Organic compounds -- part of the class 532-570 series – Organic compounds – Halogen containing
Reexamination Certificate
1997-11-13
2001-10-23
Siegel, Alan (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Halogen containing
C570S180000
Reexamination Certificate
active
06307115
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the purification of pentafluoroethane (F125) containing chloropentafluoroethane (F115) and relates more particularly to a purification process in which the F115 is removed by liquid/liquid extraction or by extractive distillation and readily recovered so that it can be converted subsequently into products that are harmless to the earth's atmosphere.
BACKGROUND OF THE INVENTION
Pentafluoroethane is one of the possible replacements for chlorofluorocarbons (CFC), which are the subject-matter of the Montreal protocol and are characterized by an exceptionally long lifetime which allows them to reach the upper layers of the atmosphere and thus play a part, under the influence of UV radiation, in destroying the ozone layer. It is thus obvious that, as a function of the various production processes, their replacements should contain only traces of these CFCs.
The replacements are generally obtained either by appropriate fluorination methods, which are not highly selective and may generate perhalo compounds of the CFC type by dismutation, or are obtained from CFCs themselves by reduction methods, in practice by hydrogenolysis reactions. Thus, pentafluoroethane (F125) may be prepared by fluorination of perchloroethylene or of its intermediate fluorination products such as dichlorotrifluoroethane (F123) and chlorotetrafluoroethane (F124) or by hydrogenolysis of chloropentafluoroethane (F115). In both cases, the F125 produced contains non-negligible amounts of F115 which, since F115 is a CFC, should be removed as fully as possible.
Now, the existence of an F115/F125 azeotrope at 21% by weight of F115 (see U.S. Pat. No. 3,505,233) with a boiling point (−48.5° C. at 1.013 bar) which is very close to that of F125 (−48.1° C.) makes it virtually impossible to separate F115 and F125 completely by distillation. The F115 can thus only be removed from the F125 via a chemical route or by physical methods involving an intermediary substance.
In Patent Application EP 508,631, which describes the production of hydrofluorocarbons (HFC) by liquid-phase chemical reduction of chloro, bromo or iodo compounds with a metal hydride or a complex of such a hydride, it is indicated that this process may be advantageous for purifying certain HFCs such as F125. With the same aim, the Japanese patent application published (Kokai) under the No. 2001414/90 uses metal redox couples in a solvent medium. Other techniques, such as that described in Journal of Fluorine Chemistry, 1991 vol. 55, pp. 105-107, use organic reducing agents such as ammonium formate in DMF medium and in the presence of ammonium persulphate.
These processes, which use reactants that are difficult to handle (metal hydrides) or that are liable to pose effluent problems, are relatively incompatible with industrial production of F125 in large tonnage.
For the industrial manufacture of F125, the technique of extractive distillation appears to be an ideal process for removing the residual F115.
In an extractive distillation process, the constituents of a binary mixture are separated using a so-called extraction column containing successively, from the boiling vessel to the head, three sections, one for depletion, the second for absorption and the third for recovery.
The binary mixture to be fractionated is injected into the top of the depletion section, whereas the intermediary substance acting as selective solvent is introduced into the top of the absorption section so as to circulate in the liquid state from its point of introduction to the boiling vessel.
The third section, known as the recovery section, serves to separate out by distillation the constituent which is least absorbed from the traces of solvent entrained under the effect of its non-zero vapour pressure.
The application of this technique to the purification of 1,1,1,2-tetrafluoroethane (F134a) is the subject of U.S. Pat. No. 5,200,431; the extraction agent used is a chlorinated solvent or an aliphatic hydrocarbon.
The application of extractive distillation to the purification of F125 is already described in U.S. Pat. No. 5,087,329, which uses as extraction agent a C
1
to C
4
fluorohydrocarbon optionally containing hydrogen and/or chlorine atoms and having a boiling point of between −39 and +50° C. According to the data in that patent, the dichlorotetrafluoroethanes (F114 and F114a) are at least three times as effective as the other compounds mentioned. Moreover, 5 of the 8 solvents mentioned are CFCs forming the subject-matter of the Montreal protocol and whose marketing should cease in the near future.
The industrial use of the process according to that patent can therefore be envisaged economically only when the extraction agent used forms part of the chain of intermediates leading to F125, that is to say, in fact, in processes for the preparation of F125 by hydrogenolysis.
In the case of the manufacture of F125 by fluorination of perchloroethylene or of its partial fluorination products (F122, F123, F124), U.S. Pat. No. 5,087,329 only allows a choice between CFCs which will no longer be commercially available and products of lower performance such as F124 or F123.
DESCRIPTION OF THE INVENTION
It has now been found that perchloroethylene is of very much higher selectivity than chlorofluoroethanes and that there is also a wide settling range for F125/F115/perchloroethylene mixtures.
Settling of these mixtures makes it possible to obtain:
a lower phase rich in perchloroethylene containing F125 enriched with F115 relative to the starting mixture of F125+F115 to be treated,
an upper phase which is rich in F125 and depleted in F115 relative to the starting mixture of F125+F115 to be treated.
The F125/F115 ratio thus becomes established in the two phases in the following way:
Upper phase>initial mixture>lower phase
The subject of the present invention is thus a process for the purification of a pentafluoroethane containing chloropentafluoroethane by extractive distillation or by liquid/liquid extraction, characterized in that perchloroethylene is used as extraction agent.
The use of perchloroethylene as extraction solvent according to the invention is particularly advantageous to apply when it is desired to purify an F125 obtained by fluorination. The reason for this is that, in this case, the perchloroethylene used as extraction solvent is in fact the starting material used in the process for obtaining F125.
The process according to the invention may be carried out according to the well-known principles of extractive distillation or of liquid/liquid extraction, working under pressures of between 2 and 20 bar absolute, the temperatures being given by the liquid/vapour equilibrium diagrams of the individual constituents and of their mixtures.
REFERENCES:
patent: 5200431 (1993-04-01), Dattani et al.
patent: 6179967 (2001-01-01), Nishimura et al.
patent: 472391 (1992-02-01), None
patent: WO 95/21147 (1995-08-01), None
Research Disclosure, vol. 360, 1994, Havant, pp. 191-193 “Methods for Separating Chloro-Carbons fro Hydrofluoroalkanes”.
Descamps Cathy
Guiraud Emmanuel
Siegel Alan
Smith , Gambrell & Russell, LLP
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