Fluidized-bed granulators. In fluidized-bed granulators, particles are supported and mixed by a heated gas. This action also induces drying. Proper gas-distributor design is required to maintain solids mixing, heat and mass transfer; and to prevent unstable operation that could cause the bed to defluidize or collapse. Batch and continuous designs are available (Figures 1a and 1b), as well as spouted-beds and coaters. Liquid binder is sprayed through an atomizing two-fluid nozzle located above, in or below the bed. Both aqueous and solvent solutions are used, which requires solvent recovery considerations. Spray distribution, atomizer design and humidity control are crucial to proper operation. Bag filters or cyclones are needed to remove entrained dust from the exit air. Batch processes in a wide range of batch sizes are used to agglomerate fine powder to produce high porosity granules. Alternatively, slurries of feed materials may be sprayed onto a bed of seed particles to produce high-strength, layered granules. Continuous fluidized beds, generally of a serpentine design, produce layered granules. Recycle of off-size material is very common and involves ancillary equipment for continuous classification, crushing and grinding. Seed material is often introduced to the initial stages to promote process stability. Spouted-bed designs
are used for coating applications, as well as precision granulation where growth is localized within the upward conveying draft tube.