A CFD-PBM model considering ethylene polymerization for the flow behaviors and particle size distribution of polyethylene in a pilot-plant fluidized bed reactor

Abstract

In large-scale industrial fluidized bed reactors (FBRs) for polyethylene (PE) production, the prediction of the interactions between polymerization, gas–solid two phase flow and particle kinetics is very challenging. This paper aims at providing a new insight into the effects of ethylene polymerization on the particle flow behaviors and particle size distributions (PSDs) of PE in a pilot-plant FBR via a CFD-PBM modeling approach. An Eulerian–Eulerian two-fluid model involving ethylene polymerization kinetics is coupled with a population balance model (PBM) to track the polymer PSDs. The particle growth, aggregation and breakage are taken into account in the PBM. The predicted pressure drop and temperature agree well with the available experimental data. Compared with the cold-model, the flow characteristics of polymer particles and the PSDs show noticeable differences in the case of considering polymerization. A slightly higher bed expansion height and lower axial particle velocity are also presented in this case. The core-regions of the core–annulus flow structures in terms of the solid concentration and particle velocity are obviously broader than that of the cold-model. Moreover, due to the polymerization reaction and mass/heat transfer effects, more PE particles are concentrated on the middle–upper areas of the FBR. As expected, particle breakage leads to a decrease in the average PE particle diameter and an increase in the bed expansion height.