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7/31/2019 Heat Transfer Equipments

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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



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



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

http://www.cbu.edu/~rprice/lectures/evap3.html


http://www.ddpsinc.com/TeachingUnits/qts5.html


http://www.endersprocess.com/proc7.html


https://engineering.purdue.edu/ProPractice/Co-Op%20Student%20Work%20Reports/Jacob%20Wooddell%20-%20BFPE.pdf


http://www.eng.auburn.edu/users/josepbe/courses/CHEN3820/EvaporatorEquipmentAndObjectives.pdf


http://www.britannica.com/EBchecked/topic/397107/multiple-effect-evaporator








http://www.britannica.com/EBchecked/topic/397107/multiple-effect-evaporator



Batch PanNatural Circulation Tubular Rising Film Tubular

Falling Film TubularRising \ Falling Film Tubular Forced Circulation Tubular

Rising \ Falling Film Plate



Backward Feed for Double Effect Evaporator

Forward feed for Double Effect Evaporator

RISING AND FALLING FILM EVAPORATOR



DOUBLE EFFECT EVAPORATOR



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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