Due to its very high salt content, seawater cannot be used directly, and two main methods are used to desalinate seawater, i.e. distillation and reverse osmosis.
1. Distillation method: distillation method is mainly used in large-scale seawater desalination treatment and heat-rich places. The reverse osmosis membrane method is widely used because of its wide applicability and high desalination rate. Reverse osmosis membrane method is first extracted from seawater, preliminary treatment, reduce the turbidity of seawater, to prevent the growth of bacteria, algae and other microorganisms, and then pressurized with a special high-pressure pump, so that seawater enters the reverse osmosis membrane, due to the high salt content of seawater, seawater reverse osmosis membrane must have a high rate of desalination, corrosion-resistant, high-pressure resistant, anti-pollution and other characteristics of seawater after the reverse osmosis membrane treatment, the salinity of seawater is greatly reduced, TDS content from 36,000 mg mg of seawater to 36,000 mg of water. TDS content from 36000 mg/l down to about 200 mg/l. The quality of desalinated water is even better than tap water, so it can be used by industry, commerce, residents and ships and vessels.
2. Reverse osmosis method: seawater with high salt content, high hardness, corrosive to the equipment, and seasonal changes in water temperature makes the reverse osmosis desalination system more complex than the conventional brackish water desalination system, engineering investment and energy consumption is also much higher. Therefore, through careful process design, reasonable equipment configuration to reduce engineering investment and energy consumption, thereby reducing the unit cost of water, and to ensure the stable operation of the system is particularly important.
Process Flow Description:
1. Pre-treatment
Whether it is seawater desalination, or brackish water desalination, feed water pretreatment is the key to ensure the long-term stable operation of the reverse osmosis system. In the development of seawater pretreatment program should be fully considered: there are a large number of microorganisms, bacteria and algae in seawater. Seawater bacteria, algae reproduction and microbial growth will not only bring a lot of trouble to the water facilities, and will directly affect the normal operation of desalination equipment and process piping. Periodic high tide and low tide, seawater with a lot of sediment, turbidity changes, easy to cause the operation of seawater pretreatment system is not stable. Seawater has a large corrosive, the system used in the equipment, valves, piping material to make certain screening, corrosion resistance should be good.
2. Sterilization of algae
Foreign desalination projects are mostly used to add liquid chlorine, NaClO and CuSO4 and other chemical reagents to sterilize algae. Taking into account the transportation and other factors, adding chemical reagents to sterilize algae have certain difficulties, in the development of equipment for this project specifically using seawater sodium hypochlorite generator. After the seawater pump to take a small share of pressurized seawater, into the sodium hypochlorite generator, under the action of DC electric field to produce NaClO, relying on the difference in the level of direct injection into the beach sinkhole, in order to kill seawater in the bacteria, algae and microorganisms.
Due to the high hardness of seawater seawater direct electrolysis to produce N aC lO must be overcome to generate electrode scaling problems. In the development process, drawing on electrodialysis frequently inverted pole (EDR) technology, that is, every 5 ~ 10m in inverted polarity of the electrode, an effective solution to the problem of sodium hypochlorite generator scaling and precipitation.
3. Coagulation filtration
Coagulation filtration is aimed at removing colloid and suspended impurities in seawater and reducing turbidity. In reverse osmosis membrane separation project is usually measured by the pollution index (FI), required to enter the reverse osmosis equipment feed water FI value <4. Due to the greater specific gravity of seawater, high pH, and seasonal changes in water temperature, the system selected FeCl3 as a coagulant, which is not affected by the temperature, alum flower is large and solid, fast settling speed and other advantages.
4. Chemical adjustment
In order to prevent the seawater desalination process due to the concentration of seawater and produce insoluble inorganic salts, such as CaCO3, CaSO4, in the reverse osmosis membrane surface and the system piping pieces on the scale precipitation, seawater in the reverse osmosis desalination system should be added before the anti-scaling agent.
Adding H2SO4 to adjust the pH value of seawater to decompose HCO-3 in seawater to prevent CaCO3 precipitation is a more common and economical method in seawater desalination. Adding (NaPO3)6 (SHMP) is an effective method to prevent CaSO4 precipitation, but (NaPO3)6 produces by-products of phosphate in scale inhibition, which will promote the growth of microorganisms, bacteria and algae, so the use of this product has certain limitations. The special polymer scale inhibitor imported from western countries has a high price, which will directly affect the operation cost of the desalination project. In this project, H2SO4 is selected as scale inhibitor to control the pH value of feed water of reverse osmosis system between 6.8 and 7.0, and at the same time, control the water recovery rate of seawater desalination system in order to prevent the precipitation of CaSO4.
Considering the composite membrane element with aromatic polyamide as the membrane material in the reverse osmosis desalination system and its poor oxidation resistance, the residual chlorine content in the feed water is required to be 0.1m g/L or less than the reductant off, so the seawater is injected with NaHSO3 before it enters into the membrane system, and the oxidation reduction potential (ORP) of the seawater before entering the reverse osmosis device is controlled so that the seawater before entering into the reverse osmosis device has the oxidation reduction potential (ORP) between 280 and 320mV. The dosage of NaHSO3 is 3 times of the residual chlorine in seawater.
5. Remove odor
Around the island waters of the seawater by the surrounding environment has a greater impact on the seawater chemical oxygen consumption (COD) in 1.7 ~ 2.5m g / L, especially in the summer and fall seasons, sometimes seawater has a greater odor and odor. Therefore, in addition to adding NaClO oxidation, additional activated carbon filters, the selection of a high mechanical strength of the fruit particles of activated carbon can effectively adsorb organic matter and odor, improve the quality of reverse osmosis water, while reducing the pollution of the reverse osmosis membrane surface to prolong the service life of the membrane.
6. Security Filtration
The security filtration uses a 316L filter with a 5µm cartridge to filter the seawater before the high pressure pump, blocking particles larger than 5µm in diameter from the seawater to ensure the safety and long term operation of the high pressure pump, the energy recovery unit and the reverse osmosis membrane element.
7. High Pressure Pump and Energy Recovery Unit
High-pressure pump and energy recovery device is for reverse osmosis desalination to provide energy conversion and energy saving important equipment, according to reverse osmosis desalination required flow and pressure selection, we choose a single-stage centrifugal pump, with 60m3 / h flow, head 640Psi; energy recovery device for HTC-300 type, with hydraulic turbine structure, can use reverse osmosis discharge of seawater concentration of seawater pressure so that reverse osmosis feed pressure by 30%, effectively reduce the water pressure. The pressure is increased by 30%, which effectively reduces energy consumption.
The whole reverse osmosis seawater desalination control system design adopts advanced computer program control at home and abroad, and the programmable controller PLC from the industrial control machine operating station forms a control system with decentralized sampling control and centralized monitoring operation. According to the process parameters set high and low pressure protection switch, automatic switching device, conductivity, flow and pressure abnormalities, can realize automatic switching, automatic interlocking alarm, shutdown, in order to protect the high-pressure pump and reverse osmosis membrane components.
Frequency conversion controls the starting and stopping of the high-pressure pump, realizing soft operation of the high-pressure pump, saving energy consumption and preventing damage to the high-pressure pump and membrane elements due to water hammer or back pressure. The program design can realize low-pressure automatic flushing before and after starting and shutting down the reverse osmosis device. Especially when shutting down, the sub-stable state of concentrated seawater will be transformed into precipitation and contaminate the membrane surface, and the low-pressure desalination water automatic flushing can replace the concentrated seawater, protect the membrane surface from contamination, and prolong the service life of the membrane.
The temperature, flow rate, water quality, output and other related parameters of the system can be displayed, stored, counted, tabulated and printed. The dynamic process flow screen in monitoring operation is clear and intuitive, and the system control simplifies manual operation to ensure that the system can run automatically, safely and reliably.
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