The most common type of granulator used in the pharmaceutical industry today is a wet granulation which is available in both a high shear and low shear design. The high shear mixers are the more intense cousins of mixer technology while the lowshear process utilizes fluidized bed technology. A typical high shear granulator consists of a main impeller on the bottom of the processing chamber and a chopper located higher up in the chamber.
The main drive shaft can enter through the top or bottom of the processing chamber but in both cases the impeller imparts shear energy by moving the product along the floor and walls of the chamber. The chopper sits above the process in a less dense zone, protruding out from the process chamber wall or roof, where it prevents agglomerates from building by returning the material back into the rotating bed of material. Binder liquid is introduced through the roof of the processing chamber, using a hydraulic nozzle or a simple fitting, and the liquid is uniformly distributed into the product mass due to the shear forces present.
Top-driven granulators are considered easier to clean since the impeller blade lifts out of the chamber and the seal is above the rotating mass. Bottom-driven granulators typically have tighter tolerances between the impeller blade and processing changers walls and floor thereby providing higher shear but the shaft seal sits in the material. Fluid beds granulators operate in a low or no shear environment. The material to be processed sits on a screen at the bottom of a cylindrical processing chamber and air is drawn up thought the bed of material.
The air entrains the particles , lifting up in to the expansion chamber, thus causing the particles to be fluidized. Binder liquid is introduced into the processing chamber via a high pressure nozzle using atomization air to create fine droplets the atomized binder droplets make contact the powder particles as they are suspended in air with no mechanical force present. As these droplets contact the suspended particles, the particles are bound together forming groups of individual particles into granules. As the particles are entrained in the airstream, rise and then fall back to the bottom of the processing chamber they constantly pass through the droplet cloud, thereby building larger granules.
A filter system above the processing zone ensures that any fines or dust particles are trapped and returned to the process, ensuring high yields with minimum product loss. Fluid bed granulation offer tremendous flexibility since the particle size can be readily controlled by varying key process parameters such as spray rate, droplet size, particle recirculation rate, air volume and air temperature among others. In addition, once binder addition is complete and the product has been fully granulated the product is then dried in the same process chamber by the heated fluidizing air, thus offering a so-called “single pot process” as compared to high shear granulators.
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