Seawater desalination temperature issue

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The boiling point of seawater within each effect must be higher than that of the secondary steam, as seawater is a saltwater solution that exhibits an increase in temperature when heated.

It needs to be 5-8°C higher; this is the boiling point elevation

Time for several main seawater desalination processes: 2009-10-24 Source: Chengdu Kaiyuan Fluid Control Engineering Co., Ltd. Editor: Dai Fulin Seawater desalination (also known as seawater desalting) is the process of separating salts from water in seawater. Removing water from seawater or eliminating the salt in seawater can both achieve the purpose of desalination. The methods of seawater desalination are also basically divided into these two major categories. 　　Among them, reverse osmosis, electrodialysis, and distillation have been widely applied commercially. ①Reverse osmosis (RO): Under pressure, the fresh water in seawater passes through a semipermeable membrane to the low-pressure side of the membrane, while other components in seawater (such as salts) are trapped on the high-pressure side of the membrane and are discharged along with the concentrated seawater, thereby achieving effective separation. In seawater desalination, an external pressure greater than the osmotic pressure of seawater is applied to the seawater side, causing the pure water in the seawater to reverse-osmosis into fresh water; this is the principle behind reverse-osmosis seawater desalination (Figure 1). To achieve the necessary desalination rate, the operating pressure is greater than 5.5 MPa. Figure 1: Reverse osmosis desalination process. ② Electrodialysis: Under the influence of a direct current electric field, ions migrate through selective ion exchange membranes, thereby allowing the electrolyte ions to be partially separated from the solution; this process is known as electrodialysis (Figure 2). Figure 2 Electroosmotic desalination process ③ Distillation method. The principle of seawater desalination by distillation is simple: seawater is heated, and fresh water is obtained after the water vapor condenses. Distillation methods can be classified into Multi-stage flash distillation (MSF), Multiple-effect distillation (MED), and Vapor compression distillation (VC), depending on the energy source, equipment, and process used. Among them, low-temperature multi-effect evaporation operates at temperatures below 70°C, which is much lower than the steam temperature of around 110°C in multi-stage flash evaporation; this helps to prevent the formation of scale caused by inorganic salts, making it one of the most suitable evaporation techniques available today. At the same time, it can utilize various forms of low-grade heat sources; for example, by integrating with thermal or nuclear power plants, it can use the back pressure of 0.02–0.04 MPa (absolute pressure) from steam turbines to extract vapor for water production, thereby reducing power generation losses and improving the efficiency of the generator set (Figure 3). Figure 3 MED process flow using the backpressure steam of a steam turbine