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Take more than product attributes into account when choosing agglomeration equipment
A virtually endless number of process options are available for agglomerating powders in the chemical process industries (CPI). These processes can be divided into two main categories: agitation and compression. Agglomeration by agitation includes granulation processes, such as fluidized-bed, disc, drum and mixer granulatore. Agglomeration by compression includes compaction processes, such as tabletting, extrusion, roll presses and pellet mills. Process choice is affected by certain feed attributes as well as a host of ancillary decisions. This article focuses on equipment selection criteria for granulation and compaction processes. The mechanisms of agglomeration were reviewed in an earlier article [1], and additional detailed information on agglomeration is available elsewhere [2-7].
Wet granulation
Granulation processes vary from low to medium levels of applied shear and stress, producing granules of low to medium, and in some cases, high density [2] . Ranked from lowest to highest levels of shear, these processes include fluidized-bed, tumbling and mixer granulatore. In wet granulation, agglomeration is promoted by the addition of binders and solvents.
Fluidized-bed granulatore. In fluidized-bed granulatore, 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 la and lb), 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...