Simulation Of Particle Size Distribution In Gas Phase Ethylene Polymerization Fluidized-Bed Reactor

In gas phase polymerization fluidized-bed reactor, polymerization rate, process stability, cost of polymer process stage and polymer properties may be affected by polymer particle size distribution (PSD). Polyethylene manufactories want to know not only the polymer's PSD in reactor and what factors affect on PSD but also how to control it for the aim of satisfying manufactories demand on PSD. This thesis aims at the research on modeling polyethylene particle size distribution of Unipol-LLDPE system.A steady-state particle size distribution model is developed based on mass balance. This model synthetically takes into account the particle growth, attrition and emulation based on solid population balance. The effects of feed catalyst particle size, catalyst's PSD, superficial gas velocity, bed temperature, concentration of ethylene, proportion of ethylene to butylene, bed pressure, concentration of butylene and hydrogen on polymer size distribution were investigated. The computation result shows the former six factors strongly affect polymer's PSD, and latter three's effect could be neglected. Comparing model predictions with experimental results on polymer's PSD shows that this model could predict the polyethylene's PSD in the industrial fluidized bed reactor with fairly good accuracy.On the basis of steady-state model, a time series model is proposed to investigate how the average polymer size changes during fluctuation of operating parameters, including olefin flowrate, butylenes flowrate, hydrogen flowrate, reacting temperature, catalyst size distribution and superficial gas velocity. Excursion of both instantaneous and cumulative average size occurs as soon as fluctuation begins. And when fluctuation stops, average size will return to its former value. Because the range of fluctuation of cumulative average size is small during the fluctuation process, the system can be considered as a steady one.Comparing how long it is between the fluctuation beginning and influence the polymer particle size happening, we find olefin and butylenes and hydrogen flowrate act on polymer particle size more slowly than catalyst size and reacting temperature and superficial gas velocity. Therefore, in order to advance the control time of polymer particle size, we should control the two proceeding parameters earlier than the three latter parameters during unsteady state operation.In part three, by analyzing how to plan operating parameters in order to fulfill the polymer manufactories demand about the PSD, a new model, named PSDcustomization model, is given. The objective isparticle swarm optimizer is selected as optimization algorithm. In the end, three cases are studied to illuminate the feasibility and validity of the model.