Evaporation as a process means boiling water with the purpose of creating vapour, while the stream the vapour is released from is concentrated and reduced in volume. The vapour formed can be condensed to produce very pure water or distillate. Evaporation is in fact the most powerful method of water purification nature has provided. Billions of cubic meters of water contaminated in different ways is every day evaporated from the earth's surface by the heat of the sun and subsequently recycled back as rain. Man has learned to apply evaporation in industrial processes.
Evaporation has been used for more than hundred years in the industry. Several different types of evaporators have been designed based on the service in question. By evaporation it is possible to concentrate solutions into heavy syrups and also to crystallize out the dissolved compounds and thus recover them in a dry form. In most of these cases evaporation has shown to be the only possible tool.
A problem of evaporation is that boiling of water to create vapour is highly energy consuming. But the good thing is that the same energy that was used for evaporation is released when the vapour condenses. Several methods to recover this heat for reuse have been developed, as Multi Effect evaporation (ME), Thermal Vapour Recompression (TVR) and Mechanical Vapour Recompression (MVR).
Industrial wastewater discharge requirements nowadays are going more and more towards totally effluent free systems. From a technical point of view, evaporation is an ideal method also for purification of industrial wastewater streams for following reasons:
- Practically complete separation of all non-volatile impurities in the water.
- The recovered condensate is in most cases so clean that it can be returned back to the process
- Valuable dry solids can be recovered
- Only small concentrate stream making it possible to dispose it off by sun drying or other methods
But still, the high cost of investment and high energy consumption of traditional evaporators has so far limited their application for wastewater treatment to only a few cases. The heat transfer surface area is the most costly part in an evaporator. In highly corrosive environments this tends to be really expensive due to use of exotic materials
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The art of evaporation can be defined by some simple equations.
Equation 1.
For heat transfer over the heat transfer surface applies:
Q = U x A x d or ΔTeff
The heat flow Q (W) is directly proportional to the surface area A (m2) and the effective temperature difference ΔTeff (°C) over the surface. U (W/m2 °C) is the heat transfer coefficient for the heat exchanger.
Equation 2.
The power consumption for an MVR compressor or fan is defined by:
P = C x MF x d or ΔT
The power consumption P (kW) is directly proportional to the mass flow of vapour MF (t/h) and the condensing temperature rise ΔT (°C) the compressor creates. C is a compressor related constant, which typically is 2,5 … 3.
Equation 3.
The difference between Δ Teff and ΔT is the Boiling Point Elevation Δ TBPE (°C) of the liquid to be evaporated:
Δ Teff = ΔT - ΔTBPE
The boiling point elevation ΔTBPE (°C) reduces the temperature difference over the heat transfer surface. It is a characteristic of dissolved substances dependent on the type and concentration of the substance. The practical effect of it is that it tends to reduce the capacity of a given heat exchanger compared to clean water.
The evaporator types are divided into different basic groups and sub-groups based on heating surface lay out and internal flow arrangement:
1. Long Tube Vertical (LTV) evaporators
- Rising film (RF)
- Falling film (FF)
2. Short Tube Vertical (STV)
- Evaporators
- Natural circulation
- Propeller calandria
3. Forced circulation (FC) evaporators
- Submerged type (crystalliser)
- Non-submerged type
4. Evaporators with miscellaneous forms of heating surface
- Plate heat exchangers
- Horizontal tube evaporators
- Agitated thin film
- Jacketed kettles
- Coiled tubes
5. Evaporators without heating surfaces
- Multi stage flash evaporators (MSF)
- Submerged combustion
LTV-evaporators
Falling Film (FF) and Rising Film (RF) evaporators are used for normal concentration of liquids with low or medium fouling properties. One main difference regarding FF evaporators is that a circulation pump normally is used to transport the liquid to the top of the tube or plate bundle. FF evaporators also need a means to distribute the liquid evenly over the entire tube bundle. The distribution may be accomplished by orifice plates, overflow weirs or spray nozzles. A special benefit with FF evaporators is that they may operate at very small temperature differences, the hold-up time is short and the capacity is easily adjustable from 0 - 100%. They are thus well suited for evaporation of heat sensitive materials, such as fruit juices and dairy products. The tube lengths in LTV-evaporators are normally 6 - 10 m, and the diameter 25 - 50 mm.
STV-evaporators
Short Tube Vertical STV-evaporators are used for similar purposes as RF-evaporators especially in cases where high headspace is not available, and for crystallizing purposes. They are normally equipped with a down comer tube of roughly the same area as the total cross sectional area of the heat transfer tubes. An agitator may be positioned in the down comer tube in order to force up the liquid flow. A typical example is the sugar-crystallizing pan in the sugar industry.
Retention times are long due to the large liquid volume in the evaporator body, a feature that has to be taken in account when handling heat sensitive liquids. Typical tube lengths are 1,0 - 3,0 m, and the diameter 25 - 75 mm for normal operation and 80 - 100 mm for crystallising purposes.
FC evaporators
FC evaporators are used for severely scaling applications, and for crystallising purposes. The submerged type, where boiling in the tube is restricted by the hydrostatic pressure or a throttling valve, is the most common one. The heat exchanger bundle may be both horizontal or vertical. Also metal plate heat exchangers are used. As no concentration of the liquid occurs during the passage through the heat exchanger, very low scaling due to supersaturation of salts is anticipated. In addition, the high turbulence at velocities of 2 - 3 m/s tends to keep the scale off the heat transfer surface. For extremely severe scaling conditions the Non-fouling Fluidised Bed Heat Exchanger design may be the choice. This technology applies continuous circulation of solid particles such as glass or metal granules through the tubes to remove scale from the tube-wall.
FC evaporators consume a considerable amount of electrical power for the circulation pump, up to 25 kW/t of evaporation. Therefore they are applied only when the evaporation is not possible with other evaporator types.
Evaporators may be equipped with plate heat exchangers of various types. The well known chevron type plate heat exchanger produced by e.g. Alfa Laval and APV can be adopted for evaporation also. Both FF and RF types are available. A special type is the new FF type thin film plastic heat exchanger produced by Windsor.
In sea water evaporation a horizontal type tubular evaporator is commonly used. The sea water is distributed on the outside of the tubes, while the vapour is condensed on the inside. As cleaning of the tubes can be done only chemically, scaling should be avoided.
Agitated thin-film evaporators employ one large heat transfer tube with an internal rotating assembly of blades to distribute the liquid in a thin film on the tube wall. They are primarily applied for very viscous liquids where the viscosity is limiting use of more traditional evaporators. The practical viscosity range is from a few hundred to several thousand centipoises. Very short retention times of only a few seconds allow concentration of heat sensitive materials. Investment cost and power use is high.
In a flash evaporator a hot liquid is introduced into a tank of low pressure. By flashing the liquid cools down while the sensible heat thus released is converted into vapour. Flash evaporators are often used in combination with other evaporators as the last stage in cases where rapid cooling of the product to lower temperatures is needed. One example of flash evaporators is Multi Stage Flash (MSF), which is commonly used in seawater desalination. The produced vapour is condensed in each stage by the incoming liquid. Thus the recovery rate is very low.
In direct contact evaporators a hot flue gas is introduced through a perforated pipe directly into the liquid in the evaporation chamber. These evaporators are often used for total destruction of the final concentrate. The vapour is normally released directly to the atmosphere.
1 comments:
Very informative post! You explained about the basic and key components of a unit of the MVR evaporation.
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