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Several Major Membrane Separation Techniques Time:2018.03.13 金沙7868com
Membrane separation techniques commonly used in biochemical processes include microfiltration (MF), ultrafiltration (UF), reverse osmosis (RO), nanofiltration (NF), electrodialysis (ED), liquid membrane (LM), etc.
Microfiltration is the use of porous thin membranes as the media for filtration and membrane filtration based on the principle of sieving. Under pressure as a driving force, solvents, water, salts and macromolecules substance can all penetrate the membrane. However, substances such as fine particles and super macromolecules, whose particle diameter is larger than the membrane pore size, are stranded, to achieve the purpose of separation and further purify the solution. Microfiltration is the most widely used and most economical technology in membrane separation technology, it mainly used in the pharmaceutical industry in bio-chemical industry.  
Ultrafiltration is based on the principle of screening, with a certain pressure difference as the driving force to separate the solvent from the solution.Compared with the microfiltration process, the ultrafiltration process is affected by the chemical properties of the pores on the surface of the membrane, solvents or small molecular weight substances can pass through the membrane pores under a certain pressure difference, the macromolecules and fine particles are trapped for separation purposes.
Ultrafiltration membranes are usually asymmetric membranes, and the size of the membrane aperture and the nature of the membrane surface play different roles for interception performance. Ultrafiltration is mainly used in the purification of concentrated macromolecular solutions, etc. And it is the most widely used in the biochemical process.
The reverse osmosis process is mainly based on the dissolution and diffusion principle of the solution, it is the membrane separation process driven by the pressure difference.It is contrary to the process of natural penetration. Both permeation and reverse osmosis are accomplished through semipermeable membranes. On the side of the concentrated solution, when the applied pressure is higher than the natural osmotic pressure, the solvent in the solution is forced to reversely permeate through the membrane and flow toward the dilute solution, thereby achieving the purpose of separation and purification. The reverse osmosis process is mainly used for the concentration of low molecular weight components, such as amino acid concentration(Glycine HGB 3075-79),Ethanol concentration (GB 679-65), etc. The size of the osmotic pressure is independent of the type of membrane and it is related to the nature of the solution.
Nanofiltration is also based on the principle of adsorption and diffusion, and it is the membrane separation process is driven by pressure difference. In addition to its own working principle, it also has the working principle of reverse osmosis and ultrafiltration. Nanofiltration, also known as low pressure reverse osmosis, is a new type of membrane separation technology. This membrane process broadens the application of liquid membrane separation. The separation performance is between ultrafiltration and reverse osmosis, and its molecular weight cut off is approximately 200 to 2000. Nanofilms are composite membranes that allow some inorganic salts and some solvents to pass through the membrane. The pressure required for the nanofiltration process is much lower than that of reverse osmosis, which has the advantage of saving power. It can cut off part of the solute that is easy to pass through the ultrafiltration membrane, and may also be penetrated by the solute that is intercepted by the reverse osmosis membrane. Its unique function can not be replaced by reverse osmosis and ultrafiltration.Nanofiltration membrane has good thermal stability, pH stability and stability to organic solvents, so it has been widely used in various industrial fields, especially the separation and purification process of pharmaceutical and biochemical industries.Nanofiltration membrane is today's most advanced membrane separation technology. The separation techniques of microfiltration, ultrafiltration, reverse osmosis, and nanofiltration do not have obvious boundary lines. They all use pressure as the driving force, and the diameters of the solutes that are truncated overlap each other in certain ranges.
Electrodialysis is the use of potential difference as the driving force, under the action of direct current, it uses the selective permeability of the ion exchange membrane to separate the electrolyte from the solution, so as to achieve the purposes of desalination, refining or purification.
The liquid membrane is a layer of emulsion particles suspended in the liquid to form a liquid membrane. According to the principle of dissolution and diffusion, these two solutions with different compositions and mutually soluble can be separated by this liquid membrane, and achieve the effect of separation and purification through the phenomenon of infiltration,  It overcomes the low selectivity and low flux of solid membranes.Liquid membranes generally consist of solvents, surfactants, and additives.According to its configuration and operation methods, it is divided into Liquid surfactant membranes and Supported membranes.

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