As discussed previously the particle temperature was given
by the surrounding air temperature (outlet temperature). As the last water is
the most difficult to remove by the conventional drying, the outlet temperature
has to be high enough to ensure a driving force (Δt or temperature difference
between particle and air) capable of removing the last moisture. That this will
very often have a detrimental effect on the particles has been discussed
earlier.
It is therefore not
astonishing that a completely different drying technology has been developed
especially to evaporate the last 2-10% moisture from the particles already
formed at that stage.
As the
evaporation will go very slowly in this range, due to the diffusion coefficient
being low, the drying equipment or after-dryer should be designed so that the
powder will get a long residence time. It can be done in a pneumatic conveying
system using hot air thus increasing the driving force. However, as a velocity
of ≈20 m/sec. is required in the duct, it takes a considerable length of duct
if it should be efficient. Another system consists of the so-called "Hot
Chamber" with tangential inlet for pro-longing the holding time. After the
drying is completed, the powder is separated in a cyclone and passed on to
another pneumatic conveying system with cold or dehumidi-fied air for cooling.
The powder is separated in a cyclone and is ready to be bagged off.
Another system for after-drying is a
VIBRO-FLUIDIZERŪ, which is a big horizontal box divided in an upper and a lower
section by a perforated plate welded to the side wall of the box. See Fig. 77.
For drying, alternatively cooling, warm and cold air is introduced into the air
plenum chamber and is distributed evenly over the whole area of a special
perforated plate, the BUBBLE PLATE™, with the following
advantages:
The air is directed downward towards the plate surface,
therefore particles will be kept moving on the plate, which has few, but large
holes and can therefore operate longer time between cleaning. Further, it has
demonstrated a very good emptying effect. See Fig. 77a.
The perforation and amount of air are determined by the
necessary air velocity needed for the fluidizing of the powder, which in turn
is determined by the nature of the powder such as the moisture content and
thermoplasticity.
The temperature is
determined according to the required evaporation duty. The hole size in the
perforated plate is chosen, so that the air velocity will be high enough to
fluidize the powder on the plate. The air velocity should not be so high that
the agglomerated powder is destroyed due to attrition. However, it can never be
avoided (and in some cases it is even desirable) that some particles,
especially the small ones, leave the fluid bed with the air. The air is
therefore passed through a cyclone or bag filter, where the particles are
separated and returned to the process.
With this new equipment in hand it is possible to evaporate
the last few per cent of moisture from the powder in a gentle way. This means
that the spray dryer can be op-erated in a different way from the one
previously described, where the powder left the chamber with the final moisture
content.
The advantage of the two-stage
drying can be summarized as follows:
The fluid bed can be designed either as a vibrating
plug-flow bed (Vibro-Fluidizer) or a static back-mix bed.