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DOUBLE EFFECT EVAPORATOR
Conservation of energy is one major consideration that needs to be well-treated in
the application of an evaporator system. This is due to the fact that the larger the
evaporation process unit, the more crucial it is to conserve energy. One of the
techniques available for this purpose is by applying double or multiple effect evaporator.
The invention of multiple effect evaporators is generally credited to Norbert Rillieux. Back
then, Rillieux developed a multiple pan evaporation system for use in sugar refining.
Double-effect evaporation generally involves processing the steam generated from
evaporation in first effect, where it is condensed in the heating element in order to
provide the heat for product evaporation in a second effect maintained at lower pressure.
The boiling temperature is usually high enough so that the evaporated water can serve
as heating medium in second effect. The first effect will however has the highest
pressure. The effects in an evaporator are usually the same in terms of size,
construction and heat transfer and unless thermal oses are significant, the capacity will
be the same as well. The steam economy rises as the number of effects used is
increased. Increasing the number of effect does increase the capital investments;therefore careful evaluation is necessary for such systems. In general, when the
evaporation rate is no lower than 3,000 lbs/h (1350 kg/h), thus application of multi-effect
evaporator should be considered.The feed for double effect evaporator is normally
transferred from first effect to another. This will in turn leads to the ultimate product
concentration to reach only in either one of the effect. However, there are three different
ways for feed introduction in double effect evaporator. The feed order is not according to
the number of effects, whereas effects are always numbered according to reducing
pressure.
The first feed way is by forward feed arrangement , where the raw feed is
introduced in the first effect and then to the second parallel to steam flows. Operating
costs are increased since the feed is raised to the highest operating temperature. The
liquor produced are however most concentrated and viscous in lowest temperature, and
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thus provides low heat transfer coefficient.On the contrary, feed is introduced in the
second effect and then is transferred to the first effect. This method is termed as
backward feed operation. Multiple pumps are needed to deal with pressure drop of the
system, and this method is suitable for viscous solution as the viscosity will be reduced
as long as the system is operated at high temperature. As a result, a high transfer
coefficient will also be attained.The third method is by parallel feed systems where the
feed stream is split and portion of them are fed to each effect. This method however is
suitable for producing substance that involves crystallization where the product is likely
to be slurry.
APPLICATIONS OF DOUBLE EFFECT EVAPORATOR IN VARIOUS INDUSTRIES
Caustic Soda
Caustic soda is usually manufactured by electrolytic process of sodium chloride
brine. The caustic soda solution produced needs to be concentrated by evaporation.
This evaporation is however difficult since caustic soda has high boiling point elevation.
Double effect evaporator is used in backward feed arrangement so that the highest
concentration is on the first effect. Besides that, caustic soda can corrode stainless steel
at high temperature, therefore the first and second effect calandrias are best fabricated
in nickel that resists corrosion. Due to high temperature, falling film tubular evaporations
are the most suitable for this application.
Brewery / Distillery Effluents
The effluents from brewery and distillery plants are often processed in
evaporators to recover the water and produce concentrated syrup, which can be sold in
liquid form, or even added to the spent grains prior to drying. Normally, falling film tubular
calandria are used for the pre evaporator with forced circulation plate or tubular
evaporators as the finisher.
As a matter of fact, the viscosity of the concentrated liquids depends on the raw material
grain. As the viscosity is getting higher, thus the evaporator will have to be cleaned even
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more frequent. Thus, on the finisher, it is preferable to use duplex heat exchangers, so
that one can be cleaned while the other is in operation.
Black Liquor
Literally, black water is caustic waste water generated at paper plants. Typically
black liquor contains 3% solids, and half of it is caustic soda. This results in merely high
boiling point elevation as the concentration increases, thus most black liquor evaporators
are double-effect evaporators.
Sugar Refining
Crystallization of very coarse refined sugar is done by using double-effect
evaporator in conjunction with thermal vapour compression. Conservation of energy is practised in this case where vacuum is used from batch pans evaporator as heating
steam. A controllable thermo-compressor increases this vapour from 0.2 bar to 0.3 bar.
In addition, vapour is supplied at approximately 0.35 bars, thus reducing the heating
steam requirements of the pan vapour.
REFERENCES
1. http://www.cbu.edu/~rprice/lectures/evap3.html
2. http://www.ddpsinc.com/TeachingUnits/qts5.html
3. http://www.endersprocess.com/proc7.html
4. https://engineering.purdue.edu/ProPractice/Co-Op%20Student%20Work
%20Reports/Jacob%20Wooddell%20-%20BFPE.pdf
5. http://www.eng.auburn.edu/users/josepbe/courses/CHEN3820/EvaporatorEquip
mentAndObjectives.pdf
6. http://www.britannica.com/EBchecked/topic/397107/multiple-effect-evaporator
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Batch PanNatural Circulation Tubular Rising Film Tubular
Falling Film TubularRising \ Falling Film Tubular Forced Circulation Tubular
Rising \ Falling Film Plate
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Backward Feed for Double Effect Evaporator
Forward feed for Double Effect Evaporator
RISING AND FALLING FILM EVAPORATOR
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DOUBLE EFFECT EVAPORATOR
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1-2SHELL & TUBE HEAT EXCHANGER CLIMBING & FALLING FILM
EVAPORATOR
DOUBLE EFFECT EVAPORATOR DOUBLE PIPE CONCENTRIC TUBE HEAT EXCHANGER
HEAT CONDUCTION STUDY APPARATUS
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1-2SHELL & TUBE HEAT EXCHANGER CLIMBING & FALLING FILM
EVAPORATOR
DOUBLE EFFECT EVAPORATOR DOUBLE PIPE CONCENTRIC TUBE HEAT EXCHANGER
HEAT CONDUCTION STUDY APPARATUS