The Internal Mechanism of Drying

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Chapter: Pharmaceutical Engineering: Drying

Extensive studies have been made to determine the nature of the forces that initially convey moisture to the surface at a rate sufficient to maintain saturation and their subsequent failure.


THE INTERNAL MECHANISM OF DRYING

Extensive studies have been made to determine the nature of the forces that initially convey moisture to the surface at a rate sufficient to maintain saturation and their subsequent failure. Movement of liquid may occur by diffusion under the concentration gradient created by depletion of water at the surface by evaporation, as the result of capillary forces, through a cycle of vaporization and condensation, or by osmotic effects. Of these, capillary forces offer a coherent explanation for the drying periods of many materials.

If a tapered capillary is filled with water and exposed to a current of air, the meniscus at the smaller end remains stationary while the tube empties from the wider end. A similar situation exists in a wet particulate bed, and the phenomenon is explained by the concept of suction potential. A negative pressure exists below the meniscus of a curved liquid surface, which is pro-portional to the surface tension, g, and inversely proportional to the radius of curvature, γ. (The meniscus is assumed to be a part of a hemisphere.) This negative pressure or suction potential may be expressed as the height of liquid, h, it will support:


where ρ is the density of the liquid.

The suction potential, hx, acting at a depth x below the meniscus will then be given by

hx = hx                    (7.6)

The particles of the bed enclose spaces or pores connected by passages, the narrowest part of which is called the waist. The dimensions of the latter will be determined by the size of the surrounding particles and the manner in which they are packed. In a randomly packed bed, pores and waists of varying sizes will be found. Thus, the radius of a capillary running through the bed varies continuously. The depletion of water in this network will be controlled by the waists because the radii of curvature will be smaller and the suction potentials greater than that for the pores. Depletion occurs in the following way. As evaporation proceeds, the water surface recedes into the waists of the top layer of particles, and a suction potential develops. The maximum suction potential a waist can develop is called its “entry” suction potential, and this will be exceeded for the larger waists by the suction potential developed by the smaller waists and transmitted through the continuous, connecting thread of liquid. The menisci in the larger waists will collapse and the pores they protect will be emptied, that is, assuming an interconnecting thread of liquid, a surface waist developing a suction potential, hs, will cause the collapse of an interior waist developing a suction potential hi and distance x below the surface if hs > hi ) + x. The liquid in the exposed pores is then lost at the surface by evaporation. This effect will continue until a waist provides an opposing suction potential that is equal to or greater than the suction potential provided at that depth by the fine surface waist meniscus. The latter then collapses, and the pore it protects is emptied.

By this mechanism, a meniscus in a fine surface waist will hold its position and deplete the interior of moisture. If sufficient full surface waists are present, the constant rate period is maintained since the stationary air film in contact with the bed can be saturated. The first falling rate period indicates that insuf-ficient full surface waists are present. Eventually, the collapse of all surface waists takes place, giving a breakdown of the capillary network supplying moisture to the surface, and the second falling rate period ensues.

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