 200.degree. C. to 500.degree. in the presence of a fluorination catalyst such as alumina, aluminium fluoride, aluminium oxyfluoride, chromia, halogenated chromia or chromium oxyhalide. The fluorination catalyst may also comprise one or more metals other than chromium or aluminium, for example zinc and/or nickel in combination with alumina and/or chromia.
However, these and other processes for the production of pentafluoroethane result in a product comprising pentafluoroethane together with impurities, in particular the chlorofluorocarbon impurity chloropentafluoroethane (CFC 115).
The removal of CFC 115 from HFA 125 by distillation is possible in theory but is impractical; the boiling points of the components are -38.7.degree. C. and -48.degree. C. respectively at atmospheric pressure and of the azeotrope thereof is -55.degree. C. at atmospheric pressure. To obtain pure 125 requires large stills with very low condenser temperatures. Moreover, the azeotropic composition is rich in 125 (around 90 wt % HFA 125) and hence azeotropic distillation is wasteful of HFA 125. The separation is even more difficult at superatmospheric pressure, where the relative volatility of HFA 125 versus CFC 115 is close to 1.0 as HFA 125 purity of 100% is approached.
We have now found that CFC 115 can be effectively removed from HFA 125 by a technique herein termed third component distillation.
According to the present invention there is provided a process for the purification of pentafluoroethane by removing chloropentafluoroethane therefrom which comprises adding to the impure pentafluoroethane a component which undergoes a non-ideal interaction with chloropentafluoroethane and/or with the azeotrope of chloropentafluoroethane and pentafluoroethane such that the volatility of chloropentafluoroethane and/or the azeotrope of chloropentafluoroethane and pentafluoroethane relative to bulk pentafluoroethane is increased and distilling the mixture.
The component added to the impure HFA 125 to increase the relative volatility of the CFC 115 is referred to hereinafter for convenience as "the third component" although it is to be understood that the invention is not limited to treatment of compositions containing only the two components HFA 125 and CFC 115.
The third component may be any compound which undergoes a non-ideal interaction with CFC 115 and/or the azeotrope of CFC 115 and HFA 125 whereby to increase the volatility of the CFC 115 relative to bulk HFA 125 and thereby enable removal of the CFC 115 from the HFA 125 by simple distillation. In general, the compound preferably will be polar and will have a boiling point below that of HFA 125 so that its effect is to increase the volatility of CFC 115 relative to bulk HFA 125 thereby enabling its removal as a top product from a distillation column o
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Corr Stuart
Ewing Paul Nicholas
Martin John Stuart
Watson Michael John
Bhat Nina
Imperial Chemical Industries plc
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