Evaporative crystallization technology with zero discharge of high-salt wastewater
With the leap-forward development of my country’s industrial technology, people’s quality of life has also been greatly improved, and many industries have higher requirements for water quality. At present, various countries are paying more and more attention to the protection of the environment, and the requirements for energy consumption are also higher. As the problem of water shortage in my country is becoming more and more serious, comprehensive utilization of wastewater and “zero discharge” have become one of the most concerned topics in the field of environmental protection.
High-salt wastewater refers to wastewater with a total salt content of more than 1%, and the sources of high-salt wastewater are very wide, whether it is chemical, pesticide or power industries.
The organic matter in high-salt wastewater will vary greatly in the types and properties of organic matter contained in different production processes, but the substances in salts are usually Cl-, SO42-, Na+ and other salts.
The above substances are all necessary for the growth of microorganisms, and the microorganisms will accelerate the enzymatic reaction during the growth period and effectively adjust the osmotic pressure. However, when the ion concentration exceeds a certain value, the microorganisms will have a certain toxic effect.
Introduction to evaporative crystallization technology
(1) Multi-effect evaporation technology.
The composition of the multi-effect forced circulation evaporator is mainly composed of two parts, namely the heat exchanger and the crystallization evaporator. The liquid is continuously circulated in the tubes in a circulating pump, heated above the boiling point of the liquid, which will continuously generate steam, which will enter the condenser for condensation. Finally, it is sent to the salt separator, so that the brine can be separated.
(2) Mechanical thermal compression technology.
The mechanical hot steam is compressed by the secondary steam in the evaporator, thereby increasing the pressure, increasing the temperature, increasing the enthalpy, and then sending it to the heating chamber for heating.
During the heating process, it must be ensured that the feed liquid is always in a boiling state. Compared with the heat exchange and evaporation of MED, the use of two devices will be more efficient in terms of efficiency, and can effectively reduce the energy consumption.
(3) Contrast MEE and MVR.
Compared with MEE, MVR can maximize the secondary steam and has a very good energy saving effect.
However, because the MVR technology must be supported by a large amount of steam at the first start-up, it is necessary to build a backup boiler in an area without a steam source, which increases the input cost to a certain extent.
In addition, MVR technology has very high requirements on power demand, and is not suitable for use in areas with insufficient power.
For equipment, MVR has very high requirements on the capability of steam compressors, because there is still a big gap between domestic machining and manufacturing levels compared with foreign countries, so there are often some loopholes in actual application. At present, most of the steam compressors used in my country are imported equipment, and the price is relatively high.
Zero discharge of high-salt wastewater by applying steam compressor technology
(1) Working principle and process
The working principle of the steam compressor technology is that the secondary steam generated by the evaporator is further compressed by the compressor, and the pressure and temperature are increased.
After that, the compressed steam will release a large amount of latent heat, which will form condensed water after condensation, and the material will continue to absorb heat and continue to generate steam, which is secondary steam.
The secondary steam will continue to be repeated, and the material will continue to evaporate, and it will continue to circulate. After starting the steam compressor device, if the vacuum degree of the evaporator is 80kPa, the pressure of the secondary steam generated by the evaporation is 20kPa, and the temperature can reach 60°C.
Through the secondary steam in the evaporator, a further step of compression is carried out, thereby increasing the pressure, increasing the temperature, increasing the enthalpy, and the steam compressor will increase the pressure by about 2kPa.
(2) Evaporative crystallization process
The evaporation process of thermal crystallization can be divided into two parts, namely evaporation concentration and evaporation crystallization. The raw water is extracted from the raw water tank, passed through the raw water pump, and then sent to the evaporator using the heat exchanger.
If the vacuum degree in the evaporator is 80kPa and the temperature reaches 60°C, the raw water will boil and evaporate, which will generate secondary steam. After the steam compressor is used to pressurize again, the condensed water is formed through the film heat exchanger. , flows into the condensed water tank, and finally transported to the clean water tank through the water pump.
During the evaporation process of the raw water in the evaporator, a concentrated liquid will be produced, some of which will be directly discharged to the collection tank for processing in the next process; some of the concentrated liquid will be pumped back to the inlet pool according to the proportion of raw water flow. .
The operator can make appropriate adjustments to the flow rate of the concentrate return.
The raw liquid in the evaporator is the cooling water on the evaporation side of the thin film heat exchanger. Under the action of the circulating water pump, the raw liquid is sprayed from the top of the thin film heat exchanger, and then enters the bottom of the evaporator. System pressure is maintained through this cycle.
For the evaporator, the interior needs to be kept in a vacuum state, which can be achieved by an external vacuum pump.
The vacuum degree of the system is kept at 80kPa, the temperature is 60℃, and the influent flow is 500L/h. The experiment of desulfurization wastewater treatment is simulated. The final results are: the conductivity of condensed water and concentrate, Cl -, SO42-, Ca2+, these substances will continue to increase with the increase of the salt content of the raw water.
And the water recovery rate will also decrease with the increase of the salt content in the raw water, and the water recovery rate is up to 40.0%; when 1 ton of simulated desulfurization wastewater is treated, the actual average energy consumption is 23.5kW· h, compared with the three-effect evaporator, it is much lower; the water recovery rate is low, and the energy consumption is also very high.
With the continuous increase of the salt content of the raw water, the water recovery rate will show a slow decrease.
If the salt content of the raw water is 2.8%, the average recovery rate of water is 40.0%; if the salt content of the raw water containing wastewater reaches 3.5%, the recovery rate of water is only about 20.0%. There is a very large gap in the water recovery rate when the raw water salt content of a single pollutant is 0.5%-0.6%.
(3) Inadequacies of steam compressor technology
In the process of system operation and management, it can be found from the analysis of the obtained experimental results that if the calcium content of the raw water is too high, scaling will occur inside the evaporator, which will affect the quality of the effluent and the quality of the water. The actual recovery rate has a very large impact.
The dirt has to be treated manually before the next work on the equipment.
Therefore, the measures to strengthen the anticorrosion of the dirt in the equipment are a topic that people in this field are very concerned about.
In fact, in the process of testing, if the operating state of the equipment is not particularly stable, the actual efficiency of the steam compressor will not be too high, which will greatly affect the final evaporation effect and the accuracy of the obtained data. big impact.
The evaporator uses a sealing ring to seal the tank. In fact, the pressure will be relatively low during operation, and the interior of the evaporator will not be able to achieve a vacuum state. Therefore, it is necessary to strengthen and improve the selection of the sealing method.
To sum up, in order to realize zero discharge of wastewater, it is necessary to pay more attention to the treatment technology of high salt water content. At this stage, evaporative crystallization technology is mainly used in chemical, pharmaceutical and other fields, which can effectively treat wastewater and realize zero discharge of wastewater. Moreover, the useful part of the chemical raw materials can be recycled and used again, which has a great environmental protection effect, thereby increasing the economic benefits of the enterprise.