Fluorination of a haloalkane by exchange of fluorine for another halogen is the most generally used technique for preparing fluorinated alkanes. It is not usually practical to react a mixture of a haloalkane with HF as the reaction proceeds sluggishly and requires very high temperatures and pressures. In 1892 Swartz synthesized CCl.sub.3 F in a liquid phase reaction in which the reaction product of SbF.sub.3 and Br.sub.2 was contacted with CCl.sub.4; Bull. Acad. R. Belg., 1892(3), 24, 309. However, it was not until 1926 that CF.sub.4, the simplest perfluorocarbon, was isolated. At just about that point, the needs of the refrigeration industry prompted considerable development in the field. The first commercial synthesis developed thereafter (about 1930) was a continuous process wherein hydrogen fluoride and haloalkanes containing halogen other than fluorine were reacted in the presence of antimony pentachloride; Daudt et al., U.S. Pat. Nos. 2,005,705 and 2,005,708. The reaction is that of replacing Cl, Br or I of the haloalkane with fluorine of the hydrogen fluoride. Most generally, the haloalkanes of choice are chloroalkanes because of their availability and their tendency to undergo fewer side reactions during the exchange reaction than their bromo or iodo analogs. The fluorination reaction may be represented by the following equation using chloroform as the illustrative haloalkane: ##STR1## wherein x is 1-3. The process is carried out by continuously cofeeding hydrogen fluoride and haloalkane into antimony pentachloride. Generally, a mixture of CHCl.sub.2 F, CHClF.sub.2 and CHF.sub.3 is obtained, the particular proportion of the fluorinated products depending upon the reactant ratios and the reaction conditions. For each mol of hydrogen fluoride undergoing the exchange reaction, one mol equivalent of hydrogen chloride is generated. Usually HF is used in excess so as to ensure maximum utilization of the haloalkane reactant. Therefore, the crude reaction product will contain HF as well as a mixture of CHCl.sub.2 F, CHClF.sub.2, CHF.sub.3 and HCl.
Despite a number of disadvantages in it, the above-described continuous process has been the primary manufacturing process for preparing virtually all of the major fluorocarbons and chlorofluorocarbons which have been manufactured since industrial production of them commenced. Since the beginning of industrial production of such products, efficient utilization of HF, its removal from products and by-products of the reaction, and its recovery for re-use have been important considerations. In addition, it is necessary to remove virtually all by-product HCl in addition to significant amounts of HF which are found in the crude reaction product as an azeotrope with the fluorinated reaction products. Another factor which complicates purification of the fluorinated reaction products resides in the fact that boiling points of some of the fluorinated products overlap the boiling point of HCl or are not far removed from the boiling point of HF. For example, in the fluorination of chloroform, the crude reaction product includes CHCl.sub.2 F (b.p. 8.9.degree. C.),
CHClF.sub.2 (b.p. -40.8.degree. C.), CHF.sub.3 (b.p. -82.degree. C.), HCl (b.p. -85.degree. C.) and HF (b.p. 19.4.degree. C.). Moreover, the production of CHF.sub.3 normally exceeds its demand, so that its overproduction constitutes an economic penalty.
Aqueous scrubbing of the reaction product can be used to remove the by-product HCl and unreacted HF; Daudt et al. U.S. Pat. No. 2,005,705. However, such an approach is an uneconomic one since it sacrifices most of the HF as waste and necessitates subsequent sale of the by-product HCl as a 30% aqueous solution; Hamilton, Advances in Fluorine Chemistry, Vol. 3, Buttersworth (1963). Separation of products from by-products and unreacted HF has been accomplished by use of series of distillation techniques. But that means of purifying the products requires a great deal of energy for refrigeration and the installation of a great deal of pressurized equipment, resulting in high capital costs for construction of manufacturing facilities and high operating costs as well. Moreover, such anhydrous pressured distillation techniques do not effect separation of fluorinated product/HF azeotropes. Further processing is required, such as scrubbing with water (U.S. Pat. No. 2,450,414) or concentrated H.sub.2 SO.sub.4 (U.S. Pat. No. 3,873,629).
The corrosivity of HF is well known. In the presence of small amounts of antimony pentachloride (used in the continuous process described above), its corrosivity increases dramatically. The reactor systems used in the prior art continuous process have been known to fail because of corrosion. That effect is attributable to the fact that in that process, HF and the chlorocarbon are continuously fed into a static charge of antimony pentachloride, a liquid under the reaction conditions, in a reactor system that contains no mechanical agitation. As a consequence, there is a very high mol ratio of HF to antimony pentachloride at the point at which the HF first comes in contact with the antimony pentachloride. At such a high HF to antimony pentachloride ratio, the corrosivity of the mixture increases dramatically.
It is therefore an object of the present invention to provide an economical, energy-efficient, continuous process for the production of fluorinated haloalkanes. It is a further object of the present invention to provide a continuous haloalkane fluorination process which will provide crude fluorinated products substantially free of hydrogen halides, in particular CHClF.sub.2 free of its azeotrope with HF, and CHF.sub.3 as well as CClF.sub.3 free of close-boiling HCl. It is a still further object of the present invention to provide a continuous haloalkane fluorination process which minimizes costs associated with the purification of reaction products. It is another object of the present invention to provide a continuous haloalkane fluorination process which results in reduced corrosion of equipment used in the process. It is still another object of the present invention to provide a continuous haloalkane fluorination process which results in waste streams that are environmentally more acceptable. Yet another object of the present invention is to provide a continuous haloalkane fluorination process in which anhydrous HCl is recovered substantially free of HF and fluorinated haloalkanes. Still another object of the present invention is to provide greater flexibility in the production of fluorinated chloromethanes in which the overproduction of low-demand fluorinated products, such as CHF.sub.3, can be controlled and minimized as desired. A still further object of the present invention is to provide a continuous haloalkane fluorination process in which the utilization of HF is essentially complete. These and other objects will be apparent from the description provided herein.
USPTO Backgrounds
USPTO Backgrounds