Static Beds of Nonporous Solids

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Chapter: Pharmaceutical Engineering: Drying

Drying of wet granular beds, the particles of which are not porous and are insoluble in the wetting liquid, has been extensively studied.


Drying of wet granular beds, the particles of which are not porous and are insoluble in the wetting liquid, has been extensively studied. The operation is presented as the relation of moisture content and time of drying in Figure 7.3A. It should be noted that the equilibrium moisture content is approached slowly. A protracted period may be required for the removal of water just above the equilibrium value. This is not justified if a small amount of water can be tol-erated in further processing and indicates the importance of establishing realistic drying requirements. 

FIGURE 7.3 (A) Moisture content versus time of drying and (B) rate of drying versus moisture content.

The stability of the solids, maintained, as shown later, at a temperature close to that of the drying air, may allow unnecessary deterioration.

The data has been converted to a curve relating the rate of drying to moisture content in Figure 7.3B. The initial heating-up period during which equilibrium is established is short and has been omitted from both figures.

Assuming that sufficient moisture is initially present, the drying rate curve exhibits three distinct sections limited by the points A, B, C, and D.

In section AB, called the constant rate period, it is considered that moisture is evaporating from a saturated surface at a rate governed by diffusion from the surface through the stationary air film in contact with it. An analogy with evaporation from a plain water surface can therefore be drawn. The rate of drying during this period depends on the air temperature, humidity, and speed, which in turn determine the temperature of the saturated surface. Assuming that these are constant, all variables in the drying equations given above are fixed and a constant rate of drying is established, which is largely independent of the material being dried. The drying rate is somewhat lower than that for a free water surface and depends to some extent on the particle size of the solids. During the constant rate period, liquid must be transported to the surface at a rate sufficient to maintain saturation. The mechanism of transport is discussed later.

At the end of the constant rate period, B, a break in the drying curve occurs. This point is called the critical moisture content, and a linear fall in the drying rate occurs with further drying. This section, BC, is called the first falling rate period. At and below the critical moisture content, the movement of moisture from the interior is no longer sufficient to saturate the surface. As drying pro-ceeds, moisture reaches the surface at a decreasing rate and the mechanism that controls its transfer will influence the rate of drying. Since the surface is no longer saturated, it will tend to rise above the wet bulb temperature.

For any material, the critical moisture content decreases as the particle size decreases. Eventually, moisture does not reach the surface that becomes dry. The plane of evaporation recedes into the solid, the vapor reaching the surface by diffusion through the pores of the bed. This section is called the second falling rate period and is controlled by vapor diffusion, a factor which will be largely

FIGURE 7.4 Drying curve for a skin-forming material.

independent of the conditions outside the bed but markedly affected by the particle size because of the latter’s influence on the dimensions of pores and channels. During this period, the surface temperature approaches the temper-ature of the drying air.

Considerable migration of liquid occurs during the constant rate and first falling rate periods. Associated with the liquid will be any soluble constituents that will form a concentrating solution in the surface layers as drying proceeds. Deposition of these materials will take place when the surface dries. Consider-able segregation of soluble elements in the cake can, therefore, occur during drying.

If the soluble matter forms a skin or gel on drying rather than a crystalline deposit, a different drying curve, shown in Figure 7.4, is obtained. The constant rate period is followed by a continuous fall in the drying rate in which no differentiation of first and second falling rate periods can be made (hence “C” is not identified in Fig. 7.4). During this period, drying is controlled by diffusion through the skin that is continually increasing in thickness. Soap and gelatin are solutes that behave in this way.

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