Three-phase model of a fluidized-bed catalytic reactor for polyethylene synthesis

Abstract

A mathematical model of a bubbling fluidizedbed reactor for the production of polyolefins is presented. The model is employed to simulate a typical, commercialscale reactor where the synthesis of polyethylene using supported catalysts is carried out. Results are used to follow the evolution of temperature within the reactor bed to avoid conditions producing polymer degradation. The fluidized bed is modeled as a heterogeneous system with a bubble gas phase and a solid-particle emulsion. The catalyst active sites are considered located within a growing, solid, ever changing particle composed of the support, the catalyst and the polymer being produced. The model sees the reactor as a three phase complex: (a) the bubble phase, transporting most of the gas entering the reactor; (b) the solid-particle phase, where polymerization takes place; and (c) the interstitial-gas phase among solid particles. Both gaseous phases move continuously upward, with different velocities, and are modeled as plug flows. For the solid-particle phase, modeling alternatives are explored, ranging from a descending plug-flow limiting case to the opposite extreme of a perfectly mixed tank related to the particle drag-effect the rising bubble produces in the bed. In the scouting process between these limits instabilities are predicted by the model. The most realistic representation of the bed is that of the two gas phases moving upward in two plugflow patterns and the solids moving with ascending and descending trajectories due to back-mixing.