EVAPORATION

Evaporation

• Evaporation is when a liquid becomes a gas without forming bubbles inside the liquid volume.
• During evaporation, only the molecules near the liquid surface are changing from a liquid to vapor.
• Evaporation is the process that occurs when the surface of a liquid is converted into a gas
• For example, evaporation occurs when a glass of water is left out overnight and the water level is found to drop.
• Another example is when steam flows through a tube that is submerged in a pool of liquid, minutes bubbles of vapor form at random points on the surface of the tube. The heat is passing through the tube surface where no bubbles form enters the surrounding liquid by convention. Then some of the heat in the liquid then flows toward the bubbles, causing evaporation from its inner surface into itself.
• The objective of evaporation is to concentrate a solution consisting of a non volatile solute and volatile solvent.
• Evaporation is conducted by vaporizing a portion of the solvent to produce a concentrated solution of thick liquor.
• Normally, in evaporation thick liquor is the voluble product and the vapor is condensed and discarded.
• Hence, Evaporation is a widely used method for the concentration of aqueous solutions, involves the removal of water from a solution by boiling the liquor in a suitable vessel, an evaporator, and withdrawing the vapor. 
• If the solution contains dissolved solids, the resulting strong liquor may become saturated so that crystals are deposited. 
• Evaporation is achieved by adding heat to the solution to vaporize the solvent. 
• The heat is supplied principally to provide the latent heat of vaporization, and, by adopting methods for recovery of heat from the vapor, it has been possible to achieve a great economy in heat utilization. 
• Whilst the normal heating medium is generally low-pressure exhaust steam from turbines, special heat transfer fluids or flue gases are also used. 
• Evaporation is differ from drying in that the residue is the liquid, sometimes a highly viscous one rather than solid.
• Evaporation is differ from distillation in that vapor usually is a single component, and even when vapor is a mixture, separation of vapor into a smaller fraction is not carried out in evaporation.

• Single effect Evaporator
• Multiple effect evaporator
• once through and circulation evaporators
• Capacity
• Economy
• Steam consumption
• Heat transfer in Evaporators
• Boiling point elevation
• Boiling point rise
• Comparison of single and multiple effect evaporators