Other Methods of Drying

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Chapter: Pharmaceutical Engineering: Drying

Apart from specialized dryers using infrared or dielectric heating, the chief method of passing heat into a drying solid, other than from a hot airstream, is by conduction from a heated surface.


OTHER METHODS OF DRYING

Apart from specialized dryers using infrared or dielectric heating, the chief method of passing heat into a drying solid, other than from a hot airstream, is by conduction from a heated surface. When a wet solid is placed in contact with a hot surface, subsequent events depend on the temperature of the surface relative to the boiling point of the liquid, the nature of the solid, and the method of heating the surface. It is assumed here that the temperature of the surface is not hot enough for convective boiling to take place.

Consider first a cake of finely divided solids saturated with water. A temperature gradient will be established through the cake and evaporation from the free surface will take place at a rate governed entirely by the rate of heat input. 


FIGURE 7.6 Drying by conduction of heat from a heated surface.

During this period, the rate of evaporation and the temperature of a particular layer of cake will be approximately constant. This will continue until capillary forces are unable to transfer liquid to the free surface at the required rate. The temperature gradients during this period are given in Figure 7.6A and B for conditions in which the shelf temperature is below and above the boiling point of the liquid, respectively.

With a comparatively low heat flux, so that the partially dried cake can conduct heat away from the hot surface at the required rate, the free surface will dry and a fictitious drying line will recede slowly into the cake, the vapor diffusing through the dry cake to the free surface. The temperature gradient during this falling rate period is shown in Figure 7.6C. If the heat flux is high, the point at which mobile water can no longer reach the surface is marked by the onset of drying in a layer adjacent to the hot surface, and vapor is forced through the wet cake above. As the solid dries, its temperature increases and a temperature gradient is established through the dry solids to the drying line that is receding upward. This is shown in Figure 7.6D. The free surface of the solid appears wet and is at a constant temperature. These conditions are destroyed when the drying line reaches the surface.

In either case, a low and falling rate of drying will persist as the absorbed water is removed. In this form of drying, the heat treatment received by the solid is not uniform but depends on its position in the cake.

A hot surface may also be used to dry solutions, such as milk or plant extracts, which do not readily give porous, crystalline solids on concentration. Apart from an initial constant rate period, when heat transfer is mainly con-vective, drying periods are ill defined. As concentration proceeds, the liquor becomes more viscous and heat transfer is mainly by conduction. Large volume changes occur between initial and final stages. It is possible to dry thin films of solution to a solid film, but if deeper layers are taken, a skin is frequently formed at the free surface that is almost impervious to the vapor. Frothing and drying to a porous, friable structure will then occur. This may also happen if, during the upward recession of the drying line, the material above is too viscous to allow the escape of vapor.

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