A computational fluid dynamics model coupled with ethylene polymerization kinetics for fluidized bed polyethylene reactor

Abstract

This work aims at exploring the effect of macro operating conditions on the distribution of temperature field and chain microstructures of polymer in a gas phase ethylene polymerization fluidized bed reactor (FBR) by coupling polymerization kinetics with computational fluid dynamics (CFD). An ethylene polymerization kinetic model in terms of the method of moments was introduced to CFD model. In the modeling framework, the Flory's distribution and custom field functions were used to predict the molecular weight distribution (MWD). The results show that the average molecular weight decreases with the increasing of reaction temperature, and MWD becomes wider because of the increase of temperature gradient. With the increase of hydrogen concentration, the average molecular weight and MWD becomes smaller and narrower, respectively. Unlike the hydrogen concentration, an increase in the ethylene concentration leads to an increase in the average molecular weight, polydispersity distribution index (PDI), and temperature. An increase of gas velocity improves the heat transfer of FBR but results in a very wide MWD when gas velocity exceeds the optimum fluidizing velocity. This model is helpful to understand how the operating conditions affect the performance of FBR and can provide theoretical guidance for the operation and design of polyolefin FBR.