Organic compounds -- part of the class 532-570 series – Organic compounds – Halogen containing
Reexamination Certificate
2000-01-28
2003-10-21
Barts, Samuel (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Halogen containing
C570S166000, C570S167000, C570S168000, C570S169000
Reexamination Certificate
active
06635790
ABSTRACT:
The present invention relates to the field of fluorinated hydrocarbons and has more particularly as subject a continuous process for the manufacture of difluoromethane from methylene chloride and hydrogen fluoride HF.
Now that chlorofluorohydrocarbons (CFCs) have been identified as one of the factors responsible for accelerating the deterioration in the stratospheric ozone layer, politicians and industrialists have been irrevocably committed to a process of substitution of CFCs. This substitution process relates to essential industrial sectors such as the food refrigeration procedure, the insulation of buildings, air conditioning, microelectronics, and the like.
The substitutes envisaged are fluorinated compounds containing hydrogen atoms but not chlorine atoms. One of these compounds, which is without effect on the ozone layer, is difluoromethane, which is known in the trade under the designation F32 and is mainly intended to replaced F22 (chlorodifluoromethane) and R502 (azeotropic mixture of F22 and chloropentafluoroethane) in the field of refrigeration, air conditioning and other applications. There is therefore interest in developing the simplest possible procedure for producing F32 in large and economically competitive amounts.
Access to F32 by the gas phase fluorination of methylene chloride, known in the trade under the designation F30, has already formed the subject of patents claiming the use of catalysts such as Cr
2
O
3
, CrF
3
, Cr/carbon, Ni/AlF
3
, and the like.
However, like that of the majority of substitutes for CFCs, the production of F32 poses serious problems because it generates a great many by-products and impurities which, after separation of the HCl and F32, are found either in the recovered HCl and F32 or in the flow to be recycled mainly comprising F30 and F31 (chlorofluoromethane).
In the case of the production of F32 by fluorination of methylene chloride, the most serious problem is posed by the generation, as intermediate compound, of large amounts of highly toxic F31. These contents can be of the order of 20% and it is therefore imperative to limit as far as possible the circulation and the residence time of this compound in the plant, as well as the unit operations involving flows containing F31.
In the catalytic fluorination of F30, the degree of conversion of F30 to F32 is limited by thermodynamics. Typically, for an HF/Organics molar ratio equal to 3 at the inlet of the reactor and a reaction temperature of 300° C., thermodynamic equilibrium corresponds to degrees of conversion of F30 of 65% and of HF of 43%. The flow emerging from the reactor thus mainly contains unconverted reactants (F30, F31 and HF), which it is essential to recycle. To do this, it is possible, in accordance with conventional techniques, to separate and then to purify the main constituents of the flow exiting from the reactor, in particular the unconverted F30, F31 and HF, in order to remove therefrom, before recycling in the reaction, the harmful impurities (such as organic by-products or water) which are generated in the reaction or introduced by the starting materials and which are capable of bringing about-deactivation of the catalyst or of causing corrosion.
During this type of manipulation, the flows to be treated are highly concentrated in F31, which requires reinforcement of the safety measures and equipment and thus an increase in costs.
The direct recycling of the flow of HF, of F30 and of F31 to the reactor after separation of the HCl and F32 produced, without prior purification, has the advantage of limiting the manipulations of flows containing concentrations of F31. This is the reason why the majority of patents describing a process for access to F32 by fluorination of F30 with anhydrous HP in the gas phase mention the direct recycling of the unreacted products (F30, F31 and HF) to the reactor after separation of the HCl and F32 produced (Patent Applications JP 5-50953/93, WO 94/21579 and WO 95/12563).
Moreover, it is known (Patent Applications JP 51-82206 and JP 49-134612) that the continuous injection of oxygen or air or chlorine can increase the lifetime of fluorination catalysts, which have a tendency to become coked or to crystallize very rapidly. However, during the synthesis of F32 in the presence of oxygen, conventionally used to maintain the catalytic activity, the recycling of the crude products to the reactor causes considerable and rapid (less than 100 hours) deterioration in the performance of the catalyst with respect to a recycling-free operation.
It is probably for this reason that the use of oxygen or chlorine for maintaining the activity of the catalyst during the gas-phase fluorination of F30 is not mentioned in any of the patents describing a process for access to F32 by fluorination of F30 with anhydrous HF in the gas phase with direct recycling of the unreacted products (F30, F31 and HF) to the reactor after separation of the HCl and F32 produced.
It has now been found that the injection of chlorine with the reactants (F30 and HF) is not only more effective than the injection of oxygen in stabilizing the catalytic activity but allows, without disadvantage, the direct recycling (without purification) of the flow of unreacted products (F30, F31 and HF).
The subject of the invention is thus a continuous process for the manufacture of F32 from F30 and HF in the gas phase in the presence of a fluorination catalyst, characterized in that the reaction is carried but in the presence of chlorine and in that the gas flow exiting from the reactor is subjected to a distillation in order to separate, at the top, a flow containing virtually all the hydrochloric acid and at least 90% of the F32 produced by the reaction and, at the bottom, a flow containing at least 90% of the unconverted reactants (F30, F31 and HF) present in the gas flow exiting from the reactor and in that the flow recovered at the distillation bottom is recycled directly to the reactor, without any purification operation.
As might have been expected, the recycling of the unconverted reactants directly to the reactor, in the absence of specific purification of the recyclate, results in a certain degree of accumulation of water and of organic by-products in this recyclate. Under stabilized operating conditions, the content of these organic by-products becomes stabilized in a stationary state. Curiously, the nature of these by-products and their content do not hinder the performance of the catalyst:
degree of conversion of F30 similar to the thermodynamic equilibrium state of the reaction CH
2
Cl
2
+2HF
CH
2
F
2
+2HCl
high selectivity for F32, typically of the order of 80 molar %.
In the implementation of the process according to the invention, this performance (activity, selectivity) remains stable for at least 1000 hours; this makes it possible to avoid frequent operations of replacement or regeneration of the catalyst, which operations result in high costs with respect to investment and operating costs. In addition, the process according to the invention is all the more safe in that it does not involve operations of purification of the flow to be recycled and thus production of effluents containing toxic F31.
The fluorination catalyst to be used for the implementation of the process according to the invention can be a bulk catalyst or a supported catalyst, the support stable in the reaction mixture being, for example, an active charcoal, an alumina, a partially fluorinated alumina, aluminium trifluoride or aluminium phosphate. Partially fluorinated alumina is understood to mean a composition which is rich in fluorine and containing mainly aluminium, fluorine and oxygen in proportions such that the amount of fluorine, expressed as AlF
3
, constitutes at least 50% of the total weight. A catalyst based on chromium is preferably used.
Mention may more particularly be made, among bulk catalysts, of chromium(III) oxide, prepared according to any one of the methods known to the person skilled in the art (sol/gel process, precipitation of the hydroxide from chromium salts, redu
Garrait Dominique
Guiraud Emmanuel
Atofina
Pennie & Edmonds LLP
Price Elvis O.
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