Drying and Evaporation

Drying and Evaporation

Difference between drying and evaporation: 

1.      The term drying usually refers the removal of relatively small amounts of water from solid or nearly solid material.

The term evaporation is usually limited to the removal of relatively large amounts of water from solutions.

2.      In most cases drying involves the removal of water at temperatures below its boiling point, whereas,

Evaporation means the removal of water by boiling a solution.

3.      In evaporation, the water is removed from the material as practically pure water vapor, mixed with other gases only because of unavoidable leaks.

Tray dryer/Compartment dryer (Static bed systems):

Principle: An insulated chamber is used in which several compartments and trays are set up. The materials, which are dried, are placed on the trays by opening a door. The door is also insulated. Inner surface of the metallic chamber should be well polished for the reflection of heat. The air is passed through an inlet and heated by a heating coil.

Operation: The trays are loaded with materials to be dried on the trucks, which are pushed into the dryer. Now air is allowed to pass through the air inlet and heated by the heating coil. A number of fans are used to circulate the hot air throughout the chamber and baffles serve these purposes.

The hot air circulates through the racks of trays and the materials become dried. Timing and constant temperature regulations are essential and are maintained by timer and thermostat. After drying the materials, air goes out through outlet (exhaust). The outlet contains a filter that prevents the loss of fine particles with exhaust air.

Applications:

1. Tray dryers are used in research laboratories and QC department for drying glass wires and small apparatus.
2. Dryers are used for drying of packaging materials, plastic caps, spoons, injectable vials, glass containers, etc.
3. Tray dryers are used in sterilization of containers.

Advantages:

1. It is easier to repair and operate.
2. It is excellent for small-scale production.
3. It has good control of heat and humidity.
4. The machine is less expensive as the initial cost is low.
5. The machine is very flexible and versatile.
6. The material movement is less, so heat of mass transfer is less.

Disadvantages:

1. Power consumption is high (approximately 1.5 hp/track).
2. Oxidation sensitive materials cannot be dried in the dryer.
3. Thermolabile substances are not used.
4. Its output is limited to dry.
5. Particles may lose by exhaust.
6. Air pollution may occur.
7. Liquid materials cannot be dried.
8. It is not suitable for large scale production.
9. Gas-solid contact is not perfect.