Bubble behaviour in fluidised beds at elevated pressures

Low density polyethylene and polypropylene are produced at large scale via the Unipol process. In this process catalyst particles are fluidised with monomer gas and grow into polymeric particles up to a size of 1¿mm. The process is operated at 20–25 bar. Pressure plays an important role in the hydrodynamics of the fluidised bed, which is reflected in bubble behaviour, particle mixing and heat transfer characteristics. Despite decades of research these effects are not completely understood. We use a state-of-the art discrete particle model (DPM) to simulate fluidisation behaviour at different pressures. The DPM is the most fundamental model suited for studying pressure effects in gas-fluidised beds, since it accurately represents the gas–particle interaction and particle–particle interaction. Fourier analysis of the bed pressure drop fluctuations and bubble properties (bubble velocity, bubble size, coalescence and break-up) obtained from a sophisticated bubble detection algorithm are presented.