Nanofiltration (NF) membrane development and application of reverse osmosis membrane about 20 years later. In the 1970s Jote adtete studied NS-300 membrane, which is the beginning of the study NF membrane. At that time, Israeli desalination companies used "hybridfiltration" to denote a membrane separation process between reverse osmosis and ultrafiltration called a loose RO membrane. Later, the United States Filmtec company called this membrane technology nanofiltration, has been in use today. After nanofiltration technology developed rapidly, the membrane module in the mid-80s commercialization. At present, nanofiltration technology has become one of the hot spots in the field of membrane separation in the world.
(1) Definition of nanofiltration So far, the understanding of the exact definition, mechanism, characteristics, etc. of nanofiltration is far from sufficient. More uniform academic interpretation of the nanofiltration membrane includes the following seven aspects:
â‘ nanofiltration membrane between the reverse osmosis and ultrafiltration membrane, the membrane surface separation may have a nano-pore structure.
â‘¡ Relative to the reverse osmosis membrane NaCI removal rates were above 95%, the general NaCI removal rate of 90% or less of the membrane can be called nanofiltration membrane.
â‘¢ reverse osmosis membrane almost all solutes have a high removal rate, while the nanofiltration membrane only has a specific solute removal rate.
â‘£ Nanofiltration pore size in more than 1nm, usually 1 ~ 2nm.
⑤ mainly to remove a nano-solute particles, molecular weight cut-off in the 200 to 1000 Dalton.
â‘¥ reverse osmosis membrane are almost polyamide material, and nanofiltration membrane material can be used a variety of materials, such as cellulose acetate, acetic acid - triacetate cellulose, sulfonated polysulfone, sulfonated polyether sulfone, aromatic polyamide composite And inorganic materials.
⑦ general nanofiltration membrane surface formation of polymer electrolyte and therefore often have a strong negative charge.
(2) The principle of nanofiltration and ultrafiltration and reverse osmosis membrane separation process is the same, nanofiltration is also the pressure difference as the driving force of the membrane separation process is an irreversible process. The separation mechanism can be described by the charge model (space charge model and fixed charge model), the pore model and the electrostatic repulsion and the three-dimensional obstruction model proposed in recent years. One of the advantages of nanofiltration compared to other membrane separations is the ability to trap small molecular weight organic matter that passes through the ultrafiltration membrane as well as to dialyze some of the inorganic salts trapped in the reverse osmosis membrane-that is, to synchronize "concentration" with desalination get on.
The transmembrane pressure differential required for NF membrane separation is generally 0.5-2.0 MPa, which is 0.5-3 MPa lower than the differential pressure that must be applied to achieve the same permeate energy with a reverse osmosis membrane. Under the same applied pressure, the flux of nanofiltration is much larger than that of reverse osmosis, while the pressure required for nanofiltration is much lower than that of reverse osmosis when the flux is constant. So when nanofiltration is used instead of reverse osmosis, the "concentration" process can be carried out more efficiently and quickly with a larger "concentration" multiple. In general, the use of nanofiltration membrane separation process, the solution of various solutes retention rate has the following rules:
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â‘ increase with the increase of molar mass;
â‘¡ given the feed concentration, with the increase of transmembrane pressure drop;
In the case of given pressure, decrease with increasing concentration;
â‘£ For anions, press NO3-, CI-, OH-, SO42-, CO42- in the order of rise.
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