Desalination of seawater is an important way to solve the shortage of freshwater resources. Associate Professor Zheng Xiaoying from Beijing University of Technology gave a detailed introduction to the most widely used commercial seawater desalination processes: low-temperature multi effect distillation, multi-stage flash distillation, and reverse osmosis. She analyzed their process principles, characteristics, development trends, and the current development status of China's seawater desalination industry. The original text was published in the "Experience Exchange" column of the sixth issue of "Water Purification Technology" in 2016. Today, let's learn about the purification process of low-temperature multi effect distillation.
The low-temperature multi effect distillation process (LT-MED) is carried out in a series of pressure decreasing evaporation chambers, and the number of evaporation chambers is called the efficiency. The efficiency of a typical LT-MED desalination plant is generally 7 or more. As shown in Figure 1, each evaporator chamber is equipped with a series of evaporator tube bundles, which have excellent heat transfer performance and certain mechanical strength. In the system, steam from an external heat source is introduced into the first effect evaporator of the seawater desalination equipment; Seawater enters from the right side of the distillation system and is partially preheated by the steam generated in the final effect as supplementary water for the distillation system. The seawater entering the system is sprayed onto the internal evaporator bundle from the top of each effect, flowing from top to bottom along the surface of the evaporator under the action of gravity and continuously absorbing heat from the surface of the evaporator, partially evaporating and generating steam. At the same time, the steam inside the evaporator is released and condensed to produce fresh water, which is collected. The secondary steam generated in the first effect of the system is removed by the mist eliminator and enters the evaporation tube of the next effect, directly serving as the heating steam for the evaporation tube of the seawater desalination equipment in the next effect. The seawater desalination equipment gradually decreases the pressure and temperature inside the chamber along the direction of steam transmission. As a result, evaporation and condensation will repeat in each effect. Only the fresh water condensed in the first effect evaporation tube will flow back to the steam generator, and the remaining effects will become the product water.
Technical characteristics
LT-MED achieves the evaporation and condensation cycle of seawater at lower temperatures (<70 ℃) by controlling the pressure in the evaporation chamber, greatly reducing the scaling and corrosion problems of seawater desalination equipment. It has a long operating life, less maintenance, and greatly increases the feasibility of using inexpensive heat transfer materials. Due to the fact that the condensation process of LT-MED occurs entirely inside the evaporator tube, the system is minimally affected by seawater quality. At lower temperature conditions, impurities in seawater hardly undergo phase transition, and the TDS of the product water can reach below 5 mg/L. LT-MED mainly utilizes low-pressure steam or waste heat from power plants, which also enables it to utilize a variety of low-level thermal energy and have energy-saving characteristics. Based on the scale of the seawater desalination equipment system, the number of evaporation chambers, and the heat transfer efficiency of the evaporation tubes, the water production ratio (mass ratio of producing fresh water to consuming steam) is 8-15, and its operating load can be 40% to 110% of the design load while maintaining the same water production ratio. This operational flexibility is an important advantage. If the steam volume decreases or increases for some reason, the impact on the operation of LT-MED is not significant. However, due to the operating temperature of LT-MED exceeding 70 ℃, this also becomes a limiting factor for the further improvement of thermal efficiency in this process technology.
Current development trends
Due to the significant reduction of scaling and corrosion in seawater at lower temperatures, LT-MED was developed by Israeli IDE in the late 1970s based on this principle. From the perspective of thermal energy utilization and conversion, the efficiency of LT-MED is higher than that of MSF, which makes LT-MED's market advantage obvious and widely used in newly built thermal process. In terms of the global market, although the number of LT-MED desalination plants is relatively small compared to MSF, the development of LT-MED has given multi effect distillation many advantages, and its water production cost is also quite competitive compared to reverse osmosis technology after using low-temperature waste heat steam. The number of completed plants is constantly increasing. The focus of future research on low-temperature multi effect distillation technology is the application of new and inexpensive materials, the expansion of equipment scale, and other technologies, aiming to further reduce equipment and operating costs, and improve competitiveness.
