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Evaporator
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Within a downstream processing system, several stages are used to further isolate and purify the desired product. The overall structure of the process includes pre-treatment, solid-liquid separation, concentration, and purification and formulation. Evaporation falls into the concentration stage of downstream processing and is widely used to concentrate foods, chemicals, and salvage solvents. The goal of evaporation is to vaporize most of the water from a solution containing a desired product.
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Within a downstream processing system, several stages are used to further isolate and purify the desired product. The overall structure of the process includes pre-treatment, solid-liquid separation, concentration, and purification and formulation. Evaporation falls into the concentration stage of downstream processing and is widely used to concentrate foods, chemicals, and salvage solvents. The goal of evaporation is to vaporize most of the water from a solution containing a desired product. After initial pre-treatment and separation, a solution often contains over 85% water. This is not suitable for industry usage because of the cost associated with processing such a large quantity of solution, such as the need for larger equipment.
Energetics
Water can be removed from solutions in ways other than evaporation, including membrane processes, liquid-liquid extractions, 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 in 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.
q 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.
Types of evaporators used today
Natural/forced circulation evaporator
Natural circulation evaporators are based on the natural circulation of the product caused by the density 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.
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 applicable to highly viscous solutions so it is frequently used in the chemical, food, and fermentation industry.
Rising film (Long Tube Vertical) evaporator
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 vapour 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+ metre); 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 favour 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.
Applications
The goal of evaporation is to concentrate a target liquid, and this needs to be achieved for many different targets today. One of the most important applications of evaporation is that on 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. It is also used as a drying process and can be applied in this way to laboratories where preservation of long-term activity or stabilization is needed (for enzymes for example). Evaporation is also used in order 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. If up to 98% of a wastes can be vaporized, industry can greatly reduce the amount of money that would otherwise be allocated towards waste handling.
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. Fouling 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.
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 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 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.
Certain vessels, such as small ships and passenger vessels are utilising the Reverse Osmosis principal for fresh water production instead of evaporators.
See also
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