Modern coal chemical industry is a new type of chemical industry in which coal is used as raw material to produce carbon chemicals and their derivatives, and it is also a large-scale coal chemical industry that has developed rapidly and gradually in recent years. With the intensive construction and development of all kinds of coal chemical projects in the country, adopting the economic development mode of high efficiency, low energy consumption, low pollution and low emission based on the environmental and resource affordability is the only acceptable sustainable development path for modern coal chemical industry. Especially in the northwest of China where there is more drought and less rain, the problems of water resources reuse and environmental protection are becoming increasingly prominent. It can be said that the environmental protection problem of coal chemical industry is, in the final analysis, the problem of development mode and energy conservation and emission reduction. In order to solve the environmental problems of coal chemical industry, we must first solve the problem of high brine treatment and discharge, and achieve "near zero discharge" of industrial wastewater is one of the important problems that need to be solved urgently in modern coal chemical industry.
1. Sources and characteristics of high brine
1.1 High saline source
The salt in modern coal chemical high-salt water mainly comes from the circulating water, the desalinated water preparation link into and concentrated, as well as the industrial wastewater treatment and reuse process to add a variety of chemicals and generated concentrated salt water. According to the analysis of existing projects, the amount of salt brought into a coal chemical project by supplementing fresh water (with the Yellow River as the water source) exceeds more than 1/2 of the salt amount of the entire system, followed by the amount of salt generated by the addition of chemical agents in the production process and water system, accounting for more than 1/3 of the total salt amount. This shows that in order to reduce the salt content of water to achieve the purpose of water saving, it can be achieved by determining a reasonable cycle multiple and dosing method, but the most difficult to solve is the 15-30% (volume percentage) concentrated brine generated after recycling industrial wastewater.
1.2 Features of high brine
According to the daily water quality test results, the high brine of coal chemical industry generally presents the characteristics of large discharge, small change in water quality, high chloride ion content, stable and generally not high water salt content. Its composition is mainly based on organic matter and inorganic salts, CODcr is generally in 60 ~ 120mg/l, TDS is generally in 1900 ~ 3000mg/l, NH3-N content is very low, the water is good, clear and transparent, no obvious odor. Due to the different coal chemical production process, the indicators of drainage water quality are slightly different, and the above water quality characteristics only represent the actual situation of the enterprise.
2. Overview of high brine treatment technology
According to the characteristics and disposal needs of high brine in coal chemical industry, there are three kinds of high brine treatment processes usually used at this stage: membrane separation technology, thermal evaporation technology and the combination process formed by the two technologies.
2.1 Membrane separation technology [1,2]
Membrane separation technology is a new type of separation technology that uses membrane to separate, purify and concentrate the target substance through the difference of the properties of each component in the mixture. At present, there are five membrane separation technologies that have been widely used in the chemical and petroleum industries, namely ultrafiltration, microfiltration, nanofiltration, dialysis and reverse osmosis. According to the size of desalting capacity, it can be preliminatively divided into microfiltration < ultrafiltration < nanofiltration < electrodialysis < reverse osmosis.
(1) ultrafiltration, microfiltration, nanofiltration membrane separation technology
Ultrafiltration, microfiltration, nanofiltration is mainly used for gas, liquid phase particles, bacteria and other pollutants interception removal, the minimum interception molecular weight can reach 80~1000Dal. In particular, the retention rate of standard organic matter and NaCl, MgSO4, CaCl2 solution can be up to 90%, which can effectively remove relatively large particles such as suspended matter (SS) and colloids, so as to achieve the purpose of purification, separation and concentration. However, the desalting effect of the above technology is not ideal, and it can generally be used as the clarification of the feed liquid, security filtration, air sterilization, separation and purification of macromolecular organic matter.
(2) Electrodialysis and reverse osmosis membrane separation technology
Electrodialysis and reverse osmosis are two commonly used methods in desalination technology. The former is a membrane separation process driven by potential difference, while the latter is a membrane separation process driven by osmotic pressure. In recent years, these two technologies have been further improved and optimized, mainly reflected in the inverse electrode electrodialysis (EDR) technology and high efficiency reverse osmosis (HERO) technology. The former effectively solves the problem of continuous and stable operation and frequent scaling by means of automatic and frequent electrode switching. The latter enables RO to remove the hardness and carbon dioxide through two-stage softening and degassing treatment under the condition of high PH, improve the scale limit of silicon, effectively control the fouling of biological and organic matter, and greatly improve the wastewater reuse rate (> 90%). Due to the high power consumption, high treatment cost, insufficient operation experience and generally low reuse water rate, EDR technology has been gradually replaced by reverse osmosis membrane separation technology with energy-saving, low treatment cost, large scale and mature technology. However, reverse osmosis membrane separation technology also has problems that need to be solved urgently, such as membrane pollution, blockage, corrosion and short service life, especially when the feedwater TDS is higher than 6000mg/l, its desalination rate will drop sharply [3].
2.2 Hot evaporation technology
The thermal evaporation technology is mainly for the evaporation and concentration of saline wastewater with a salt content of about 4% (mass fraction) or higher, and its characteristics are mainly manifested in: (1) the general use of physical methods for evaporation and concentration, and sometimes chemical methods (incineration, advanced oxidation, etc.); ② The amount of wastewater treated is generally not large, and some are even small; (3) Treatment costs and energy consumption are generally high; (4) The amount of solid waste is large, the composition is complex, and it can not be effectively recycled and reused. Thermal evaporation technology mainly includes multi-effect evaporation, mechanical compression reevaporation, film distillation and so on.
(1) Multi-effect evaporation (MED) technology Multi-effect evaporation is to let the heated brine evaporate in a number of series evaporators, the steam evaporated from the former evaporator as the heat source of the next evaporator and condensed into fresh water, each evaporator is called "one effect". Under normal circumstances, the series number (effect number) of the circulating evaporator is 3 to 4. According to the different technological conditions, there are four main technological processes: parallel flow method, countercurrent method, advection method and mixed flow method. In the wastewater treatment, multi-effect evaporation is mainly suitable for the separate treatment of high salt and high organic content wastewater, and at the same time, with membrane technology to achieve a full range of "zero discharge" process.
(2) Mechanical compression reevaporation (MVR) technology
The high energy efficiency steam compressor is used to compress the secondary steam generated by the evaporation system, increase the enthalpy of the secondary steam, and introduce the secondary steam into the original evaporation system as a heat source for recycling [4]. The technology greatly reduces the consumption of evaporator steam, the supplementary steam is only used for the system heat loss and the temperature difference between incoming and outgoing materials required enthalpy supplement, the energy saving effect is equivalent to the ten-effect evaporation system, is currently the world's most widely used and advanced evaporator technology.
(3) Membrane distillation (MD) technology
Membrane distillation is a new separation technology driven by vapor pressure difference, that is, through the process of cold and hot side phase transformation, the mixture is separated or purified. Compared with the traditional distillation method and other membrane separation technologies, the technology has the advantages of low operating pressure, low operating temperature and high separation efficiency, and can make full use of solar energy, waste heat and waste heat as heat sources. According to the different condensation modes on the downstream side of the membrane, membrane distillation technology can be divided into four forms: contact, air gap, air sweep and vacuum membrane distillation [5].
In recent years, membrane distillation technology has been developed to a certain extent, but there are still the same problems as membrane separation technology, such as: membrane pollution, scale blockage, etc., the application field is not very wide, and the technical problems that can be commercialized still need to be further solved.
2.3 Combination technology of membrane separation and thermal evaporation
With the deepening of national and local environmental protection policies and requirements for coal chemical wastewater discharge, the process combination technology of high brine treatment has been rapidly developed and researched, and the mature route of diversified and collaborative treatment has been steadily developed. The biggest advantage of the combined process is that the process has more selectivity, good water quality adaptability, and different treatment methods can be formed according to the actual conditions of desalting scale, water quality requirements, geographical and climatic conditions, technology and safety, investment source and management system. The process mainly adopts the combination technology of limestone softening, ultrafiltration, reverse osmosis and thermal evaporation. Among them, the limestone softening pretreatment process adds PAM dosing system, high efficiency precipitator, neutralization tank and secondary filtration system, which can further improve the flocculation, sedimentation and separation of precipitated salts, and has a certain degree of CODcr removal ability. The process combination of ultrafiltration and reverse osmosis is currently widely used in salt removal technology, the treatment effect is obvious, the operation is relatively stable, suitable for the retreatment and reuse of saline wastewater with TDS < 6000mg/l, the reuse water rate can reach more than 70%, and the service life of the membrane can reach 3 years. The discharged concentrated brine can be recycled through DM (butterfly vibrating membrane) device, its biggest advantage is that the membrane pollution control effect is good, water quality adaptability, low energy consumption, sewage recovery rate of up to 85%, and at the same time set up mechanical compression and re-evaporation system and salt separator, so that the brine can be completely separated, to achieve "near zero discharge" treatment needs.
3 Main problems of high brine treatment [6]
The main problems of high brine treatment are as follows:
1) Solve the fouling and corrosion problems of membrane and evaporation system.
The main causes of fouling and corrosion of the membrane and evaporation system are the deposition, growth and crystallization of colloidal substances, microorganisms and inorganic salts on the surface of the membrane, as well as the high concentration of chloride ions and the water quality of low PH value. Although some control measures have been adopted at present, the methods to fundamentally solve such problems are not mature, and most of them are still in the research stage.
2) Technology application production cost control.
Whether it is membrane separation technology or thermal evaporation technology, there are prominent problems of high input, high consumption and high energy consumption, and the economic cost of high brine treatment is huge. It can be simply said that the current solution to high saline emissions is mainly to exchange more energy consumption for pollutant emission reduction. Therefore, so that the high water treatment system can really run, it must consider its operating costs.
3) The solution of the "three" (reduction, resource, harmless) problem of solid waste.