Fluidized-bed granulators (e.g. Aeromatic, Glatt) have a similar design and operation to fluidized-bed driers, i.e. the powder particles are fluidized in a stream of air, but in addition granulation fluid is sprayed from a nozzle on to the bed of powders. It is ideal for a wide range of both heat sensitive and non-heat sensitive products.

Working Principle of Fluidized Bed Granulators

Heated and filtered air is blown or sucked through the bed of unmixed powders to fluidize the particles and mix the powders; fluidization is very efficient mixing process. Granulating fluid is pumped from a reservoir through a spray nozzle positioned over the bed of particles.The fluid causes the primary powder particles to adhere when the droplets and powders collide. Escape of material from the granulation chamber is prevented by exhaust filters, which are periodically agitated to reintroduce the collected material into the fluidized bed. Sufficient liquid is sprayed to produce granules of the required size, at which point the spray is turned off but the fluidizing air continued. The wet granules are then dried in the heated fluidizing airstream.

Advantages of Fluidized Bed Granulation 

All the granulation processes are performed in one unit, saving labour costs, transfer losses and time. Another advantage is that the process can be automated once the conditions affecting the granulation have been optimized. Fluid bed drying is suited for powders, granules, agglomerates, and pellets with an average particle size normally between 50 and 5,000 microns. Very fine, light powders or highly elongated particles may require vibration for successful fluid bed drying.

Disadvantages of Fluidized Bed Granulation 

The equipment is expensive and optimization of process (and product) parameters affecting granulation needs extensive development work, not only during initial formulation work but also during scale-up from development to production. Similar development work for the traditional process and that using high-speed granulators is not as extensive.

This long and very product-specific development process has proved to be a serious problem with fluidized-bed granulation in the pharmaceutical industry.T here are numerous apparatus, process and product parameters that affect the quality of the final granule. Each formulation presents its own individual development problems, has led to fluidized-bed granulation not reaching its full potential in pharmaceutical production.

Apparatus parametersProcess ParametersProduct Parameters
Air distribution plateBed loadType of b i nder
Shape of granulator bodyFluidizing air flow rateQuantity of binder
Nozzle heightFluidizing air temperatureBinder solvent
Positive or negative operationFluidizingair humidityConcentration of granulating solution
Scale-upAtomization : Nozzle type, Spray angel, spraying regime, Liquid flow rate, Atomizing air flow rate, Atomizing air pressure, Droplet sizeTemperature of granulation solution

Starting materials : Fluidization and Powder hydrophobicity