Abstract : In the production systems of chemical, coal and other industries, a large amount of saline wastewater is generated. Under the traditional governance model, the wastewater treatment effect is poor, and it cannot meet the basic requirements of wastewater discharge, and it will have an irreversible impact on the surrounding environment, especially the soil and groundwater. The application of multiple-effect evaporation technology can better solve this problem. Based on a brief description of the operation mechanism and development status of multiple-effect evaporation technology, this paper analyzes the main influencing factors in the treatment of saline wastewater, and puts forward the key points of technology application in combination with practice, so as to provide reference for relevant technology applications.
The development of the coal chemical industry has promoted the large-scale use of water resources. The wastewater discharged after use contains a lot of salt. Direct discharge will increase the risk of environmental pollution and is not conducive to ensuring the safety of drinking water for residents. Multiple-effect evaporation of concentrated brine can effectively reduce the above-mentioned phenomena and achieve pollution prevention.
In order to promote zero discharge of high-salinity wastewater, research on multiple-effect evaporation technology for concentrated brine has been carried out. The total salt content of saline wastewater exceeds 3.5%, which is widely found in chemical, metallurgical, petroleum, power and coal industries, and is an important type of pollution source in environmental protection. Traditional wastewater treatment mainly uses biological methods. Although the overall cost investment is relatively low, the actual treatment effect is relatively limited, and inorganic salts cannot be recycled. The application of multiple-effect evaporation technology can effectively improve the shortcomings of traditional processes through repetitive evaporation processes.
1. Main Sources and Characteristics of Wastewater in the Coal Chemical Industry
Coal chemical wastewater is a chemical product that uses coal as the main raw material, undergoes systematic chemical treatment, slowly converts coal, and separates various chemicals and fuels from solids, liquids, and gases. Then, through a systematic process, valuable chemical products are formed in the market. The composition of coal is very complex, so the processing technology used is also different, resulting in a very complex composition of coal chemical wastewater. In the wastewater of the coal chemical industry, there are as many as more than 300 pollutants, in addition to the main components ammonia, phenol, tar, phenol, cyanide, COD, etc. These components need to be treated and dissolved using efficient wastewater treatment technology. Otherwise, it will have a very serious impact and pollution on the ecological environment and land.
The composition and main sources of coal chemical wastewater are manifested in many aspects, but this article only analyzes them from the following three aspects: First, the degradation of coal chemical wastewater is very difficult. Due to the presence of many refractory organic substances such as biphenyl and long-chain hydrocarbons in coal chemical wastewater, the difficulty of coal chemical wastewater treatment is directly increased; second, the turbidity and chroma of coal chemical wastewater increase rapidly. The direct reason for this result is that many pollutants are generated in each link during the wastewater treatment process of the coal chemical industry, and a chemical reaction occurs when combined with the wastewater of the coal chemical industry, making the chroma relatively concentrated, which will increase the chroma of coal chemical wastewater and indirectly make it difficult to treat and decompose coal chemical wastewater. Third, coal chemical wastewater contains a large amount of pollutants. Because the coal chemical production process is very complex, many types of pollutants will appear in each process, and they are all concentrated in the wastewater of the coal chemical industry, resulting in many types of pollutants in the wastewater, which greatly increases the difficulty of wastewater treatment. Therefore, relevant processing departments should use professional coal chemical treatment technology to decompose and process it more effectively.
2. Necessity of Treating High-Salinity Wastewater in the Coal Chemical Industry
2.1 The Primary Task of Environmental Protection
With the continuous development of society, environmental protection has become the focus of global social development. China's industry as a whole is still dominated by heavy industries such as coal chemical industry, consuming a large amount of water resources and producing tens of thousands of tons of industrial wastewater every day. If not handled properly, it will seriously affect the natural environment and cause a series of serious consequences. Therefore, the reasonable treatment of industrial wastewater is the primary task of environmental protection.
2.2 An Inevitable Requirement for Sustainable Development
In 2003, China proposed the "China's Agenda 21 for Sustainable Development in the Early 21st Century" (hereinafter referred to as the "Agenda"). The "Agenda" puts forward the sustainable development goals of China's social development at the present stage. It points out that in the process of economic development, we must adhere to the principle of sustainable development, pay attention to coordination, and continuously promote the harmonious coexistence between man and nature. According to the data from the United Nations survey, the water resources that humans can directly or indirectly use on the earth are extremely scarce, and freshwater resources account for 0.007% of the total water resources on the earth. Water resources, as a necessity for human life, are as important to humans as oxygen. Therefore, in the process of social development and life, we must maintain a thrifty and cherishing attitude. For industries with high water consumption such as coal chemical industry, how to improve water resource utilization rate and reduce water pollution is crucial.
3. Operation Mechanism and Development Status of Concentrated Brine Multiple-Effect Evaporation Technology
Multiple-effect evaporation refers to the fact that as the pressure decreases, the steam temperature also decreases, resulting in a temperature difference and a heat transfer effect. If the boiling point of the solution in the heater is lower than the temperature of the secondary steam, the secondary steam will condense to generate heat, heat the solution until the solution boils and evaporates. In the specific application process, the following methods should be adopted: In the process of multiple-effect evaporation, the pressure of the heater solution must first be reduced to increase the pressure of the heating steam and reduce the pressure of the secondary steam in the last effect! To ensure that the pressure between each effect can be reduced in a stepped manner, so as to form a temperature difference and ensure that each effect solution can absorb the latent heat of the secondary steam of the previous effect to achieve the purpose of boiling evaporation.
3.1 Operation Mechanism of Multiple-Effect Evaporation Technology
The basic principle of the application of multiple-effect evaporation technology is to use the function of the condenser to condense and extract pollutants such as inorganic salts in wastewater, thereby purifying the water quality and meeting the discharge standards. In the current multiple-effect evaporation operation system, the operation mechanism is mainly reflected in the following three aspects: First, the optimization of steam utilization is achieved by adding additional evaporation units, and the utilization level of redundant steam is improved through the circulation mechanism. Second, sensors and other equipment are added to key parts such as evaporation tanks and pipelines, and the automatic control system is used to balance the operating pressure at different times, improving the safety of the evaporation process. Third, by optimizing the plate structure, the initial temperature of the wastewater is improved, effectively saving input steam, improving the purification level, and achieving better wastewater treatment effects.
3.2 Development Status of Multiple-Effect Evaporation Technology in Saline Wastewater
Single-effect evaporation and vacuum multiple-effect evaporation technologies have a history of over 100 years, initially used in seawater desalination. In China's traditional process system, multiple-effect evaporation technology is mainly based on evaporation and crystallization processes. Only with the increasing maturity of MVR technology did it begin to become the main application form of multiple-effect evaporation technology. MVR is a single evaporator integrating multiple-effect falling film evaporators. Through staged evaporation and repeated actions, the set concentration requirements are ultimately achieved. Multiple-effect distillation (MED) evaporation technology is an extension of this, connecting multiple horizontal or vertical pipes with membrane evaporators in series. According to the processing requirements, it is divided into several effect groups, and multiple evaporations and concentrations are performed on the effect groups to achieve the purpose of water desalination. In the current application functions of multiple-effect evaporation technology, the main focus is on optimizing system design, manufacturing, and operating parameters to achieve a better overall heat transfer coefficient. At the same time, through economic cost control, the cost investment of enterprises using multiple-effect evaporation technology is reduced, and the application range of multiple-effect evaporation technology is continuously expanded.
4. Influencing Factors of Multiple-Effect Evaporation of Concentrated Brine in Coal Chemical Wastewater Treatment
4.1 Influence of the Number of Effects
In the treatment of saline wastewater, the basic requirements for applying multiple-effect evaporation technology are to make full use of heat energy and the utilization level of secondary steam, reduce overall steam consumption, and the number of effects is the most critical factor affecting steam consumption. In the same multiple-effect evaporation operation system, as the efficiency increases, the heat exchange area of each effect evaporator will increase accordingly, and the evaporation amount of each effect will decrease. However, in actual operation, higher efficiency does not necessarily mean better actual results. Improving efficiency requires increased investment in equipment and other aspects. Therefore, in actual construction, it is necessary to comprehensively consider technical limitations, investment costs, and processing target requirements to select the appropriate number of effects.
4.2 Influence of Concentration Ratio (CR)
In the multiple-effect evaporation process, the gradual increase in the concentration ratio will gradually reduce the steam consumption and cause the GOR to slowly increase, gradually improving the performance of multiple-effect evaporation. However, under high salinity conditions, the corrosion rate and scaling rate of the operating equipment will also increase significantly. Therefore, when treating saline wastewater, it is not possible to simply pursue an excessively high concentration ratio, but to set the concentration ratio within a reasonable range according to the actual operating conditions of the equipment.
4.3 Influence of the Final Effect Evaporation Temperature
In the initial application of multiple-effect evaporation technology, more attention should be paid to the heat transfer temperature difference index. In the case of a larger heat transfer temperature difference, the heat transfer driving force of the system is also larger, which will affect the overall thermal performance. Therefore, when the feed rate remains unchanged, the consumption of heating steam can be reduced by changing the heat transfer temperature difference of the system. When the final effect evaporation temperature remains constant, the higher the number of effects, the larger the total heat exchange area of the evaporator, but when the final effect evaporation temperature continuously increases, the GDR increment brought by the total heat exchange area of the evaporator gradually decreases, and the effect becomes less and less obvious.
4.4 Influence of the Feed Preheater
The use of a preheater significantly increases the temperature of the feed brine before entering the evaporator under secondary steam heating, reducing steam consumption during the evaporation process. At the same time, fully utilizing the secondary steam from the final effect to heat the feed brine can effectively reduce energy loss in the operation of the entire system. While maintaining the amount of freshwater used unchanged, it can effectively reduce steam consumption, increase GOR, and reduce the overall operating cost of the multiple-effect evaporation system. From this aspect, it is necessary to add a feed preheater link in the multiple-effect evaporation system to improve the evaporation effect and the economic performance of the system operation.
5. Application Control Points of Concentrated Brine Multiple-Effect Evaporation Technology
5.1 Technical Control of Crystallization and Desalination
In the application system of multiple-effect evaporation technology, the design and optimization of the evaporator link are important factors in improving the evaporation effect. At the same time, technical application control of related links should also be carried out. Taking the desalination link as an example, due to the different salt content of different wastewaters and deviations in the existence of other types of impurities, different types of evaporators should be selected according to the specific subdivided wastewater types, or the circulating evaporation process should be improved. This can remove some of the solvent during the heating process and improve the evaporation effect. By adding auxiliary crystallization or desalination equipment in the evaporation stage, crystals can be removed in time during the series connection, thereby achieving a better desalination effect. For example, in the multiple-effect evaporator system of some enterprises, a negative pressure device can be added to improve the water production ratio of the evaporator system. When the working steam pressure is 0.30 MPa, the energy saving level can be improved by more than 45%, achieving a good energy saving and consumption reduction effect.
5.2 Membrane Separation Technology Control
Membrane separation technology analyzes the components of saline wastewater before it enters the multiple-effect evaporation circulation system and uses a thin film to screen the wastewater and separate the permeable substances in advance. Through the application of nanofiltration, ultrafiltration, and electrodialysis membrane separation technologies, not only unnecessary energy loss during the multiple-effect evaporation process is reduced, but also the corrosive effect of pollutants on the evaporation system is reduced, extending the service life of the equipment. Taking ultrafiltration separation technology as an example, it can effectively retain colloidal subunits and reduce the corrosive effect of pollutants on the evaporation system, extending the service life of the equipment. Taking ultrafiltration separation technology as an example, it can effectively retain colloidal substances in wastewater, leaving only solid suspended substances in the wastewater. The application of reverse osmosis separation technology can select the appropriate membrane type according to the characteristics of the membrane itself, the concentration of saline wastewater, the acidity and alkalinity of the liquid, and other parameters to achieve a better desalination effect in advance and improve the operation level of the subsequent multiple-effect evaporation system.
5.3 Improvement of Composite Treatment Level
Although the application of multiple-effect evaporation technology has greatly improved the treatment effect of saline wastewater and achieved good environmental protection effects, due to the complexity of application scenarios and industrial production types, the types of impurities in saline wastewater are also constantly increasing. In this case, a single treatment process is difficult to meet the actual requirements of wastewater treatment. In this case, composite treatment steps need to be adopted, and the wastewater needs to be concentrated to a corresponding degree to achieve the actual salt separation effect. Taking the humidification-dehumidification technology currently used abroad as an example, the saline wastewater is first introduced into the humidifier, the air is converted into saturated wet steam, and then enters the condensation section for water-vapor separation, finally achieving a good treatment effect.
In summary, industrial development has brought immeasurable benefits to urban and human societal development. However, while developing industry, the principles of environmental protection and sustainable development must also be implemented. In the coal chemical industry, in particular, special attention must be paid to water resource protection and the treatment of industrial wastewater. Continuously improving and perfecting relevant technologies, moving towards green, intensive, and sustainable development, will contribute to environmental protection and sustainable development. Currently, the application level of multi-effect evaporation technology in the treatment of saline wastewater is continuously improving, and the overall treatment effect is also continuously improving, laying a good foundation for the environmental protection work of enterprises. However, from an overall perspective, the energy loss ratio in the treatment process is still relatively high, and the economic cost invested by enterprises in the saline wastewater stage is still relatively high, becoming a key link in the control of the overall operating cost of enterprises. Strengthening technological research in this field and comprehensively optimizing multi-effect evaporation technology are fundamental requirements for improving the economic and ecological benefits of enterprises.
