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Application of Three - effect Evaporator in High Salinity Wastewater Treatment

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Application of Three - effect Evaporator in High Salinity Wastewater Treatment

                                                                             Zhao xiaokun (Beijing Institute of Mechanical and Electrical Engineering High Technology Co., Ltd., Beijing 100027)

Abstract: Based on the analysis of various high salinity wastewater treatment technologies, the three-effect evaporator desalting method is introduced. It has the advantages of mature technology, wide range of wastewater, small footprint, fast processing and energy saving. China has a greater development prospects. The technology in practical engineering applications, there are still some difficulties, such as high processing costs, equipment life is short, need a large amount of steam, etc., to be resolved.

Key words: high salinity wastewater; three-effect evaporator; SBR process
CLC number: X703 Document code: A Article ID: 1006-5377 (2011) 11-0037-04


1 Introduction

High salinity wastewater is a wastewater containing at least 3.5% of the total dissolved solids TDS (Total Dis-Solution Solid) and organic matter, including high salt domestic wastewater and high salt industrial wastewater. Mainly from the direct use of seawater industrial production, domestic sewage and food processing plants, pharmaceutical plants, chemical plants and oil and gas collection and processing. In addition to organic pollutants, these wastewater also contains a large amount of inorganic salts such as Cl, SO4, Na, Ca and other ions. These high-salt, high-organic wastewater, if not disposed of directly discharge, is bound to water biological, drinking water and industrial and agricultural production of water have a great harm. The common characteristics of this kind of concentrated wastewater are: can not simply use biochemical treatment, and the physical and chemical treatment process is more complex, high processing costs, sewage treatment industry is recognized as difficult to treat wastewater [1].

2 High salt water treatment technology

On the high salinity wastewater treatment technology, has been studied at home and abroad for decades, the current commonly used methods include: biological, SBR process and three-effect evaporator desalination method.

2.1 biological method

Biological treatment is one of the most commonly used methods of wastewater treatment, with a wide range of applications, strong adaptability and other characteristics [2]. Chemical wastewater, such as dyes, pesticides, pharmaceutical intermediates and other high salt content of wastewater, serious pollution, must be processed to discharge. Moreover, the composition of such waste water is complex, do not have the value of recycling, the use of other methods of high cost, so biological treatment is still the preferred method. Inorganic salts play an important role in promoting enzyme reaction, maintaining membrane balance and regulating osmotic pressure during microbial growth, but the salt concentration is too high and will inhibit the growth of microorganisms. The main reason is that:
 (1) salt concentration is too high osmotic pressure, so that microbial cell dehydration caused by cell protoplasm separation;
 (2) the high salt content due to salting out the role of dehydrogenase activity decreased;
 (3) high chloride ion concentration on the toxic effects of bacteria;
 (4) As the density of sewage increases, the activated sludge is easy to float.
To this end, the biological treatment of high-salt wastewater needs to be diluted, usually at low salt concentrations (salt concentration less than 1%) operation, which will result in waste of water resources, and because of the huge processing facilities will also increase investment, operating costs improve. With the increasingly tense water resources, the state introduced the protection of water resources laws and fees and measures to high salt wastewater treatment enterprises to bring a burden.

2.2 SBR process

SBR is the abbreviation of the Sequencing Batch Reactor Activated Sludge Process. It is a kind of activated sludge sewage treatment technology which is operated by intermittent aeration. It is also called batch batch activated sludge method. Different from the traditional sewage treatment process, SBR technology uses time division of the operation mode instead of space division of the operation mode, non-stable biochemical reaction instead of steady-state biochemical reaction, standing ideal precipitation instead of the traditional dynamic precipitation. Its main feature is the operation of the orderly and intermittent operation. SBR technology is the core of the SBR reaction pool, the pool set homogenization, sinking, biodegradation, secondary sink and other functions in a pool, no sludge backflow system.
SBR process advantages are:
 (1) pool anaerobic, aerobic in an alternating state, the purification effect is good;
 (2) the operation effect is stable, the sewage in the ideal static state precipitation, takes a short time, high efficiency, good water quality;
(3) impact load, the pool has a stranded treatment of water, dilution of the sewage, the role of buffer;
 (4) the process of the process can be based on water quality, water to adjust, run flexible;
 (5) less processing equipment, simple structure, easy operation and maintenance management;
 (6) There is DO and BOD5 concentration gradient in the reaction tank, which can effectively control the expansion of activated sludge.
 (7) SBR system itself is also suitable for modular construction methods, conducive to the expansion of waste water treatment plant and transformation;
 (8) through proper control of the operation mode, to achieve aerobic, anoxic, anaerobic state alternately, with good nitrogen and phosphorus removal effect;
 (9) the process is simple, low cost.

Although the SBR process has so many advantages in wastewater treatment engineering, there are some difficulties in the treatment of highly salty wastewater, which needs to be overcome. The main difficulties are:
 (1) the increase of salt content in wastewater has great influence on the nitrification capacity of wastewater treatment system.
 (2) more salt in the wastewater, the buoyancy is large, not easy to precipitate;
 (3) most of the high salt containing waste water containing harmful organic matter and other impurities, can not be removed by SBR process;
 (4) SBR process automation requirements of a high degree;

(5) post-processing equipment requires more, such as disinfection equipment, contact pool volume, and drainage facilities such as drainage pipes are demanding.

2.3 Three-effect evaporator desalination method
Evaporation is one of the modern chemical unit operations, that is, by heating the solution of part of the solvent vaporization and removal, in order to improve the concentration of the solution, or to create conditions for solute precipitation [4]. Three-effect evaporator desalination method is the use of concentrated crystallization system will waste liquid inorganic salt by evaporation method to remove the way. The three-effect evaporator is composed of three evaporators in series with each other. At low temperature (about 90 ℃), the heating steam is introduced into the first effect, heating the waste liquid, and the generated steam is introduced into the second effect as the heating steam, The waste is evaporated at a lower temperature than the first effect, and this process is repeated until the last effect. The first effect of condensate back to the heat source, the other condensate condensate as a desalinated water output, a steam input, you can evaporate out of the water several times. At the same time, high-salt wastewater is concentrated from the first effect to the final effect, in the final effect to achieve supersaturation and crystallization, thus achieving salt and wastewater solid-liquid separation. During the treatment of salt-containing wastewater, the salt-containing wastewater enters the three-effect concentrated crystallization device, and is separated into desalinated water (desalinated water may contain trace low boiling point organic matter) and concentrated liquid crystal liquid after three-step evaporative condensation. Inorganic salts and some organic matter can be crystallized and separated, incineration treatment for inorganic salt residue; can not crystallize the organic matter concentrated waste can be used drum evaporator, the formation of solid waste, incineration; desalinated water can be returned to the production system to replace the softened water to be used. The three-effect evaporator desalination method has the advantages of mature technology, can handle a wide range of waste water, small footprint, fast processing speed and energy saving. With the development of chemical industry, more and more high salt wastewater needs to be treated, Evaporator desalination method will be more and more widely used.

3 three - effect evaporator
3.1 Three-effect evaporator application range
The three-effect evaporator can be used in the treatment of chemical production, food processing plants, pharmaceutical production, oil and gas collection and processing enterprises in the process of production process of high salt water, suitable treatment of wastewater salinity of 3.5% to 25% (Mass percentage), COD concentration of 2000 ~ 10,000ppm.
Composition and principle of three - effect evaporator
The three-effect evaporator consists of three groups of evaporators, condensers, salt separators and auxiliary equipment in series with each other (as shown). The three groups of evaporators are operated in series to form a three-effect evaporator. The entire evaporation system uses continuous feeding, continuous discharge of the production mode. The high-salt wastewater first enters the forced circulation crystallization evaporator. The crystallization evaporator is equipped with a circulating pump, and the waste water enters the evaporative heat exchange chamber. In the evaporative heat exchange chamber, the external vapor liquefaction generates the latent heat of vaporization and heats the waste water. Due to the large pressure in the evaporative heat transfer chamber, the waste water is heated to overheat in the evaporative heat exchange chamber at a pressure higher than the boiling point of the normal liquid. After the heated liquid enters the crystallization evaporation chamber, the rapid drop in the pressure of the waste water causes some of the waste water to flash, or quickly boil. The vapor after evaporation of the wastewater enters the two-effect forced circulation evaporator as the power vapor to heat the second-effect evaporator, and the non-evaporated waste water and the salt are temporarily stored in the crystallization evaporation chamber. In the case of negative pressure, the high salinity wastewater is controlled by one effect to the second effect and the three effect flows in succession. The waste water is continuously evaporated, and the waste water is continuously evaporated in the waste water. Salt concentration is getting higher and higher, when the salt in the waste water over the saturated state, the water will continue to precipitate the salt into the lower part of the evaporation chamber crystallization chamber. The salt is continuously sent to the vortex salt separator. In the spin salt separator, the solid salt is separated into the salt storage tank, and the separated wastewater enters the second force forced cycle evaporator. The whole process
 Cycle to achieve the final separation of water and salt.

The condenser is connected with a vacuum system which removes the uncondensed gas generated in the evaporation system, maintains the condenser and the evaporator in a negative pressure state, and improves the evaporation efficiency of the evaporation system. Under the action of negative pressure, the secondary steam generated by the waste water in the three-effect forced circulation evaporator automatically enters the condenser, and the secondary steam generated by the waste water is rapidly converted into condensed water under the cooling of the circulating cooling water. Condensate can be used in a continuous way of water, recycling to reuse pool.

Three - effect evaporator process flow chart

Application Example of Three - effect Evaporator
4.1 processing objects and processing technology
The main components of high salt wastewater are 15% sodium chloride solution, the wastewater pH value is 6 ~ 8, the wastewater COD is 50,000ppm. Handling capacity of 3t / h. According to the characteristics of high salinity wastewater, the process design according to the three-effect evaporator.

 According to the calculation, the main technical parameters of the three-effect evaporator are as follows:

The actual steam consumption Q = 1200kg / h (inlet pressure 0.3 ~ 0.4MPa); one effect evaporator heat transfer area S = 80m2, the actual steam consumption Q = 1200kg / h (inlet pressure 0.3 ~ 0.4MPa) Vacuum degree P = -0.03MPa; two-effect evaporator heat transfer area S = 80m2, vacuum P = -0.06MPa; three-effect evaporator heat transfer area S = 80m2, vacuum P = -0.085MPa; Consumption of Q = 40t / h; condensing cooling area A = 240m 2; unit total power P = 25kW; unit covers an area of 10m × width 5m × high 4m. According to the process, fully consider the corrosiveness of waste water on the equipment, and in the premise of satisfying the use of the principle of cost savings as much as possible, the system equipment selection is as follows:
1) evaporator body selection carbon steel heavy anti-corrosion, can withstand 120 ℃ or less
 Acid, alkali, salt solution corrosion;

2) heater selection Ta1 titanium tube;

3) Condenser cooler tube selection 316L stainless steel;

4) out of the screw pump selection 316L stainless steel;

5) recovery of water tank and flash tank selection of carbon steel spray anti-corrosion coating;

6) process pipeline, pipe fittings, valve selection 316L stainless steel + PPR material;

7) crystallization tank selection of carbon steel heavy anti-corrosion.

4.2 Processing results and problems
The high salinity wastewater is treated by the three-effect evaporator, resulting in the crystallization of the salt, the organic concentrated liquid and the desalinated water, the crystallization salt and the organic matter concentrated waste to the hazardous waste disposal center. The desalination water is used in the production use. Through the operation of the system found that despite the three effects of steaming
 Hair machine can effectively deal with high salt water, but there are still some problems need to be overcome, mainly in:
 (1) high cost of wastewater treatment. As the treatment of waste water is more corrosive, so the choice of equipment need to consider corrosion resistance, high cost.
 (2) the whole package of equipment in the process of serious corrosion, short life. In the design of the three-effect evaporator, although the choice of corrosion-resistant materials, but can not avoid corrosion, there is still a shorter service life of the problem, the need for timely replacement.
 (3) three-effect evaporator treatment of high salt water requires a lot of steam, many places do not have the conditions.
 (4) through the three-effect evaporator treatment of high salt water also need to be sent to the hazardous waste disposal center for further processing.


5 Conclusion

High salt wastewater must be properly treated before returning to the environment. Practice has proved that the traditional wastewater treatment method is not suitable for dealing with high salt water. In many high salinity wastewater treatment technology, the three-effect evaporator desalination method has a mature technology, can handle a wide range of wastewater, small footprint
 Fast speed, energy saving, etc., in the country has a larger development prospects. Although the three-effect evaporator has the disadvantages of high processing cost, short service life and large amount of steam, the application of this technology in the field of high salt water treatment will be further expanded with the further development of technology.

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