A multiple-effect evaporator, as defined in chemical engineering, is an apparatus for efficiently using the heat from steam to evaporate water. In a multiple-effect evaporator, water is boiled in a sequence of vessels, each held at a lower pressure than the last. Because the boiling point of water decreases as pressure decreases, the vapor boiled off in one vessel can be used to heat the next, and only the first vessel (at the highest pressure) requires an external source of heat. While in theory, evaporators may be built with an arbitrarily large number of stages, evaporators with more than four stages are rarely practical.

The multiple-effect evaporator was invented by the African-American engineer Norbert Rillieux. Although he may have designed the apparatus during the 1820s and constructed a prototype in 1834, he did not build the first industrially practical evaporator until 1845 . Originally designed for concentrating sugar in sugar cane juice, it has since become widely used in all industrial applications where large volumes of water must be evaporated, such as salt production and water desalination.

The Evaporation Process

In the evaporation process, concentration of a product is accomplished by boiling out a solvent, generally water. The recovered end product should have an optimum solids content consistent with desired product quality and operating economics.

Types of Evaporator

The more common types of evaporators include:

• Batch pan
• Forced circulation
• Natural circulation
• Wiped film
• Rising film tubular
• Rising/falling film tubular
• Plate equivalents of tubular evaporators
• Falling film tubular
  

BATCH PAN

Next to natural solar evaporation, the batch pan is one of the oldest methods of concentration.

It is somewhat outdated in today's technology, but is still used in a few limited applications, such as the concentration of jams and jellies where whole fruit is present and in processing some pharmaceutical products.

With a batch pan evaporator, product residence time normally is many hours. Therefore, it is essential to boil at low temperatures and high vacuum when a heat sensitive or thermo-degradable product is involved.

Heat transfer is improved by agitation within the vessel.

large temperature differences cannot be used for fear of rapid fouling of the heat transfer surface. Relatively low evaporation capacities, therefore, limit its use.

NATURAL CIRCULATION

Evaporation by natural circulation is achieved through the use of a short tube bundle within the batch pan or by having an external shell and tube heater outside of the main vessel.

The external heater has the advantage that its size is not dependent upon the size or shape of the vessel itself. As a result, larger evaporation capacities may be obtained.

The most common application for this type of unit is as a re-boiler at the base of a distillation column.

RISING FILM TUBULAR

The rising film principle was developed commercially by using a vertical tube with steam condensing on its outside surface.

Liquid on the inside of the tube is brought to a boil, with the vapor generated forming a core in the center of the tube. As the fluid moves up the tube, more vapor is formed resulting in a higher central core velocity that forces the remaining liquid to the tube wall. Higher vapor velocities, in turn, result in thinner and more rapidly moving liquid film.

The development of the rising film principle was a giant step forward in the evaporation field, particularly in product quality.

Higher HTC's resulted in reduced heat transfer area requirements and consequently, in a lower initial capital investment.

RISING/FALLING FILM TUBULAR

The rising/falling film evaporator has the advantages of the ease of liquid distribution of the rising film unit coupled with lower head room requirements.

The tube bundle is approximately half the height of either a rising or falling film Evaporator.

The vapor/liquid separator is positioned at the bottom of the calandria.

FORCED CIRCULATION

The forced circulation evaporator was developed for processing liquors which are susceptible to scaling or crystallizing.

Liquid is circulated at a high rate through the heat exchanger, boiling being prevented within the unit by virtue of a hydrostatic head maintained above the top tube plate. As the liquid enters the separator where the absolute pressure is slightly less than in the tube bundle, the liquid flashes to form a vapor.

The main applications for a forced circulation evaporator are in the concentration of inversely soluble materials, crystallizing duties, and in the concentration of thermally degradable materials which result in the deposition of solids.

FALLING FILM TUBULAR

Falling film distribution generally is based around use of a perforated plate positioned above the top tube plate of the calandria.

Spreading of liquid to each tube is sometimes further enhanced by generating flash vapor at this point.

The falling film evaporator does have the advantage that the film is 'going with gravity' instead of against it. This results in a thinner, faster moving film and gives rise to an even shorter product contact time and a further improvement in the value of HTC.

The falling film evaporator does not have a driving force limitation–permitting a greater number of evaporator effects to be used within the same overall operating limits.

It is feasible to have as many as 10 or more effects.