Drying of Hygroscopic Products in Rotary Dryers
Characteristics Drying Curve of Hydroscopic Products
products are those which readily retain moisture based on temperature
and humidity. They can be dried to their equilibrium moisture content
in accordance with their desorption isotherms as related to air
humidity and product temperature.
The group of hydroscopic products includes wood, cellulose, paper,
food stuff, and other organic substances. When a hydroscopic product
particle is suspended in a drying medium of constant physical
properties such as air of constant temperature, pressure and relative
humidity and with constant relative velocity between particle
and air, the characteristics drying curve for hydroscopic products
is found to consist of three phases.
First Drying Phase
||The first drying phase
shows an almost constant rate of evaporation during which the particle
essentially behave as a water droplet and has free surface moisture.
The rate of drying is determined by physical properties of the gas
stream such as relative velocity between particle and air, and temperature
and relative humidity of the drying medium.
||B. Second Drying
||When the maximum hydroscopic
moisture on the particle surface is reached a rapid decrease in
the drying rate occurs. The drying zone recedes into the interior
of the product and is governed by diffusion and capillary properties
of the product.
||C. Third Drying
||This phase is reached
when the entire product has hydroscopic moisture content. The drying
rate approaches a value of zero asymptotically in accordance with
its equilibrium moisture content. The drying rate in this region
depends only on the diffusion conditions of the product.
of the Triple Pass Drying Drum
pass drying drum was developed in an attempt to adjust physical
properties of the drying medium to the three phase drying curve
of hydroscopic products in a concurrent drying process.
Co., Inc. has advanced the conventional triple pass drum by incorporating
a stepped inner cylinder which is of relatively small diameter at
its inlet and increased to a relatively large diameter at its outlet
end. As a result, the cross sectional area of the inner cylinder
increases along the drying path. Although the Intermediate Cylinder
is of constant diameter, the cross-sectional area of the intermediate
pass also increases along the drying path since there the product
moves along the exterior of the inner cylinder towards the smaller
diameter at the inlet end of this cylinder.
The outer cylinder cross sectional area of the drum is constant
and is larger than that of the other cylinders. This gives a gentle
finish time for the product.
In the drum incoming wet product is fed into a flash drying region
at the inlet of the inner cylinder with its high air velocities.
This results in high evaporation rates as well as in great differences
in axial advancements between small and large particles and between
light and heavy particles. The lighter and dryer particles are
quickly accelerated into the lower temperature of the expanding
cylinder while the wet and heavy particles receive longer exposure
in this area. This results in good adjustment of exposure time
to particle size and particles of all sizes at the outlet end
of the drying drum.
The flash drying section of the drum also explains why much higher
air inlet temperatures can be used without causing damage to the
product to be dried.
It has to be remembered that the rotary drying drum is not only
a heat exchanger but it is also a conveyor. The higher drying
rate coupled with optimum dwell time in the stepped inner cylinder
results in overall higher dryer efficiency over conventional dryers.