chemical engineering Chemical engineering is an engineering field which deals with the study of the operation and design of chemical plants as well as methods of improving production. Chemical engineers develop economical commercial processes to convert raw materials ...

, a multiple-effect evaporator is an apparatus for efficiently using the heat from

steam Steam is water vapor, often mixed with air or an aerosol of liquid water droplets. This may occur due to evaporation or due to boiling, where heat is applied until water reaches the enthalpy of vaporization. Saturated or superheated steam is inv ...

to evaporate water. Water is boiled in a sequence of vessels, each held at a lower

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and eve ...

than the last. Because the boiling temperature of water decreases as pressure decreases, the

vapor In physics, a vapor (American English) or vapour (Commonwealth English; American and British English spelling differences#-our, -or, see spelling differences) is a substance in the gas phase at a temperature lower than its critical temperature,R ...

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. The multiple-effect evaporator was invented by the American ( Louisiana Creole) engineer

Norbert Rillieux Norbert Rillieux (March 17, 1806 – October 8, 1894) was a Louisiana Creole inventor who was widely considered one of the earliest chemical engineers and noted for his pioneering invention of the multiple-effect evaporator. This invention w ...

. 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 Sugar is the generic name for sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, include glucose Glucose is a sugar with the Chemical formula#Molecular formula, molecul ...

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 Desalination is a process that removes mineral components from saline water. More generally, desalination is the removal of salts and minerals from a substance. One example is Soil salinity control, soil desalination. This is important for agric ...

. Multiple-effect evaporation commonly uses sensible heat in the condensate to preheat liquor to be flashed. In practice the design liquid flow paths can be somewhat complicated in order to extract the most recoverable heat and to obtain the highest evaporation rates from the equipment. While in theory, evaporators may be built with an arbitrarily large number of stages, evaporators with more than four stages are rarely practical except in certain applications. Multiple-effect evaporation plants in sugar beet factories have up to eight effects; sextuple-effect evaporators are common in the recovery of black liquor in the

kraft process The kraft process (also known as kraft pulping or sulfate process) is a process for conversion of wood into wood pulp, which consists of almost pure cellulose fibres, the main component of paper. The kraft process involves treatment of wood chip ...

for making wood pulp. Entrainment of the product in the solvent causes a number of issues including firstly decrease in the amount of product recovered, secondly potential damage or fouling of the evaporation lines and steam chest of the next stage, and thirdly problems dealing or disposing of the solvent.

Types

Forward feed

In forward feed the condensate and the feed-concentrate move in the same direction. In this case the vacuum pressure can be sufficient to pull the feed through the system.

Backward feed

Here the concentrate flows in the opposite direction to the condensate. Pumps are required between stages, however the design has less heating costs.

Parallel feed

In the simplest version of this design the feed goes to each stage.

References

External links