The Working Principle of Membrane Filter

The microporous separation process is to use the size selectivity of the membrane to separate components under the action of the fluid pressure difference to trap the particles and macromolecular solutes that can be retained by the membrane pores, while the particles or small molecular solutes that cannot be retained by the die pores through the membrane.

Microporous membrane operations include dead-end filtration (vertical flow filtration) and cross-flow filtration (tangential flow filtration). Dead-end filtration is mainly used for fluids with small solid content and general processing scale, and most of the membranes are made disposable. The commonly used filter mobile phase and disposable syringe filter membrane are dead-end filters.

Principle of microfiltration

Membrane filter principle construction and working. The filtration mechanism of the microporous membrane is mainly interception. The interception can be divided into the following types: mechanical interception, adsorption interception, bridging interception, and an interception in the network inside the membrane. As shown below:

1. Mechanical interception: that is, sieving. It means that the membrane has the function of retaining impurities such as particles that are larger than their pore size or have a similar pore size.
2. Physical action or adsorption and interception: In addition to considering the sieving effect, the influence of other factors should also be considered, including the influence of adsorption and electrical properties.
3. Bridging effect: At the entrance of the hole, particles can also be trapped because of the bridging effect.
4. The network interception effect of the network membrane. This retention traps the particles inside the membrane rather than on the surface of the membrane.

It can be seen from the above that the sieving effect is very important, and the interaction between impurities such as particles and the pore wall is sometimes more important than the pore size.

Microfiltration is mainly used to separate macromolecules, colloidal particles, proteins, and other particles. Their separation mechanism is based on the physical and chemical properties of molecules or particles, the physical and chemical properties of the membrane used, and the interaction between them (such as size, shape, and electrical properties) to achieve separation.

The microfiltration process generally has three stages:

1. Initial stage: Particles larger than the pore diameter of the membrane are trapped on the surface of the membrane, and particles smaller than the membrane enter the die hole, some of which are adsorbed in the membrane hole due to various forces, reducing the effective diameter of the membrane hole ;
2. In the middle stage of filtration: the particles begin to form a filter cake layer on the surface of the membrane, and the adsorption inside the membrane pores gradually tends to be saturated;
3. Later stage: As more particles are trapped on the surface of the membrane, the adsorption inside the membrane pores tends to be saturated, and the particles begin to block the membrane pores, which eventually stabilizes the membrane flux and then decreases continuously.

Through the above three processes, it can be understood that the adsorption of the filter membrane can reach saturation during the dissolution process, and it becomes more and more difficult to filter as the number of filtrations increases.