EVAPORATION
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Evaporation is the removal of solvent as vapor from a solution or slurry.
For the overwhelming majority of evaporation systems the solvent is water.
DIFFER THAN DISTILLATION, DRYING AND CRYSTALISATION
In evaporation, no attempt is made to separate components of the vapor. This distinguishes
evaporation from distillation.
Evaporation is distinguished from drying in that the residue is always a liquid.
Evaporation differs from crystallization in that evaporation is concerned with concentrating a
solution rather than producing or building crystals.
WHAT AN EVAPORATOR DOES
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Transfer large amounts of heat to the solution with a minimum amount of metallic surface area.
This requirement, more than all other factors, determines the type, size, and cost of the
evaporator system
Achieve the specified separation of liquid and vapor and do it with the simplest devices
available.
Make efficient use of the available energy. This may take several forms. Evaporator performance
often is rated on the basis of steam economy-pounds of solvent evaporated per pound of steam
used
Heat is required to raise the feed temperature from its initial value to that of the boiling liquid,
to provide the energy required to separate liquid solvent from the feed, and to vaporize the
solvent. The greatest increase in energy economy is achieved by reusing the vaporized solvent
as a heating medium.
EVAPORATOR ELEMENTS
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Three principal elements are of concern in evaporator design: heat transfer, vapor-liquid
separation, and efficient energy consumption.
The units in which heat transfer takes place are called heating units or calandrias. The vaporliquid separators are called bodies, vapor heads, or flash chambers.
LIQUID CHARACTERISTICS
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CONCENTRATION - 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 - 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.
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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.
SCALING - Scaling is the growth or deposition on heating surfaces of a material which is either
insoluble or has a solubility that decreases with an increase in temperature.
FOULING - Fouling is the formation of deposits other than salt or scale.
CORROSION - Corrosion and erosion are frequently more severe in evaporators than in other
types of equipment because of the high liquid and vapor velocities, the frequent presence of
suspended solids, and the concentrations required.
PRODUCT QUALITY - Product quality may require low holdup and low temperatures. Low-holdup-time requirements may eliminate application of some evaporator types.
Other fluid properties must also be considered. These include: heat of solution, toxicity,
explosion hazards, radioactivity, and ease of cleaning.