1. Field of the Invention
This invention relates to the purification of 1,1,1,2-tetrafluoroethane by causing at least one unsaturated impurity contained in the 1,1,1,2-tetrafluoroethane to react with hydrogen fluoride (HF).
More particularly, this invention relates to a method for the purification of 1,1,1,2-tetrafluoroethane (hereinafter referred to briefly as "HFC-134" or "CF.sub.3 CH.sub.2 F"), which has been attracting attention as a prospective substitute for CFC-12, which is a refrigerant used extensively in automobile air conditioners, refrigerators, etc. that is hazardous to the environment, particularly, the ozonosphere.
2. Description of the Prior Art
CF.sub.3 CH.sub.2 F is produced using a method that comprises fluorinating by using a chromium type catalyst 1,1,1-trifluoro-2-chloroethane (hereinafter referred to as "HCFC-133a" or "CF.sub.3 CH.sub.2 Cl") which is produced on a commercial scale as a raw material for trifluoroethanol [KOKOKU (Japanese Examined Patent Publication) No. 43-10601 and U.S. Pat. No. 4,158,675], a method that comprises adding hydrogen fluoride to trifluoroethylene (CF.sub.2 .dbd.CHF) (KOKOKU No. 62-23728), and a method that comprises causing 2,2-dichloro-1,1,1,2-tetrafluoroethane (CF.sub.3 CCl.sub.2 F) or 2-chloro-1,1,1,2-tetrafluoroethane (CF.sub.3 CHClF) to react with hydrogen in the presence of a palladium catalyst (KOKOKU No. 56-38131) have been known.
When these methods are adopted for the production of CF.sub.3 CH.sub.2 F, the relevant reactions entail by-productions of various impurities depending on the used catalyst, reaction conditions, etc.
The impurities occurring as by-products in the reactions include unsaturated impurities such as CF.sub.2 .dbd.CClF, CC1F.dbd.CHCl, CF.sub.2 .dbd.CHCl, CHF.dbd.CClF, CF.sub.2 .dbd.CHF, and CHCl.dbd.CHF, chlorofluorocarbons such as CCl.sub.2 F.sub.2, CH.sub.2 C1F, CH.sub.2 Cl.CClF.sub.2, CF.sub.3 CHCl.sub.2, and CF.sub.3 CHClF, and hydrofluorocarbons such as CF.sub.3 CHF.sub.2, CF.sub.3 CH.sub.3, and CHF.sub.2 CHF.sub.2, for example.
Of these impurities, the produced CF.sub.3 CH.sub.2 F tolerates the presence of hydrofluorocarbons when their contents are small but does not tolerate the presence of unsaturated impurities and chlorofluorocarbons even when their contents are extremely small. These impurities, therefore, are removed from the product by fractional distillation, for example.
The impurities that have boiling points close to the boiling point of CF.sub.3 CH.sub.2 F and the impurities that occur in the form of an azeotrope, however, are extremely difficult to be removed by distillation. Particularly, the unsaturated impurities persist as trace impurities in the product even after distillation.
Various processes have been proposed so as to overcome the difficulties encountered in the distillation.
(1) In the conventional method for the production of HFC-134a, trichloroethylene as a raw material and HF are introduced into a first reactor. The produced gas is composed predominantly of HCFC-133a, hydrogen chloride (HC1), and unreacted HF.
When this produced gas is introduced as it is into a second reactor, and since it contains hydrogen chloride in a large amount, the reaction that ensues forms a disadvantageous equilibrium, as represented by the formula (1), and produces virtually no HFC-134a, the desired product. EQU CF.sub.3 CH2Cl+HF.about.CF.sub.3 CH.sub.2 F+HCl (1)
This gas, therefore, is subjected to separation and removal of hydrogen chloride by using a purifying system.
The remaining gas, either as it is or after replenishment of HF, is introduced into the second reactor. The gas produced in the second reactor is a mixture consisting of unreacted HCFC-133a and HF, desired HFC-134a, by-products comprising mainly unsaturated impurities, and hydrogen chloride.
This gas is supplied as it is to a third reactor and subjected therein to a reaction for the addition of HF to the unsaturated impurities. The gas produced in the third reactor is forwarded to a purifying system for the purpose of separation and removal of hydrogen chloride. The remaining gas is forwarded to a separating and purifying system to effect isolation of HFC-134a as the product aimed at. The HCFC-133a and HF, which are expelled from the HFC-134a, are recycled to the second reactor.
This process entails a decline in the efficiency of the reaction of the unsaturated impurities because the outlet gas from the second reactor is introduced into the third reactor in a form still containing HC1. It also suffers from a disadvantage in that the reactor is required to have a larger capacity because the effluent gas contains the HCFC-133a and HF in a large amount relative to the desired HFC-134a. The solution of this problem necessitates interposition of a purifying system between the second reactor and the third reactor and consequently entails an addition to the cost of equipment.
(2) Other processes have been proposed by the inventions disclosed in EP-O 446 869-A1, EP-O 449 614-A2, and EP-O 449 617-A2, for example.
In accordance with these processes of production, HCFC-133a and HF are supplied to a second reactor and the product of the reaction therein is a mixture consisting of the unreacted starting materials of HCFC-133a and HF, desired product of HFC-134a, by-products of HCFC-1122 (CF.sub.2 .dbd.CHCl) and the like, and hydrogen chloride.
This mixed gas is supplied as it is to a first reactor. At the same time, trichloroethylene as the raw material and HF are supplied thereto. The trichloroethylene reacts with HF to produce HCFC-133a and hydrogen chloride and HCFC-1122 reacts with HF and consequently converts into HCFC-133a.
The produced gas that occurs in the first reactor, therefore, is a mixture that consists of HCFC-133a, HFC-134a, HF, hydrogen chloride, a small amount of trichloroethylene, and other by-products. This produced gas is forwarded to a purifying system for the separation and removal of hydrogen chloride and for the subsequent separation of HFC-134a. The remaining HCFC-133a and HF are returned to the second reactor.
Though this procedure is characterized by diluting the reaction gas prior to the reaction in consideration of large exothermicity of the trichloroethylene reaction with HF, it entails the following drawbacks.
(a) In the above method (2), since the hydrogen chloride-containing effluent gas from the second reactor is wholly introduced into the first reactor, the concentration of hydrogen chloride within the first reactor is increased and the efficiency of the reaction of the unsaturated impurities by the procedure is lowered compared with the conventional method of (1).
(b) Further, since the produced gas from the second reactor is wholly introduced into the first reactor, the amount of gas to be received in the first reactor is increased and the reaction system requires an increased capacity compared with that required by the conventional method.