A centrifuge is a machine that uses centrifugal force to separate liquid and solid particles or components of a mixture of liquid and liquid. The centrifuge is mainly used to separate the solid particles in the suspension from the liquid; or to separate the two liquids of different density and immiscible in the emulsion (for example, separating the cream from the milk); it can also be used to exclude Liquid in wet solids, such as wet clothes in a washing machine; special ultra-speed tubular separators can also separate gas mixtures of different densities; use different density or granularity of solid particles to set different velocity in liquid, some sedimentation Centrifuges can also classify solid particles by density or particle size.
History development
In ancient China, people used one end of a rope to hold a clay pot, and held the other end of the rope, rotating the pottery jar, and generating centrifugal force to squeeze out the honey in the pot. This is the early application of the centrifugal separation principle.
Industrial centrifuges were born in Europe. For example, in the middle of the 19th century, there were three-legged centrifuges for textile dehydration and top-suspension centrifuges for separating crystal sugar from sugar factories. These earliest centrifuges were batch operated and manually drained.
Due to the improvement of the slag discharge mechanism, a continuously operating centrifuge appeared in the 1930s, and the intermittent operation of the centrifuge was also developed due to the realization of automatic control.
Industrial centrifuges can be divided into three types: filtration centrifuge, sedimentation centrifuge and separator according to structure and separation requirements.
The centrifuge has a cylinder that rotates at a high speed about its own axis, called a drum, which is usually driven by an electric motor. After the suspension (or emulsion) is added to the drum, it is rapidly driven to rotate at the same speed as the drum, and the components are separated under the action of centrifugal force and discharged separately. Generally, the higher the drum speed, the better the separation.
The centrifugal separator has two working principles: centrifugal filtration and centrifugal sedimentation. Centrifugal filtration is the centrifugal pressure generated by the suspension under the centrifugal force field, acting on the filter medium, causing the liquid to pass through the filter medium to become the filtrate, and the solid particles are trapped on the surface of the filter medium to achieve liquid-solid separation; centrifugal sedimentation is utilized The principle of rapid sedimentation and stratification of the components with different densities (or emulsions) in the centrifugal force field enables liquid-solid (or liquid-liquid) separation.
There is also a type of separator for experimental analysis that can perform liquid clarification and solid particle enrichment, or liquid-liquid separation. These separators have different structural types operating under atmospheric pressure, vacuum, and freezing conditions.
An important indicator for measuring the separation performance of a centrifuge is the separation factor. It represents the ratio of the centrifugal force to the gravity of the separated material in the drum. The larger the separation factor, the faster the separation is usually and the better the separation effect. The separation factor of the industrial centrifugal separator is generally 100 to 20000, the separation factor of the super-speed tubular separator can be as high as 62000, and the separation factor of the analytical super-speed separator is up to 610000. Another factor that determines the processing capacity of the centrifuge is the working area of ​​the drum, which is also large in processing area.
The filter centrifuge and the sedimentation centrifuge mainly rely on increasing the diameter of the drum to enlarge the working surface on the circumference of the drum; in addition to the circumferential wall of the drum, the separator has additional working surfaces, such as discs and chambers of the disc separator. The inner cylinder of the separator significantly increases the settlement working surface.
In addition, the more difficult it is to separate the solid particles in the suspension, the fine particles carried away in the filtrate or the separation liquid will increase. In this case, the centrifugal separator needs a higher separation factor to effectively separate the suspension; When the viscosity of the liquid is large, the separation speed is slowed down; the difference in density of the components of the suspension or the emulsion is large, which is advantageous for centrifugal sedimentation, and the centrifugal filtration of the suspension does not require density difference of each component.
The centrifuge should be selected according to the size and concentration of solid particles in the suspension (or emulsion), the difference in density between solid and liquid (or two liquids), the viscosity of the liquid, the characteristics of the filter residue (or sediment), and the requirements for separation. The comprehensive analysis is carried out to meet the requirements for the moisture content of the filter residue (sludge) and the clarity of the filtrate (separation liquid), and which type of centrifugal separator is initially selected. Then according to the amount of processing and the automation requirements for the operation, determine the type and specifications of the centrifuge, and finally verify by the actual test.
Generally, for suspensions containing particles larger than 0.01 mm in size, a filter centrifuge can be used; for fine or compressible deformation of the suspension, a sedimentation centrifuge should be used; for the suspension, the solid content is low, the particles are small and When the liquid clarity is high, a separator should be used.
The future development trend of centrifugal separators will be to strengthen separation performance, develop large centrifugal separators, improve slag discharge mechanism, increase dedicated and combined drum centrifuges, strengthen separation theory research and study centrifugal separation process optimization control technology.
The enhanced separation performance includes increasing the rotation speed of the drum; adding a new driving force during the centrifugal separation process; speeding up the slag pushing speed; increasing the length of the rotating drum to prolong the time of centrifugal sedimentation separation. The development of large-scale centrifugal separators, mainly to increase the diameter of the drum and the use of double-sided drums to improve the processing capacity, so that the equipment investment, energy consumption and maintenance costs of processing unit volume of materials are reduced. In theoretical research, it mainly studies the fluid flow conditions in the drum and the formation mechanism of the filter residue, and studies the calculation method of the minimum separation degree and the processing capacity.
Principle of centrifugation
When the suspension containing fine particles is left stationary, the suspended particles gradually sink due to the action of the gravitational field. The heavier the particles, the faster the sinking, whereas the particles with a lower density than the liquid will float. The rate at which particles move under the gravitational field is related to the size, morphology, and density of the particles, and is related to the strength of the gravitational field and the viscosity of the liquid. Particles of the size of red blood cells, a few micrometers in diameter, can be observed under normal gravity.
In addition, the substance is accompanied by a diffusion phenomenon when it settles in the medium. Diffusion is unconditional and absolute. The diffusion is inversely proportional to the mass of the material, and the smaller the particle, the more severe the diffusion. The settlement is relative, and if it is conditional, it must be subjected to external forces to move. The settlement is proportional to the weight of the object, and the larger the particle, the faster the sedimentation. For particles smaller than a few micrometers such as viruses or proteins, they are in a colloidal or semi-colloidal state in solution, and it is impossible to observe the sedimentation process by gravity alone. The smaller the particles, the slower the sedimentation, and the more severe the diffusion phenomenon. Therefore, it is necessary to use a centrifugal machine to generate a strong centrifugal force in order to force these particles to overcome the diffusion to produce a settling motion.
Centrifugation is the use of the powerful centrifugal force generated by the high-speed rotation of the centrifuge rotor to accelerate the sedimentation velocity of the particles in the liquid, and separate the sedimentation coefficient and the buoyant density of the sample.
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