Page 165 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

6.2 Components of an evaporation system 163

Some phenomena affecting the performance and operation of evaporators are discussed. These
depend both on the process fluid and affect the choice and design of evaporator.
Salting, Scaling, and Fouling e These are three distinct phenomena to be minimized/eliminated
in evaporators. The choice of the material of construction of evaporators often depends on these.
Salting is deposit and growth of material on cooler surfaces in an evaporator. This is common in
crystallizing evaporators as the crystal solubility increases with temperature. Salting is reduced by
keeping the liquid in contact with a large surface of the crystallizing solid.
Scaling is also deposition and growth of a solid on heat transfer surface and happens for solutes
whose solubility decreases with increase in temperature. It may also be the result of an irreversible
chemical reaction in the evaporator. It is managed in the same manner as salting. Forced circulation
evaporators are preferred over evaporators with boiling induced circulation for processes having high-
scale formation tendency.
Fouling is deposition other than salts and scales on the heat transfer surface that can be due to
corrosion, solid matter entering with feed and deposits from condensing vapors.
Foaming and Entrainment: Foaming is caused by colloids, surface tension depressants, finely
divided solids and dissolved air or gases in the liquid. Defoaming techniques use chemical, thermal, or
mechanical methods. Correct choice and dosage of chemical antifoams (vegetable oil, fatty acid,
sulfonated castor oil, etc.) are quite effective and drastically reduce/eliminate foaming. Thermal
defoaming causes foam collapse by increasing or decreasing the temperature. At a high temperature,
foams may collapse due to a decrease in surface tension, solvent evaporation, or chemical degradation
of the foam-producing agents. This is often effected by the use of steam jets impinging on the foam
surface or operation at a low liquid level so that the hot surface can break the foam on contact.
A reduction in surface elasticity may be responsible for foam breakage by lowering the temperature.
Mechanical methods use tensile shear, or compressive forces to destroy foams, for example,
impingement at high velocity against a baffle as done in a long vertical tube, agitated film, and forced
circulation evaporators. The choice of defoaming technique depends on the process to which it is
applied and the convenience with which it may be applied.
Entrainment is carryover of liquid droplets by the exiting vapor and is considerably reduced by
providing sufficient disengagement height of say 1.8 m, above the boiling liquid. In spite of this, the
addition of some form of de-entrainer is common. The upturned pipe, deflector, and the tangential
separator shown in Fig. 6.3 are mechanical contraptions for reducing entrainment. An upturned pipe
has the simplest construction and is normally for adopted small equipment. Deflector type is most
common in use. Climbing-film evaporators are fitted with tangential separators. A good entrainment
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separator will limit liquid carryover to 10e20 kg/10 kg of vapor.
Corrosion and erosion are common in evaporators due to (A) the high liquid and vapor velocities
encountered, (B) presence of solids in suspension and (C) necessary concentration differences.
Splashing losses are usually insignificant if a reasonable disengagement height is provided above
the liquid level. This height depends on the vigor of boiling and is usually 2.4e3.5 m in short-tube
vertical evaporators and less in forced circulation and long-tube vertical evaporators where liquid
motion is retarded by baffle or by centrifugal motion.