Centrifugation
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| Technology details | |
| Name: | Centrifugation |
| Category: | Pre-processing (Separation technologies), Post-processing (Separation technologies) |
| Feedstock: | All materials |
| Product: | Separated products |
Centrifugation is a mechanical separation process which involves the use of the centrifugal force to separate particles from a solution or liquids of different densities according to particle size, shape, density, viscosity and rotor speed. The more dense components of the mixture migrate away from the axis of the centrifuge, while the less dense components of the mixture migrate towards the axis. Next to the separation of solids from liquid, it is possible to obtain separation between two liquids of different densities as well, given that the density difference is large enough.
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Origin and composition
The centrifugation method is used to separate two miscible substances. The most common application is the separation of solids from highly concentrated suspensions, which is used in the treatment of sewage sludges for dewatering where less consistent sediment is produced. In the food industries, special centrifuges can process a continuous stream of particle-laden liquid.
Pre-treatment
For a centrifugation, in general no specific pre-treatment is needed since it is used to separate different fractions within a process chain. Sometimes it is combined with other separation technologies like filtration.
Process and technologies
There is a correlation between the size and density of a particle and the speed at which the particle separates from a heterogeneous mixture, when the only force applied is gravity. The larger the size and the larger the density of the particles, the faster they separate from the mixture. By applying a larger effective gravitational force to the mixture, like a centrifuge does, the separation of the particles is accelerated. This way, in industrial and lab settings, particles that would naturally separate over a long period of time can be separated much faster.
The centrifugation speed is specified by the angular velocity, usually expressed as revolutions per minute (rpm), or acceleration expressed as g. The conversion factor between rpm and g depends on the radius of the centrifuge rotor. The particles' settling velocity in centrifugation is a function of their size and shape, centrifugal acceleration, the volume fraction of solids present, the density difference between the particle and the liquid, and the viscosity.
The sedimentation of particles can be explained by Stoke's law. The equation can be used to calculate the velocity of sedimentation based on five parameters:
From the Stokes equation, five important behaviours of particles can be explained:
- The rate of particle sedimentation is proportional to the particle size
- The sedimentation rate is proportional to the difference in density between the particle and the medium.
- The sedimentation rate is zero when the particle density is the same as the medium density.
- The sedimentation rate decreases as the medium viscosity increases.
- The sedimentation rate increases as the gravitational force increases.
Disc-stack centrifugation
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Disc-stack centrifugation is used for removing suspended solids from a liquid having a lower specific gravity than the solids. The solids content of the feed is usually in the range of 0.1-10 V/V%.
Basket centrifugation
Basket centrifuges are often called centrifugal filters or clarifiers. The basket centrifuge uses centrifugal force to generate a pressure which forces the liquid through the caked solids, the filter cloth, the backing screen, and finally the basket perforations. The filter cloth retains the solid particles inside the rotating basket while the permeating liquid is continuously discharged.
Solid bowl centrifugation
Products
The remaining liquid that lies above the precipitate is called a supernatant or supernate. The precipitate and the supernatant can then be further processed or are the final product.
Post-treatment
The post-treatment of the precipitate and/or the supernatant is depending on the next steps within the production chain.
Technology providers
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| Technology name: | Technology category: | Pre-processing (Separation technologies), Post-processing (Separation technologies) | |
| TRL: | Capacity: | l·h-1 | |
| Processable volume: | L | Relative centrifugal force: | g |
| Rotor type: | Temperature: | °C | |
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Open access pilot and demo facility providers
Patents
Currently no patents have been identified.
References
- Centrifugation in Wikipedia
- Centrifuge in Wikipedia