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 principle and process of multi-stage flash evaporation seawater desalination technology.
The principle of multi-stage flash evaporation (MSF) process is to flash seawater in a series of flash evaporation chambers with gradually decreasing pressure. The number of flash evaporation chambers is called the number of stages, and the ultra large MSF seawater desalination plant can reach 50 stages. As shown in Figure 2, seawater first passes through the condenser tube bundle at the top of the flash evaporation chamber, where it is preheated by the gradually warming steam generated by each stage of the flash evaporation chamber, and finally reaches the brine heater. The heating steam of the brine heater comes from the thermal power plant. The seawater is heated to the highest temperature (90-115 ℃) in the heater and then sent to the first stage flash evaporation chamber. The pressure in the flash evaporation chamber is lower than the saturated vapor pressure corresponding to seawater, that is, the temperature at which seawater is sent into the flash evaporation chamber is higher than the boiling point temperature under pressure conditions in the flash evaporation chamber. Therefore, seawater will quickly boil until the temperature drops to the boiling point. Part of the seawater evaporates and the steam generated is removed by the mist eliminator to remove dissolved salt substances. Finally, it condenses on the surface of the inlet condenser tube bundle at the top and generates fresh water for collection. The temperature of seawater without evaporation decreases and flows into the next flash evaporation chamber with lower pressure to continue the flash evaporation process. This repeated evaporation and condensation process continues to produce seawater desalination water.
Technical characteristics
MSF is more traditional, with mature and reliable technology, safe and stable operation, and its device has the characteristics of being large and super large. Due to the multi-stage flash distillation process, which separates the heating and evaporation of seawater, the scaling phenomenon of MSF equipment is relatively mild and easy to remove compared to early multi effect distillation processes. Large MSFs are typically designed as concentrated water circulation systems, where a portion of the concentrated water is recycled as supplementary water to the system. Circulating a certain proportion of concentrated water can reduce the demand for steam and seawater, as well as the use of scale inhibitors and defoamers. The water level and water production rate of MSF are influenced by various factors. Usually, when all other factors are the same, the water production rate of the vertical condensing tube bundle MSF system is about 1/2 to 1/3 of the water level, while the water production rate of the parallel condensing tube bundle MSFF system is about 1/4 of the liquid level. Degassing treatment is an important pre-treatment process in the circulating MSF system, which can remove dissolved gases such as oxygen, nitrogen, and carbon dioxide from the influent. If the inlet water is not degassed, these dissolved gases will be released during the flash evaporation process. These gases have relatively poor thermal conductivity, which will reduce the heat transfer rate of the condenser tube bundle. Meanwhile, carbon dioxide and oxygen can also accelerate equipment corrosion. MSF has high power consumption; This device has limited operational flexibility and is not suitable for situations with significant changes in water production.
Current development trends
Since the 1960s, MSF has rapidly occupied the Middle East market, marking the beginning of large-scale application of seawater desalination technology. At present, in terms of total production capacity, it remains the second largest seawater desalination process in the global market, with its development and application mainly concentrated in the Middle East and Gulf regions. Compared with LT-MED, this technology has higher energy consumption and is less commonly used in emerging seawater desalination markets such as China. In recent years, MSF has had almost no large-scale construction projects, and has been replaced by LT-MED thermal technology. With the rise of the global seawater desalination industry and the development and application of new technologies, MSF's future market share is expected to further shrink.
