APV Dryer Handbook - Umbc

THE AGGLOMERATION MECHANISM
Two particles can be made to agglomerate if they are brought into contact and at
least one of them has a sticky surface. This condition can be obtained by one or
a combination of the following:
1. Droplet humidification whereby the surface of the particles is uniformly
wetted by the application of a finely dispersed liquid.
2. Steam humidification whereby saturated steam injected into the powder
causes condensation on the particles.
3. Heating – for thermoplastic materials.
4. Addition of binder media, i.e., a solution that can serve as an adhesive
between the particles.
The steam condensation method usually cannot provide enough wetting without
adversely heating the material and is used less frequently on newer systems.
After having been brought into a sticky state, the particles are contacted under
such conditions that a suitable, stable, agglomerate structure can be formed. The
success of this formation will depend on such physical properties as product
solubility and surface tension, as well as on the conditions that can be generated
in the process equipment.
For most products, possible combinations of moisture and temperature can be
established as shown in Figure 17. Usually, the window for operation is further
narrowed down by the specifications for product characteristics. Once the
agglomerate structure is created, the added moisture is dried off and the powder
cooled below its thermoplastic point.
AGGLOMERATION EQUIPMENT
While slightly different in equipment design and operation, most commercially
available agglomeration processes are fundamentally the same. Each relies on the
formation of agglomerates by the mechanism already described. This is followed
by final drying, cooling and size classification to eliminate the particle
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