Drying in fluidized beds with immersed heating elements

The drying of six different fractions of γγ-Al2O3 with particle diameters between 50 and 1800μm has been investigated in a batchwise operated, lab-scale fluidized bed equipped with a cylindrical, vertically or horizontally placed heater. Resulting drying curves and heater-to-bed heat transfer coefficients are compared with various model versions, which are derived by combination of well established modules for fluidized bed drying, indirect heat transfer and intraparticle kinetics. In this way, good agreement with the experimental results is obtained for large particles. For small particles, a strong dependence of heat transfer on solid moisture content is observed, in the sense of an enhancement in comparison to the behaviour of the dry product. The part of this effect due to phase change (latent heat sink) in the interior of drying particles could be identified and analysed by extension and modification of existing model modules. This analysis points out limitations of such modules and still remaining modelling challenges. For all types of product, a very significant intensification of the drying process by immersed heating elements is demonstrated.