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Zero liquid discharge of wastewater hinges on salt separation; reduced discharge should be the mainstream approach.

Zero liquid discharge of wastewater hinges on salt separation; reduced discharge should be the mainstream approach.

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2023

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Stainless Steel | What are the differences between 304, 304L, 316, and 316L stainless steel? How are they used respectively?

Stainless Steel | What are the differences between 304, 304L, 316, and 316L stainless steel? How are they used respectively?

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2023

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A Brief Discussion on the Evaporation and Concentration Process and Equipment for Lithium Sulfate Solution in New Energy Lithium Batteries

A Brief Discussion on the Evaporation and Concentration Process and Equipment for Lithium Sulfate Solution in New Energy Lithium Batteries

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2023

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Minister of Ecology and Environment: Prioritize the governance of new pollutants in national basic research and technological innovation

Minister of Ecology and Environment: Prioritize the governance of new pollutants in national basic research and technological innovation

New pollutants have diverse sources, involve numerous industries, have long industrial chains, and pose significant challenges in terms of R&D for treatment technologies, requiring cross-departmental and cross-field collaborative governance. China's new pollutant control efforts are still in their infancy, with weaknesses in the foundation, insufficient resource allocation, and severe deficiencies in governance capabilities. Recently, at a press conference held by the State Council Information Office, Minister Huang Runqiu of the Ministry of Ecology and Environment stated that the Ministry will, in the next step, work with relevant departments to strengthen technological support, making new pollutant control a key area of national basic research and technological innovation, and focusing on breakthroughs in key core technologies. It is understood that new pollutants generally refer to chemical substances with environmental persistence, bioaccumulation, and biotoxicity. These substances pose relatively hidden environmental and health risks, and even at low doses in the environment, they may pose risks and hazards to people's health, environmental health, and environmental safety. In May 2022, the General Office of the State Council issued the "Action Plan for the Governance of New Pollutants", systematically arranging and deploying the governance of new pollutants. Minister Huang Runqiu introduced that the Ministry of Ecology and Environment, together with relevant departments, has comprehensively promoted the implementation of new pollutant control work, achieving positive results. First, we have taken stock of the situation. We have screened more than 4,000 chemical substances with high hazards and high environmental detection rates, conducted a preliminary survey of their production and use, and gradually grasped their distribution. Second, we have assessed environmental risks. Chemical substances that have been banned or restricted internationally but are still produced and used in China are prioritized in the assessment plan, and their full life-cycle environmental risk assessments are conducted. Third, we strictly adhere to risk bottom lines. Together with relevant departments, we have issued the "List of Key Controlled New Pollutants (2023 Edition)", implementing control measures such as prohibition, restriction, and emission limits for 14 new pollutants with prominent environmental risks. For example, short-chain chlorinated paraffins used in the production of student backpacks, plastic running tracks, and children's toys will be eliminated by the end of this year. This year, two major categories of perfluorinated compounds of international concern will also be eliminated, and the use of perfluorooctanoic acid will be restricted, prohibiting its use in waterproof and oil-proof clothing, baking paper, gastroscopes, etc. Fourth, we have strengthened supervision and law enforcement. Joint law enforcement inspections were carried out with market supervision departments, and last year 15 illegal production cases of persistent organic pollutants used in building insulation materials were investigated and dealt with. Fifth, we have strengthened guidance and assistance. Currently, 31 provinces and the Xinjiang Production and Construction Corps have issued provincial-level action plans for the governance of new pollutants. Minister Huang Runqiu mentioned that, in the next step, the Ministry of Ecology and Environment, together with relevant departments, will, on the one hand, strengthen technological support, making new pollutant control a key area of national basic research and technological innovation, and focusing on breakthroughs in key core technologies. On the other hand, effectively prevent environmental and health risks from new pollutants, follow the concept of full life-cycle environmental risk management, strengthen the modernization of the new pollutant governance system and capabilities, and make every effort to ensure ecological and environmental safety and the health of the people. 01 What exactly are new pollutants? Currently, there is no authoritative definition of new pollutants internationally or domestically. From the perspective of environmental management, new pollutants generally refer to pollutants that have recently been discovered or brought to attention, pose risks to the ecological environment or human health, have not yet been included in management, or whose risks cannot be effectively controlled by existing management measures. According to the "Action Plan for the Governance of New Pollutants", new pollutants of widespread international and domestic concern mainly include persistent organic pollutants, endocrine disruptors, and antibiotics controlled by international conventions. The main sources of new pollutants are the production and use of toxic and harmful chemicals. Compared with "conventional pollutants" with a relatively complete management system, "new pollutants" mainly refer to substances that are not yet regulated by laws, regulations, and standards, or for which regulations are incomplete. Currently, internationally, the main new pollutants of concern include: endocrine disruptors (EDCs), pharmaceuticals and personal care products (PPCPs), perfluorinated compounds (PFCs), brominated flame retardants (BFRs), drinking water disinfection by-products, antibiotics, and microplastics. In daily life, these substances are not unfamiliar to us: Typical substances of endocrine disruptors include: phthalates, bisphenol A, polybrominated diphenyl ethers, etc. Typical substances of antibiotics include: macrolides, tetracyclines, quinolones, sulfonamides, chloramphenicol, etc. Pharmaceuticals and personal care products (PPCPs): are very common in medicine cabinets, girls' makeup tables, and bathrooms, including more than 5,000 substances such as drugs, diagnostic agents, musk, and opacifiers. Perfluorinated compounds: representative products include perfluorooctanesulfonic acid and perfluorooctanoic acid, widely used in textiles, coatings, leather, synthetic detergents, cookware manufacturing, paper food packaging materials, etc. Microplastics: Microplastics refer to plastic fragments and particles less than 5 mm in diameter. Some microbeads in toothpaste, bath scrubs, and sunscreens contain microplastics. Non-degradable plastics discarded into the natural environment, after a long time of degradation and fragmentation, produce "ubiquitous" microplastics. 02 Characteristics of new pollutants New pollutants have five main characteristics: serious harm, hidden risks, environmental persistence, widespread sources, and complex governance. In addition, new pollutants also have characteristics such as "difficult to degrade, difficult to replace and reduce emissions, and involving multiple and wide-ranging fields". Serious harm: New pollutants have various biotoxicities, manifested in organ toxicity, neurotoxicity, reproductive and developmental toxicity, immunotoxicity, endocrine-disrupting effects, genotoxicity, etc. However, while we must fully understand their harmfulness, we should not be overly panicked. Hidden risks: The short-term harm of most new pollutants is not obvious, and they may have existed in the environment or have been used for many years without being considered harmful substances, but when their harmfulness is discovered, they have already entered environmental media through various pathways. Taking antibiotics as an example, low concentrations of antibiotics that exist in environmental media such as river water and soil, as well as in the human body for a long time, can lead to drug resistance in some microorganisms, which may spread in the environment through respiration, food, drinking water, excretion, agricultural irrigation, etc., thus threatening human health. Environmental persistence: refers to the fact that in the environment, it has a long half-life, and can accumulate through the food web, causing adverse effects on human health and the environment. To determine whether a substance is a POPs, scientific judgment criteria should be established. The judgment criteria recommended by ICCA (International Council of Chemical Associations) include: Persistence criterion: judged by half-life (t1/2), 180d in water bodies, and 360d in sediments and soil; Bioaccumulation criterion: judged by bioconcentration factor, BCF>5000; Criterion for long-distance migration and return to Earth: half-life 2d (in air) and vapor pressure in the range of 0.01~1kpa; Criterion for judging the presence of a substance in remote, low-concentration areas: the mass concentration of the substance in water bodies is greater than 10ng/L. Widespread sources: New pollutants are diverse, involving a wide range of industries, covering industrial production, daily consumption, military fire fighting, and many other fields, as well as numerous industries such as pharmaceuticals, chemicals, agricultural planting, aquaculture, textiles, construction, plastics processing, automobiles, aerospace, electronics and electrical appliances, fire-fighting foam, and garbage incineration. Complex governance: For new pollutants with persistence and bioaccumulation, even if discharged into the environment at low doses, they may harm the environment, organisms, and human health, requiring high levels of governance. New pollutants involve many industries, have long industrial chains, and the development of substitutes and alternative technologies is difficult, requiring cross-departmental and cross-field collaborative governance and implementation of full life-cycle environmental risk control. 03 Dynamically carrying out "screening-evaluation-control" of new pollutants In June this year, the Ministry of Ecology and Environment announced the "Announcement on Environmental Risk Control Requirements for 5 Types of Persistent Organic Pollutants such as Polychlorinated Naphthalenes", making provisions for the elimination or restriction of 5 types of persistent organic pollutants (POPs). To date, China has completely eliminated 23 types of persistent organic pollutants controlled by the Stockholm Convention on Persistent Organic Pollutants, and the emission intensity of dioxins in major industries nationwide has significantly decreased. China's governance of new pollutants has entered the "fast lane". New pollutants discharged into the environment have characteristics such as biotoxicity, environmental persistence, and bioaccumulation, posing significant risks to the ecological environment or human health, but are not yet included in management or existing management measures are insufficient to effectively control their risks. It is understood that most new pollutants have various biotoxicities such as organ toxicity, neurotoxicity, reproductive and developmental toxicity, immunotoxicity, endocrine-disrupting effects, carcinogenicity, and teratogenicity. At the same time, new pollutants can migrate over long distances with air and water flow, and after enrichment through the food chain, they accumulate in organisms for a long time. Liu Guozheng, director of the Solid Waste and Chemical Management Technology Center of the Ministry of Ecology and Environment, said that unlike conventional pollution control, the complexity of new pollutant control lies in the large number of types of toxic and harmful chemical substances, their widespread sources, and hidden environmental risks. When determining the control measures for each new pollutant, it is necessary to closely combine the specificity of environmental risks and find targeted environmental management countermeasures. In May 2022, the General Office of the State Council issued the "Action Plan for the Governance of New Pollutants" (Guobanfa [2022] No. 15), making comprehensive deployments for the governance of new pollutants and clarifying the overall approach to the governance of new pollutants in China. The "Action Plan" takes effective prevention of environmental and health risks from new pollutants as its core, adheres to the working principles of precise pollution control, scientific pollution control, and law-based pollution control, follows the concept of full life-cycle environmental risk management, comprehensively promotes the environmental risk management of new pollutants, and implements investigation and assessment, classified governance, and full-process environmental risk control. The plan proposes to build a "screening, evaluation, and control" system for environmental risk management of toxic and harmful chemical substances, as well as a full-process control system of "prohibition, reduction, and treatment". That is: through the conduct of chemical substance environmental risk screening and assessment, accurately screening and evaluating new pollutants that need to be重点管控, scientifically formulating and legally implementing full-process environmental risk control measures, including prohibition and restriction at the source of production and use, process reduction, and end-of-pipe treatment. Specifically: (1) Conduct investigations and monitoring to assess the environmental risk status of new pollutants. (2) Strictly control the source, preventing the generation of new pollutants. (3) Strengthen process control to reduce the emission of new pollutants. (4) Deepen end-of-pipe treatment to reduce the environmental risks of new pollutants. The "screening-evaluation-control" tiered identification and classified management system for new pollutant governance refers to the relevant units conducting investigations and monitoring, screening out new pollutants with higher environmental risks, scientifically assessing their environmental risks, and then implementing full-process environmental risk control measures for their source prohibition and restriction, process reduction, and end-of-pipe treatment. Liu Guozheng said that since people's understanding of the harm of new pollutants is a continuous process of deepening, the screening of new pollutants and environmental risk assessment need to be continuously promoted. 04 Green substitutes contribute to the governance of new pollutants For many years, China has vigorously developed green substitutes and alternative technologies for new pollutants, effectively supporting the work of new pollutant governance. Taking hexabromocyclododecane (HBCD) as an example, HBCD is a polybrominated white solid substance that can be added as a flame retardant to other materials. Studies have found that HBCD itself is toxic and easily enters the body, and the large-scale and long-term accumulation of HBCD may also increase the risk of cancer. "After repeated research and experimental verification by experts, it was found that methyl octabromodiphenyl ether and brominated SBS (brominated styrene-butadiene-styrene block copolymer) can effectively replace HBCD, and have industrial production conditions and low environmental risks. Enterprises that originally used HBCD to produce insulation boards only need to simply adjust the process parameters to use substitutes for production, and the product quality meets national requirements. Now the production capacity of these two substitutes is continuously expanding, and both production and sales are booming." Liu Qiang, director of the Shandong Provincial Solid Waste and Hazardous Chemicals Pollution Control Center, said. 05 Establishing a technological support system for the governance of new pollutants According to incomplete statistics, after the release of the "Action Plan for the Governance of New Pollutants", about 30 regions across the country have issued local action plans for the governance of new pollutants. The governance of new pollutants is accelerating nationwide. However, it is undeniable that China's governance of new pollutants is still in its infancy, facing difficulties and challenges such as high governance difficulty, high technical complexity, and insufficient scientific understanding. "Faced with new challenges and new prevention and control needs, we should take effective prevention of environmental and health risks from new pollutants as the core, aiming at building a risk assessment and control technology system for new pollutants, establishing a complete risk assessment methodology, and identifying key risk sources, and carry out a series of basic theoretical research and key technology development." Academician Hou Lian, a member of the Chinese Academy of Engineering, emphasized that we should vigorously develop efficient and sensitive new pollutant detection technologies, achieve the identification of new pollutants, research on the list of key controlled new pollutants, conduct research on the biotoxicity and health risk assessment system of new pollutants, develop green and efficient practical removal technologies for new pollutants, and develop and build a smart water network supported by big data technology and covering the transformation and migration system of new pollutants. Academician Jiang Guibin, a member of the Chinese Academy of Sciences and a researcher at the Institute of Eco-Environment, Chinese Academy of Sciences, said that faced with more complex new pollutant environmental problems, China needs to establish a relatively complete technological innovation mechanism to promote the research and development and application of new pollutant governance technologies, improve the effectiveness of new pollutant governance, and ensure the continuous improvement of the ecological environment. For example, developing green substitutes through blockchain and deep learning technologies; developing toxicity testing and prioritization screening systems for new pollutants by combining artificial intelligence and automation technologies, etc.

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2023

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08

【梳理】Multi-effect evaporation, MVR, multi-stage flash evaporation

【梳理】Multi-effect evaporation, MVR, multi-stage flash evaporation

From emission reduction to emission limitation and then to zero emission, wastewater discharge standards are gradually increasing. To achieve "zero emission", the key is to realize the complete recovery of high-salt wastewater, which essentially means separating water and salts in wastewater. Currently, concentration technology, crystallization technology, and technologies that combine these two technologies are widely used to achieve zero emission of high-salt wastewater recovery. Of course, sometimes, depending on the actual situation of high-salt wastewater, pre-treatment technology may need to be added before the main technology to provide better processing conditions for subsequent processes. Concentration, as the core process of resource treatment of high-salt wastewater, is divided into thermal concentration and membrane concentration according to different treatment objects and applicable scopes. Among them, thermal concentration technology is suitable for treating wastewater with high TDS and COD up to hundreds of grams per liter. It concentrates ions in high-salt wastewater by heating, mainly including multi-stage flash MSF, multi-effect evaporation MED, and mechanical vapor recompression evaporation MVR. Multi-stage flash (MSF) Multi-stage flash technology started in the 1950s. It involves heating high-salt wastewater to a certain temperature and then flashing it in a series of containers with gradually decreasing pressure, and then condensing the steam to obtain fresh water. Multi-stage flash MSF, as the earliest distillation technology applied, has mature technology, reliable operation, and is suitable for large-scale applications. However, its thermodynamic efficiency is relatively low, energy consumption is high, and scaling and corrosion of equipment limit the MSF primary steam temperature, affecting the operating cost. This technology will not be described in detail in this article. We will focus on multi-effect evaporation MED and mechanical vapor recompression evaporation MVR. Multi-effect evaporation (MED) 1. Basic principle: The principle of multi-effect evaporation (hereinafter referred to as MED) is to connect multiple evaporators in series. The secondary steam from the previous evaporator is used as the heating steam for the next evaporator, and the heating chamber of the next evaporator is the condenser of the previous evaporator. Three-effect evaporator process flow In a multi-effect evaporation system, fresh steam is only added to the first effect. The secondary steam produced at the top of the previous evaporation tower is directly used as the heating medium for the reboiler of the next evaporation tower. The evaporation towers after the first effect do not need to introduce fresh steam, and the steam at the top of the last effect tower can be used as a low-pressure heat source. Therefore, its biggest advantage is that it can significantly reduce the consumption of fresh steam by repeatedly using the vaporization and condensation of secondary steam. 2. MED evaporator types There are many types of MED evaporators, which are classified into four categories and fifteen types according to evaporation pressure, evaporator type, number of effects, and material flow direction: According to steam pressure: atmospheric evaporation, pressure evaporation, and vacuum evaporation; According to evaporator type: tube evaporator, plate evaporator, and tube-plate combined evaporator; According to the number of effects: two-effect, three-effect, four-effect, five-effect, and six-effect evaporation; According to material flow: parallel flow, countercurrent flow, mixed flow, and cross flow. So, how should MED evaporators be selected? Three principles: 1. Countercurrent and mixed flow are better than parallel flow systems. Countercurrent multi-effect evaporation has the lowest energy consumption, and parallel flow multi-effect evaporation has the highest energy consumption; the characteristics of mixed flow multi-effect evaporation systems are relatively better than parallel flow multi-effect evaporation systems. 2. The number of evaporation effects is not always better. When the number of effects increases, the efficiency of heat utilization also decreases. Considering that the increase in the number of effects will increase the investment in equipment, there should be an optimal point for the actual number of effects used. For example, for some high-boiling point systems, only two-effect or three-effect evaporators can be used. 3. Consider factors such as material properties, heat balance, and non-condensable gas retention to select the evaporation pressure. Studies have shown that the pressure of each effect is related not only to the material and heat balance of the evaporator but also to the properties of the material and the degree of throttling of non-condensable gases at the top and bottom of each effect. 3. Advantages and disadvantages of MED 1. The advantages of MED are mainly reflected in the following 5 aspects: Simple pretreatment, less chemical reagent consumption, only scale inhibitors need to be added. Short heating time, mostly using double-sided heat transfer method of inner tube condensation and outer tube boiling, small heat transfer area, high heat transfer coefficient. Large operating flexibility, the system can provide 40%~110% of the design value of product water, while multi-stage flash and reverse osmosis do not have such large operating flexibility. Good treatment effect, the salt analysis is thorough during the treatment process, and after cooling, more than 90% of the salt in the coolant can be removed, making it difficult for microorganisms to be inhibited by salt. High operational reliability, the whole process uses fully automated operation, and the pressure inside the tube is greater than the pressure outside the tube during operation. Even if corrosion of the heat exchange tube occurs, the cooling water will not contaminate the product water. 2. The disadvantages of MED are mainly reflected in the following 3 aspects: Scaling inside the tube, cleaning is required every 10 days, and timely descaling treatment is needed. Increased number of effects, low steam utilization rate. When the number of effects increases, the heat transfer temperature difference loss of each effect increases. For example, the ratio of steam consumption per ton of water evaporated is 1.1 for one effect, 0.57 for two effects, 0.4 for three effects, 0.3 for four effects, and 0.27 for five effects, and the production capacity of the equipment decreases. 4. Three common technical problems and countermeasures of multi-effect evaporation MED Generally speaking, multi-effect evaporation MED often encounters three major problems: foaming in the device, scaling of the evaporator, and corrosion of the equipment by salt ions in the last effect steam. 1. For the foaming problem in the device, the solution is: Physical defoaming mainly includes high-temperature and low-temperature defoaming methods, ultrasonic defoaming methods, liquid spraying defoaming methods, and mechanical vibration methods. Although physical defoaming is effective when the processing volume is particularly large, its equipment and operating costs are high; Chemical defoaming mainly refers to the use of defoamers, but its use is affected by the high price of defoamers, high production cost, and complex production process; Mechanical defoaming mainly uses rotation to change the pressure and shear force acting on the bubbles to achieve defoaming. Because of its low cost and good defoaming effect, it is currently more popular. 2. For the scaling problem of the evaporator, the solution is: Researchers have conducted acid washing and neutral cleaning on the scale samples (sodium sulfate and calcium carbonate) on the outer wall of the evaporator and acid washing on the scale samples (calcium carbonate) on the inner wall of the last effect heat exchanger. The hanging plate analysis found that the average corrosion rate of each effect hanging plate is less than 1 g/m2·h, and the total corrosion amount is less than 10 g/m2. It is worth mentioning that this method is superior to the "Quality Standard for Chemical Cleaning of Industrial Equipment" (HG/T2387-2007) and the "Standard for Preparation, Cleaning, and Evaluation of Corrosion Samples". 3. For the problem of equipment corrosion by salt ions in the last effect steam, the solution is: Low-chloride ion content condensate water can be used for low-temperature, timed, and quantitative replacement and supplementation, and high-efficiency corrosion inhibitors can be added to the circulating water. Mechanical vapor recompression evaporation (MVR) 1. Basic principle Mechanical vapor recompression technology (hereinafter referred to as MVR) is an energy-saving technology that uses the secondary steam and its energy generated by the evaporation system itself to raise the low-grade steam to high-grade steam heat source through the mechanical work of the compressor. In this way, it circulates to provide thermal energy to the evaporation system, thereby reducing the demand for external energy. MVR process flow In this system, the heat source in the preheating stage is provided by the steam generator until the material starts to evaporate and produce steam. The secondary steam produced by heating the material is compressed by the compressor into high-temperature and high-pressure steam. The high-temperature and high-pressure steam produced here is used as the heating heat source. The material in the evaporation chamber is continuously evaporated by heating, and the high-temperature and high-pressure steam passing through the compressor is cooled into condensate water through continuous heat exchange, which is the treated water. The compressor, as the heat source of the entire system, realizes the conversion of electrical energy into thermal energy, avoiding the dependence and intake of live steam from the outside world. 2. MVR system equipment composition It is not difficult to see from the MVR evaporation process flow that the MVR evaporation system is composed of various equipment connected in series. The equipment must be cleverly matched in terms of thermodynamics and heat transfer to achieve the best effect for the entire system. The main equipment in the system includes the following 4: 1. Compressor. There are two types of MVR compressors: Roots compressors and centrifugal compressors. Roots blowers are often used to compress small-flow steam. They are volumetric compressors, which provide small air volume, large temperature rise, and are suitable for materials with small evaporation capacity and large boiling point elevation. Centrifugal compressors are pressure-difference blowers, which provide small pressure difference, large flow rate, small temperature rise, uniform exhaust, and no pulse in the airflow. They are suitable for materials with large evaporation capacity and small boiling point elevation. In general, the stability of centrifugal compressors is better than that of Roots compressors, but centrifugal compressors sometimes experience surge, which can lead to instability of the compressor. 2. Evaporator. The types of evaporation treatment devices are generally divided into rising film evaporation and falling film evaporation. They are mainly selected according to the characteristics and energy consumption of the treated material. Currently, falling film evaporation is mainly used in China. 3. Heat exchanger. In the MVR heat pump evaporation process, the heat exchangers used are mostly wall-type heat exchangers. In this type of heat exchanger, the cold and hot fluids do not directly contact but exchange heat through the wall. Commonly used wall-type heat exchangers in production include: tube-type heat exchangers, corrugated heat exchangers, and spiral heat exchangers. 4. Gas-liquid separator. The gas-liquid separator is the place where the material and secondary steam are separated. Its main function is to aggregate the solution in the mist into droplets and separate the droplets from the secondary steam. It is worth mentioning that the design of the separator should fully consider factors such as evaporation capacity, evaporation temperature, material viscosity, and separator liquid level. 3. Technical advantages of MVR 1. Compared with traditional evaporation systems, the MVR system only needs to introduce live steam as a heat source at startup. When secondary steam is generated and the system is stable, it does not need an external heat source. The energy consumption of the system is only the energy consumption of the compressor and various pumps, so the energy-saving effect is quite significant. 2. The energy consumption of the MVR evaporator system is mainly the electricity consumption of the compressor, the operating cost is greatly reduced, the maintenance cost is low, and because the system does not require industrial steam, the safety risks are lower, and the operation is simple. 3. Under the same evaporation treatment capacity, the floor area required by the MVR evaporator is much smaller than that of traditional multi-effect evaporation equipment. 4. Common technical problems in MVR treatment of high-salt wastewater Although MVR technology has played a good role in the treatment of high-salt wastewater, some technical problems still affect the operation effect. 1. System scaling problem Scaling on the heat exchanger wall is one of the main reasons for the decrease in system evaporation efficiency. This is mainly because the heating heat source uses secondary steam, and scaling and coking will reduce the heat transfer effect, reducing the evaporation capacity per unit time, which will further affect the available amount of compressed secondary steam and have a greater impact on production capacity. Due to the particularity of the MVR evaporator, it is common that the equipment cannot be cleaned on time, which is one of the reasons for the instability of production capacity. 2. Temperature rise problem The temperature rise problem in the MVR system is an important factor affecting its application in the treatment of saline wastewater. When MVR technology is used to treat high-concentration saline wastewater, due to its high concentration and large boiling point elevation, the corresponding steam compressor needs to increase a higher temperature to overcome the impact of the boiling point elevation, which puts higher requirements on the compressor and significantly increases the system energy consumption. Studies have shown that when using MVR evaporation technology, the reasonable temperature rise range is 8℃～20℃. If the boiling point elevation exceeds 18℃, the MVR technology will lose its advantage. 3. Material properties matching problem for MVR Due to the different sources of industrial wastewater, MVR needs to be selected according to the properties of different materials. Material property analysis mainly includes: the components contained in the material; whether crystallization precipitates during the evaporation process; the viscosity, specific heat, density, and boiling point elevation of the material. The parameters of a single material can be obtained by consulting relevant tables, but industrial high-salt wastewater is mostly a mixed liquid, and its relevant data can only be estimated by simulation. Therefore, accurately analyzing and calculating the material properties is the key factor to ensure the normal operation of the MVR device. Generally speaking, For materials with a large increase in boiling point temperature, single-effect MVR evaporation is generally used; High-concentration materials need to use forced circulation to prevent the material flow rate from being too slow and causing coking; Heat-sensitive materials require that the residence time in the evaporator be as short as possible. In summary, current evaporation technologies are widely used, but they also have problems such as high energy consumption, high operating costs, and easy scaling and blockage. Therefore, high efficiency and energy saving must be considered, and multi-effect evaporation MED and mechanical vapor recompression evaporation MVR are recommended high-efficiency energy-saving technologies. Among them, the initial investment of the MVR evaporation device is relatively large, and the energy consumption is low. However, with the continuous improvement of technology and production processes of domestic steam compressors, the price is also continuously decreasing; the number of effects of multi-effect evaporation devices increases, so the investment in multi-effect evaporation devices will also increase, but energy consumption can also be reduced to a certain extent. Therefore, both MVR evaporation devices and multi-effect evaporation devices have certain relative advantages, and multi-faceted comparisons should be made based on applicability, investment, operation, consumption, labor, and land occupation.

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2023

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08

Triple-effect evaporator composition principle and application in high-salt wastewater treatment

Triple-effect evaporator composition principle and application in high-salt wastewater treatment

Evaporation is one of the modern unit operations in chemical engineering. It involves using heat to vaporize and remove part of the solvent in a solution to increase the solution's concentration or create conditions for solute precipitation. The triple-effect evaporator desalination method uses a concentration crystallization system to remove inorganic salts from wastewater through evaporation. Triple-effect evaporators consist of three evaporators connected in series. Low-temperature (around 90℃) heating steam is introduced into the first effect, heating the wastewater inside. The generated steam is then introduced into the second effect as heating steam, causing the wastewater in the second effect to evaporate at a lower temperature than the first effect. This process is repeated until the last effect. The condensate from the first effect returns to the heat source, while the condensate from the other effects is collected and output as desalinated water. One unit of steam input can evaporate multiple times the amount of water. Simultaneously, the high-salt wastewater is concentrated successively from the first effect to the last effect. In the last effect, it reaches supersaturation and crystallizes, achieving solid-liquid separation of salt and wastewater. In the treatment of saline wastewater, the saline wastewater enters a triple-effect concentration crystallization device. Through the triple-effect evaporation and condensation concentration crystallization process, it is separated into desalinated water (desalinated water may contain trace amounts of low-boiling-point organic matter) and concentrated slurry wastewater; inorganic salts and some organic matter can be crystallized and separated, incinerated to produce inorganic salt waste residue; organic matter that cannot be crystallized is concentrated into wastewater and can be treated using a rotary evaporator to form solid waste residue, which is then incinerated; desalinated water can be returned to the production system to replace softened water. The triple-effect evaporator desalination method has the advantages of mature technology, a wide range of wastewater treatment, small footprint, fast processing speed, and energy saving. With the development of the chemical industry, more and more high-salt wastewater needs to be treated, and the application of the triple-effect evaporator desalination method will become increasingly widespread. Triple-effect Evaporator 1. Application Range of Triple-effect Evaporators Triple-effect evaporators can be used to treat high-salt wastewater generated during the production processes of chemical production, food processing plants, pharmaceutical production, and oil and natural gas collection and processing enterprises. Suitable wastewater has a salt content of 3.5%～25% (mass percentage) and a COD concentration of 2000～10,000ppm. 2. Composition and Principle of Triple-effect Evaporators Triple-effect evaporators mainly consist of three evaporators connected in series, a condenser, a salt separator, and auxiliary equipment (as shown in the figure). The three evaporators operate in series to form a triple-effect evaporator. The entire evaporation system uses a continuous feed and continuous discharge production method. High-salt wastewater first enters a single-effect forced circulation crystallization evaporator. The crystallization evaporator is equipped with a circulation pump that pumps the wastewater into the evaporation heat exchange chamber. In the evaporation heat exchange chamber, externally connected steam liquefies to generate latent heat of vaporization, heating the wastewater. Because the pressure in the evaporation heat exchange chamber is relatively high, the wastewater is heated to superheat at a pressure higher than the normal boiling point of the liquid. After the heated liquid enters the crystallization evaporation chamber, the pressure of the wastewater rapidly decreases, causing some of the wastewater to flash evaporate or boil rapidly. The steam from the evaporated wastewater enters the second-effect forced circulation evaporator as power steam to heat the second-effect evaporator. The unevaporated wastewater and salts remain temporarily in the crystallization evaporation chamber. The first, second, and third-effect forced circulation evaporators are connected by balancing pipes. Under the action of negative pressure, the high-salt wastewater flows successively from the first effect to the second and third effects. The wastewater is continuously evaporated, and the salt concentration in the wastewater increases. When the salt content in the wastewater exceeds saturation, the salt in the water continuously precipitates into the salt collection chamber at the bottom of the evaporation crystallization chamber. The salt suction pump continuously sends the salt-containing wastewater to the vortex salt separator. In the vortex salt separator, the solid salt is separated and enters the salt storage tank. The separated wastewater enters the second-effect forced circulation evaporator for heating. The entire process is repeated to achieve the final separation of water and salt. The condenser is connected to a vacuum system that removes the uncondensed gases generated in the evaporation system, maintaining a negative pressure in the condenser and evaporator to improve the evaporation efficiency of the evaporation system. Under the action of negative pressure, the secondary steam generated by the wastewater in the third-effect forced circulation evaporator automatically enters the condenser, and under the cooling of the circulating cooling water, the secondary steam generated by the wastewater quickly turns into condensate. The condensate can be continuously discharged and recovered to the recycled water tank. Application Examples of Triple-effect Evaporators 1. Treatment Object and Process The main component of the high-salt wastewater is a 15% sodium chloride solution. The wastewater pH is 6～8, and the wastewater COD is 50,000ppm. The treatment capacity is 3t/h. Based on the characteristics of the high-salt wastewater, the process design is based on a triple-effect evaporator. According to the calculation, the main technical parameters of the triple-effect evaporator are determined as follows: Evaporation amount Q=3000kg/h (3000kg of water evaporated per hour); Actual steam consumption Q=1200kg/h (inlet pressure 0.3～0.4MPa); First-effect evaporator heat transfer area S=80m2, vacuum degree P=-0.03MPa; Second-effect evaporator heat transfer area S=80m2, vacuum degree P=-0.06MPa; Third-effect evaporator heat transfer area S=80m2, vacuum degree P=-0.085MPa; Circulating cooling water consumption Q=40t/h; Condensation cooling area A=240m2; Total unit power P=25kW; Unit footprint is 10m long × 5m wide × 4m high. According to the process, fully considering the corrosiveness of the wastewater, and based on the principle of saving costs while meeting usability, the system equipment materials are selected as follows: 1) The evaporator body is made of carbon steel with heavy corrosion protection, which can withstand the corrosion of acid, alkali, and salt solutions below 120℃; 2) The heater uses Ta1 titanium tubes; 3) The condenser tube bundle uses 316L stainless steel; 4) The discharge screw pump uses 316L stainless steel; 5) The recycled water tank and flash tank use carbon steel with spray-coated anti-corrosion paint; 6) Process pipes, fittings, and valves use 316L stainless steel + PPR materials; 7) The crystallization tank uses carbon steel with heavy corrosion protection. 2. Treatment Results and Existing Problems After treatment with a triple-effect evaporator, the high-salt wastewater produces crystallized salt, concentrated organic wastewater, and desalinated water. The crystallized salt and concentrated organic wastewater are sent to a hazardous waste disposal center for centralized incineration, and the desalinated water is reused in production. The operation of this system has revealed that although the triple-effect evaporator can effectively treat high-salt wastewater, there are still some problems that need to be overcome, mainly: a. High wastewater treatment cost. Because the wastewater to be treated is mostly corrosive, the selection of equipment materials needs to consider corrosion resistance, resulting in higher costs. b. Severe corrosion and short lifespan during the operation of the entire equipment. Although corrosion-resistant materials are used as much as possible in the design of the triple-effect evaporator, corrosion cannot be completely avoided, and the equipment has a relatively short lifespan and needs to be replaced promptly. c. The treatment of high-salt wastewater using a triple-effect evaporator requires a large amount of steam, which is not feasible in many places. d. High-salt wastewater treated by a triple-effect evaporator still needs to be sent to a hazardous waste disposal center for further treatment. Conclusion High-salt wastewater must be properly treated before it can be returned to the environment. Practice has proven that traditional wastewater treatment methods are not suitable for treating high-salt wastewater. Among the many high-salt wastewater treatment technologies, the triple-effect evaporator desalination method has the advantages of mature technology, a wide range of wastewater treatment, small footprint, fast processing speed, and energy saving, and has great development prospects in China. Although triple-effect evaporators have drawbacks such as high treatment costs, short equipment lifespan, and high steam requirements, with further technological development, the application of this technology in the field of high-salt wastewater treatment will further expand.

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