Fluidized-bed reactors have conventionally been employed extensively for producing an unsaturated nitrile by reacting ammonia, an oxygen-containing gas, and propylene, isobutylene, or tert-butyl alcohol by gas-phase catalytic reaction.
A fluidized-bed reactor usually has a gas dispersing device, a cooling coil, etc. in a lower part thereof. It further has a cyclone in an upper part thereof to conduct catalyst/gas separation.
The technique of feeding ammonia, methanol, and an oxygen-containing gas in the course of the whole contact time of a fluidized-bed reactor to cause ammoxidation reaction is known as disclosed in JP-B-54-8655. (The term "JP-B" as used herein means an "examined Japanese patent publication".) There is a description in the above reference to the effect that in a fluidized-bed reactor having an inner diameter of 2 inches and a height of 2 m, the ammoxidation reaction of methanol can be conducted without lowering the yield of acrylonitrile. In U.S. Pat. No. 5,288,473 is given an example in which a larger reactor for acrylonitrile is packed with about 18 t of a propylene ammoxidation catalyst. However, in a so-called commercial reactor having a diameter of 3 m or larger, production of the target .alpha.,.beta.-unsaturated nitrile is inhibited because the methanol and the oxygen-containing gas which have been fed are mixed with the circulating fluidized bed catalyst and thus reach the region where ammoxidation reaction occurs between propylene, isobutylene, or tert-butyl alcohol and ammonia, and because methanol has a higher reaction rate than propylene, etc. Consequently, in industrial apparatuses, the feeding of methanol and an oxygen-containing gas results in a reduced yield of an .alpha.,.beta.-unsaturated nitrile as compared with the case in which these ingredients are not fed. Sufficient investigations have not been made so far on measures for solving the above problem.
Furthermore, when an industrial fluidized-bed reactor having an inner diameter of 3 m or larger which employs a molybdenum-bismuth catalyst support on silica is used in such a manner that methanol and an oxygen-containing gas are fed thereto in the course of the whole contact time of the fluidized-bed reactor to cause ammoxidation reaction, then an oxide of molybdenum, which is a main constituent element of the catalyst, deposits on the feed openings of a methanol dispersing pipe with the lapse of time. This deposition finally results in clogging of the feed openings of the methanol dispersing pipe to pose an industrially serious problem that unreacted ammonia cannot be stably diminished. On measures for solving this problem also, sufficient investigations have not been made so far.
An object of the present invention is to provide a method of diminishing unreacted ammonia in an industrial fluidized-bed reactor used for producing an .alpha.,.beta.-unsaturated nitrile through reaction of propylene, isobutylene, or tert-butyl alcohol, wherein unreacted ammonia is reacted with methanol and an oxygen-containing gas both fed in the course of the whole contact time of the fluidized-bed reactor to thereby diminish unreacted ammonia without lowering the yield of the .alpha.,.beta.-unsaturated nitrile. Another object is to provide an industrial method of stably diminishing unreacted ammonia by preventing the clogging of the feed openings of a methanol dispersing pipe with a molybdenum oxide which clogging occurs when the above reaction is conducted in an industrial fluidized-bed reactor having an inner diameter of 3 m or larger and filled with a molybdenum-bismuth catalyst supported on silica.