The present invention relates to the activation of fluid bed oxidation catalysts. More particularly the present invention relates to the activation of fluid bed oxidation catalysts useful in the preparation of maleic anhydride from 4-carbon atom hydrocarbons, including n-butane.
Oxidation catalysts containing the mixed oxides of vanadium and phosphorus have been utilized to produce maleic anhydride from 4-carbon atom hydrocarbons, such as n-butane. Methods have been investigated for activating, or increasing the catalytic activity of these catalysts. It is taught in the literature to "condition" vanadium phosphate-containing maleic anhydride catalysts under the flow of a low level of hydrocarbon in air, such as 0.2 volume percent to 2 volume percent hydrocarbon in air at temperatures of 300.degree. C. to 600.degree. C., as in U.S. Pat. No. 4,171,316.
U.S. Pat. No. 3,985,775 discloses the preparation of vanadium phosphorus mixed oxide catalysts prepared by a method requiring heating the catalyst precursor at about 350.degree. C. to 410.degree. C. to drive off at least a portion of hydration, and then at a higher temperature. This method is taught as suitable for use in fixed bed operation.
Attempting to "condition" a fluid bed catalyst with low levels of hydrocarbon in air under normal operating conditions has been found to have little beneficial effect.
Activation procedures for fixed bed catalysts are not readily applicable for use with fluid bed catalysts. In fixed bed operation, a characteristic exotherm or hot spot is generated in the catalyst bed within the reactor vessel. A substantial portion of the total reaction takes place at this hot spot. The location of the hot spot is dependent upon the location of the oxygen and feed stock inlets, the dimensions of the vessel, and operating conditions such as flow rates, temperature and pressure. Fixed bed activation procedures generally may concentrate in the hot spot of the catalyst bed, having little effect on the remainder of the catalyst in the reactor.
In fluid bed operation, such a hot spot is not generated. Catalyst particles are not localized, but move throughout the reactor vessel, and substantially the entire portion of the catalyst within the reactor vessel contacts the feed to contribute to the reaction.