(70e) Multiphase Flow Modelling Approach for the LLDPE Fluidized Bed Reactors

Abstract

Gas phase polymerization in fluidized bed reactors (FBR), such as those used in the production of linear low density polyethylene (LLDPE), is a well-recognized technology for polyolefin production. Despite the complexity of reacting systems, FBRs have advantages in transporting solids in and out of the reactor, and fast reaction occurs due to high mass and transfer rates between the gas and solid phases. In LLDPE, the particle growth is influenced by catalyst particles, gas reactants (ethylene, co-monomer) and inert condensing agents in producing different products (grades) within the same reactor. The phenomena occurring in commercial size FBRs in reacting conditions cannot fully be explained by experiments on a small, bench scale. Although such experiments provide insight into particles behavior (such as flow profiles and softening) and kinetics, but fail to provide information that is fully transferrable to industrial scale systems. In addition, it can be challenging to model or conduct experiments in commercial size FBRs due to their large and multi scale, particles dynamic behavior, and turbulent flow. This can be attributed to the large number of particles that need to be accounted for in order to get results that are representative enough of the reactor dynamics in a bigger scale.

In this work, polyethylene (PE) softening measurements and lab scale 2D fluidization experiments were conducted on different LLDPE reactor products to determine particle properties, bubble properties, and the process conditions influencing their behavior in FBR. The experimental data were fitted into an Aspen-based FBR simulation module to deepen understanding of fluidization fundamentals. The industrial scale FBR for LLDPE production was also modelled using Aspen plus and multiphase flow computational fluid dynamics (CFD) tools, by incorporating hydrodynamics and reaction kinetics, and the results were validated with the available reactor data. The multiphase flow modeling results provided critical information on fluidization characteristics and thermal behavior, focusing on gas and particle distributions within the reactor. All these approaches (bench scale experimentation, and Aspen and CFD modeling) proved useful and showed how challenges, such as gas distribution in the reactor, bubble types and reactor hot spots, etc., can be traced back to the small particle (catalyst) behavior, which is shown on a small scale. This information was useful in assisting with the optimization of the industrial FBR performance.