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An evaporator is a device used to turn the
liquid A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a (nearly) constant volume independent of pressure. As such, it is one of the four fundamental states of matter (the others being solid, gas, ...
form of a chemical substance, such as water, into a
vapor In physics, a vapor (American English) or vapour (British English and Canadian English; see spelling differences) is a substance in the gas phase at a temperature lower than its critical temperature,R. H. Petrucci, W. S. Harwood, and F. G. Her ...
.


Uses


Air conditioning and refrigeration

Some
air conditioner Air conditioning, often abbreviated as A/C or AC, is the process of removing heat from an enclosed space to achieve a more comfortable interior environment (sometimes referred to as 'comfort cooling') and in some cases also strictly controlling ...
s and
refrigerator A refrigerator, colloquially fridge, is a commercial and home appliance consisting of a thermally insulated compartment and a heat pump (mechanical, electronic or chemical) that transfers heat from its inside to its external environment so th ...
s use a compressed liquid with a low boiling point, such as
chlorodifluoromethane Chlorodifluoromethane or difluoromonochloromethane is a hydrochlorofluorocarbon (HCFC). This colorless gas is better known as HCFC-22, or R-22, or . It was commonly used as a propellant and refrigerant. These applications were phased out under ...
or R-410A, that vaporizes within the system while emitting heat to its surroundings.


Food industry and synthetic chemistry

Evaporators are used to concentrate a solution. One example is the climbing/falling film plate evaporator, which is used to make
condensed milk Condensed milk is cow's milk from which water has been removed (roughly 60% of it). It is most often found with sugar added, in the form of ''sweetened condensed milk'' (SCM), to the extent that the terms "condensed milk" and "sweetened condense ...
. Similarly, reduction (cooking) is a process of evaporating liquids from a solution to produce a "reduced" food product, such as wine reduction. Evaporation is the main process behind
distillation Distillation, or classical distillation, is the process of separating the components or substances from a liquid mixture by using selective boiling and condensation, usually inside an apparatus known as a still. Dry distillation is the he ...
, which is used to concentrate alcohol, isolate liquid chemical products, or recover
solvents A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...
in chemical reactions (
green chemistry Green chemistry, also called sustainable chemistry, is an area of chemistry and chemical engineering focused on the design of products and processes that minimize or eliminate the use and generation of hazardous substances. While environmental che ...
). The fragrance and
essential oil An essential oil is a concentrated hydrophobic liquid containing volatile (easily evaporated at normal temperatures) chemical compounds from plants. Essential oils are also known as volatile oils, ethereal oils, aetheroleum, or simply as the o ...
industry uses distillation to purify compounds. Each application uses specialized devices.


Chemical engineering

In the case of
desalination Desalination is a process that takes away mineral components from saline water. More generally, desalination refers to the removal of salts and minerals from a target substance, as in soil desalination, which is an issue for agriculture. Salt ...
of seawater or in Zero Liquid Discharge plants, the reverse purpose applies; evaporation removes the desirable
drinking water Drinking water is water that is used in drink or food preparation; potable water is water that is safe to be used as drinking water. The amount of drinking water required to maintain good health varies, and depends on physical activity level, ...
from the undesired solute/product, salt. Chemical engineering uses evaporation in many processes. For example, the multiple-effect evaporator is used in
kraft pulping 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 ...
, the process of producing wood pulp from wood.


Marine

Large ships usually carry evaporating plants to produce fresh water, thus reducing their reliance on shore-based supplies. Steamships must be able to produce high-quality distillate in order to maintain boiler-water levels. Diesel-engine ships often utilize
waste heat Waste heat is heat that is produced by a machine, or other process that uses energy, as a byproduct of doing work. All such processes give off some waste heat as a fundamental result of the laws of thermodynamics. Waste heat has lower utilit ...
as an energy source for producing fresh water. In this system, the engine-cooling water is passed through a
heat exchanger A heat exchanger is a system used to transfer heat between a source and a working fluid. Heat exchangers are used in both cooling and heating processes. The fluids may be separated by a solid wall to prevent mixing or they may be in direct conta ...
, where it is cooled by concentrated seawater (brine). Because the cooling water (which is chemically treated fresh water) is at a temperature of , it would not be possible to flash off any water vapor unless the pressure in the
heat exchanger A heat exchanger is a system used to transfer heat between a source and a working fluid. Heat exchangers are used in both cooling and heating processes. The fluids may be separated by a solid wall to prevent mixing or they may be in direct conta ...
vessel was dropped. To alleviate this problem, a brine-air ejector
venturi pump A vacuum ejector, or simply ejector is a type of vacuum pump, which produces vacuum by means of the Venturi effect. In an ejector, a working fluid (liquid or gaseous) flows through a jet nozzle into a tube that first narrows and then expands i ...
is used to create a vacuum inside the vessel. Partial evaporation is achieved, and the vapor passes through a demister before reaching the condenser section. Seawater is pumped through the condenser section to cool the vapor sufficiently to precipitate it. The distillate gathers in a tray, from where it is pumped to the storage tanks. A salinometer monitors salt content and diverts the flow of distillate from the storage tanks if the salt content exceeds the alarm limit. Sterilization is carried out after the evaporator. Evaporators are usually of the shell-and-tube type (known as an Atlas Plant) or of the plate type (such as the type designed by
Alfa Laval Alfa Laval AB is a Swedish company, founded in 1883 by Gustaf de Laval and Oscar Lamm. The company, which started in providing centrifugal separation solutions for dairy (see Separator (milk)), now deals in the production of specialised produ ...
). Temperature, production and vacuum are controlled by regulating the system valves. Seawater temperature can interfere with production, as can fluctuations in engine load. For this reason, the evaporator is adjusted as seawater temperature changes and shuts down altogether when the ship is maneuvering. An alternative in some vessels, such as naval ships and passenger ships, is the use of the
reverse osmosis Reverse osmosis (RO) is a water purification process that uses a partially permeable membrane to separate ions, unwanted molecules and larger particles from drinking water. In reverse osmosis, an applied pressure is used to overcome osmotic ...
principle for fresh-water production instead of using evaporators.


Energetics

Evaporation Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase. High concentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when h ...
, or
vaporization Vaporization (or vaporisation) of an element or compound is a phase transition from the liquid phase to vapor. There are two types of vaporization: evaporation and boiling. Evaporation is a surface phenomenon, whereas boiling is a bulk phenomenon ...
, is an endothermic phase transition process that is thoroughly understood in the field of
thermodynamics Thermodynamics is a branch of physics that deals with heat, work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by the four laws ...
. It is intimately related to the
vapor pressure Vapor pressure (or vapour pressure in English-speaking countries other than the US; see spelling differences) or equilibrium vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed pha ...
of the liquid and surrounding pressure, in addition to the
enthalpy of vaporization The enthalpy of vaporization (symbol ), also known as the (latent) heat of vaporization or heat of evaporation, is the amount of energy (enthalpy) that must be added to a liquid substance to transform a quantity of that substance into a gas. T ...
.


Types of evaporators

Evaporators work using the same principle design. A heat source is in contact with the liquid causing it to evaporate. The vapor is removed entirely (like in cooking), or it is stored for reuse (like in a refrigerator) or a product for isolation (essential oil).


Rotary evaporator

Rotary evaporators use a vacuum pump to create a low pressure over a solvent while simultaneously rotating the liquid flask to increase surface area and decrease bubble size. Typically, the vapor is passed over a cold finger or coil so that the vaporized material does not damage the pump. The rotary evaporator is best used for removing solvent from solutions containing the desired product that will not vaporize at the operating pressure to separate the volatile components of a mixture from non-volatile materials.


Natural/forced circulation evaporator

Natural circulation evaporators are based on the natural circulation of the product caused by the
density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematicall ...
differences that arise from heating (
convection Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the c ...
). A chamber containing a solution is heated, and the vaporized liquid is collected in a receiving flask.


Falling film evaporator

This type of evaporator is generally made of ) tubes enclosed by steam jackets. The uniform distribution of the solution is important when using this type of evaporator. The solution enters and gains velocity as it flows downward. This gain in velocity is attributed to the vapor being evolved against the heating medium, which also flows downward. This evaporator is usually applied to highly
viscous The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the in ...
solutions, so it is frequently used in the chemical, sugar, food, and fermentation industries.


Rising film (long tube vertical) evaporator

This type of evaporator is useful in concentrating a solution. The operation is very similar to that of a calandria where the liquid is boiled inside vertical tubes by applying
heat In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is ...
to the outside of the tubes. The produced solvent vapor presses the liquid against the walls of the tubes forming a thin film which moves upwards with the vapor. The vapor may be released from the system while the liquid may be recirculated through the evaporator to further concentrate the
solute In chemistry, a solution is a special type of homogeneous mixture composed of two or more substances. In such a mixture, a solute is a substance dissolved in another substance, known as a solvent. If the attractive forces between the solvent ...
. In many cases, the tubes of a rising film evaporator are usually between 3 and 10 meters (10-33 ft) in height with a diameter of between 25 and 50 millimeters (1-2 in). Sizing this type of evaporator requires a precise evaluation of the actual level of the liquid inside the tubes and the flow rates of the vapor and film.


Climbing and falling-film plate evaporator

Climbing and falling-film plate evaporators have a relatively large surface area. The plates are usually
corrugated The term corrugated, describing a series of parallel ridges and furrows, may refer to the following: Materials *Corrugated fiberboard, also called corrugated cardboard *Corrugated galvanised iron, a building material composed of sheets of cold-r ...
and are supported by the frame. During evaporation, steam flows through the channels formed by the free spaces between the plates. The steam alternately climbs and falls parallel to the concentrated liquid. The steam follows a co-current, counter-current path with the liquid. The concentrate and the vapor are fed into the separation stage, where the vapor is sent to a condenser. This type of plate evaporator is frequently applied in the dairy and fermentation industries since they have spatial flexibility. A negative point of this type of evaporator is its limited ability to treat viscous or solid-containing products. There are other types of plate evaporators that work with only climbing film.


Multiple-effect evaporators

Unlike single-stage evaporators, these evaporators can be composed of up to seven evaporator stages (effects). The energy consumption for single-effect evaporators is very high and is most of the cost for an evaporation system. Putting together evaporators saves heat and thus requires less energy. Adding one evaporator to the original decreases energy consumption by 50%. Adding another effect reduces it to 33% and so on. A heat-saving-percent equation can estimate how much one will save by adding a certain number of effects. The number of effects in a multiple-effect evaporator is usually restricted to seven because, after that, the equipment cost approaches the cost savings of the energy-requirement drop. Two types of feeding can be used when dealing with multiple-effect evaporators. Forward feeding occurs when the product enters the system through the first effect at the highest temperature. The product is then partially concentrated as some water is transformed into vapor and carried away. It is then fed into the second effect, which is slightly lower in temperature. The second effect uses the heated vapor created in the first stage as its heat source (hence the saving in energy expenditure). The combination of lower temperatures and higher viscosity in subsequent effects provides good conditions for treating heat-sensitive products, such as enzymes and proteins. In this system, an increase in the heating surface area of subsequent effects is required. Another method is using backwards feeding. In this process, the dilute products are fed into the last effect with the lowest temperature and transferred from effect to effect, with the temperature increasing. The final concentrate is collected in the hottest effect, which provides an advantage in that the product is highly viscous in the last stages, so the heat transfer is better. In recent years, multiple-effect vacuum evaporator (with
heat pump A heat pump is a device that can heat a building (or part of a building) by transferring thermal energy from the outside using a refrigeration cycle. Many heat pumps can also operate in the opposite direction, cooling the building by removing ...
) systems have come into use. These are well known to be energetically and technically more effective than systems with mechanical vapor recompression (MVR). Due to the lower boiling temperature, they can handle highly corrosive liquids or liquids which are prone to forming incrustations.multiple-effect evaporators with heat pump
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Agitated thin film evaporators

Agitated thin-film evaporation has been very successful with difficult-to-handle products. Simply stated, the method quickly separates the volatile from the less volatile components using indirect heat transfer and mechanical agitation of the flowing product film under controlled conditions. The separation is normally made under vacuum conditions to maximize ∆T while maintaining the most favorable product temperature so that the product only sees equilibrium conditions inside the evaporator and can maximize volatile stripping and recovery.


Problems

Technical problems can arise during evaporation, especially when the process is used in the food industry. Some evaporators are sensitive to differences in viscosity and consistency of the dilute solution. These evaporators could work inefficiently because of a loss of circulation. The pump of an evaporator may need to be changed if the evaporator needs to be used to concentrate a highly viscous solution.
Fouling Fouling is the accumulation of unwanted material on solid surfaces. The fouling materials can consist of either living organisms (biofouling) or a non-living substance (inorganic or organic). Fouling is usually distinguished from other surf ...
also occurs when hard deposits form on the surfaces of the heating mediums in the evaporators. In foods, proteins and polysaccharides can create such deposits that reduce the efficiency of heat transfer. Foaming can also create a problem since dealing with excess foam can be costly in time and efficiency. Antifoam agents are to be used, but only a few can be used when food is being processed.
Corrosion Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engi ...
can also occur when acidic solutions such as citrus juices are concentrated. The surface damage caused can shorten the long life of evaporators. The quality and flavor of food can also suffer during evaporation. Overall, when choosing an evaporator, the qualities of the product solution must be taken into careful consideration.


See also

* Flash evaporation * Vacuum evaporation *
Centrifugal evaporator A centrifugal evaporator is a device used in chemical and biochemical laboratories for the efficient and gentle evaporation of solvents from many samples at the same time, and samples contained in microtitre plates. If only one sample required eva ...
*
Rotary evaporator A rotary evaporator (rotovap) is a device used in chemical laboratories for the efficient and gentle removal of solvents from samples by evaporation. When referenced in the chemistry research literature, description of the use of this technique and ...
* Vapor-compression evaporator *
Evaporative cooler An evaporative cooler (also known as evaporative air conditioner, swamp cooler, swamp box, desert cooler and wet air cooler) is a device that cools air through the evaporation of water. Evaporative cooling differs from other air conditioning s ...
* Pumpable ice technology *
Circulation evaporator Circulation evaporators are a type of evaporator, evaporating unit designed to separate mixtures unable to be evaporated by a conventional evaporating unit. Circulation evaporation incorporates the use of both heat exchangers and flash separation u ...


References

*Fennema, Owen R., Marcus Karel, and Daryl B. Lund. Physical Principles of Food Preservation. Marcel Deker, Inc. New York and Basel, 1975. *Krijgsman, Ir J., Principal Scientist and Research Project Manager, Gist-brocades, Delft and Delft University of Technology, Delft, and The Netherlands. Product Recovery in Bioprocess Technology. Butterworth-Heinemann, 1992. {{HVAC de:Verdampfer