For specific system conditions, water flux and desalination rate are the characteristics of reverse osmosis membranes, and there are many factors affecting the water flux and desalination rate of FRP membrane housing pressure vessels, including pressure, temperature, Influencing factors such as recovery rate, influent salinity, and pH value.
(1) Influence of pressure
Reverse osmosis inlet pressure directly affects the membrane flux and desalination rate of the reverse osmosis membrane. As shown in the figure, the increase in membrane flux has a linear relationship with the reverse osmosis inlet pressure; the desalination rate has a linear relationship with the inlet pressure, but when the pressure reaches a certain value, the change curve of the desalination rate tends to be flat, and the desalination rate no longer increases.
(2) Temperature effect
The desalination rate decreases with the increase of reverse osmosis inlet water temperature. The water flux increases almost linearly. The main reason is that as the temperature increases, the viscosity of water molecules decreases and the diffusion capacity is strong, so the flux of produced water increases; as the temperature increases, the rate of salt penetration through the reverse osmosis membrane will also increase, so the desalination rate will decrease. Raw water temperature is an important reference index for the design of the reverse osmosis systems. For example, when a power plant undergoes reverse osmosis engineering technical transformation, the raw water temperature is calculated at 25 ° C during design, and the calculated inlet pressure is 1.6MPa, while the actual temperature of the system is only 8 ° C, and the inlet pressure must be increased to 2.0MPa. Ensure the design flow of freshwater. The consequence is that the energy consumption of the system operation increases, the life of the internal sealing ring of the membrane assembly of the reverse osmosis device becomes shorter, and the maintenance of the equipment is increased.
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(3) Influence of salt content
The salt concentration in water is an important indicator that affects the membrane osmotic pressure. As the salt content of the influent increases, the membrane osmotic pressure also increases. As shown in Figure 3-18, when the reverse osmosis inlet pressure is constant, the salt content of the inlet water increases. Because the increase in osmotic pressure offsets part of the inlet water driving force, the flux becomes lower and the desalination rate Lower.
(4) Impact of recovery rate
The increase in the recovery rate of the reverse osmosis system will make the salt content of the membrane element inlet water higher in the direction of the water flow, resulting in an increase in the membrane osmotic pressure, which will offset the driving force of the reverse osmosis inlet pressure, thereby reducing the water production. Flux. The increase of the salt content of the membrane element inlet water increases the salt content in the freshwater, thereby reducing the desalination rate. As shown in Figure 3-19, the trend of recovery rate on membrane flux and desalination rate.
In the system design, the maximum recovery rate of FRP RO membrane housings does not depend on the limitation of osmotic pressure but often depends on the composition and content of the salt in the raw water. Dissolved salts such as calcium carbonate, calcium sulfate, and silicon will scale during the concentration process.
(5) Influence of pH
Different types of membrane elements have a wide range of applicable pH values. For example, the cellulose acetate membrane has a stable water flux and desalination rate in the pH range of 4 to 8, and the pH range is lower than 4 or higher than 8. Within, it is more affected. At present, most of the membrane materials used in industrial water treatment are composite materials, and the adaptable pH range is wide (the pH value can be controlled in the range of 3 to 10 under continuous operation), and the membrane flux and desalination rate within this range relatively stable.
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