Parametric study of n-butane oxidation in a circulating fluidized bed reactor

DuPont's n-butane oxidation process uses VPP catalyst and CFB technology to produce maleic anhydride. Reactor operating conditions fall within the butane flammability envelope, and localized combustion reactions were experienced during operation. Experimental and theoretical investigations of local reaction behavior demonstrated pronounced pressure, temperature, and residence time effects on the onset of combustion and led to operating strategies to improve reactor productivity. DuPont's n-butane oxidation process uses a proprietary attrition-resistant vanadyl pyrophosphate (VPP) catalyst and circulating fluidized bed (CFB) technology for the selective oxidation of n-butane to maleic anhydride. In the redox mode, n-butane feed plus recycle gas are fed to the riser side of the CFB, and air is fed to the regenerator. To optimize both activity and selectivity, oxygen may be fed to the riser side together with n-butane. These conditions fall within the n-butane flammability envelope, and localized combustion reactions were experienced during operation of a pilot plant utilizing this reactor technology. Complementary experimental and theoretical investigations were conducted using a laboratory fluidized bed reactor and an elementary step kinetic model to study the local reaction behavior near an oxygen sparger placed in a fluidized bed of solid VPP catalyst and to evaluate the effect of operating conditions on the onset of combustion in the homogeneous gas phase regions of the reactor. The laboratory fluidized bed reactor was operated at temperatures of 320–390 °C, pressures of 3–4 bar, freeboard residence times of 4–8 s, and n-butane and oxygen compositions of 4–10 and 1–6%, respectively. For the low solids regions of the reactor, both experimental and modeling results demonstrated pronounced pressure and temperature effects on the onset of combustion in this range of operating conditions and that decreasing the residence time delays the onset of complete combustion to longer time or higher relative molecular oxygen compositions. Kinetic model predictions also showed a significant influence of reactant composition and reactor pressure on oxidation induction time.