Spray Granulation

Spray granulation (Ref 2) forms granules by atomizing a liquid or a binder solution into a fluidized powder bed (Fig. 1). Fluidization is achieved by directing a heated gas, which is usually air, through a distributor at the bottom of the powder bed. The gas imparts a vigorous motion to the particles, which prevents the formation of large lumps. The binding liquid is usually sprayed into the powder bed with a two-fluid nozzle. Spray granulators can be designed to operate in either a batch or a continuous mode.

Fig. 1 Fluidized bed spray granulator. Source: Ref 3

The formation of granules in a spray granulator occurs through the random nucleation of small seed agglomerates, followed by the growth of these seeds to the desired size. Growth occurs either by the layering of powder onto the seeds or by the agglomeration of seeds to form larger granules. Granule growth by seed agglomeration forms irregular shapes.

Granule size increases as the fraction of the bed exposed to the binding liquid is reduced and as the spray nozzle is adjusted to give coarser droplets (Ref 3). Increasing the intensity of agitation of the bed (with a higher gas velocity) decreases the size of the granules. There is an upper limit on granule size because of the tendency of the powder bed to defluidize. However, spray granulation can form larger granules than is usually possible by spray drying because of longer residence times.

Spray granulation is commonly used in the pharmaceutical industry to prepare feedstock for tablet presses. It is used less frequently in the metal powder industry. However, both spray drying and fluid bed conversion are scaleable technologies and together provide the means for producing bulk quantities of nanophase composite powders at low manufacturing cost. A few examples of fluidized bed granulation are described below.

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