Page 320 - Biosystems Engineering

Bioseparation Pr ocesses 297

In modern systems, vapor compression (mechanical as well as
thermal) techniques are used wherein energy use is extremely good.

9.3.7 Drying and Crystallization
The term drying usually infers the removal of relatively small amounts
of water from a solid or nearly solid material to reduce the content of
residual liquid to an acceptably low value. Drying is different from
evaporation in that the latter is limited to the removal of a large
amount of water from solution. In drying processes, the major empha-
sis is usually on the solid product. In most cases, drying involves the
removal of water at temperature below its boiling point, whereas
evaporation means removal of water by boiling the material. Another
distinction is that in evaporation, water is removed from the material
as practically pure water vapors, mixed with other gases only because
of unavoidable leaks. In drying, on the other hand, water is usually
removed by circulating air or some other gas over the material to
carry away the water vapors; in some drying processes, however, on-
carrier gas is used.

Methods of Removing Liquid
Liquids may be removed in the following ways:
1. Deposition of water or liquid as ice
2. Decomposition of water, that is, liquids can be drawn off by
means of NaCl, CaCl , and the likes
2
3. Precipitation as hydrates, for example, CaCl , P O , and CuSO
2 2 5 4
4. Absorption technique
5. Adsorption, such as drying of air by silica gel
6. Mechanical separation, for example, removing moisture by
squeezing and centrifuging
7. Vaporization, air drying, vacuum drying, and so on

Classifying Dryers
Classification of dryers is based on the form of the material being
handled. Thus, for

1. Materials in sheets (or masses) carried through on conveyors or
trays: Examples are atmospheric compartment and vacuum
tray (batch) tunnel dryer (continuous).
2. Granular or loose materials: Examples are standard rotary, roto-
louver (rotary dryers), turbodryers and conveyor dryers, or
filter dryer combination.
3. Material in continuous sheets: Examples are cylinder dryers and
festoon dryers.