In the environment of increasingly strict requirements for the development of "green industry", "zero discharge" of coal chemical wastewater has gradually become a development trend, and high-salt wastewater treatment has become a key breakthrough point to achieve this goal. Compared with biochemical treatment, exchange resin separation technology, ultrafiltration, reverse osmosis and other membrane technologies, the use of evaporation technology to treat high-concentration salt wastewater in coal chemical industry is mature and widely used, and it is more suitable for producing a large amount of reclaimed water from organic wastewater with high salinity.
With the rapid rise of the modern economy, the mutual constraint between industrial development and resources and environment has become the focus of worldwide attention. Water resources, as an important raw material, recycling medium, and emission carrier of products and wastes in the industrial production process, require a huge amount. China's industrial water use is facing problems such as low utilization rate, low wastewater discharge efficiency, and imbalance between the level of industrial development and the distribution and utilization of water resources. According to surveys, China's industrial water waste is serious, and the reuse rate is about 40%, which is only 1 2 of developed countries. In the past 10 years, China's nearly 1 3 regions adopted the gap between the level of industrial wastewater treatment and the advanced level is getting bigger and bigger, with the "three red lines", "four basic systems" and the new "water ten" issued and implemented, China has strengthened the supervision and accountability mechanism for wastewater treatment and water resource utilization, industrial wastewater only to achieve simple standard discharge has been unable to meet the current stage of resource utilization and ecological protection standards, high efficiency, conservation, "zero discharge" has become the current trend of industrial water.
Industrial wastewater is mainly used in petrochemical, coal mining, printing and dyeing, papermaking and other industries, generally containing organic matter, suspended solids, colloids, microorganisms and soluble salts, etc., with complex composition, long treatment process, great difficulty and comprehensive methods. Most of the industrial wastewater has removed most of the insoluble solids, organic matter and toxic and harmful substances through the early physicochemical pretreatment, biochemical treatment and advanced treatment processes, and finally discharged the total dissolved solids (TDS) mass fraction of more than 8% of the high-concentration salt wastewater treatment process has become the key link to achieve "zero discharge" of wastewater.
Status quo and characteristics of high-salt wastewater treatment
At present, although there are various treatment methods for high-salinity wastewater, they have strict requirements for wastewater raw materials, and many processes cannot meet the conditions for treating high-salinity wastewater. Taking the biological removal of organic matter in high-salt wastewater as an example, the removal rate of COD and total nitrogen decreased significantly with the increase of salinity, while the salinity tolerance of acclimatized activated sludge could only reach 5% at most.
Although ultrafiltration, reverse osmosis and other membrane technologies or ion exchange resins can obtain a high water rate for the treatment of high-concentration salt wastewater, the high concentration of salt ions in the wastewater will cause serious corrosion to the membrane or resin, and with the continuous increase of concentrate concentration and viscosity, the organic matter in the wastewater and Ca2+, Mg2+ and other easily scaling ions are easy to block the membrane pores or resin, and cause non-renewable pollution and damage to the membrane or resin.
At present, membrane technology is mostly applied to the primary concentration process of salty wastewater with less organic matter, and the COD mass concentration is reduced to less than 20mg L by pretreatment and membrane filtration, and the decolorized wastewater is desalination by multi-stage countercurrent inverted electrodialysis device, and the wastewater ** rate is more than 85%.
The counterflow of fresh water and concentrated water in the membrane stack effectively inhibits the diffusion of concentration difference between the concentrated water chamber and the fresh water chamber, and also slows down the rate of fouling and scaling on the membrane surface. The removal rate of Cl- in wastewater decreased significantly with the increase of brine salinity, and the polarization of the concentration difference on the membrane surface was aggravated. The problems faced by electrodialysis technology in the treatment of high-concentration salt wastewater are still concentrated in the technical aspects closely related to membrane materials, such as membrane fouling, corrosion and cleaning.
The heating and evaporation process is widely used in the treatment of high-salt wastewater with complex composition such as electric power, petrochemical, coal chemical industry and oil production, and the technology is mature, and it is suitable for treating wastewater concentration with a salinity of more than 8%. The hot high-salt wastewater is continuously evaporated and concentrated, and the solid-phase salts are precipitated from it, and the solvent is evaporated and converted to the gas phase and then condensed to continue to be recycled. There are various forms of heating evaporation, among which multi-effect evaporation and mechanical compression evaporation are widely used.
Coal chemical industryZero wastewater dischargeDeal with the current situation and existing problems
Background of high-concentration salt wastewater treatment in coal chemical industry: The distribution of resources in China is characterized by poor oil, less gas and more coal, and the traditional coal chemical industry using coal coking, coal calcium carbide and coal gasification to produce natural gas and urea, as well as new coal chemical processes such as coal liquefaction and coal gasification to produce alcohol ether fuel and olefins, have long become the hot spots of energy utilization research.
However, the huge water demand of coal chemical industry, and the serious imbalance of water resources in the region where the production enterprises are located, coupled with the increasingly serious environmental resource problems in recent years, the emission control of industrial production waste has been increased at home and abroad, and the "zero discharge" of coal chemical wastewater has become the ultimate development trend of wastewater treatment. According to its composition, coal chemical wastewater can be divided into two categories: organic wastewater and saline wastewater, among which the treatment process of high-concentration brine products obtained in the later stage of saline wastewater treatment has become the key to achieve "zero discharge" of wastewater.
Coal chemical concentrated salt wastewater is generally prepared by "softening + high-efficiency membrane concentration" technology, and its TDS mass concentration can reach 50000 80000mg L. Although the membrane treatment process has a high water rate, it is found that on the one hand, if the COD mass fraction in the waste liquid is higher than 6 10-5, the membrane surface is easy to scale, and the performance is significantly reduced. On the other hand, the increase of salt content in wastewater will aggravate the concentration polarization phenomenon, and the corrosive nature of Cl-plasma in the water will seriously affect the membrane efficiency and service life.
Coal chemical high-concentration salt wastewater treatment methods: The treatment methods of high-concentration salt wastewater include ash flushing method, incineration method, deep well perfusion method, evaporation crystallization method, etc.
The ash flushing method is to spray concentrated salt wastewater in the form of spray in the plant area to achieve the purpose of dust reduction and ash removal. The application of this method has been limited due to the limited amount required in the area and the volatilization of organic matter in wastewater is easy to cause secondary pollution.
The incineration method is a process in which the incinerator is used to carbonize and solidify the high-concentration salt wastewater at high temperature, and finally discharge the waste gas and salt-based waste residue. The fuel energy consumption of this method is huge, the heat energy utilization rate is low, and about 1 3 heat is lost from the flue gas, and it is generally economical to treat high-salt wastewater with more than 10% organic components.
The underwater incineration evaporation device uses a pipeline burner to vaporize the wastewater, and the waste heat is transferred to the external water body for preheating, and the heat energy utilization rate can be increased to more than 99%. However, this method still has problems such as serious equipment corrosion and unstable operation, and has not been applied to coal chemical wastewater treatment.
The deep well perfusion method has been used in the United States, Mexico and other countries, but due to the limitations of geological conditions and ecological environment, the method has not been approved for implementation in China.
At present, the high-concentration salt water and quality salt separation technology has become the key technology to overcome the bottleneck of "zero discharge" of coal chemical wastewater, while the evaporation crystallization method is mainly used for high-concentration salt wastewater in industry. Evaporation crystallization technology is divided into natural evaporation and mechanical evaporation, of which mechanical evaporation can be divided into multi-effect evaporation, mechanical compression evaporation, multi-effect flash evaporation, membrane distillation and other methods.
Natural evaporation
Natural evaporation refers to the process of discharging high-concentration wastewater into the evaporation pond and using solar energy to evaporate the waste liquid, and converting the high-concentration wastewater and volatile organic matter in the evaporation pond into the gas phase to finally obtain salt slag. The evaporation pond evolved from the sun-dried salt field in the salt industry, which has the advantages of low energy consumption, simple operation and long service life, and has outstanding performance in the high-concentration wastewater treatment process of coal chemical industry.
Natural evaporation is seriously affected by the external meteorological environment, the evaporation is much greater than the rainfall, and the volatilization of organic matter and the enrichment of heavy metal ions in the evaporation process will cause secondary pollution to the surrounding environment to a certain extent. Although the investment cost of the construction of the evaporation pond is low, due to the huge evaporation area, the economic cost of the actual operation must be comprehensively considered while the process method of the high-concentration wastewater of the evaporation pond is selected.
In order to reduce the evaporation footprint, experts proposed a mechanical atomization evaporation method to increase the gas-liquid contact area, which can increase the evaporation efficiency by 14 30 times, and reduce the evaporation area to 10% of the original. An industrial park in Inner Mongolia has achieved good results by adopting this method.
Multi-effect evaporation
Multi-effect evaporation (MED) is connected in series with multiple evaporators, heating wastewater is concentrated to obtain solid phase salt, and the heating steam required by the next evaporator comes from the secondary steam of the previous evaporator, and the evaporation efficiency is the number of steam utilization. From the aspects of cost saving and energy reduction, the efficiency of multi-effect evaporation is generally 3 4 levels.
The energy consumption of multi-effect evaporation itself is high, but if it is combined with a coal chemical project that produces a large amount of low-pressure steam as a by-product, it can achieve comprehensive and efficient utilization of the energy of the whole plant. The horizontal tube falling film evaporator with high phase change thermal efficiency is combined with the vertical pipe falling film evaporator, the horizontal pipe falling film evaporator adopts negative pressure evaporation, and the wastewater is preheated by using the secondary steam of the final effect of the vertical pipe falling film evaporator, and then transported to the vertical pipe falling film evaporator for evaporation crystallization, which can realize the graded utilization of heat energy, and the amount of high-pressure steam can be reduced by at least 30%, and the energy consumption is significantly reduced.
Multi-effect flash evaporation
Multi-effect flash evaporation (MSF) is proposed to solve the problem of serious fouling in the multi-effect evaporation process, which uses low-temperature flash evaporation technology to send the hot waste to the flash chamber with reduced temperature and pressure for concentration, and the final material liquid is concentrated and discharged in the form of salt slurry. MSF has mature technology, stable operation, low requirements for raw materials, and is suitable for large-scale chemical wastewater treatment. However, due to its low pressure operating conditions, it will produce higher power consumption, energy consumption is higher than that of multi-effect evaporation, and the operation flexibility is small, which is not suitable for the situation of large changes in the flow rate of the treated material liquid.
Mechanical compression evaporation
Mechanical compression evaporation (MVR) is one of the most advanced evaporation technologies in the world, using a compressor to improve the grade of secondary steam, recycling steam to improve heat utilization, and greatly reducing the demand for external heat sources. Its low energy consumption, small footprint, low operating cost, simple operation, high degree of automation and other characteristics make it widely favored in the field of evaporation crystallization, with high practical performance, used to treat high-salt wastewater can effectively avoid corrosion, scaling, foaming and other problems. Compared to multi-effect evaporation, mechanical compression improves the utilization of steam during evaporation.
The MVR process can be combined with high-efficiency membrane concentration technology, and its water ** rate can reach 90%. After research, the series of multiple MVR devices in series to form a two-effect or multi-effect mechanical compression evaporation process can effectively reduce energy consumption, because the heat exchange area and compressor power are opposite by the heat transfer temperature difference and the discharge concentration, so the selection of the appropriate heat transfer temperature difference is the key to the efficient and energy-saving operation of the effective control system.
Compared to multi-effect evaporation, mechanical compression improves the utilization of steam during evaporation. At present, from the perspective of total economic cost, policy encouragement and environmental protection, MVR technology has a good momentum of development.
Membrane distillation
Membrane distillation (MD) technology uses the air pressure difference on both sides of the hydrophobic microporous membrane as the driving force, and the solute converted from the liquid phase to the gas phase is diffused to the cold side of the membrane due to heat, and condensed into the liquid phase to achieve water resources and wastewater concentration. The membrane distillation technology has a high water rate, good production water quality, and low equipment cost compared with MED. However, in practical application, membrane distillation still faces problems such as loss of latent heat of phase change, wetting and leakage of hydrophobic membranes, membrane drying and membrane fouling, which affects the stability of membrane distillation technology and greatly increases the operating cost.
The combination of membrane distillation and evaporation crystallization has successfully achieved the highest standard salt from high-salt wastewater, and realized the recycling of wastewater. The membrane distillation-crystallization process formed by the coupling of membrane distillation technology and thermal evaporation crystallization technology has become a research hotspot, which separates solvent evaporation and solute crystallization and has good sealing, and the membrane distillation device can provide a large heat transfer area and effectively reduce the footprint of the equipment.
Membrane crystallization process has been researched and developed in wastewater treatment and salts, from single salt NaCl high-salt wastewater and Na2SO4 high-salt wastewater to complex high-salt organic wastewater, it is believed that with the improvement of membrane materials and the development of membrane-related processes, membrane distillation-crystallization technology can create a new world for the treatment of high-concentration salt wastewater.
Integrated process of "zero discharge" of saline wastewater
Most of the evaporation and desalination of concentrated salt wastewater is used to produce industrial production reuse water at the cost of consuming a large amount of steam, and reducing the amount of steam used to heat the material liquid to be evaporated can directly reduce the evaporation cost. For this purpose, the high-efficiency membrane separation technologies such as ultrafiltration and reverse osmosis and the thermal evaporation process are connected in series to form a new integrated device for concentrated salt wastewater treatment, which has stronger industrial adaptability.
On the one hand, increasing the concentration of the wastewater to be treated, i.e., reducing the amount of vaporization of the waste liquid, can directly reduce the amount of generated steam used during evaporation. The design includes a pretreatment system, a reuse and reduction system and a zero-discharge system for the treatment of reverse osmosis concentrate or high-salt complex wastewater, using an immersion microfiltration device as a reverse osmosis unit component, and sequentially using medium pressure, high pressure and ultra-high pressure step-by-step pressurized reverse osmosis device to concentrate and improve the wastewater, and the wastewater entering the evaporator is only 5% of the original wastewater, which greatly reduces the heat consumption of the evaporation system, and the operating cost is reduced to at least 1 4 before.
On the one hand, reducing the evaporation temperature of the waste liquid and using negative pressure evaporation can also effectively reduce steam consumption. For the reverse osmosis concentrate, the negative pressure evaporation is used to reduce the boiling point of the wastewater, and the porous concave-convex inclined plate is used to form a thin layer of liquid film in the preheated wastewater, which increases the gas-liquid contact area, and at the same time, the induced draft fan is turned on to enhance the air flow in the evaporation chamber and strengthen the evaporation rate. After the saline wastewater is evaporated to saturation, it is sent to the crystallizer to precipitate solid-phase salt, and the wastewater rate can reach 80%, the operation is stable, and the evaporation energy consumption is effectively reduced, which can be applied to large-scale industrial wastewater treatment.
From the macro perspective of the entire coal chemical wastewater treatment process, the treatment of high-concentration salt wastewater is a key part of determining the "zero discharge" of wastewater, and the connection and contradiction between energy consumption, resource recycling, turning waste into treasure and production economy are concentrated in this link. It is of great significance for the development of the entire coal chemical industry and even the industry to grasp the problems and deficiencies of high-concentration salt wastewater treatment in the coal chemical industry and optimize and improve it. It is believed that with the advancement of industrial technology, the realization of "zero discharge" of wastewater is just around the corner.
In the field of mother liquor drying at the end of zero discharge of coal chemical wastewater, Weishengda Environmental Protection has successfully implemented typical projects such as Ningxia Coal Chemical Reuse Evaporation Mother Liquor Crystallization and Drying Project, Henan Coal Tar Processing Crude Phenol High Salt Wastewater Reduction Project.