Drying

In the drying process volatile liquids are separated from solid materials by vaporizing the liquid and removing the vapor. During the process, heat is supplied to vaporize the liquids. Drying accounts for a large part of the energy usage in the chemical industry. Dryers can be separated into five main categories, depending on the method of heat transfer [3, 16]:

• convective dryers;

• radiant heat dryers;

• high-frequency dryers;

In a convective dryer the vaporization of liquid is carried out by direct contact of the wet solid and a hot gas stream. In contact dryers the heat is conducted from a heated surface to the wet solid. The principle of radiation is exploited in radiant heat dryers, in which heat is supplied from a radiation source. High-frequency drying is characterized by dielectric heating of the wet solid for example, microwave drying. Finally, in freeze dryers the removal of water is carried out by sublimation of ice from the solid at temperatures below 0 °C and reduced pressure.

The most popular type of large scale dryer in chemical industry is the convective dryer, used to perform evaporation of water. Additionally, dryers that evaporate solvents can be found in polymer, fine chemical and pharmaceutical processes [16].

The ratio of the heat required to vaporize the solvent in relation to the heat actually consumed can be defined as the thermal efficiency of dryers. Contact dryers have the highest efficiency. However, the thermal efficiency of many con-vective dryers is often low. Thermal efficiency of convective dryers can be enhanced by improved operation and control and by heat recovery from exhaust gases.

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