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As early as 1877, there was a report that “a drum that could be rotated, after the drum had been rotated for a certain period of time, the fresh cream floats on the surface and then was scraped off”. In 1879, in Sweden, Laval invented the first separator to separate fat from milk. The separator is mainly used to separate the solid particles and the liquid in the suspensions, or to separate the two different density and immiscible liquids in the emulsion (for example, separate the cream from milk). In modern dairy processing, the separator is a very important equipment in dairy production.
1. Principle of Separation
Historically, separation is a recent invention. But many years ago, the technology of separating one substance from another was a natural process of gravity settling. Sedimentation occurs all the time in nature. Gravity sedimentation is a primitive technology for separating fat from milk in dairy production. It is used to separate solid particles or liquid in the mixture of two or more phases.
Milk is a solid (impurity) and two liquid phases (skimmed milk, milk fat) complex dispersion system. After the extruded milk is placed in a container for a period of time, the fat ball floats upward and gathers on the surface of the milk to form a layer of fresh cream. The diameter of the fat ball is 3 μ m at 35 ℃, and the floating speed is about 0.6 mm/h, which can not meet the needs of production.
2. Structure of Separator
In the past, the separator had a cylinder rotating at high speed around its axis, which was called a drum. After the milk was fed into the drum, it was rapidly rotated, and separated under the action of centrifugal force. The earliest separators were operated intermittently and manually. At that time, the drum was only an empty cylinder. Later, conical discs were added in the drum. The increase of the discs enlarged the settlement area, shortened the settlement distance, significantly improved the separation effect and the processing capacity. The application of this technology led to the rapid development of the disc separator.
3. Transmission of Disc Separator
The rotation of the discs in the drum is that: the discs on the distributor are driven to rotate by the motor through the worm wheel, cylinder, and the distributor fixed on the cylinder.
4. Separation Process of Milk in Disc Separator
In the disc separator, the discs are fixed on the distributor. The milk enters the vertical distribution holes which are a certain distance from the edge of the disc from the cylinder. Under the action of centrifugal force, the particles or droplets (fat balls) in the milk are separated according to their density relative to the continuous medium (skimmed milk). The separation process is as follows:
(1) The density of the fat balls is small, they moves upward and inward along the separation channels, converges in the center of the axis, and then be discharged through the outlet of the cream.
(2) The skimmed milk moves downward and outward along the separation channels. After reaching the edge of the discs, it moves upward through the channels between the discs with no holes at the top and the cone cover, and then it is discharged through the outlet of the skimmed milk.
(3) The impurities with high density in milk are thrown to the sediment chamber around the separator along the separation channels, and be discharged regularly. Modern disc separators all have automatic slag discharge system, a certain volume of water is added into the drainage device as “ balance water”. When the water is discharged from the bottom of the slipper piston, the slipper piston immediately drops and the sediment is discharged.
5. Factors Affecting the Separation Effect
Considering from the disc separator, the main influencing factors are:
(1) Rotating speed. The greater the rotating speed, the better the separation effect.
(2) The shape and number of the discs.
(3) The diameter of the drum. The larger the diameter, the better the separation effect.
Considering from the milk, the main influencing factors are:
(1) The diameter of the fat ball. The larger the diameter, the better the separation effect.
(2) The content and size of the impurities in the milk.
(3) The flow rate of the milk. The small the flow rate, the higher the degreasing efficiency.
(4) Preheating the milk before separation will improve the separation effect. After preheating, the diameter of the fat balls will increase, and the density difference between the fat balls and the skimmed milk will increase.
(5) The viscosity of the skimmed. According to the Stokes law, when the viscosity decreases, the floating speed will increase, and the separation effect will also increase.
6. Application of Disc Separator in Production
6.1 Milk Clarification
The disc separator is used to remove the impurities in milk. In the process of milking, the milk is polluted by fodders, dust, cow hairs and insects. The density of these impurities is higher than that of milk. They are thrown into the sediment chamber and discharged regularly, so as to clarify the milk.
6.2 Degreasing
The fat in milk is removed by the disc separator to form fresh cream and skimmed milk. Fresh cream is made into cream and anhydrous cream, skimmed milk is made into skimmed dairy products.
6.3 Standardization
In order to meet the requirements of dairy products standard in actual production, the proportion of fat and non fat milk solids in raw milk was adjusted by the disc separator, so as to meet the proportion of finished products. In cheese production, if the milk fat content is too high, the texture will be loose, and if the milk fat content is too low, the texture will be too hard. Therefore, it is necessary to adjust the milk fat content by the disc separator.
6.4 Bactofugation
The density of bacteria in milk, especially the spores with heat resistance, is higher than that of milk, which can be removed by the disc separator. Bactofugation is a process of separating bacteria from milk by a specially designed sealed separator, especially separating the spores formed by some species of bacteria.
Bactofugation has been proved to be an effective method to reduce the number of spores in milk. Because the proportion of spores is larger than that of milk, the milk is usually separated into bacteria-free parts and concentrated parts containing bacteria and spores. The latter accounts for 3% of the incoming materials into the separator.
7. Precautions When Using Disc Separator
(1) When selecting the separator, it should be considered according to the actual situation and the production capacity, so as to improve the utilization rate of the equipment and reduce the kinetic energy consumption.
(2) When working, the disc separator rotates at a very high speed, so it should be installed on a solid foundation. The rotating spindle should be vertical to the horizontal plane, and all components should be installed accurately. If necessary, rubber band shall be equipped on the foundation to play a buffering role. For the rotating part, it is necessary to replace the oil regularly and clean up the dirty oil to prevent impurities from mixing.
(3) Before starting, the transmission machinery and fasteners must be checked to see whether the rotation direction is reversed, in order to prevent damage to the machine. Observe whether the motor and the centrifugal clutch of the horizontal axis are concentric and flexible. If necessary, carry out a idle test, and listen whether there is abnormal noise.
(4) The continuous operation time should be determined by the physical property and impurity content of the material. Generally, the separator needs to be cleaned every two or four hours. In order to ensure the quality, it is better to equip two separators and work out a detailed operation plan and operation schedule. If it is found that the separated materials do not meet the specified indicators, the machine should be adjusted. After adjusting, if it is not effective, the machine should be stopped for inspection.
(5) Pay attention to and often check whether the peripheral seal of the pump and the suction pipe is tight to prevent air mixing.
(6) After operation, the machine should be opened to clean up for the next use.
8. Conclusion
The disc separator plays an irreplaceable role in dairy production. It can clarify, degrease, standardize and disinfect the milk, so it is a key equipment in dairy production.
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