Single Particle Modeling During Olefin Gas Phase Polymerization Process

Particle overheating is one of the most important phenomena in heterogeneous catalysis olefin gas phase polymerization reactors, resulting in particle agglomeration and worsely blocking FBR. It is impossible to effectively measure the ture temperature and monomer concentration inside polymer particle during polymerization at present. This paper developed single particle models to examine intra-particle concentration, temperature and particle growth changes with time. The objective is to give some suggestions in preparing catalyst and operating polymerization reactors so as to avoid the appearance of serious particle overheating and ensure the stability of reacors.Steady state and dynamic particle models were developed. Steady state models are simple steady state particle model, steady state mass transfer model, steady state mass and heat transfer model, whose model equations are algebric equations or nonlinear second order differential equations, using analytical or numerical menthod to solve them. The dynamic models were dynamic mass transfer model and dynamic mass and heat transfer models, whose equations are second order partial differential equations, using polylayer method to discrete and solve them.The results of steady state model show that: particle surface concentration is higher than that of the center and center temperature is higher than surface temperature; intra-particle temperature profile is uniform; as particle radius increases, particle temperature, concentration and temperature gradients decrease; boundary layer mass transfer resistance has little influence on the result; the rate expression should consider the dilution effect of catalyst active site inside particle.The dynamic particle model suggests: as reaction progresses, concentration gradient inside particle decreases; particle temperature distribution is uniform, reaches the highest within seconds and decreases below the melting point soon later; particle radius increases fast at the beginning of reaction and slow down afterawards. Parameters analysis shows that catalyst activity (pre-exponential factor of propagation reaction) and catalyst radius influence the model result the most. For high activity catalyst, metal load concentration and catalyst radius should not be too large at the same time, or it will result in serious particle overheating causing particle sticking and aggolormeration, meanwhile, the polymer radius could not grow large enough.The initial seconds of reaction have significant influence on polymer particle overheating, growth and morphology, and the key of avoiding particle overheating, agglomeration and blocking is controlling of the initial reaction period. Pre-polymerization is an effective method.