Multi-stage flash evaporation: suitable for large-scale desalination plants
Multi-stage flash evaporation technology has the characteristics of large single unit capacity, high engineering investment, large power consumption and small equipment operation flexibility. The development trend of this technology is to improve the unit’s single-machine water production capacity, reduce unit power consumption, and improve heat transfer efficiency. Due to the high operating temperature of the system (approximately 120°C), in order to prevent equipment corrosion, the raw seawater needs to be filtered, deoxygenated and carbon dioxide removed before entering the multi-stage flash evaporation device. This technology is suitable for large-scale seawater desalination projects that use low-grade steam from thermal power plants or nuclear power plants as heat sources, and is suitable for large and ultra-large seawater desalination plants. At present, the world’s seawater desalination plants still have the largest output by the multi-stage flash evaporation process, and due to the energy policy, they are mainly used in the Gulf countries. As early as 1990, Tianjin Dagang Power Plant introduced two sets of multi-stage flash desalination plants with a daily output of 3000 tons from Envirogenics of the United States.
What factors should be considered in the selection of seawater desalination process?
The desalination water quality requirements determine the seawater desalination process plan. The above three mainstream processes are different in product water purity. Among them, the low-temperature multi-effect distillation process has the highest purity. The product water can be used as feed water for high-pressure boilers after simple treatment, and can also be used for high-end drinking water, physiological The development of salt water and medical water. For example, Guohua Cangdong Power Plant adopts the cooperation model of “water production by enterprises, underwriting by the government”. After the desalinated product water is piped to nearby steel plants, the fresh water is not treated in any way, and part of it is used as desalinated water or softened water for boilers. Water replenishment, part of it is mixed with Yellow River water to adjust the water quality and used for circulating cooling. Therefore, users can choose a single or combined seawater desalination program according to different water quality requirements. For example, thermal film coupling and quality water supply are also currently more promising seawater desalination projects.
Engineering characteristics are an important basis for the selection of seawater desalination technology. Generally, a separate seawater desalination plant is suitable for reverse osmosis. If there is steam or waste heat, the low-temperature multi-effect distillation process is more economical and reliable. Although reverse osmosis is suitable for large-scale, medium-sized and small-scale seawater desalination projects, the frequent cleaning and membrane changes of reverse osmosis make equipment maintenance large and operating costs high. Therefore, its final production cost is not compared with thermal method. Too much advantage.
Low-temperature multi-effect distillation and multi-stage flash evaporation are suitable for larger seawater desalination systems. For desalination projects of 100,000 tons and above, multi-stage flash evaporation technology is safer, more reliable and economical. When constructing seawater desalination projects, when determining the desalination process and process parameters, boundary conditions such as raw seawater quality, water intake and drainage methods, pretreatment plans, product water quality, and concentrated drainage disposal must also be considered. In short, there is no unique or optimal process choice for seawater desalination projects. It should be demonstrated and selected based on the characteristics of the project.
