The use of a two-zone fluidized bed reactor (TZFBR) has been proposed in previous works for several reactions, such as the oxidative dehydrogenation of butane, as a new system for carrying up catalytic oxidation. Although the oxidative dehydrogenation is a reaction of clear interest, it has not been employed industrially because the achieved selectivities are not enough.
On the other hand, the oxidation of butane to maleic anhydride is already being done industrially, and therefore improvements in the process economy will result in a more competitive production. In the TZFBR two different zones appear: in one zone the catalyst is reduced, providing oxygen from the lattice during the oxidation of the hydrocarbon; in the second zone the catalyst is reoxidized with oxygen from the gas phase. By a careful selection of the operating conditions the hydrocarbon is oxidized with low or null gas phase oxygen, which can give improvements in terms of selectivity or safety. This may be an interesting alternative to the circulating fluidized bed that is being employed in some catalytic oxidations, with the reaction in a reactor and the catalyst reoxidation in another reactor. The TZFBR avoids the use of cumbersome standpipes connecting both reactors, since the two zones are located in the same vessel.
In such way two functions are made in a single reactor: the catalyst is oxidized in one zone and the hydrocarbon is oxidized, with low or null oxygen concentration in the gas phase in the other zone. The TZFBR may be advantageous in respect to the circulating fluidized bed reactor when the desired residence time of the catalyst in the reduction zone is larger than a few seconds.
Maleic anhydride (MA) has become an important chemical intermediate, finding applications in a large variety of products, from acids to surfactants. Most of the MA is utilized for the production of unsaturated polyether resins, which represents about 60 % of its total outlets.
There has been a considerable interest in the selective oxidation of n-butane since Bergman and Frisch1 disclosed that this reaction could be catalyzed by Vanadium phosphorus catalysts. The use of n-butane as a feedstock in the commercial production of maleic anhydride began in 1974 at Monsanto, using a fixed bed reactor system. By late 1985 there was no commercial manufacture of maleic anhydride in the United States by other than n- butane based processes. Currently, all new processes to obtain maleic anhydride are based on n-butane oxidation scheme. They only differ by the type of reactor used, the procedure for recovering the effluent (water versus organic quenching), and the final purification system. Packed and Fuidized bed have been the preferred reactors to develop the n-butane partial Oxidation. Fixed bed is a well known technology where improvements in selectivity are only possible by improving the catalyst and there are other drawbacks associated with the use of packed beds in the MA production, such as the hot spots formation and the dilution conditions which must be used to avoid flammability limits (maximum concentration of butane in air around 1–2%).
