Before explaining the principle and characteristics of membrane separation, let’s first understand the scope of membrane separation:
There are three types of solid objects in liquids:
Membrane separation refers to the separation of different solutes in a solution. Each solute is composed of different molecules. Therefore, membrane separation is also a kind of molecular separation. Choose the corresponding membrane pore size according to the molecular weight of the separated substance.
Many people regard membrane as a sieve like filter paper or filter cloth. Large molecules are intercepted and small molecules pass through. In fact, the principle of membrane separation is far from simple. There are many collapse factors, such as the shape of molecules and gels, as well as the influence of surface chemistry. They include the attraction and repulsion of solute solvent molecules with the membrane, the preferential adsorption of water and solutes on the membrane, and the negative separation of some special substances.
Membrane separation is different from filtration. In addition to not being a simple sieving, another feature of it is that the separated solute flows parallel to the membrane surface. Also called lateral flow. In traditional filtration, the liquid flows perpendicular to the filter medium. The advantage of lateral flow in membrane separation is that it can reduce the concentration polarization on the membrane surface and the gel layer (the so-called membrane fouling). When the flow velocity of the solute on the membrane surface reaches 2 m/s, turbulence can be generated, which plays a role in cleaning the membrane surface, so that the attenuation of the water flux (that is, the permeation rate) during the separation process becomes slow.
Membrane separation is divided into three forms: microfiltration, ultrafiltration, and reverse osmosis due to the different ranges of the solutes to be separated:
Therefore, when separating different solutes, you should first know the molecular weight of the separated substance, and consider which membrane to choose according to its molecular weight; the common specifications of ultrafiltration are MW 500, 10,000, 30,000, 50,000, 100,000, and 150,000. . The specifications of reverse osmosis and nanofiltration are classified according to their desalination filtration. Desalting filter 99%, 95%, 90%, 80%, 60%, etc. These are only theoretical divisions. In fact, we often encounter situations where the effective separation membrane used in industry has a pore size much larger than the molecular weight of the compound to be separated.
The separation rate of any kind of separation membrane is not 100%. This is because the actual pore size of many small molecules in the lateral flow of the solute is not very uniform, but the calibration pore size is more. At the same time, there are some pores smaller or larger than this pore size, which is the so-called pore size distribution curve. The pore size distribution curve is steeper. It shows that the higher the resolution of this film, the better the performance.
The porosity of the membrane and the pore size distribution are also another important parameter to identify the quality of the membrane. The so-called porosity refers to the number of pores per unit area. The higher the porosity, the greater the water flux of the membrane, the faster the separation speed of the membrane. according to our experience. The difference in water flux of membranes of the same material and molecular weight is as much as three or four times, so it is very important to choose membranes with high porosity in industrial production.
Use membranes to separate different solutes. Not all solutes of different molecular weights can be separated. There must be a sufficient difference between them, that is, the difference is more than 10 times. The greater the difference in molecular weight, the more thorough the separation. This is determined by the characteristics of the membrane and the pore size distribution.