For every product, there
is a representative curve that describes the drying characteristics for that
product at a specific temperature, velocity and pressure conditions. This curve
is referred to as the drying curve for a specific product. Variations in the
curve will occur principally in rate relative to carrier velocity and temperature.
The curve is extremely valuable in understanding unusual behaviour associated
with the drying of each unique product. The drying process can be divided into
three periods:
- (a) Constant drying rate period.
- (b) First falling drying rate period.
- (c) Second falling rate period.
(a) Constant Drying Rate Period
In a constant drying rate
period, material or mass of material contain much of water that liquid surface
exists which dries similarly to an open-faced body of water. Diffusion of moisture
from within the droplet maintains saturated surface conditions and as long as
this lasts, evaporation takes place at a constant rate. When a solid is dried
under constant drying conditions, the moisture content (MC) typically falls.
The graph is linear at first, then curves and eventually levels off. Constant
rate drying period (B-C) will proceed until free moisture appears from the
surface, the moisture removal rate will then become progressively less. At CMC
the drying rate ceases and remains constant. During the constant rate period,
the moisture from the interior migrates to the surface by various means and is
vaporized.
As the moisture content
is lowered, the rate of migration to the surface is also lowered. If drying
occurs at too high temperatures, the surface forms closely packed shrunken
cells which are sealed together. This acts as a barrier to moisture migration
and tends to keep the moisture sealed within. This condition is known as ‘case
hardening’. The constant rate period is characterized by a drying independent
of moisture content. During this period, the solid is so wet that a continuous
film of water remains over the entire drying surface, and this water acts to
lower the drying rate. The temperature of the wetted surface attains the wet-bulb
temperature.
Fig.1: A Typical Drying Curve for a Solid (Drying Rate Vs. Free Moisture)
Web Bulb Temperature
(WBT): WBT is the steady-state temperature shown by the
thermometer whose bulb is covered with a wet wick and from which water is
evaporating into a high-velocity air stream. The quantity of water evaporated
is not high enough to alter the temperature and humidity of the air stream. The
air is blown at high velocity (minimum 300 m/min) to cause evaporation of water
from the wick. Evaporation requires latent heat. This heat comes from the surface
of the glass bulb of a thermometer. So the temperature of the glass bulb
decreases. The heat comes from the temperature difference between Tw
and Ta (large). It is the case of simultaneous heat and mass
transfer. This heat is latent heat for the phase change of water to water
vapour.
(b) Falling Rate Periods
The constant rate period
ends when the migration rate of water from the interior of the surface becomes
less than the rate of evaporation from the surface. The period after the
critical point is called ‘the falling rate period’. Following this point, the
surface temperature rises, and the drying rate falls off rapidly. The falling
rate period takes a far longer time than the constant rate period, even though
the moisture removal may be much less. The drying rate approaches zero at some
equilibrium moisture content.
Drying in falling rate
period involves two processes:
- (a) Movement of moisture within the material to the surface.
- (b) Removal of the moisture from the surface.
The method used to
estimate drying rates and drying times in the falling rate period depends on
whether the solid is porous or non-porous. In a non-porous material, once there
is no superficial moisture, further, drying can occur only at a rate governed
by diffusion of bulk moisture to the surface. In a porous material, another
mechanism appears, and drying takes place in the bulk of the solid instead of
at the surface.
(i) First falling drying
rate period: The moisture content at the end of the
constant rate period (point c), is the ‘critical moisture content’. At this
point, the surface of the solid is no longer saturated, and the rate of drying
decreases with the decrease in moisture content. At point C, the surface
moisture film evaporates fully, and with the further decrease in moisture
content, the drying rate is controlled by the rate of moisture movement through
the solid.
(ii) Second falling
drying rate period: Period C to D represents conditions when
the drying rate is largely independent of conditions outside the solid. The
moisture transfer may be due to any combination of liquid diffusion, capillary
movement, and vapour diffusion.
Effect of Shrinkage: A
factor that often greatly affects the drying rate is the shrinkage of the solid
as moisture is removed. Rigid solids do not shrink appreciably, but colloidal
and fibrous materials do undergo shrinkage. The most serious effect is the development
of a hard layer on the surface that is impervious to the flow of liquid or
vapour moisture and slows down the drying rate. In many materials, if drying
occurs at too high a temperature, a layer of closely packed, shrunken cells,
which are sealed together, forms at the surface that presents a barrier to
moisture migration. Another effect of shrinkage is to cause the materials to
warp and change their structure. Sometimes, to decrease these effects of
shrinkage, it is desirable to dry with moist air. This decreases the rate of
drying so that the effects of shrinkage on warping or hardening at the surface
are greatly reduced.
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