The Drying of Hygroscopic Products in Rotary Dryers


I. Characteristics Drying Curve of Hydroscopic Products
Hydroscopic 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.

  A. 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 Phase
  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 Phase
  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.
II.Development of the Triple Pass Drying Drum
The triple 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.
III.The Stepped Drum
Duske Engineering 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.