A variety of equipment can be used for wet granulation mainly differing in the degree of shear applied by these on the material inside. Thus, the classification of the granulators can be done as low shear, medium shear and low shear granulators. Fluidized-bed granulators or conical screw granulators are examples of low shear granulators. These usually accommodate higher batch sizes; however, these cannot be used when granule size required is less than 1mm. In fluidized-bed granulators, air is used to agitate the particles while spraying binder through a nozzle located above, inside, or below the powder bed. The agitation is less aggressive, and the granules are typically more porous. A conical screw granulator or a Nauta mixer imparts gentle mechanical mixing to the powder by the rotation and revolution of the screw in the conical vessel. The binder solution is sprayed from the top and under constant motion, granules are obtained. Mixer granulators such as planetary mixers cause agglomeration by mixing the powder bed with an agitator while adding the liquid binder. The intensity of the motion produces granules that are smaller, denser, and often more spherical than granules from fluidized-bed granulators. The added intensity reduces the amount of liquid required, leading to shorter drying times. Examples include planetary mixers. High-speed mixer granulators, or high-shear mixer granulators, are most extensively in the pharmaceutical industry because they are capable of producing granules that are small and dense, with high strength making them ideal for blending and tabletting. Additionally, a wide range of materials can be processed in these including cohesive, fluffy powders or sticky binder solutions. High-shear mixer granulators are usually operated in batch mode with a vertical mounted mixing impeller. Characteristic features of a typical high-shear granulator include a stainless-steel granulator bowl and a central impeller blade, used to agitate the powder and promote densification. A chopper blade is also be located off-centre to help break apart large agglomerates and allow uniform binder distribution.
