Application of multi-effect evaporation technology in the treatment of high-salinity wastewater

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Multi-effect evaporation processors are primarily used to treat industrial wastewater with high concentrations, high color intensity, and high salt content. At the same time, recover by-products generated during the treatment of wastewater. Low steam consumption, low evaporation temperature, high concentration ratio – more rational, more energy-efficient, and more efficient. What is high-salt wastewater: High-salt wastewater refers to wastewater with a total salt content of at least 1%, and it mainly originates from chemical plants as well as activities related to the extraction and processing of oil and gas. This type of wastewater contains various substances (including salts, oils, organic heavy metals, and radioactive materials). However, high-salt wastewater differs from other types of wastewater in that the composition of these wastewaters does not vary much; the salts present are mainly ions such as Cl-, SO42-, Na+, and Ca2+. Although these ions all provide a favorable growth environment for microorganisms, if their concentrations are too high, they can inhibit microbial growth and have toxic effects on them. It is mainly manifested by high salt concentration, high osmotic pressure, and the sharpness of cellular protoplasm resulting from dehydration of microbial cells ； Salting out reduces dehydrogenase activity ； High chloride levels are toxic to bacteria ； A high salt concentration increases the density of the wastewater, causing the activated sludge to float to the surface and be lost, which severely affects the purification efficiency of the biological treatment system. Treatment methods for high-salt wastewater: To deal with high-salt wastewater, approaches are taken from both physicochemical and biological perspectives. The main treatment methods include concentration and evaporation, membrane osmosis desalination, electrochemical desalination, and biochemical treatment using salt-tolerant bacteria. 1. Concentration and evaporation treatment method. Advantages: High processing capacity, low requirements for water quality ； Disadvantage: High operating costs. 2. Membrane osmosis treatment method. Advantages: Simple principle, suitable only for the treatment of small amounts of high-salt wastewater ； Disadvantages: The equipment is delicate, prone to clogging and contamination, and unable to handle large amounts of wastewater. 3. Electrodesalination method. Advantages: Simple principle, suitable only for the treatment of small amounts of high-salt wastewater ； Disadvantage: It can only handle salts present in wastewater. 4. Biochemical treatment using salt-tolerant bacteria. Advantages: Lower cost, average performance ； Disadvantages: It has high requirements for water quality and is greatly affected by organic matter in wastewater. With current technology, only the concentration and evaporation method can ideally treat high-salinity wastewater. Technical characteristics of multi-effect evaporation: Multi-effect evaporation is the earliest developed technology for seawater desalination; it has now evolved into a relatively mature technique for evaporating wastewater. It overcomes the problem of severe scaling and is gradually being applied to the treatment of high-salinity water. The multi-effect process has the following technical characteristics: 1. The heat transfer process in multi-effect evaporation involves boiling and condensation, which are types of two-phase heat transfer; as a result, the heat transfer coefficient is very high. For the same temperature range, the heat transfer area required for multi-effect evaporation is smaller than that for multi-stage flash evaporation. 2. Multi-effect evaporation requires less power consumption. Since multi-stage flash evaporation relies on the sensible heat absorbed by brine to produce fresh water, and the latent heat is much greater than the sensible heat, producing the same amount of fresh water requires a much larger circulation volume in multi-stage flash evaporation compared to multi-effect evaporation; therefore, it demands more energy consumption. 3. Multi-effect evaporation has a large operating flexibility; it can operate normally within a load range of 110% to 40%, without this leading to a decrease in the water production ratio. The process flow for saline wastewater involves the saline water first entering a condenser for preheating and degassing, after which it is divided into two streams. One stream is returned to the sea as cooling water, while the other serves as feed for the distillation process. The feed brine, to which scale inhibitor has been added, is introduced into the latter stages of the evaporator. The liquid feed is evenly distributed over the top tubes of the evaporator via nozzles, and then flows downward along these top tubes in the form of a thin film; part of the water evaporates by absorbing the latent heat of the condensed steam inside the tubes. The secondary steam condenses into product water in the next stage, while the remaining liquid is pumped to the next stage of the evaporator, where the operating temperature is slightly higher than that of the previous stage; the process of spraying, evaporation, and condensation is repeated in this new stage. The remaining liquid is pumped to the high-temperature stage, and finally exits the device in the form of a concentrate in the stage with the highest temperature. Saturated steam is fed into the evaporation tubes of the first effect where it condenses; meanwhile, the brine outside the tubes generates secondary steam in an amount roughly equal to that of the condensed steam. Since the operating pressure of the second effect is lower than that of the first effect, the secondary steam enters the heat transfer tubes of the next effect after passing through the vapor-liquid separator. The evaporation and condensation processes are repeated in each stage, with essentially equal amounts of distilled water being produced in each stage; the steam from the final stage is condensed in a condenser by brine. The condensate from the first effect returns to the steam generator, while the condensate from the remaining effects goes into the product water tanks; these product water tanks are connected to each other. Due to the different pressures in each effect, the product water flashes off, returning heat to the evaporator. In this way, the product water flows in a stepped manner and is flash-cooled stage by stage, and the recovered heat can improve the overall efficiency of the system. The cooled product water is pumped to the product water storage tank by the product water pump. The product water produced in this way is pure water with an average salt content of less than 5mg/L. The concentrated brine flows in a stepped manner from the first effect into a series of concentrated brine flash tanks, where the superheated concentrated brine is flashed to recover its heat. After flash cooling, the concentrated brine is finally pumped back into the sea by a concentrated brine pump. Non-condensable gases accumulate in the condenser and are removed by a vacuum pump. The technical advantages of low-temperature multi-effect evaporation can be seen from its aforementioned principles. These advantages include the following: 1. Due to the low operating temperature, corrosion and scaling of the equipment can be avoided or reduced. 2. Due to the low operating temperature, it is possible to make full use of the low-temperature waste heat from power plants and chemical plants. For low-temperature multi-effect evaporation technology, steam with a low quality grade of 50°C–70°C can serve as an ideal heat source, which helps to **reduce the impact of drawing backpressure steam on power generation in power plants. 3. The pretreatment of saline-containing feedwater is simpler. Another major advantage of operating the system at low temperatures is that it **simplifies the pretreatment process of saline water**. Before entering the low-temperature multi-effect device, brine only needs to be filtered through a screen and a small amount of scale inhibitor added, whereas acid degassing is required in multi-stage flash evaporation. 4. The system has high operational flexibility. During peak periods, this desalination system can provide product water at 110% of the designed capacity ； During low-demand periods, this desalination system can steadily supply 40% of the rated amount of product water. 5. The system has low power consumption. The power consumption of low-temperature multi-effect systems for transporting liquids is very low, at around 0.9–1.2 kWh/m3. This can **reduce the cost of producing desalinated water, which is particularly important in areas with high electricity prices. 6. The system has a high thermal efficiency. A temperature difference of over 30 degrees allows for a heat transfer coefficient of 12 or more, thereby achieving a water production ratio of around 10. 7. The system is safe and reliable to operate. In a low-temperature multi-effect system, vapor condenses inside the tubes while the liquid film outside the tubes evaporates. Even if the heat transfer tubes become corroded and punctured, allowing leakage to occur, the concentrated brine will not flow into the product water because the pressure on the vapor side is higher than that on the liquid film side; at most, only a small amount of vapor will leak, which affects the amount of water produced. Refining and chemical enterprises possess large amounts of low-temperature waste heat that can be utilized. Fresh water produced through low-temperature multi-effect evaporation technology can be reused in various processes, such as for replenishing circulating water. This approach enables the resourceful use of wastewater while also facilitating the efficient utilization of low-temperature waste heat. Therefore, by introducing low-temperature multi-effect evaporation technology into the water treatment sector of petrochemical enterprises, and taking advantage of its high water production ratio and good water treatment quality, it is possible to combine the utilization of low-temperature waste heat with advanced treatment of petrochemical wastewater. This approach helps to address issues such as the difficulty in desalinating wastewater with high salt content and the high energy consumption associated with such treatment. In the multi-effect evaporation process, the counter-current flow pattern features the same direction of flow for both the solution and steam, moving sequentially from the first effect to the last effect. The feed liquid is pumped into the first stage; due to the pressure difference between stages, it flows automatically to the next stage for further processing (a transfer pump is required if solids are formed or the solution’s viscosity is high during concentration). The finished liquid is pumped out from the last stage. The pressure in the latter effect is low, and the boiling point of the solution is relatively low as well; therefore, as the solution moves from the preceding effect to the latter one, it evaporates on its own due to superheating, a phenomenon known as flashing. As a result, the latter effect may generate more secondary steam than the former effect; however, because the concentration in the latter effect is higher and the operating temperature is lower, its heat transfer coefficient is lower than that of the former effect. Often, the heat transfer coefficient of the first effect is much higher than that of the last effect. The co-current process is suitable for processing materials that are thermosensitive at high concentrations. In the counter-current feeding process, the feed liquid is added at the last stage and pumped to the previous stage in one step; the finished liquid is discharged from the first stage, with the feed liquid and steam flowing in opposite directions. As the solvent evaporates and the solution concentration gradually increases, the evaporation temperature of the solution also rises accordingly; as a result, the concentrations of the solutions in each stage are relatively similar, which in turn leads to similar heat transfer coefficients across these stages. However, when the solution is transferred from the subsequent stage to the preceding stage, the temperature of the liquid is lower than the boiling point of that stage; therefore, heating may be necessary at times, otherwise the amount of secondary steam generated will gradually decrease. Generally, the counter-current feeding process is suitable for processing materials whose viscosity varies significantly with temperature and concentration, but it is not suitable for processing heat-sensitive materials. In the cross-flow feeding process, the feed liquid is introduced into each stage, and the finished liquid is drawn out from each stage as well. This process is used for the evaporation of saturated solutions (or solutions with high concentrations). Crystals precipitate in each effect, allowing for their timely separation. This method can also be used to concentrate two or more aqueous solutions simultaneously. The counterflow feeding process is also known as the mixed-flow process. It is a combination of parallel and counterflow processes. The characteristic of cross-flow is that it combines the advantages of co-current and counter-current flow while avoiding their disadvantages. However, it is complex to operate, and sophisticated automatic control instruments are required to ensure its stable operation. Conditions for choosing the co-current process: the wastewater feed has a low viscosity, does not contain large amounts of substances with low boiling points, there is no need to use the counter-current mode for preliminary condensation, and this approach does not affect the heat transfer coefficient. Secondly, the salt concentration in the wastewater feed is not high; the co-current feeding mode is only selected when the concentration is extremely high.