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-FLUIDIZER®, 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:

Fig. 77  Vibro-Fluidizer in sanitary design

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 manufacturing method prevents crevices. The BUBBLE PLATE™ is therefore sanitary, and as such accepted by USDA.

Fig. 77a  BUBBLE PLATE^TM

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:

• Higher capacity/kg drying air
• Better economy
• Better product quality such as:
  - good solubility
  - high bulk density
  - low free fat
  - low content of occluded air
• Less powder emission

The fluid bed can be designed either as a vibrating plug-flow bed (Vibro-Fluidizer) or a static back-mix bed.