Rotating Shape Granulators

Ample opportunity exists for fine-tuning a granulation process in these machines. Apparatus variables that can be adjusted are shell speed, bar speed, bar size, and bar design. Process changes can be made in mix time, liquid addition time, bed temperature, internal vessel pressure, and spray droplet size. 

The blade shape is critical for the liquid addition method. As the blades rotate, they carve out a toroidal void volume in the flowing powder bed. The liquid exiting the disk coats the interior of the torus. Proper vessel loading ensures that no liquid impinges on the wall of the vessel. 

Disk diameter is critical. A large-diameter disk imparts more energy as torque is a function of diameter. The larger diameter offers more circumference for liquid to be evacuated and carves out additional void volume in the material. The sweep volume of the disks is considered a major factor in properly scaling up a granulator (15). 

Particle motion is so complex when granulating that it becomes difficult to determine the optimum rate of liquid addition. Stamm and Paris (16) studied rates of 5, 10, and 20 mL/min in a fixed shell, helicoidal mixer with vertical bar, and determined that the best results occurred at the slowest rate. 

When one extends the liquid addition period, the bar continues to abrade the granules. Favorably, any particles that have been fractured can be rewetted and the fracture sruface can easily bind to another particle. This mechanism tends to hold down the oversized fraction without the expected increase in the undersized fraction. 

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Droplet size in a fluid bed has been measured by capturing droplets on a slide covered with viscous oil. The size has been determined to be in the 20-100 pm range (20). Less delicate techniques in the rotating shape granulators have shown droplet sizes of about 250 pm. Agland and Iverson provide an experimental study showing the relation of liquid droplet size to granule characteristics such as size and penetration of liquid (21). 

Heuristic 51 Particle-size enlargement. Use compression with rotary compression machines to convert powders and granules into tablets of up to 1.5 inches in diameter. Use extruders with cutters to make pellets and wafers from pastes and melts. Use roll compactors to produce sheets from finely divided materials the sheets are then cut into any desired shape. Use rotating drum granulators and rotary disk granulators with binders to produce particles in the size range of 2 to 25 mm. 

Granules are particles with a more or less spherical shape. The diameter of these particles can be between 2-30 mm. Granulation is widely used in other fields of technology, if spherical particles are needed. The principle of the shaping method is best described by the snowball effect. A round dish (see Fig. 8.10) is used, which is rotating about an inclined axis, the angle of inclination of which is variable. Small particles are fed into the dish. At the same time a cohesive slurry... 

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