Hybrid systems

Each of the drying systems described so far is best suited for specific feed characteristics and to produce specified qualities in the final product. There are however, many instances in the CPI where no single system can handle the requirements of both the initial feed and the final product. Under these circumstances, one can use a two-stage system in which one drying process precedes another.

Using a flash dryer to accomplish the removal of surface moisture and sending the flash-dryer product to a fluid bed for removal of bound moisture is a typical example. Such two-stage systems can, of course, be configured in many possible combinations. However, of greater interest are hybrid systems in which two or more fundamental types of drying systems are incorporated into a single process unit.

Fluidized spray dryers

One of the most successful systems combines a spray dryer with a fluid bed (Figure 7). This process converts a solution or suspension directly into an agglomerated product, which is non-dusty, free flowing and usually redispersible. The upper part of the system works as a spray dryer, atomizing the liquid and contacting it with heated air. However, the temperature and humidity conditions are maintained at a designated level to prevent complete drying of the droplets. The partially dried particles are then fed into the fluid bed portion of the drying chamber and allowed to agglomerate.

Drying gas is introduced not only near the atomization section in the top, but also into the fluid bed at the bottom. All of the gas exits the drying chamber from the roof area. As a result, all the fine particles leaving the system have to pass through the atomized liquid, thereby providing further opportunity for agglomeration.

Finally, any fines that do escape with the spent drying gas are captured in cyclones or bag collectors and returned to the drying chamber. Atomized droplet size, gas flow to the two parts of the drying chamber, temperatures and fluidized layer level are the parameters that influence the final particle-size distribution [7]. Mean particle sizes between 100 and 500 μm are typical for powders produced by this method.

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