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Introduction to seawater desalination technology: Seawater desalination refers to the process of converting seawater into fresh water through desalination treatment. Broadly speaking, they are divided into two main categories: distillation methods and membrane methods. The most commonly used distillation methods include Multi-Stage Flash (MSF), Multi-Effect Distillation (MED), and Vapor Compression (VC) ； Membrane processes include Electro dialysis (ED) and Reverse Osmosis (RO). The principles of various desalination technologies, analyses of their advantages and disadvantages, and their applicable ranges are described as follows: 1. Seawater desalination by distillation 1.1 Multi-effect distillation technology (MED) 1.1.1 Technical overview Based on the highest boiling temperature in multi-effect distillation, it can be divided into low-temperature multi-effect distillation and high-temperature multi-effect distillation. High-temperature multi-effect distillation allows for more heat transfer stages to be arranged, thereby achieving a higher water production rate and thus higher thermal efficiency. However, due to the relatively high evaporation temperature of the brine in the first few stages, scaling tends to form on the surface of the heat transfer tubes, corrosion occurs rapidly, high material standards are required for the equipment, frequent cleaning of the equipment is necessary, and high requirements are also placed on the pretreatment process. To address the drawbacks of high-temperature multi-effect distillation, low-temperature multi-effect distillation technology was developed. Its characteristic is that the evaporation temperature of the brine does not exceed 70°C, which reduces equipment corrosion and scaling problems. Additionally, thanks to the use of inexpensive heat transfer materials, a larger heat transfer area can be accommodated within the same investment level, allowing for a high water production ratio even at low operating temperatures (around 10). 1.1.2 Main advantages and disadvantages of low-temperature multi-effect distillation: High thermal efficiency; a temperature difference of just over 30 degrees is sufficient to achieve more than 12 heat transfer stages, resulting in a water production ratio of around 10 ； It offers high operational flexibility; it can operate normally within a load range of 110% to 40%, without causing a decrease in the water production ratio ； Low power consumption, ranging from 0.9 to 1.2 kWh per ton of water (excluding seawater extraction) ； The pretreatment process is simpler than multi-stage flash evaporation and reverse osmosis; since the acid addition and degassing steps are omitted, the consumption of chemical reagents is much lower than that in multi-stage flash evaporation ； The system is safe and reliable to operate; even in the event of a leak in the heat transfer tubes, only a slight reduction in production occurs, without any impact on water quality. Compared to multi-stage flash evaporation, it is possible to use steam at lower temperatures, which helps improve the thermal efficiency of the process. During low-temperature operation, the specific volume of steam is high, resulting in larger equipment size and thus higher costs ； Due to high-temperature and high-salinity treatments, the corrosion resistance requirements for equipment materials increase; even at a water production ratio of around 10, approximately 0.1 ton of steam is still required to produce one ton of fresh water, resulting in high production costs. 1.1.3 Scope of application: Large-scale seawater desalination projects (combined heat and power projects) that utilize the low-temperature heat source from the backpressure turbines in thermal power plants or nuclear power plants ； Steam turbines use low-pressure extraction steam to produce high-quality water, which is used to supply water to high and medium pressure boilers ； Producing high-quality fresh water using waste heat from factories ； Desalination projects with raw water concentrations higher than seawater ； Combining it with reverse osmosis to create a hybrid process helps reduce water costs, etc. 1.2 Multi-stage Flashing Method (MSF) 1.2.1 Basic Principle Multi-stage flashing is short for multi-stage flash distillation. Flash distillation involves heating seawater to a certain temperature and then introducing it into a flash chamber. The pressure in this chamber is kept below the saturated vapor pressure corresponding to the temperature of the seawater. When the hot seawater enters the chamber, some of the water with a lower temperature absorbs the excess heat as latent heat of vaporization, resulting in rapid evaporation; meanwhile, the temperature of the hot seawater drops. The vapor that forms condenses to yield the desired fresh water. Multi-stage flash evaporation is based on this principle: multiple flash chambers are connected in series to form a multi-stage system, allowing seawater to flow through several chambers with gradually decreasing pressures, thereby undergoing evaporation and cooling step by step until it reaches the lowest temperature at the final stage. The condensate from each stage is collected as a product. 1.2.2 Main advantages and disadvantages: Large capacity per unit, with a maximum of 50,000 tons per day ； The water salinity of the product is low, generally ranging from 3 to 10 mg/l. However, its engineering cost is high, being twice that of the reverse osmosis method ； High power consumption, with a value of 3.5–4.5 kWh per ton of water ； The equipment has limited operational flexibility; this flexibility ranges from 80% to 110% of its designed value, making it unsuitable for applications where the water production volume needs to vary ； When the heat transfer tubes are corroded, it will contaminate the water quality. 1.2.3 Scope of application The applicable conditions for multi-stage flash evaporation and multi-effect distillation are basically the same; under the same water production ratio, multi-stage flash evaporation requires a hotter heat source, and its power consumption per ton of water is higher than that of multi-effect distillation. 1.3 Vapor Compression Distillation Technology (VC) 1.3.1 Basic Principle Unlike the first two technologies, which directly reuse the secondary steam generated during the evaporation process, this technology uses a mechanical compressor to slightly compress this steam, increasing its pressure before feeding it back into the system. An increase in steam pressure corresponds to an increase in its saturation temperature; thus, once it is fed into the system, it can be used as a heating source, thereby creating a closed-loop cycle. In addition to the steam compression cycle mentioned above, vacuum distillation also features a heat recovery cycle. In this circuit, the feed seawater, after appropriate pretreatment, is divided into two streams. The first stream of liquid is fed into the brine heat exchanger, where it lowers the temperature of the brine from its boiling point to a level slightly above that of the feed water, after which it is discharged ； Another stream of fluid enters the product water heat exchanger, reducing the temperature of the product water from the condensation temperature of the steam to a level slightly above the temperature of the feed water, before it is discharged into the product water storage tank. The two streams of fluid recover their heat during heat exchange with the concentrated brine and product water, simultaneously raising their own temperature to just below the boiling point before entering the vapor condenser. There, the seawater exchanges heat with the steam carried away by the non-condensable gases, raising its temperature to the evaporation temperature of the brine; thereafter, it enters the evaporator together with the recycled concentrated brine to produce fresh water. 1.3.2 Main advantages and disadvantages: The low operating temperature reduces the heat dissipation of the system, lowers heat losses, and decreases the system’s energy consumption. The power consumption for low-temperature vacuum distillation is only 6–11 KWh/m3, depending on the scale of the installation; this makes it competitive in areas with high fuel prices ； Due to the low evaporation temperature, corrosion and scaling of the heat transfer surfaces are avoided and reduced, thereby extending the operational cycle and service life of the system ； Due to the low corrosion rate, inexpensive heat transfer materials can be used, reducing the cost of the device ； The total salt content in the product water is less than 5 mg/l, making post-treatment very simple ； Low-temperature vacuum distillation comes with its own packaging; it is pre-installed, wired, and tested in the workshop before being delivered for transportation, resulting in very little on-site work required. Negative pressure distillation operates under vacuum conditions and requires a high level of sealing for the system ； The heat transfer coefficient of low-temperature distillation is low, and compared to atmospheric pressure distillation, vacuum distillation requires a larger evaporation area. 1.3.3 Scope of application: Primarily for small and medium-sized applications, suitable for areas with limited water sources and difficult steam supply. 3.2 Membrane-based seawater desalination technology 3.2.1 Electrodialysis technology (ED) 3.2.1.1 Basic principle The process in which ions migrate through selective ion exchange membranes under the influence of a direct current field, thereby allowing the electrolyte ions to be partially separated from the solution, is known as electrodialysis. 3.2.2 Reverse Osmosis Technology (RO) 3.2.2.1 Basic Principle A semi-permeable membrane that allows only water to pass through while preventing salts from doing so is used to separate fresh water from saltwater; fresh water naturally passes through this membrane to the side containing saltwater, and this phenomenon is known as osmosis. When the liquid level on the saltwater side reaches a certain height, the natural tendency of osmosis is counteracted by this pressure, thus achieving equilibrium. This equilibrium pressure is the osmotic pressure of the system; if a pressure greater than the osmotic pressure is applied to the saline side, the water in the saline will pass through the semipermeable membrane to the fresh water side, thereby achieving desalination. 3.2.2.2 Main advantages and disadvantages: Reverse osmosis is a process without phase change, thus it requires low energy consumption; by employing energy recovery techniques, the energy consumption per ton of fresh water can be reduced to below 3.0 kWh ； It has a short construction period, allows for modular design, offers flexible unit sizes, and requires lower investment costs compared to distillation methods ； The device is compact and occupies less space ； It is simple to operate, flexible, has a short startup time, allows for the adjustment of water flow volume as needed, and is easy to maintain. The disadvantage is that the pretreatment requirements for reverse osmosis are strict; the reverse osmosis membranes need to be replaced regularly. When the seawater temperature is low, to maintain a constant water production rate, it is necessary to increase the number of reverse osmosis membranes or use heat exchangers to utilize waste heat, which in turn increases the cost. 3.2.2.3 Scope of application: Suitable for the desalination of seawater and brackish water on large, medium, and small scales; large-scale desalination plants can be constructed by connecting multiple units in parallel. 3.3 Comparison of seawater desalination technologies 3.1 Comparison of seawater desalination technologies Comparison items SWRO Distillation High temperature Low temperature MSF MED and TVC MVC Purity of produced water 200~500ppm