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Application of Multistage Centrifugal Pump in Seawater Desal

Source:未知|Popularity:0|release date:2020-12-09 09:29:01
1. The mainstream seawater desalination process-reverse osmosis
With the growth of the world's population and economic development, the global shortage of freshwater resources has become increasingly serious. Countries continue to strengthen the research and development of seawater desalination technology.
At present, the mainstream international water desalination technologies in Shanghai include thermal and membrane methods, each of which contains a variety of technologies. Among many desalination technologies, reverse osmosis is characterized by low investment, low energy consumption, less land occupation, and convenient operation. It has quickly occupied most of the market in less than 50 years. Compared with other technologies, reverse osmosis desalination process does not undergo phase change and is one of the most energy-efficient desalination technologies.
Reverse osmosis technology is a pressure-driven membrane separation technology. When a semi-permeable membrane is used to isolate salt water and fresh water, fresh water will spontaneously permeate through the semi-permeable membrane to the salt water side. If an external force is applied to the salt water side to hinder the flow of fresh water, the permeation speed will decrease. When the pressure increases to a certain value, the osmosis phenomenon stops completely, and the fluid on both sides of the membrane is in equilibrium. At this time, the equilibrium pressure is called osmotic pressure (standard seawater osmotic pressure is about 2.5MPa). If the salt water side is further pressurized, it will cause the salt water side The flow of fresh water in reverse osmosis is called "reverse osmosis".
Only when the operating pressure of the feed seawater is greater than the osmotic pressure of the seawater can the freshwater in the seawater be separated by the semipermeable membrane. Generally, the operating pressure of reverse osmosis desalination process is between 6 and 8 MPa.
2. Application of multistage centrifugal pumps for reverse osmosis desalination
It can be seen from the above process flow chart that in the high-pressure reverse osmosis system, the reverse osmosis membrane requires a high-pressure pump to output a pressure of 6~8MPa to separate fresh water from seawater. The flow rate of the high-pressure pump is equal to the output of product water per hour. Therefore, the required high-pressure pump flow rate can be known according to the product water output. The multistage centrifugal pump model required in seawater desalination projects: flow rate 25m³/h-500m³/h, head 500n-800m.
3. Material selection of high pressure pump for reverse osmosis process
For stainless steel materials with low alloy content and no molybdenum, although the surface has a relatively dense oxide film, pitting corrosion is prone to occur due to the effect of chloride ions, which leads to stress corrosion. Among stainless steel materials, Mo-containing materials have better resistance to pitting corrosion than those without Mo. Because the more Mo content, the better the pit corrosion resistance performance. When the Mo content in the steel is ≥3%, it can reach The effect of completely preventing the penetration of Cl- into the material matrix. The main role of Ni in austenitic stainless steel is to form and stabilize austenite, so that the steel can obtain a completely austenitic structure. It can improve the toughness of the material while improving the oxidation and corrosion resistance of the steel. However, in general austenitic steel, the Ni can not play the role of anti-pitting corrosion in the environment with chloride ion corrosion.
From the perspective of metal composition, 304 stainless steel does not contain Mo (molybdenum) element, while 316 material contains molybdenum element, so the corrosion resistance of 316 material is more obvious when the chloride ion content is high.
Since the chloride ion content in seawater is generally 2000㎎/L or even higher, and in the reverse osmosis process, the reverse osmosis membrane intercepts 60% of the feed seawater flow, which belongs to high-strength brine. This part of the high-strength brine is passed through the pressure recovery device The content of chloride ions is higher when mixed with seawater, so the content of molybdenum in general 316 material is not enough to resist the corrosion of chloride ions on the pump body. In many engineering cases, high-pressure pumps are made of 2507 duplex stainless steel.