Evaporation Plants

Evaporation is a type of vaporization of a liquid that occurs from the surface of a liquid into a gaseous phase that is not saturated with the evaporating substance. A great variety of evaporator designs are developed for specialize applications, these can be grouped into following basic types.

Evaporation Systems

Falling Film Evaporation

Principle of Operation:

The liquid to be concentrated is fed to the top of the heating tubes and is distributed evenly over distributor plate & flows down inside tube walls as a thin film.
The liquid film starts boiling due to the external heating of the heating tubes and is partially evaporated. The downward flow, caused initially by gravity, is enhanced by the parallel, downward flow of the vapor formed.
Residual film liquid and vapour is separated in vapor liquid separator (VLS).

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Rising Film Evaporation

Principle of Operation:

In rising film evaporator liquid being evaporated is fed from the bottom into long tubes and heated with steam condensing on the outside of the tube from the shell side.
This is to produce steam and vapour within the tube bringing liquid inside to a boil.
The vapour produced then presses the liquid against the walls of the tubes and causes the ascending force of this liquid.
As more vapours are formed, the centre of the tube will have a higher velocity which forces the remaining liquid against the tube wall forming a thin film which moves upwards.

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Forced Circulation Evaporation

Principle of Operation:

Liquid is circulated at high flow rate through heat exchanger, boiling being prevented within the unit by virtue of hydrostatic head maintained above top tube plate.
As liquid enters separator where absolute pressure is slightly less than in tube bundles, liquid flashes to form vapors.
Vapors are then passed to next effect and liquid is kept in continuous circulation. Once desired concentration is achieved product is taken out from final effect.
Steam or vapors are used as heating media in forced circulation evaporation.
To maintain a good heat transfer within the heat exchanger it is necessary to have a high recirculation flow rate.
Calendria can be arranged either horizontally or vertically depending on the specific requirements in each case.

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Thermal & Mechanical vapor re-compressor (TVR & MVR)

Thermal separation processes like evaporation is an energy intensive process. During continuous up-gradation in systems to make it more energy efficient, led it to multiple-effect plants, then to thermal vapour recompression and mechanical vapour recompression systems.

In each case, the decision on whether a vapour recompression system or Mechanical vapour recompression should be installed, must be made on the basis of an efficiency study.

A. Mechanical vapor recompressor (MVR):

In MVR all the process vapours leaving the evaporator are recompressed to the pressure of the corresponding heating steam pressure/temperature used to heat the evaporator.
The amounts of heat to be dissipated are considerably reduced, with the evaporator itself re-utilising the energy normally dissipated through the condenser cooling water.
The energy required to drive the compressor is only the energy difference between the process vapor and the recompressed vapours, making such system energy efficient.
Depending on the operating conditions of the plant, a small quantity of additional steam or the condensation of a small quantity of excess vapour may be required to maintain the overall evaporator heat balance and to ensure stable operating conditions.
Low pressure steam will be required for preheating when starting up the process.
The compressor can be either positive-displacement type (lobes or screw) or centrifugal type with one or several stages of compression, or fans. Due to their simplicity and maintenance friendly design, centrifugal fans are often used.

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B. Thermo vapor recompressor (TVR):

Wherever steam is available at a high pressure as compared with the pressure needed in the evaporator, the use of a thermal vapour recompressor gives the same steam/energy saving as an additional evaporation effect.
During thermal vapour recompression, a fraction of the process vapor from the boiling chamber is recycled and recompressed in a steam jet ejector with high pressure live steam, called motive steam, to a higher pressure and temperature as required for the heating of the evaporator.
As we recycle of process vapour, the amount of live steam required for heating is lower and flow of process vapour going to the subsequent effect or condenser is therefore reduced, decreasing the cooling water demand.

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