Discrete particle simulations of high pressure fluidization

Low density polyethylene and polypropylene are produced at large scale via the Unipol process. In this process catalyst particles are fluidized with monomer gas which 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 fluid 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 use 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. The simulation results were analysed with spectral analysis of the pressure drop fluctuations and analysis of the porosity field. In order to study the bubble behaviour, a sophisticated bubble detection algorithm was developed. From this algorithm, gas bubble characteristics, such as bubble velocity and bubble size are obtained. The simulation results show increasing emulsion porosity and decreasing bubble porosity with increasing pressure. In other words, the bubble-emulsion structure becomes less distinct. The determined bubble velocity is very well in accordance with empirical correlations for low pressures, and decreases at elevated pressures.