Multiple Effect Evaporation remains one of the popular methods used for the concentration of aqueous solutions. Water is removed from a solution by boiling the liquor in an evaporator and withdrawing the vapour. If the solution contains dissolved solids, the resulting strong liquor may become saturated so that crystals are deposited.
Evaporation process comprises three main steps:
1. Pre-heating of a solution prior to evaporation
2. Removal of water (solvent) as vapour by steam heating
3. Condensing the vapour removed
Evaporation is carried out by supplying heat to the solution to vaporise the solvent. The heat is supplied basically to provide the latent heat of vaporisation and by adopting methods for recovery of heat from the vapour, it has been possible to achieve great economy in heat utilisation. The normal heating medium is generally low pressure steam (1 to 1.5 kg/cm2g).
An industrial evaporator systems generally comprises:
1. A heat exchanger to supply sensible heat and latent heat of evaporation to the feed. Saturated steam is usually used as the heating medium.
2. A separator in which the vapour is separated from the concentrated liquid phase.
3. A condenser to effect condensation of the vapour and its removal from the system.
There are two main types of ways of improving steam economy in evaporators. One is to use a multiple effect evaporator, the other is to use mechanical vapour recompression.
If an evaporator, fed with steam at 399 K with a total heat of 2714 kJ/kg, is evaporating water at 373 K, then each kilogram of water vapour produced will have a total heat of 2675 kJ. If this heat is allowed to go to waste, by condensing it in a tubular condenser or by direct contact in a jet condenser for example, such a system makes very poor use of steam. The vapour produced is, however, suitable for passing to the calandria of a similar unit, provided the boiling temperature in the second unit is reduced so that an adequate temperature difference is maintained. This can be effected by applying a vacuum to the second effect in order to reduce the boiling point of the liquor. This is the principle used in the multiple effect systems .