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In general, drying a solid means the removal of relatively small amounts of water or other liquid from the solid material to reduce the content of residual liquid to an acceptably low value. Drying is usually the final step in a series of operations, and the product from a dryer is often ready for final packaging (Unit Operations of Chemical Engineering, McCabe, Smith, Harriot, 5th Edition, McGrawHill).


Industrial drying is an energy intensive process which is carried out in a wide range of industries, including chemicals, foodstuffs, minerals, paper, textiles and many others.


  • Principles of drying: (Unit Operations of Chemical Engineering, McCabe, Smith, Harriot, 5th Edition, McGrawHill)

Because of the wide variety of materials that are dried in commercial equipment and many types of equipment that are used, there is no single theory of drying that covers all materials and dryer types. Variations in shape and size of stock in moisture equilibria, in the mechanism of flow moisture through the solid and in the method of providing the heat required for the vaporization-all prevent a unified treatment. General principles used in a semiquantitative way are relied upon.

  • Feed of driers: (Unit Operations of Chemical Engineering, McCabe, Smith, Harriot, 5th Edition, McGrawHill)

The solids to be dried may be in many different forms- flakes, granules, crystals, powders, slabs, or continuous sheets- and may have widely differing properties. The liquid to be vaporized may be on the surface of the solid, as in drying salt crystals; it may be entirely inside the solid, as in solvent removal from a sheet of polymer-, or it may be partly outside and partly inside. The feed to some driers is a liquid in which the solid is suspended as particles or is in solution. The dried product may be able to stand rough handling and a very hot environment or it may require gentle treatment at low or moderate temperatures. Consequently, a multitude of types of driers are on the market for commercial drying. They differ chiefly in the way the solids are moved through the drying zone and in the way heat is transferred.

  • Classification of driers: (Unit Operations of Chemical Engineering, McCabe, Smith, Harriot, 5th Edition, McGrawHill)

There is no simple way of classifying drying equipment. Some driers are continuous and some operated batchwise; some agitate the solids and some are essentially unagitated. Operation under vacuum may be used to reduce the drying temperature. Some driers can handle almost any kind of material, while others are severely limited in the type of feed they can accept. A major division may be made between (1)driers in which the solid is directly exposed to a hot gas (usually air) and (2)driers in which heat is transferred to the solid from an external medium such as condensing steam, usually through a metal surface with which the solid is in contact. Driers that expose the solids to a hot gas are called adiabatic or direct driers; those in which heat is transferred from an external medium are known as nonadiabatic or indirect driers. Driers heated by electric, radiant or microwave energy are also nonadiabatic. Some units combine adiabatic and nonadiabatic drying; they are also known as direct-indirect driers.

  • Drying equipment: (Unit Operations of Chemical Engineering, McCabe, Smith, Harriot, 5th Edition, McGrawHill)

There are many types of commercial driers available. Here, only a small number of the important types will be mentioned:

1. Driers for Solids and Pastes:

Typical dryers for solids and pastes include tray and screen-conveyor driers for materials that cannot be agitated and tower, rotary, screw-conveyor, fluid-bed and flash driers where agitation is permissible.

2. Driers for Solutions and Slurries:

A few types of driers evaporate solutions and slurries entirely to dryness by thermal means. Typical examples are spray driers, thin-film driers and drum driers.



  • Temperature
  • Pressure
  • Medium
  • Drying material characteristics (e.g. moisture, temperature sensitivity)