PEPT visualisation of particle motion in a tapered fluidised bed coater

Abstract

Detailed knowledge of particle motion is essential in better understanding fluidised bed coating processes. This paper reports on the use of Positron Emission Particle Tracking (PEPT) in order to visualise and quantify the features of the powder movement inside a laboratory-scale GPCG-1 fluidised bed coater (Glatt GmbH, Germany) with a tapered (conical) chamber. In all cases, a clear circulating motion, upwards in the centre and downwards along the walls, was revealed. This circulating pattern lends itself to operations such as particle coating, where controlled application of the coating onto the particles is essential. In contrast to the fluidisation patterns observed in cylindrical fluidised beds or spouted beds without a draft tube, radial movement was stronger and occurred at all heights in the powder bed. By introducing a binary nozzle and increasing the nozzle atomisation air pressure, the circulating motion of the fluidised particles in a tapered vessel was found to be accelerated. When spraying a coating solution downwards onto the fluidised powder particles, effects on the particle motion, depending on coating solution properties and process parameters, were observed. Besides visualisation, PEPT was also used to derive quantitative information such as bed height and particle circulation time, as well as to investigate the size of the coating zone. Generally, it was demonstrated that PEPT can be successfully used to characterise the particle motion in a tapered fluidised bed under both drying and coating conditions.