Static Beds of Porous Solids

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Chapter: Pharmaceutical Engineering: Drying

The drying curve obtained when the particles that compose the bed are them-selves porous.


STATIC BEDS OF POROUS SOLIDS

The drying curve obtained when the particles that compose the bed are them-selves porous is shown in Figure 7.5. It differs from the curve obtained with nonporous materials in that the constant rate period is shorter. The rate of drying may be higher and is almost independent of particle size. The critical moisture content is a function of both pore size and particle size. During the first falling rate period, the rate of drying falls steeply because, it is thought, of the drying of the surface granules. The second falling rate period is influenced by the diffusion of moisture from within the particles.


FIGURE 7.5 Drying curves for a tablet granulation dried in a tray drier (A) moisture content versus time and (B) drying rate versus moisture content.

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