What are the principles of NF(nanofiltration)?
The principle of nanofiltration is the same as that of membrane separation processes such as ultrafiltration and reverse osmosis. Nanofiltration is also a membrane separation process driven by pressure difference, which is an irreversible process. The separation mechanism can be described by the charge model (space charge model and fixed charge model), pore model, and electrostatic repulsion and steric hindrance models that have only been proposed in recent years. Compared with other membrane separation processes, an advantage of nanofiltration is that it can retain small molecular weight organics that pass through the ultrafiltration membrane, and can dialyze some of the inorganic salts retained by the reverse osmosis membrane—that is, it can synchronize "concentration" and desalination. conduct.
The transmembrane pressure difference required for NF membrane separation is generally 0.5 to 2.0 MPa, which is 0.5 to 3 MPa lower than the pressure difference that must be applied to achieve the same osmotic energy with reverse osmosis membranes. Under the same applied pressure, the flux of nanofiltration is much larger than that of reverse osmosis, and when the flux is constant, the pressure required by nanofiltration is much lower than that of reverse osmosis. Therefore, when nanofiltration is used instead of reverse osmosis, the "concentration" process can be carried out more efficiently and quickly, and a larger "concentration" multiple can be achieved.
In the membrane separation process using nanofiltration membranes, the retention rates of various solutes in the solution are as follows:
①Increase with the increase of molar mass;
②In the case of a given feed concentration, it increases with the increase of the transmembrane pressure difference;
③In the case of a given pressure, it decreases with the increase of concentration;
④ For the anion, it increases in the order of NO3-, CI-, OH-, SO42-, CO42-.
⑤ For cations, it increases in the order of H+, Na+, K+, Ca2+, Mg2+, and Cu2+.
