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 = h – x (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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