Page 321 - Biosystems Engineering

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4. Pastes and sludge or caking crystals: Examples are atmospheric
and vacuum dryers (agitator dryer).
5. Materials in solution: Examples are atmospheric and vacuum
dryers and spray dryers.
6. Special methods: Modern techniques are used, such as infrared
dryer, fielectric dryer, greeze dryer, pneumatic dryer, fluid-
ized dryer, and the like.

Drying Conditions
Drying conditions can be constant or variable. Constant drying con-
ditions relate to the constancy of temperature, humidity, air velocity,
method of exposure of the material, and so on.
Drying Mechanism
The moisture (or solvent) in wet material occurs in two types, bound
and unbound moisture. During the drying of material at a specified
drying condition, the free moisture gets vaporized and reaches an
equilibrium value. To get a bone-dry material, the temperature has to
be raised considerably. Such products may absorb equilibrium water
again as soon as the material is taken out of the oven, however.
The rate of the drying curve can show different trends. For some
materials, the rate is constant throughout. In some cases, there is an
initial constant-rate period, which is followed by a falling (the point of
intersection gives critical moisture content) rate. Again, there are a few
materials for which there are two falling rates (linear and nonlinear).
When the surface of the material is sufficiently wet, a constant rate of
drying takes place. When diffusion of liquid from internal pores to the
surface becomes predominant, heat flows to the insides of the solid
mass when evaporation and subsequent diffusion take place.
The drying rate increases with increasing temperature and veloc-
ity of air if the slab thickness increases—the drying rate falls off. If the
air is sufficiently humid, there is a less concentrated gradient, and so
the drying rate can fall off sharply.
Internal Mechanism of Liquid Flow
Internal liquid flow can occur due to the following:
1. Diffusion in continuous homogeneous solids
2. Capillary flow in granular and porous solid
3. Flow caused by shrinkage and pressure gradient
4. Flow caused by gravity
5. Flow caused by the vaporization–condensation process
Crystallization
Crystallization is the formation solid particles within a homogeneous
phase. It may occur as the formation of solid particles in a vapor, as in