Concentration
The properties of the feed to an evaporator may exhibit no unusual problems. However, as the liquor is concentrated, the solution properties may drastically change. The density and viscosity may increase with solid content until the heat transfer performance is reduced or the solution becomes saturated. Continued boiling of a saturated solution may cause crystals to form which often must be removed to prevent plugging or fouling of the heat transfer surface. The boiling point of a solution also rises considerably as it is concentrated.
Foaming
Some materials may foam during vaporization. Stable foams may cause excessive entrainment. Foaming may be caused by dissolved gases in the liquor, by an air leak below the liquid level, and by the presence of surface-active agents or finely divided particles in the liquor. Many antifoaming agents can be used effectively. Foams may be suppressed by operating at low liquid levels, by mechanical methods, or by hydraulic methods.
Temperature Sensitivity
Many chemicals are degraded when heated to moderate temperatures for relatively short times. When evaporating such materials special techniques are needed to control the time/temperature characteristics of the evaporator system.
Salting
Salting refers to the growth on evaporator surfaces of a material having a solubility that increases with an increase of temperature. It can be reduced or eliminated by keeping the evaporating liquid in close or frequent contact with a large surface area of crystallized solid.
