Evaporator
Encyclopedia
An evaporator is a device used to turn (or allow to turn) the liquid form of some chemical into its gaseous form. For example, an evaporator is used in an air conditioning system to allow the compressed cooling chemical (for example, Freon) to evaporate from liquid to gas, absorbing heat in the process.
is widely used to concentrate foods and chemicals, and to salvage solvents. When concentrating, the goal of evaporation is to vaporize most of the water from a solution containing a desired product. In the case of desalination of sea water, the reverse purpose applies: evaporation removes the desirable drinking water from the undesired product, salt.
One of the most important applications of evaporation is that in the food and drink industry. Many foods that are made to last for a considerable amount of time or food that needs a certain consistency, like coffee
, need to go through an evaporation step during processing. In the pharmaceutical industry, evaporation eliminates excess moisture which makes a more easily handled product and improves product stability. For example, preservation of long-term activity or stabilization of enzymes in laboratories are greatly assisted by evaporation. It is also used to recover expensive solvents such as hexane which would otherwise be wasted.
Another example of evaporation is in the recovery of sodium hydroxide in kraft pulping. Cutting down waste handling cost is another major application of evaporation for large companies. Legally, all producers of waste must dispose of the waste in a methods that abides by environmental guidelines; these methods are costly. By removing moisture through vaporization, industry can greatly reduce the amount of wasted product that must be processed.
s, crystallization
, and precipitation
. Evaporation can be distinguished from some other drying methods in that the final product of evaporation is a concentrated liquid, not a solid. It is also relatively simple to use and understand since it has been widely used on a large scale. In order to concentrate a product by water removal, an auxiliary phase is used which allows for easy transport of the solvent (water) rather than the solute. Water vapor
is used as the auxiliary phase when concentrating non-volatile
components, such as proteins and sugars. Heat is added to the solution and part of the solvent is converted into vapor. Heat is the main tool in evaporation, and the process occurs more readily at high temperature and low pressures.
Heat is needed to provide enough energy
for the molecules of the solvent to leave the solution and move into the air surrounding the solution. The energy needed can be expressed as an excess thermodynamic potential of the water in the solution. Leading to one of the biggest problems in industrial evaporation, the process requires enough energy to remove the water from the solution and to supply the heat of evaporation. When removing the water, more than 99% of the energy needed goes towards supplying the heat of evaporation. The need to overcome the surface tension
of the solution also requires energy. The energy requirement of this process is very high because a phase transition must be caused; the water must go from a liquid to a vapor.
When designing evaporators, engineers must quantify the amount of steam needed for every mass unit of water removed when a concentration is given. An energy balance must be used based on an assumption that a negligible amount of heat is lost to the system’s surroundings. The heat that needs to be supplied by the condensing steam will approximately equal the heat needed to heat and vaporize the water. Another consideration is the size of the heat exchanger which affects the heat transfer rate.
A = heat transfer area
q = overall heat transfer rate
differences that arise from heating. In an evaporator using tubing, after the water begins to boil
, bubbles will rise and cause circulation, facilitating the separation of the liquid and the vapor at the top of the heating tubes. The amount of evaporation that takes place depends on the temperature difference between the steam and the solution. Problems can arise if the tubes are not well-immersed in the solution. If this occurs, the system will be dried out and circulation compromised. In order to avoid this, forced circulation can be used by inserting a pump to increase pressure and circulation. Forced circulation occurs when hydrostatic head prevents boiling at the heating surface. A pump can also be used to avoid fouling that is caused by the boiling of liquid on the tubes; the pump suppresses bubble formation. Other problems are that the residing time is undefined and the consumption of steam is very high, but at high temperatures, good circulation is easily achieved.
In this type of evaporator, boiling takes place inside the tubes, due to heating made (usually by steam) outside the same. Submergence is therefore not desired; the creation of water vapor bubbles inside the tube creates an ascensional flow enhancing the heat transfer coefficient. This type of evaporator is therefore quite efficient, the disadvantage being to be prone to quick scaling of the internal surface of the tubes. This design is then usually applied to clear, non-salting solutions.
Tubes are usually quite long (4+ meters); sometimes a small recycle is provided. Sizing this type of evaporator is usually a delicate task, since it requires a precise evaluation of the actual level of the process liquor inside the tubes. Recent applications tend to favor the falling film pattern rather than this one.
The number of effects in a multiple-effect evaporator is usually restricted to seven because after that, the equipment cost starts catching up to the money saved from the energy requirement drop.
There are two types of feeding that can be used when dealing with multiple-effect evaporators. Forward feeding takes place when the product enters the system through the first effect, which is at the highest temperature. The product is then partially concentrated as some of the water is transformed into vapor and carried away. It is then fed into the second effect which is a little lower in temperature. The second effect uses the heated vapor created in the first stage as its heating source (hence the saving in energy expenditure). The combination of lower temperatures and higher viscosities in subsequent effects provides good conditions for treating heat-sensitive products like enzymes and proteins. In using this system, an increase in the heating surface area of subsequent effects is required. Another way to proceed is by using backward feeding. In this process, the dilute products is fed into the last effect with has the lowest temperature and is 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 considerably better.....
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 the 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
can also occur when acidic solutions such as citrus juices are concentrated. The surface damage caused can shorten the long-life of evaporators. Quality and flavor of food can also suffer during evaporation. Overall, when choosing an evaporator, the qualities of the product solution need to be taken into heavy consideration.
as an energy source for producing fresh water. In this system, the engine cooling water is passed through a heat exchanger, where it is cooled by concentrated sea water (brine). Because the cooling water (which is chemically treated fresh water) is at a temperature of 70-80 degrees C, it would not be possible to flash off any water vapour unless the pressure in the heat exhanger vessel was dropped. To alleviate this problem, a brine-air ejector venturi is used to create a vacuum inside the vessel. Partial evaporation is achieved and the vapour passes through a demister before reaching the condenser section. Sea water is pumped through the condenser section to cool the vapour 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. Sterilisation 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
). Temperature, production and vacuum are controlled by regulating the system valves. Sea water temperature can interfere with production, as can fluctuations in engine load. For this reason, the evaporator is adjusted as seawater temperature changes, and shut down altogether when the ship is manoeuvring.
An alternative in some vessels, such as naval ships and passenger ships, is the use of the Reverse Osmosis
principle for fresh water production instead of evaporators.
Uses
As stated above, an evaporator can be used to transfer heat. It can also be used to remove water or other liquids from mixtures. EvaporationEvaporation
Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....
is widely used to concentrate foods and chemicals, and to salvage solvents. When concentrating, the goal of evaporation is to vaporize most of the water from a solution containing a desired product. In the case of desalination of sea water, the reverse purpose applies: evaporation removes the desirable drinking water from the undesired product, salt.
One of the most important applications of evaporation is that in the food and drink industry. Many foods that are made to last for a considerable amount of time or food that needs a certain consistency, like coffee
Coffee
Coffee is a brewed beverage with a dark,init brooo acidic flavor prepared from the roasted seeds of the coffee plant, colloquially called coffee beans. The beans are found in coffee cherries, which grow on trees cultivated in over 70 countries, primarily in equatorial Latin America, Southeast Asia,...
, need to go through an evaporation step during processing. In the pharmaceutical industry, evaporation eliminates excess moisture which makes a more easily handled product and improves product stability. For example, preservation of long-term activity or stabilization of enzymes in laboratories are greatly assisted by evaporation. It is also used to recover expensive solvents such as hexane which would otherwise be wasted.
Another example of evaporation is in the recovery of sodium hydroxide in kraft pulping. Cutting down waste handling cost is another major application of evaporation for large companies. Legally, all producers of waste must dispose of the waste in a methods that abides by environmental guidelines; these methods are costly. By removing moisture through vaporization, industry can greatly reduce the amount of wasted product that must be processed.
Energetics
Water can be removed from solutions in ways other than evaporation, including membrane processes, liquid-liquid extractionLiquid-liquid extraction
Liquid–liquid extraction, also known as solvent extraction and partitioning, is a method to separate compounds based on their relative solubilities in two different immiscible liquids, usually water and an organic solvent. It is an extraction of a substance from one liquid phase into another liquid...
s, crystallization
Crystallization
Crystallization is the process of formation of solid crystals precipitating from a solution, melt or more rarely deposited directly from a gas. Crystallization is also a chemical solid–liquid separation technique, in which mass transfer of a solute from the liquid solution to a pure solid...
, and precipitation
Precipitation (chemistry)
Precipitation is the formation of a solid in a solution or inside anothersolid during a chemical reaction or by diffusion in a solid. When the reaction occurs in a liquid, the solid formed is called the precipitate, or when compacted by a centrifuge, a pellet. The liquid remaining above the solid...
. Evaporation can be distinguished from some other drying methods in that the final product of evaporation is a concentrated liquid, not a solid. It is also relatively simple to use and understand since it has been widely used on a large scale. In order to concentrate a product by water removal, an auxiliary phase is used which allows for easy transport of the solvent (water) rather than the solute. Water vapor
Vapor
A vapor or vapour is a substance in the gas phase at a temperature lower than its critical point....
is used as the auxiliary phase when concentrating non-volatile
Volatility (chemistry)
In chemistry and physics, volatility is the tendency of a substance to vaporize. Volatility is directly related to a substance's vapor pressure. At a given temperature, a substance with higher vapor pressure vaporizes more readily than a substance with a lower vapor pressure.The term is primarily...
components, such as proteins and sugars. Heat is added to the solution and part of the solvent is converted into vapor. Heat is the main tool in evaporation, and the process occurs more readily at high temperature and low pressures.
Heat is needed to provide enough energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
for the molecules of the solvent to leave the solution and move into the air surrounding the solution. The energy needed can be expressed as an excess thermodynamic potential of the water in the solution. Leading to one of the biggest problems in industrial evaporation, the process requires enough energy to remove the water from the solution and to supply the heat of evaporation. When removing the water, more than 99% of the energy needed goes towards supplying the heat of evaporation. The need to overcome the surface tension
Surface tension
Surface tension is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects to run on the water surface...
of the solution also requires energy. The energy requirement of this process is very high because a phase transition must be caused; the water must go from a liquid to a vapor.
When designing evaporators, engineers must quantify the amount of steam needed for every mass unit of water removed when a concentration is given. An energy balance must be used based on an assumption that a negligible amount of heat is lost to the system’s surroundings. The heat that needs to be supplied by the condensing steam will approximately equal the heat needed to heat and vaporize the water. Another consideration is the size of the heat exchanger which affects the heat transfer rate.
A = heat transfer area
q = overall heat transfer rate
How an evaporator works
The solution containing the desired product is fed into the evaporator and passes a heat source. The applied heat converts the water in the solution into vapor. The vapor is removed from the rest of the solution and is condensed while the now concentrated solution is either fed into a second evaporator or is removed. The evaporator as a machine generally consists of four sections. The heating section contains the heating medium, which can vary. Steam is fed into this section. The most common medium consists of parallel tubes but others have plates or coils. The concentrating and separating section removes the vapor being produced from the solution. The condenser condenses the separated vapor, then the vacuum or pump provides pressure to increase circulation.Natural/forced circulation evaporator
Natural circulation evaporators are based on the natural circulation of the product caused by the densityDensity
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
differences that arise from heating. In an evaporator using tubing, after the water begins to boil
Boil
A boil, also called a furuncle, is a deep folliculitis, infection of the hair follicle. It is always caused by infection by the bacterium Staphylococcus aureus, resulting in a painful swollen area on the skin caused by an accumulation of pus and dead tissue...
, bubbles will rise and cause circulation, facilitating the separation of the liquid and the vapor at the top of the heating tubes. The amount of evaporation that takes place depends on the temperature difference between the steam and the solution. Problems can arise if the tubes are not well-immersed in the solution. If this occurs, the system will be dried out and circulation compromised. In order to avoid this, forced circulation can be used by inserting a pump to increase pressure and circulation. Forced circulation occurs when hydrostatic head prevents boiling at the heating surface. A pump can also be used to avoid fouling that is caused by the boiling of liquid on the tubes; the pump suppresses bubble formation. Other problems are that the residing time is undefined and the consumption of steam is very high, but at high temperatures, good circulation is easily achieved.
Falling film evaporator
This type of evaporator is generally made of long tubes (4–8 meters in length) which are surrounded 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 flows downward as well. This evaporator is usually applied to highly viscous solutions so it is frequently used in the chemical, food, and fermentation industry..Rising film (Long Tube Vertical) evaporator
Tubes are usually quite long (4+ meters); sometimes a small recycle is provided. Sizing this type of evaporator is usually a delicate task, since it requires a precise evaluation of the actual level of the process liquor inside the tubes. Recent applications tend to favor the falling film pattern rather than this one.
Plate evaporator
Plate evaporators have a relatively large surface area. The plates are usually corrugated and are supported by 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 in relation to the liquid. The concentrate and the vapor are both fed into the separation stage where the vapor is sent to a condenser. Plate evaporators are frequently applied in the dairy and fermentation industries since they have spatial flexibility. A negative point of this type of evaporator is that it is limited in its ability to treat viscous or solid-containing products.Multiple-effect evaporators
Unlike single-stage evaporators, these evaporators can be made of up to seven evaporator stages or effects. The energy consumption for single-effect evaporators is very high and makes up 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 the energy consumption to 50% of the original amount. Adding another effect reduces it to 33% and so on. A heat saving % equation can be used to estimate how much one will save by adding a certain amount of effects.The number of effects in a multiple-effect evaporator is usually restricted to seven because after that, the equipment cost starts catching up to the money saved from the energy requirement drop.
There are two types of feeding that can be used when dealing with multiple-effect evaporators. Forward feeding takes place when the product enters the system through the first effect, which is at the highest temperature. The product is then partially concentrated as some of the water is transformed into vapor and carried away. It is then fed into the second effect which is a little lower in temperature. The second effect uses the heated vapor created in the first stage as its heating source (hence the saving in energy expenditure). The combination of lower temperatures and higher viscosities in subsequent effects provides good conditions for treating heat-sensitive products like enzymes and proteins. In using this system, an increase in the heating surface area of subsequent effects is required. Another way to proceed is by using backward feeding. In this process, the dilute products is fed into the last effect with has the lowest temperature and is 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 considerably better.....
Problems
Technical problems can arise during evaporations, especially when the process is applied to 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. FoulingFouling
Fouling refers to the accumulation of unwanted material on solid surfaces, most often in an aquatic environment. The fouling material can consist of either living organisms or a non-living substance...
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 the 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
Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...
can also occur when acidic solutions such as citrus juices are concentrated. The surface damage caused can shorten the long-life of evaporators. Quality and flavor of food can also suffer during evaporation. Overall, when choosing an evaporator, the qualities of the product solution need to be taken into heavy consideration.
Marine Use
Large ships usually carry evaporating plants to produce fresh water, thus reducing their reliance on shore-based supplies. Steam ships must be able to produce high quality distillate in order to maintain boiler-water levels. Diesel engined ships often utilise waste heatWaste heat
Waste heat sometimes called Secondary heat or Low-grade heat refers to heat produced by machines, electrical equipment and industrial processes for which no useful application is found. Energy is often produced by a heat engine, running on a source of high-temperature heat...
as an energy source for producing fresh water. In this system, the engine cooling water is passed through a heat exchanger, where it is cooled by concentrated sea water (brine). Because the cooling water (which is chemically treated fresh water) is at a temperature of 70-80 degrees C, it would not be possible to flash off any water vapour unless the pressure in the heat exhanger vessel was dropped. To alleviate this problem, a brine-air ejector venturi is used to create a vacuum inside the vessel. Partial evaporation is achieved and the vapour passes through a demister before reaching the condenser section. Sea water is pumped through the condenser section to cool the vapour sufficiently to precipitate it. The distillate gathers in a tray, from where it is pumped to the storage tanks. A Salinometer
Salinometer
A salinometer is a device designed to measure the salinity, or dissolved salt content, of a solution.Since the salinity affect both the electrical conductivity and the specific gravity of a solution, a salinometer often consist of an ec meter or hydrometer and some means of converting those...
monitors salt content and diverts the flow of distillate from the storage tanks if the salt content exceeds the alarm limit. Sterilisation 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
Alfa Laval AB is a Swedish company, founded in 1883 by Gustaf de Laval and Oscar Lamm. The company is a leading producer of specialized products and solutions used to heat, cool, separate and transport such products as oil, water, chemicals, beverages, foodstuffs, starch and...
). Temperature, production and vacuum are controlled by regulating the system valves. Sea water temperature can interfere with production, as can fluctuations in engine load. For this reason, the evaporator is adjusted as seawater temperature changes, and shut down altogether when the ship is manoeuvring.
An alternative in some vessels, such as naval ships and passenger ships, is the use of the Reverse Osmosis
Reverse osmosis
Reverse osmosis is a membrane technical filtration method that removes many types of large molecules and ions from solutions by applying pressure to the solution when it is on one side of a selective membrane. The result is that the solute is retained on the pressurized side of the membrane and...
principle for fresh water production instead of evaporators.
See also
- Flash evaporationFlash evaporationFlash evaporation is the partial vapor that occurs when a saturated liquid stream undergoes a reduction in pressure by passing through a throttling valve or other throttling device. This process is one of the simplest unit operations...
- Vacuum evaporationVacuum evaporationVacuum evaporation is the process of causing the pressure in a liquid-filled container to be reduced below the vapor pressure of the liquid, causing the liquid to evaporate at a lower temperature than normal...
- Centrifugal evaporatorCentrifugal evaporatorA 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 evaporation then a rotary evaporator is most often used...
- Rotary evaporatorRotary evaporatorA rotary evaporator is a device used in chemical laboratories for the efficient and gentle removal of solvents from samples by evaporation...
- Vapor-compression evaporator
- Evaporative cooler
- Pumpable ice technologyPumpable ice technologyPumpable ice technology is a technology to produce fluids or secondary refrigerants also called coolants with a viscosity of water or jelly and the cooling capacity of ice...