Mathematical modelling of batch and continuous well-mixed fluidised bed dryers

A multiparticle model for fluidised bed dryers is presented. The bed is divided into a number of layers, each connected to a single particle representing all particles in that layer. A plug-flow assumption is made for the gas phase which enables the profiles of temperature and humidity in the bed to be calculated. The particles are moved around randomly in the vertical direction to account for the intense particle mixing and the variant drying conditions in the bed. For continuous beds, one of the particle layers is substituted at certain time intervals with ‘fresh’ particles representing the feed. With this treatment, the effect of the residence time distribution on the outlet moisture content is accounted for. A diffusion model is applied for the single particle drying kinetics in the falling rate period. Two different materials were studied (zerolite and wheat) and the model predictions showed good agreement with experimental data. A simple model was also shown to be valid for continuous drying of small particles where no internal moisture gradients where formed during the drying.