APV Dryer Handbook - Umbc

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3. The wetted particles sink into the water.
4. The granules disintegrate into their original smallest particles, which disperse
in the water.
5. The small, dispersed particles dissolve in the water.
It is important to realize that the total time required for all these steps should be
the criteria used to evaluate a product’s instant properties. It is not unusual to see
products characterized only on their wettability and sinkability. This neglects the
importance of the dispersion and dissolution steps, the time for which may vary
considerably with different agglomeration methods.
For powders produced by spray drying, there are a number of ways in which the
agglomeration can be accomplished in the spray dryer itself. This often is referred
to as the “straight-through” process and is illustrated in Figure 16. Note that fine
powder from the cyclone is conveyed up to the atomizer where it is introduced
into the wet zone surrounding the spray cloud. Cluster formation will occur
between the semi-moist, freshly produced particles and the recycled fines. The
agglomerated product is then removed from the bottom of the drying chamber,
cooled and packaged. This method produces a degree of agglomeration that
is sufficient for many applications.
An alternative approach to agglomeration is referred to as the re-wet method.
This is characterized
FEED
EXHAUST
AIR
by processing an
BAG already existing fine
COLLECTOR
dry powder into an
SPRAY CHAMBER
agglomerate, using
fluidized bed technology.
FINES RETURN
SYSTEM
TO
POWDER
STORAGE
Figure 16. Straight-through agglomeration