Dynamic Modeling of Polyethylene Grade Transitions in Fluidized Bed Reactors Employing Nickel−Diimine Catalysts

Dynamic simulations of grade transitions are conducted for an industrial fluidized bed polyethylene reactor using a nickel-diimine catalyst. The reactor process model incorporates a kinetic model that predicts polymer properties for nickel-diimine ethylene polymerization. The relationships between reactor process variables and the end polymer properties are determined to be more complex than for conventional early transition-metal catalysts. In this respect, the kinetic model is demonstrated to be an effective tool for mapping out the reactor steady-state operating regions for a range of polyethylene grades and for examining process issues specific to using these catalysts. Temperature, monomer concentration, and hydrogen composition are the primary manipulated variables for achieving grade transitions in polymer density and melt index. Transitions involving changes in temperature require continual simultaneous adjustment of hydrogen and monomer feed rates to maintain the desired gas composition. Thus, where possible, it is desirable to keep the temperature constant while only changing monomer and hydrogen concentrations when making product transitions to simplify reactor operation. Quick venting and overshoot strategies are demonstrated to aid in improving product transition times. The risk of particle melting is heightened because both temperature and monomer concentration have a large influence on the branching density and the melting point of the polymer.