Highly saline wastewater is usually defined as wastewater with a total dissolved solids (TDS) mass fraction greater than 3.5%. In addition to inorganic salts in the form of Cl-, SO42- and Na+ ions, this type of wastewater also contains ions such as Ca2+, Mg2+, NH4+, HCO3-, which are prone to chemical change into scale, and impurities such as chemical oxygen demand (COD) and suspended solids (SS). 5% of the total volume of wastewater was generated in China in 2015, with an emission of about 997.5 million tons. If high-salt wastewater enters the water body directly, it will eutrophicate the water body and algae will multiply rapidly, which will lead to the deterioration of water quality and the death of fish and other organisms in large quantities.
In November 2007, the State promulgated the National Environmental Protection Eleventh Five-Year Plan, which requires the promotion of wastewater recycling in key industries such as iron and steel, electric power, chemical industry and coal, and efforts to achieve less or zero discharge of wastewater. In February 2017, the National Energy Administration issued the "Thirteenth Five-Year Plan for Demonstration of Coal Deep Processing Industry", which requires new demonstration projects without sewage bodies to resourcefully utilize high-salt wastewater by using technologies such as crystallization and salt separation.
Zero discharge" treatment of high salt wastewater at home and abroad generally adopts evaporation pond natural evaporation, multi-effect evaporation crystallization, mechanical steam recompression evaporation process. The natural evaporation of evaporation ponds is to evaporate water by using the natural evaporation of solar energy, so that the salt is left at the bottom of the pond for regular cleaning. Mechanical steam recompression evaporation process because of steam compressor temperature rise 8 ~ 10 ℃, often used with falling film evaporator, mainly for high salt wastewater evaporation concentration (TDS content is not greater than 240,000mg / L), multi-effect evaporation crystallization can be for high salt wastewater salt content and water changes at any time to adjust the amount of steam to achieve control evaporation, so the stability of operation and operational flexibility than mechanical steam Recompression evaporation has more advantages.
1. How to prevent tube blocking and scaling in wastewater evaporator?
Answer: There are two general reasons for pipe blocking: one is the deposition of crystal salt, the other is the scaling of calcium and magnesium ions, etc. Therefore, the prevention of pipe blocking should also start from these two aspects.
First, choose the right evaporation form is to prevent the first step of blocking the tube, falling film evaporator and other membrane evaporator is not suitable for the occasion of crystal generation, environmental protection bee if these occasions use falling film and other film evaporator, blocking the tube is inevitable. Easy to produce crystal occasion should use the forced circulation type evaporator or scraper evaporator, which forced circulation evaporator because evaporation area is big, the operation is convenient.
Second, is the evaporation pipeline setting is also reduce the salt important factor, design the pipeline without dead corner, accord with the salt flow transportation and deposition principle, ensure that all crystallization salt to collect the salt collector, finally from the salt discharged. If the scaling is caused by calcium, magnesium ions etc., the measures that can be taken are
1, the wastewater into a degree of softening, reduce the concentration of calcium, magnesium ions.
2.Adopt forced circulation type evaporator.
3.Adopt gypsum crystal seed method to prevent scale or slightly add some scale inhibitor.
4, the evaporation equipment for routine cleaning.
2. Evaporator running cost and how to reduce running cost?
Answer: method one: using multi-effect evaporation process, each increase an effect, the operating cost decreases, but the initial investment increases, for wastewater evaporation salt, generally will not exceed three effects, at most four effects.
Method two: thermal compression type evaporator TVR, can reduce the energy consumption of one effect.
Method three: mechanical vapour compression type evaporator, reduce the operating cost to a large extent.
Method four: do a good job of evaporation equipment external insulation, reduce the loss of heat.
Method five: condensate sensible heat and latent heat and use. Use the preheater to recover the sensible heat of the
Condenser and improve the feed temperature of the evaporating raw liquid; use the flash
Evaporation System to recover the latent heat of the condensate.
3. Evaporation out of the salt situation?
Answer: There are generally two ways to evaporate salt: one is centrifuge salt, centrifuge salt with low water content; the other is the salt discharger with crystal tank salt, salt in the form of crystal precipitation, less external water.
4. evaporator material selection?
Answer: The choice of evaporator material is closely related to the composition of the waste water. The salts can be divided into chloride ion salts (such as sodium chloride and ammonium chloride) and non-chlorine ion salts (sodium sulphate, ammonium sulphate, sodium carbonate, nitrate, etc.).
Chloride ions in order of preference for corrosion resistance: titanium, duplex stainless steel, carbon steel, plain stainless steel
Chloride ion according to the order of preference for cost performance: carbon steel, titanium, duplex stainless steel, the second most common stainless steel
Non-heat exchange equipment, but also the choice of carbon steel enamel, tetrafluoro, low temperature can choose polypropylene, glass steel, etc.
Non-chlorine salt corrosion resistance priority order of choice: stainless steel 316L, stainless steel 304, carbon steel
Non-chlorine ion according to the order of priority selection of cost-effective: stainless steel 304 or stainless steel 316L, followed by carbon steel low temperature can be used UPVC, PE polypropylene, glass fiber reinforced plastic, etc.
5. How to choose the right evaporator?
Answer: For salt evaporation, priority is given to forced circulation type evaporator, if the salt concentration is low, you can also use the way of front falling film evaporator + forced circulation evaporator. For the evaporation of other non-salts, the descending film evaporator is preferred.
6. What is the removal rate of COD in the evaporation process?
Answer: The COD in wastewater is mainly determined by the amount of organic matter in the wastewater, and the organic matter contained in the wastewater may be high boiling point or low boiling point. If the organic matter with high boiling point enters the solid waste and waste liquid system with a piece of salt during evaporation, the COD of the condensate drops; if the organic matter with low boiling point in the wastewater enters the condensate system with condensate during evaporation, the COD of the condensate does not drop. Therefore, the removal rate of COD in the evaporation process is related to the specific composition of organic matter in the wastewater. If the owner can provide the specific composition and content of organic matter in the wastewater, the basic judgment can be made on the removal rate of COD in the evaporation process.
7. What is the difference between MVR and multi-effect evaporation steam reuse?
Answer: MVR and multi-effect evaporation are both designed to reduce the operating cost of wastewater evaporation, the basic principle of MVR is to recompress the secondary steam, increase the pressure and temperature of the secondary steam, reuse the secondary steam to heat the heat exchanger, and finally achieve energy saving.
The basic principle of multi-effect evaporation is that the secondary steam enters the next-effect evaporator for heating, realizing the reuse of steam. The number of effects of multi-effect evaporation is often limited due to various factors, and MVR saves steam consumption compared to multi-effect evaporation.
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Source: Xianji.com
