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Detailed analysis of the principles and classification of membrane filtration technology
Detailed analysis of the principles and classification of membrane filtration technologyI. MicrofiltrationThe first thing we need to see is microfiltration technology, which is one of the membrane technologies that use the static pressure difference as a driving force to filter and separate using the sieving effect of the membrane. Microfiltration membranes are characterized by their neat and uniform porous structure design, in which particles smaller than the membrane pores will pass through the membrane under the action of the static pressure difference, while particles larger than the membrane pores will be intercepted on the surface of the membrane, thus achieving effective separation. In addition, microfiltration membranes are homogeneous porous films with a thickness of 90 - 150 μm, filtering particle sizes of 0.025 - 10 μm, and operating pressures of 0.01 - 0.2 MPa. The separation principle of this technology has several different methods as follows. 1. Mechanical interception: as we have described above, the membrane intercepts particles larger than its pore size or equivalent; 2. Physical: the use of adsorption and electrical properties; 3. Bridging: contaminated particles are intercepted because of the bridging effect; 4. Internal retention: as the name implies, the particles are intercepted inside the membrane. II. Ultrafiltration The so-called ultrafiltration means that under a certain pressure, the solution containing small molecules passes through the surface of the supported membrane, in which the solvent and small molecules of solute will pass through the membrane, while the large molecules are intercepted and recovered as a concentrate. Ultrafiltration membranes have a filtration particle size between 5 - 10nm and an operating pressure between 0.1 - 0.25MPa. III. Reverse Osmosis Reverse osmosis, which can also be called hyper filtration, is a reverse process of osmosis in which the solvent in the original solution is compressed to the other side of a semi-permeable membrane by adding a higher pressure than the osmotic pressure to the side of the liquid to be filtered. Reverse osmosis membranes have a filtration particle size between 0.2 - 1.0nm and an operating pressure between 1 - 10MPa. The filtration principles of reverse osmosis are generally the following 1. Capillary flow mechanism: This is proposed for the cellulose acetate membrane, after the sewage enters the cellulose acetate membrane, due to the hydrogen bonding with the oxygen atoms of the carboxyl group and the composition of the combined water, the smaller the pore size at this time, the more firmly combined; 2. Diffusion model: the semi-permeable membrane is used as a completely dense neutral interface, where water and solutes are adsorbed onto the surface of the membrane and then diffuse through the membrane; 3. Pore opening and closing principle: the polymer under a certain pressure will change the disorderly Brownian motion, and then produce vibration, so that the distance between them decreases until the ions are difficult to pass, thus achieving the separation from water. IV. Nanofiltration This is a membrane separation technology developed based on reverse osmosis. The interception particle size of a nanofiltration membrane is generally between 0.1 - 1nm, the operating pressure is 0.5 - 1MPa, the interception molecular weight is 200 - 1000, and it has good separation performance for organic small molecules in water with a molecular weight of hundreds. V. Dialysis The so-called dialysis is a process of separation of different substances under the impetus of concentration difference with the help of membrane diffusion. VI. Electrodialysis Under the action of a DC electric field, the difference in potential is used as the driving force to separate electrolytes from the solution by using the permeability of ion exchange membranes, thus realizing the desalination, concentration, and purification of the solution. |
