Bioseparation Techniques Centrifugation Dr. Tarek Elbashiti Assoc. Prof. of Biotechnology

Centrifugation A centrifuge is a device that separates particles from suspensions or even macromolecules from solutions according to their size, shape and density (weight per unit volume) by subjecting these dispersed systems to artificially induced gravitational fields. Centrifugation can only be used when the isolated material is denser than the medium in which they are dispersed.

Table 6.1 lists the densities of different biological substances that are usually separated by centrifugation.

Based on data shown in Table 6 Based on data shown in Table 6.1 one may wrongly assume that proteins and nucleic acids would settle faster than cells and organelles. Biological macromolecules in aqueous solution exist in an extensively hydrated form i.e. in association with a large number of water molecules. Hence the effective densities of these substances in solution are only slightly higher than that of water. Also, these macromolecules are significantly smaller than cells.

The substances listed in Table 6 The substances listed in Table 6.1 would settle at extremely low velocities under gravity and hence separation would not be feasible. In a centrifugation process, these settling rates are amplified using an artificially induced gravitational field.

Process Functions There are several process functions using centrifuges in biotech separation. These are listed below. 1. Separation (solid/liquid, solid/liquid/liquid and solid/solid/liquid separation) Centrifugation can be used for solid-liquid separation provided the solids are heavier than the liquid.

Centrifuge can also be used to separate a heavy phase, and two lighter liquid phases, with one of the lighter phases being lighter than the other. Solids can be lighter than liquid and separation is by flotation of the dispersed solid phase.

2. Clarification- minimal solids in liquid product Centrifuge can be used to clarify the discharge separated lighter liquid phase. The objective is to minimize the discrete suspended solids in the light continuous phase. Usually, only fine submicron biosolids are left uncaptured by centrifugation and they escape with the discharged light phase.

3. Classification -sort by size and density Centrifuge is used to classify solids of different sizes. One of the several possible applications is to classify crystals of different size range, with the finer submicron sizes leaving with the light phase and retaining only the larger sizes in the separated heavy phase. Either of the separated solids can be the product.

For example, the larger crystals can be the product crystals while the finer crystals are returned to the crystallizer to grow to larger crystals. Another similar application is to classify smaller size cell debris in the light liquid phase from the heavier products after homogenizing cells.

4. Degritting- remove oversized and foreign particles Degritting is similar to classification where unwanted particles, larger or denser, are rejected in the sediment, with product (smaller or less dense) overflowing in the lighter liquid phase. Another situation is where smaller unwanted particles are rejected in the light liquid phase, and valuable heavier solids are settled with the heavier phase.

5. Thickening or concentration- remove liquid, concentrate solids Centrifuge is frequently used to concentrate the solid phase by sedimentation and compaction, removing the excess liquid phase in the overflow or centrate. This reduces the volume of the product in downstream processing.

6. Separation and repulping - remove impurities by washing or diluting With a concentrated suspension containing contaminants such as salts and ions, it is diluted and washed so that the contaminants are dissolved in the wash liquid. Then, the suspension is sent for centrifugation to remove the spent wash liquid with dissolved contaminants or finely suspended solids. Subsequently, the product can be further concentrated by centrifugation.

The abovementioned processes can be combined to achieve several objectives concurrently or in series. Cells, sub-cellular components, virus particles and precipitated forms of proteins and nucleic acids are easy to separate by centrifugation. When macromolecules such as proteins, nucleic acids and carbohydrates need to be separated, normal centrifuges cannot be used and special devices called ultracentrifuges which generate very strong artificial gravitational fields are used.

Centrifuges are classified into two categories: I. Laboratory centrifuges II. Preparative centrifuges I. Laboratory centrifuge Laboratory centrifuges are used for small-scale separation and clarification (i.e.removal of particles from liquids). Typical liquid volumes handled by such devices are in the range of 1-5000 ml.

The principle of separation by centrifugation is shown in Fig. 6.1. There are two types of rotors: fixed angle rotors and swing out rotors. A fixed angle rotor holds the centrifuge in a fixed manner at particular angle to the axis of rotation. Swing out rotors hold the tubes parallel to the axis of rotation while the rotor is stationary but when the rotor is in motion, the tubes swing out such that they are aligned perpendicular to the axis of rotation.

When the centrifuge tubes are spun, the centrifugal action creates an induced gravitational field in an outward direction relative to the axis of rotation and this drives the particles or precipitated matter towards the bottom of the tube. Typical rotation speeds of laboratory centrifuges range from 1,000-15,000 rpm.

The magnitude of the induced gravitational field is measured in terms of the G value: a G value of 1000 refers to an induced field that is thousand time stronger than that to gravity. The G value which is also referred to as the RCF (relative centrifugal force) value depends on the rotation speed as well as the manner in which the centrifuge tubes are held by the rotor:

It is virtually impossible to make very exact calculations for a laboratory centrifugation process. This is due to the fact that it usually takes a certain amount of time after start-up for the rotation speed of the centrifuge to reach the operating value. Similarly it takes a certain amount of time for the rotation speed to decrease from the operating speed to zero at the end of the process.

The settling particles go through different G value zones while moving toward the bottom of the centrifuge tube.

II. Preparative centrifuge Preparative centrifuges can handle significantly larger liquid volumes than laboratory centrifuges, typically ranging from 1 litre to several thousand litres. Preparative centrifuges come in a range of designs, the common feature in these being a tubular rotating chamber. Tubular Bowl Centrifuge For particle size ranges of 0.1 to 200 micrometer and up to 10% solids in the in-going slurry.

A simple diagram of the most common type of preparative centrifuge (the tubular bowl centrifuge)

The suspension to be centrifuged is fed into such a device from one end while the supernatant and precipitate are collected from the other end of the device in a continuous or semi-continuous manner. Typical rotating speeds for preparative centrifuges range from 500 - 2000 rpm. The motion of a particle at any point within a tubular centrifuge in the radial direction is governed by the following force balance equation:

The force used on a particle = frictional force beared by the particle The particle would continue to settle due to centrifugal acceleration till the two forces are balanced. Thus the centrifuge may be altered to use for: a. Light-phase/heavy-phase liquid separation. b. Solids/light-liquid phase/heavy-liquid phase separation. c. Solids/liquid separation

2. Disk Stack or Multichamber Centrifuge For slurry of up to 5% solids of particle size 0.1 to 200 micrometer diameter. A disc stack centrifuge is a special type of preparative centrifuge which is compact in design and gives better solid-liquid separation than the standard tubular bowl centrifuge. Fig. 6.6 shows the working principle of a disc stack centrifuge. The feed enters from the top of the device and is distributed at the bottom of the disk bowl through a hollow drive shaft.

The particles are thrown outer and these come into contact with the angled disc stack. Once this happens they slide down the disc, are collected at the periphery of the bowl and discharged from the device in the form of a slurry. The liquid flows up the device along the central regions and is discharged from the top. The smaller particles collect in the outer chambers where they are subjected to greater centrifugal forces (the greater the radial position of a particle, the greater the rate of sedimentation).

Although these vessels can have a greater solids capacity than tubular bowls and there is no loss of efficiency as the chamber fills with solids, their mechanical strength and design limits their speed to a maximum of 6500 rpm for a rotor 46-cm diamter with a holding capacity of up to 76 cubic dm.

3. Ultracentrifuge An ultracentrifuge is a special type of centrifuge in which the rotor rotates at a much higher speed than a standard centrifuge. Typical rotation speeds in ultracentrifuges range from 30000 rpm-50000 rpm. An ultracentrifuge is usually used for separating macromolecules from solvents or for fractionating mixtures of macromolecules. Ultracentrifuges are used for analytical as well as for preparative applications.

An analytical ultracentrifuge (AUC) is mainly used for studying the properties of macromolecules as well as for analyzing complex mixtures of macromolecules. Preparative ultracentrifuges are used to purify macromolecules such as proteins and nucleic acids based on their physical properties such as size, molecular weight, density and mobility.

The high rotating speeds used in ultracentrifuges can generate considerable amount of heat. Therefore cooling arrangements are required in these devices. An ultracentrifuge is also an angled spin tube, and with a titanium rotor that provides mechanical integrity, i.e. for high shear and yield strengths.

It can go up to 500,000-1,000,000 g for separating very small particles, particles and liquid with a small density difference, and/or separation in a viscous liquid phase. Theodore Svedberg invented the analytical ultracentrifuge in 1923, and won the Nobel Prize in Chemistry in 1926 for his research on colloids and proteins using the ultracentrifuge.

4. Decanter Centrifuge The figure below shows the countercurrent flow decanter, or solid-bowl centrifuge.

After accelerating in the rotating feed compartment or accelerator, feed slurry is introduced to the annular pool. Under high centrifugal force, the heavier solids migrate radially outwards towards the bowl, displacing the lighter liquid to the pool surface at a smaller radius.

Decanter centrifuge is most often used for clarification of liquid containing high concentrations of solids, removing solids from liquid. A decanter centrifuge is a sedimentation centrifuge for separation of suspended solids from one liquid. The characteristic which distinguishes a decanter centrifuge from other types of centrifuges such as disc stack separators is that it has a cono-cylindrical rotor equipped with a conveyor for continuous unloading of sedimented solids.

Decanter centrifuges are used in a wide range of applications where their ability to achieve both good clarity and low moisture in the discharged solids is appreciated. They are very versatile as they can handle both large and small particles as well as a wide variety of solids concentrations. The performance of the centrifuge depends on various operating variables, such as the feed rate, pool depth, rotation speed or G-force, and they should be optimized for a given process.