Simulation And Optimization On Acrylonitrile Fluid-bed Reactor

The competition among acrylonitrile(AN) trades is on cost because the acrylonitrile technology is relatively ripe. It is the main way for the acrylonitrile factories to increase their competition power by optimizing operations of AN reactor and reducing unit consumption of propylene. The studing acrylonitrile plant increased its capacity from 25000 t/a to 40000 t/a by adopting new techniques. After enlarging capacity, the structural parts such as the distributor of reactor, cyclone separator should be changed, so the reactor behavior also changed which influenced the reaction recovery ratio. In this article, the AN fluidized-bed reactor was modeled and optimized which based on reaction mechanism. The off-line optimization of reactor operating conditions was carried out. Meanwhile, the problem about extra high of catalyst bed was analysed, and the scheme to solve such problems were proposed.The appropriate reaction network and mathematical models of the kinetics were selected based on the literatures, and the parameters in the kinetics were corrected by using conversion of propylene at reactor exit. Under different operating conditions, the kinetics model showed the good fitness with plant data. The model for the fluidized height was developed which parameters were determined by plant data. The result shows that the model was good to predict the practical operating conditions.The result of kinetic simulation shows that the propylene ammonia-oxidation is irreversible and rapid. The ratio by materials and the progressing control of reaction are the major factors to determine the yield. The reaction of propylene is fast at the beginning stage of reaction and is completed at the middle part of fluidized bed. In the upper part of reactor (catalyst dilute phase part), the major reactions proceed slowly, and are accompanied by the over-oxidation of acrylonitrile. The height computation model of fluidize-bed showed that the bed height is mainly influenced by catalyst filling amount, linear speed of reaction gas and particle size of catalyst.On the purpose of controlling progress of reaction to increase the yield ofacrylonitrile, this article proposed the following practicable ways of optimization. The first is to assure rational ratio of gas to propylene, and ammonia to propylene, to supply as much as possibly the crystal-lattice oxygen and reactive ammonia at the bottom of reactor to maximize the conversion of propylene to acrylonitrile, enhance the selectivity of catalyst to AN. The second is to keep reasonable line speed and contact time, assure as higher conversion ratio and selectivity. The third is to keep the effective distribution of hot-moving tubes in the concentrated phase of catalyst bed, restrain the over-oxidation of acrylonitrile in the upper part of reactor.