Numerical analysis of the effects of liquid evaporation on particle fluidization and agglomeration dynamics in a top-spray gas-solid fluidized bed

This research investigates the interaction between particle agglomeration, liquid evaporation, and fluidization dynamics within a top-spray fluidized bed. The study employs a coupled CFD-DEM approach that integrates a liquid bridge force model alongside a simplified model for the transition from liquid bridges to solid bridges during evaporation. The results reveal that the region of rapid moisture evaporation is predominantly situated near the bed wall. Initially, particles located in the upper-middle region of the bed are entrained into the spray zone due to bubble-induced elutriation. These particles subsequently descend into the near-wall region, characterized by elevated evaporation rates. The interaction of bridging forces among particles leads to region-specific fluidization behaviors: particles in the central region primarily experience fluidization and agglomerate disintegration, while those near the wall exhibit localized recirculation that facilitates agglomeration. The granulation process proceeds in a staged manner, with larger agglomerates gradually settling at the bottom of the bed, while individual particles and smaller agglomerates continue circulating until a more uniform agglomerate size distribution is achieved. Notably smaller agglomerates serve as nuclei for the formation of larger agglomerates. Furthermore, an increased spray rate extends the residence time of particles within the agglomeration region, thereby enhancing the liquid bridge forces and allowing sufficient time for the transition from liquid to solid bridges. This mechanism ultimately enhances the overall degree of agglomeration within the fluidized bed.