New technologies for seawater desalination

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With the development of the world economy, the demand for freshwater is increasing steadily. 28 countries have been classified as having water shortages or severe water scarcity, and our country is among them. With eyes turned to the inexhaustible seawater, a new type of water treatment technology—seawater desalination—was developed, which will change the situation of freshwater scarcity. Currently, thanks to advances in production technology, desalination techniques have become increasingly sophisticated, and the costs have dropped significantly; in some areas, they are now close to or even lower than the cost of tap water. This has also played a key role in the widespread use of seawater desalination in regions facing water shortages worldwide. It is reported that large steel companies in some coastal areas of our country are beginning to utilize seawater as a source of water for their operations in order to address the growing shortage of freshwater resources. Steel companies require a large amount of fresh water in their production processes. According to statistics from 2006, China consumed an average of 6.56 cubic meters of fresh water per ton of steel produced, whereas advanced foreign steel companies used 3.36 cubic meters in 2005. Currently, direct seawater cooling in China (to use seawater for \"cooling\") and seawater desalination are already mature technologies. On May 15, 2007, Siemens VAI held its first media summit after the merger in Linz, Austria, where the Siemens Industrial Systems and Technology Services group (I&S) introduced new technologies for seawater desalination to meet the needs of the steel industry for process water. CNN Water Technology, a Siemens subsidiary in Beijing, is installing a MED (multi-effect distillation) system at the steel plant of Shougang Jingang Iron and Steel Joint Venture Co., Ltd. The daily treatment capacity of this water treatment system will reach 25,000 tons. The Shougang Caofeidian project has taken seawater utilization into account, and the Baosteel Zhanjiang project is also exploring the use of seawater. For commercial use, freshwater treatment technologies can be divided into two main categories: distillation and membrane processes. Distillation can be further divided into Multi-Stage Flash (MSF) and Multi-Effekt Distillation (MED) (Note by the author: this is the seawater desalination system installed at the steel plant of Shougang Jingang Iron and Steel Joint Venture Co., Ltd. by Siemens VAI). ) and Vapor Compression (VC) are the three methods. The membrane process methods mainly include Electro Dialysis (ED), Reverse Osmosis (RO), and Nanofiltration (NF), among others. Among the methods used for seawater desalination worldwide, reverse osmosis (RO) ranks first, with a daily production volume of 14.091 million tons (43.5%). The multi-stage flash method (MSF) came in second, accounting for 39.2%, with a daily production volume of 12.692 million tons. Electrodialysis (ED) ranked third with a daily production of 1.462 million tons (4.5%). Others include the vapor compression method (VC), multi-effect distillation method (MED), nanofiltration, and so on. Multi-stage flash distillation and reverse osmosis are the mainstream methods in the field of seawater desalination. The principle of multi-stage flash distillation for seawater desalination is based on the fact that a liquid turns into vapor at its boiling point; this process converts the water in the solution into vapor, thereby separating it from the salts dissolved in the solution. Flashing reduces the boiling point by lowering pressure, generating steam; fresh water can be produced each time this steam is condensed. The multi-stage flash evaporation process is mainly divided into two systems: one is the heating zone, which is used for preheating the feed seawater; steam is generally used as the heat source, and after condensation, it returns to the boiler. The other is the flashing zone, which is a multi-stage flashing and recovery area, typically ranging from 16 to 50 stages. The pressure in each stage of the evaporation chamber decreases successively, and seawater flashes from one stage to another; no external heating is required in this area. The vaporized water vapor rises and undergoes heat exchange with the cold feed seawater in the coil at the upper end of the evaporation chamber, condensing to form fresh water. The last evaporation chamber is evacuated using a steam ejector, causing the surface of the brine to boil.