Drying Definition. Drying is a unit operation in which a solvent, generally water, is separated from a solution, semisolid material or cake/solid pastes by evaporation.

In the drying process, the heat is transferred simultaneously with the mass, but in the opposite direction.

Drying Process Description. The moisture content of a material is usually expressed as a weight percentage on a dry basis. The moisture may be present as:

• Free moisture. This is the liquid in excess ofthe equilibrium moisture content for the specific temperature and humidity condition of the dryer. Practically, it is the liquid content removable at a given temperature and humidity.

• Bound moisture. This is the amount of liquid in the solids that exhibits a vapor pressure less than normal for the pure liquid.

In the drying of materials it is necessary to remove free moisture from the surface as well as bound moisture from the interior. The drying characteristics of wet solids can be described by plotting the rate of drying against the corresponding moisture content. A typical drying curve is shown in Fig. 1 and it can easily be seen that this is subdivided into four distinct sections:

The curved portion, AB, is representative of the unsteady state period during which the solid temperature reaches its steady state value, ts. AB may occur at decreasing rate as well as at the increasing rate shown.

The critical moisture content is thus identified as the average moisture content of the solid at the instant the first increment of dry area appears on the surface of solid.

The critical moisture content depends upon the ease of moisture movement through the solid, and hence, upon the pore structure of the solid, sample thickness and drying rate. Segment BC is the constant-rate period. During this period, the drying is controlled simultaneously by heat and mass transfer applied to a liquid-gas interface in dynamic equilibrium with a bulk gas phase.

Figure 1. Drying rate curve.

Moisture flow from within the material to the surface is fast enough to maintain a completely wet surface. The surface temperature reaches the wet-bulb temperature. The rate of drying can be expressed as:

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