Mechanical agitator granulators as the name implies are granulators that by virtue of agitator speed, sweep volume, or bed pressures, generate relatively lower shear which has implications on the operating conditions and resultant granule properties. Mechanical agitator granulators can also be referred to as low shear granulators.
During granulation process, the active compound and other excipients are combined with a binder solution to form granules with a fixed composition through rotating low-shear forces via low-power-per-unit-mass with addition of a liquid.
Mechanical agitator granulators are generally characterized by;
- Longer run time
- Lower density and higher porosity granule yield than those of high-shear granules.
- Their tendency of compressing the granules and reduced void volume as a result of their design.
- Their requirement of additional binder solution for granulation processes.
Subclasses of mechanical agitator granulators
The subclasses of wet low-shear granulator/ mechanical agitator granulators are primarily distinguished from one another by the geometry and design of the shear inducing components. Granulators which operate by mechanical agitation include the following;
- Ribbon or paddle blender.
- Planetary mixer.
- Orbiting screw mixers
- Sigma blade mixer
Shear is induced by either rotating impeller, reciprocal kneading action, or convection screw action.
i. Ribbon or Paddle Blender
Ribbon blender popularly used as a dry mixer can serve as a very reliable granulator if a dry paste is formulated for the machine. The following is worthy of note when using a ribbon or paddle blender as a granulator;
- Most ribbon blenders are not designed to withstand the shift torque required for granule formation and for this reason one should make sure that the granulator shaft is strong enough before carrying out a granulation process in a ribbon blender designed for dry mix.
- When the kneading time is extended (above normal), the granules are densified and this leads to increased moisture exposure, increased particle size, and reduced porosity.
- The tendency of the material to adhere to the side well is pronounced in the ribbon blender.
- Paddles decrease sticking problems encountered in ribbon blenders and it also decreases the torque required for granulating and thus can handle wetter paste.
- Paddle blenders are often used as a continuous granulator and have both lower torque and more applications when compared to a continuous ribbon blender.
The two popular ribbon or paddle blender used as granulators are;
- The topogranulator: This is a batch-style ribbon blender with the ability to either compress or mechanically fluidize the granulation.
- The turbulizer: This is a continuous paddle granulator whose unit provides adjustable mixing, shear, and impact action based on the revolution per minute of the shaft and the angle of the impact blade.
ii. Planetary Mixer
Planetary mixers which are mainly used to mix medium viscous to high viscous media can also be used as granulating equipment. When used as a granulator, planetary motion of the mixer is generated by rotating the agitator off an assembly in a direction opposite to that of the rotation of the agitator assembly as it moves around the bowl. Despite the representations of planetary by different commercial names, like Hobart granulator, kitchen Aide granulator, etc, they still have the following basic makeup;
- Planetary motion
- Removable bowl
- Top-drive agitator
Planetary mixer tends to be better of mixing drying powders in a horizontal plane than in a vertical plane and so may require a distinct dry blending step prior to wet granulation.
iii. Orbiting Screw Granulator
Orbiting screw blender which is mainly used for dry mixing of powders when modified is used as a very gentle granulator mixer. The following modifications along with a chopper in the sidewall make orbiting screw blender an effective granulator for powders, slurries, suspensions, and pastes;
- The unit is fitted with a spray nozzle mounted on the agitator. This nozzle adds liquids to the dry powder.
- A sintered metal plate which allows entry of compressed air through the skin of the mixer or exhaust moisture to obtain drying.
iv. Sigma Blade Mixer
Sigma blade mixer is a compressive granulator designed for kneading, mixing, and homogenizing wet, pasty and high viscous materials. The two sigma shaped blades which sit horizontally and parallel at the bottom of the mixing trough rotates in opposite directions at different speeds.
This rotatory movement of the blades causes the material to be moved from blade to blade and from end to end generating /resulting in a figure-eight pattern of movement that produces a thorough powder mix. When used as a low shear granulator, the sigma blade mixer is proceeded by a dry mixer and the unit generates liquid added to the powder during granulation which facilitates the formation of uniform granules with binder matrix and good binder distribution.
- Dilip M. Parikh (2010). Handbook of Pharmaceutical Granulation Technology, Third Edition. CRC Press, Taylor and Francis Group, LLC.
- Jim Litster, and Bryan Ennis (2004). The Science and Engineering of Granulation Processes. Kluwer Academic Publishers, Dordrecht, Netherland.
- Larry, L. A. and Stephen, W. H. (2008), Pharmaceutical Dosage Form: Tablet. CRC Press, Taylor and Francis Group, LLC.