Centrifuge

All unapproved Citizendium articles may contain errors of fact, bias, grammar etc. A version of an article is unapproved unless it is marked as citable with a dedicated green template at the top of the page, as in this version of the 'Biology' article. Citable articles are intended to be of reasonably high quality. The participants in the Citizendium project make no representations about the reliability of Citizendium articles or, generally, their suitability for any purpose. This article is currently being developed as part of an Eduzendium student project in the framework of a course entitled BEE 4640 Bioseparation Processes at Cornell University. The course homepage can be found at CZ:Cornell_University_2010_BEE_4640_Bioseparation_Processes. For the course duration, the article is closed to outside editing. Of course you can always leave comments on the discussion page. The anticipated date of course completion is 21 December 2010. One month after that date at the latest, this notice shall be removed. Besides, many other Citizendium articles welcome your collaboration! Note to course participants: Looking forward to some insightful and useful articles from your collaborations.

Centrifuge - a mechanical device that decreases sedimentation time through the use of centripetal acceleration.

Types of centrifuges

Centrifuges fall into three different categories based on size: laboratory, process-scale, and human-sized. Laboratory centrifuges are the smallest of the three and are primarily used for cell sedimentation and purification. Industrial sized centrifuges are significantly larger than laboratory centrifuges and are utilized for large separation processes. Yet the largest centrifuges are human-sized and are scientifically utilized by space agencies and biomedical research to simulate high gravity conditions.

Laboratory centrifuges

Laboratory Centrifuges are frequently used in various scientific protocols such as DNA and protein purification. The most common laboratory centrifuge is the bench top centrifuge because they are multipurpose and have removable rotors. Laboratory centrifuges can spin up to about 20,000 rpm.

For more information:

Gas Centrifuge

Sucrose Gradient Centrifuge

Micro Centrifuge

Ultra Centrifuge

Clinical Centrifuge

Bench top Centrifuge

Process scale centrifuges

Biological and chemical processing plants use large-scale centrifuges for separation and sedimentation. The two most common separation centrifuges are tubular bowl centrifuges and disk stack centrifuges. Large gas chromatography centrifuges are also used in Uranium purification.

Tubular bowl centrifuge

The tubular bowl centrifuge operates by allowing fluid to enter the bottom of the centrifuge and exit out the top. Particles separated from the centrifuge are collected on the side of the bowl and need to be cleaned after processing. Fortunately, the bowl is usually easy to remove and wash. Another application for the tubular bowl centrifuge is the separation of a light liquid from a heavy liquid because of the density difference.

Disk stack centrifuge

The disk stack centrifuge is fed from the top into a basin and the clarified liquid is removed from the top after passing through a series of disks. Similar to the tubular bowl centrifuge, the dense particles are captured on the side of the centrifuge. Some disk stack centrifuges have solid discharge valves that can clean the sides and prevent buildup.

Human-sized centrifuge

Moving to a larger scale, human-sized centrifuges are used for high gravity training by NASA and entertainment value in carnival rides such as the Gravitron. Human sized centrifuges spin much slower than their smaller counter parts as sedimentation is undesired.

(PD) Photo: National Aeronautics and Space Administration (NASA)
NASA's 20 g centrifuge.

Equations governing centrifuges

Sedimentation velocity

• The speed at which a particle falls out of solution in Earth’s gravitational field.

$V_{g}={\frac {\pi d_{p}\Delta \rho g}{18\mu }}$

Where: d_p - diameter of the particle (cm), ∆ρ – the difference in densities between the particle and the solvent (g/cm³), g – gravitational constant (cm/s²), μ - viscosity of the solvent (g/cm*s)

RCF: “Relative Centrifugal Force” or g’s

• The strength of the centrifugal acceleration in multiples of gravitational acceleration

$\ RCF={\frac {r*(2\pi \omega )^{2}}{g}}\,$

Where: r – radius of the centrifuge (cm), ω – angular velocity (cm*rad/s), g – gravitational constant (cm/s²)

Particle velocity

• The speed at which the particle falls out of solution in the centrifuge

$V_{r}=V_{g}*RCF$

Retention flow rate

• In an process scale centrifuge, the flow rate at which all the particles will sediment out of solution.

$Q_{r}et=V_{g}*\Sigma$

Where: V_g - sedimentation velocity (cm/s), ∑ - sigma factor (cm²)

Sigma Factor

• $\Sigma$ is the operation constant representing the geometry and speed of the centrifuge.

For tubular bowl centrifuge:

$\Sigma ={\frac {(\pi L(r_{o}^{2}-r_{i}^{2})\omega ^{2})}{gln(r_{o}/r_{i})}}$

Where: L – length of the column (m), r_o - outer radius of centrifuge (cm), r_i - inner radius of centrifuge (cm), ω – angular velocity (cm*rad/s), g – gravitational constant (m/s²)

For a disk stack centrifuge:

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \Sigma = \frac{2\pi n(r_o^3-r_i^3) ω^2}{(3g \tan(\theta)}}

Where: ω – angular velocity (cm*rad/s), n – number of discs, r_o - outer radius of disks (cm), r_i - inner radius of disks (cm), θ - angle between disc and vertical (rad), g – gravitational constant (cm/s²)