Organic compounds -- part of the class 532-570 series – Organic compounds – Halogen containing
Reexamination Certificate
2001-02-21
2001-10-02
Siegel, Alan (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Halogen containing
Reexamination Certificate
active
06297412
ABSTRACT:
The present invention relates to a process for purifying difluoromethane useful as e.g. a refrigerant.
Production of difluoromethane is, in many cases, carried out by a reaction method of fluorinating dichloromethane in a gas phase or a liquid phase in the presence of various catalysts such as antimony fluoride. In such production of difluoromethane, hydrogen fluoride (hereinafter referred to also as HF) is used in excess relative to dichloromethane as the starting material. Accordingly, in the reaction product obtained by the above reaction, a large amount of unreacted HF remains in addition to difluoromethane as the desired product, and it is necessary to remove HF from the reaction product.
In such separation of HF from the above reaction product, it is economically necessary to recover a large amount of HF contained in the reaction product simultaneously and to return it to the reaction system for re-use. However, difluoromethane and HF are mutually dissolved, and not only they are hardly separable by phase separation, but also they form an azeotropic mixture, whereby it is difficult to obtain difluoromethane of high purity containing no HF by simple distillation.
Heretofore, as a method of separating such a mixture comprising difluoromethane and HF, a method of employing sulfuric acid as an extractant for HF, is known as disclosed in W098/08789.
In the above-mentioned conventional method of employing sulfuric acid as an extractant, in order to recycle HF extracted with sulfuric acid, it is necessary to distill it to separate sulfuric acid and recover HF, whereby there will be a problem due to its strong corrosiveness.
The present inventors have conducted a study for a method of separating HF from a mixture comprising difluoromethane and HF, which does not have the above-mentioned problem, and as a result, have arrived at the present invention.
Namely, the present invention provides a process for purifying difluoromethane, which comprises subjecting a mixture comprising difluoromethane and hydrogen fluoride to extraction treatment with at least one extractant selected from the following (a) and (b) to separate it by liquid separation into an extractant layer comprising the difluoromethane and the extractant as the main components, and a hydrogen fluoride layer comprising the hydrogen fluoride as the main component, and separating the difluoromethane from the extractant layer:
(a) Dichloromethane
(b) Chlorofluoromethane.
The substances of the above (a) and (b) as extractants to be used in the present invention, correspond to a starting material for producing difluoromethane by fluorinating dichloromethane with HF and an intermediate compound obtained at the time of the production, respectively. They have structures similar to difluoromethane as the desired product. Accordingly, their physical properties were expected to be similar to those of difluoromethane. However, contrary to such expectation, the present inventors have found that they show a small mutual solubility to HF, while they show a large mutual solubility to difluoromethane.
Thus, according to the present invention, at least one substance selected from the above (a) and (b), is used as an extractant, whereby high purity difluoromethane and HF can be efficiently extracted and separated from a mixture comprising difluoromethane and HF, and it has further been found that the separated HF can be re-used by returning it to the reaction system for producing difluoromethane, without separating the extractant contained therein. This is possible since the extractants to be used in the present invention, are the starting material and the intermediate for difluoromethane, and the extractants dissolved in HF can be converted to difluoromethane as the desired product, by returning them to the reaction system.
Now, the present invention will be described in detail with reference to the preferred embodiments.
The mixture comprising difluoromethane and HF to be treated in the present invention, is the one comprising difluoromethane and HF as the main components, wherein the respective components are present in the form of liquid and/or gas. The proportions of difluoromethane and HF contained in the mixture are not particularly limited, but typically, the mixture contains from 5 to 1,000 parts by weight, preferably from 10 to 100 parts by weight, of HF, per 100 parts by weight of difluoromethane. If the amount of HF is smaller than 5 parts by weight, the effect of using the extractant tends to be small. On the other hand, if the amount of HF exceeds 1,000 parts by weight, the amount of difluoromethane dissolved in the HF layer tends to be large, whereby the separation efficiency tends to be poor.
A typical example of the mixture comprising difluoromethane and HF to be treated by the present invention, is a reaction product obtained by fluorinating dichloromethane with HF in the presence of various catalysts such as antimony fluoride. This reaction product contains, in addition to difluoromethane and HF, small amounts of the substances of (a) and (b) to be used as extractants in the present invention and e.g. hydrogen chloride, but the presence of such additional components will not adversely affect the operation of the present invention. It is rather advantageous that the substances of (a) and (b) are contained, since the amount of the extractant to be used, may be reduced.
The extractant to be used in the present invention is at least one member selected from (a) dichloromethane and (b) chlorofluoromethane. Such an extractant can be obtained as an intermediate during the production of difluoromethane, but may of course be supplied from exterior of the system. As the extractant of the present invention, it is particularly preferred to use dichloromethane (a) among (a) and (b), for the reason that the solubility of HF is low. Of course, the substances of (a) and (b) may be used in combination, and other extractants may be used in combination.
In the present invention, the amount of the extractant is preferably from 0.3 to 30 times by a molar ratio to the difluoromethane. If the amount of the extractant is smaller than 0.3 time, the extraction efficiency tends to be poor, and the difluoromethane tends to remain in the HF layer. On the other hand, if it exceeds 30 times, the efficiency for recovery of difluoromethane by distillation tends to be poor. Particularly preferably, the amount of the extractant is from 1 to 10 times.
In the present invention, the extraction treatment with the extractant can be carried out by adopting a method of contacting the extractant with the mixture comprising difluoromethane and HF as intimately as possible. Such a contacting method is not particularly limited. For example, stirring or contacting by means of a line mixer can be employed. The liquid separation is carried out by a known method. The extraction and the liquid separation may be conducted at the same time by using the same apparatus, or may be conducted separately by means of separate apparatus.
The temperature for the extraction and the liquid separation is preferably from −40 to 50° C., which is the temperature at which the extractant containing HF and difluoromethane after the extraction, will be liquefied. If the temperature is lower than −40° C., the cost for the cooling installation tends to be high, such being undesirable. On the other hand, if it exceeds 50° C., the treating pressure is required to be maintained to be high, whereby the cost for the apparatus tends to be high, such being undesirable. Particularly preferably, the extraction and the liquid separation are carried out at a temperature of from −30 to 30° C. The pressure is related to the temperature, but it is preferably from 0.1 to 3 MPa, particularly preferably from 0.1 to 1 MPa.
The above extraction treatment is followed by liquid separation into two layers i.e. an extractant layer comprising the difluoromethane and the extractant as the main components, and a HF layer comprising HF as the main component.
The extractant layer is preferably subjecte
Suzuki Yasuhiro
Yanase Koichi
Yokoyama Takaaki
Asahi Glass Company Limited
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Siegel Alan
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