Experimental spray zone characterization in top-spray fluidized bed granulation

Abstract

In fluidized bed granulation wetting and coating of particles depends on the atomization of a liquid binder agent. Droplet distribution and the subsequent micro-scale processes of droplet deposition on particle surfaces as well as drying of layers and liquid bonds between aggregated particles lead to a subdivision of the process space into two major compartments, a spray zone and a drying zone. By using a self-constructed, simple conductivity probe spray patterns inside the fluidized bed are located. The spatial demarcation of the compartments, which is dependent on the fluidization and spray conditions, is deduced. Particularly, nozzle height and nozzle gas flow rates influence the expansion of the spray zone and its intrusion into the bed. The presented results show by means of the particle residence time for the two considered compartments that an increased nozzle mass flow rate leads to significantly accelerated particle flow in the spray zone, and the fluidization velocity of the gas forces a faster particle re-circulation behavior in the entire fluidized bed. Consequently, process time for wetting and drying is reduced. By using a flat fluidized bed with rectangular cross section in combination with image-based acquisition techniques, particle velocities and solid volume fractions have been acquired. Comparing the results of particle circulation patterns with data obtained in cylindrical equipment shows that information can be transferred from the quasi-2D configuration to real 3D geometries.