On the relationship between operating pressure and granular temperature: A discrete particle simulation study

Low density polyethylene and polypropylene are produced at large scale via the UNIPOL(TM) and SPHERIPOL(TM) process. In this process catalyst particles are fluidized with monomer gas that reacts with the catalyst particles to form polymeric particles up to a size of 1 mm. The process is typically operated at pressures of 20 to 25 bar. Pressure impacts the hydrodynamics of the fluidized bed as it influences the bubble behaviour, particle mixing and heat transfer characteristics. Despite decades of research on fluidized beds these effects are not completely understood. In order to gain more insight in the effects of operating pressure on the fluidization behaviour we have performed full 3D discrete particle simulations. We used a state-of-the-art discrete particle model (DPM) to simulate fluidization behaviour at different pressures. In our model the gas phase is described by the volume-averaged Navier-Stokes equations, whereas the particles are described by the Newtonian equations of motion. The DPM accurately accounts for the gas-particle interaction, which is necessary for capturing the pressure effect. In order to study the pressure effect on the granular temperature, we analysed seven simulations with operating pressures ranging from 1 to 64 bar. It was found that the granular temperature increases with pressure. This is mostly caused by the increased porosity at elevated operating pressures. The granular temperature is anisotropic: it is larger in the vertical direction. Also the pressure dependency of the granular temperature is larger in the vertical direction. (C) 2007 Elsevier B.V. All rights reserved.