APV Dryer Handbook - Umbc

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E F F I C I E N T
E N E R G Y
U T I L I Z A T I O N
I N D R Y I N G
It is generally necessary to employ thermal methods in order to achieve
a product that is termed commercially dry. Thermal dryers, therefore, are
an important unit operation in many industries. Products such as pigments,
baby formula, kaolin and instant coffee usually have to be processed in dryers
to obtain the final product.
The commercial drying process can take place in a number of different types
of dryers as described in the previous section. However, the thermodynamics
of drying is the same regardless of the actual device. Water or another liquid
such as a solvent has to be evaporated from a solid by the application of heat.
Generally, the drying takes place in an air atmosphere, although some
specialized dryers may use nitrogen or even superheated steam as
the atmosphere.
The most important parameter that governs the pretreatment and the dryer design
is the “cost per unit weight of dried product.”
While drying is an extremely energy intensive operation, there are techniques that
can be used to minimize the energy costs per unit output of product, including:
■ Minimizing the water content of the feed prior to feeding to the dryer.
■ Maximizing the temperature drop of the drying gas. This implies maximum
inlet and minimum outlet temperature.
■ Employing the maximum possible recirculation of the drying gas.
■ Considering the possibility of two stage counter flow drying.
■ Utilizing the heat in the discharge air to preheat incoming air.