Principles of Evaporation

Evaporation is a unit operation. It comes under the unit operation of simultaneous heat and mass transfer. Evaporation means the removal of solvent as vapour from a solution or slurry under such conditions that no attempt is made to separate components of the vapour. In majority of evaporation process the solvent is water and latent heat for evaporation is supplied by condensing steam and heat from the steam is transmitted to the solution indirectly through the metallic surfaces.

Basic Factors of Evaporation: For effective evaporation of a liquid, two essential things must be provided.

 Necessary heat must be supplied: The heat may be supplied by direct exposure to the liquid or indirectly by transmission through suitable solid retaining wall. The vapour evolved must be removed continuously: The removal of vapour may be carried out either by mixing it with some inert gas or as undiluted vapour.

Three principal elements involved in an evaporator are:

•  Heat transfer
•  Vapour-liquid separation
•  Energy utilization

The unit in which heat transfer takes place is called heating element or calendria.The term calendria is also used to describe a particular type of evaporator. The,vapour-liquid separators are variously called bodies, vapour heads or flash chambers.The term body is also used to denote an evaporator comprising of one heating element and one vapour head. An effect is one or more bodies boiling at the same pressure. A multiple effect evaporator is one in which vapour from one effect is used as heating medium for another effect boiling at a lower pressure. The requirements for optimum functioning of any evaporator are as follows:

•  It must transfer maximum amount of heat.
•  It must efficiently separate vapour from the residual liquid.
•  It must make an efficient use of the available heat or mechanical energy as is economically feasible.