Centrifuges
A centrifuge is a piece of equipment, generally driven by a motor,
that puts an object in rotation around a fixed axis, applying force
perpendicular to the axis.
The centrifuge works using the sedimentation principle,
where the centrifugal force is used to separate substances of greater
and less density.
Terminology
Pellet - Material that has accumulated on the bottom of a tube
after centrifugation (if any) is called the "pellet“. Material
comprising the pellet is said to have "sedimented".
Supernatant - The overlying fluid is called the "supernatant
solution" or simple the "supernate“.
Relative centrifugal force or Centrifugal effect - The force that
the sample is subjected to, is expressed as some number times
the force of gravity, or relative centrifugal force (RCF).
Critical speed - Critical speed is the speed when the frequency of
rotation matches the natural frequency of the rotating part
and any vibration induced by slight imbalance in the rotor is
highly magnified, resulting in high stresses and even failure of
the equipment.
Almost all centrifuges operate above the critical speed.
General Principles of Centrifuges
Basis of separation:
-Size –Shape -Density
Methodology:
- Utilizes density difference between the particles and the medium in
which these are dispersed
- Dispersed systems are subjected to artificially induced gravitational
fields.
- The centrifugal force causes the sedimentation of heavier solid particles.
Supernatant
Induced
gravitational
field
Suspension
In process
Precipitate
Separation
complete
Centrifugal Force
Let us consider a body of mass m rotating
in a circular path of radius r at a velocity
v. The force acting on the body in a radial
direction is given by:
F=
mv 2
r
Here, F = centrifugal force
m = mass of the body
v = velocity of body
r = radius of circle of rotation
The same body is experiencing gravitational force. It can be expressed as:
G = mg
where, G = gravitational force , and
g = gravitational constant
The centrifugal effect is the ratio of the two forces, so that:
C=F/G
= mv2 / mgr
= v2 / gr
But:
v = 2πrn
Where n = speed of rotation.
F / G = (2πrn)2 / gr
= 4π2r2n2 / gr
= 2 π2n2d / g ------------------------ (1)
Where d = diameter of rotation.
So,
The gravitational constant has a value of 9.807 m/s2, so that equation (1)
may be simplified to:
Centrifugal effect = 2.013 n2d
Provided that n is expressed in s-1 and d is in meter.
Materials of Construction
A centrifuge is comprised of two major components:
1.
The drive mechanism:
The drive mechanism is the source of rotary motion, and is
powered by an electric motor, by air pressure, or by oil
turbines, depending upon the type of centrifuge.
2.
The Rotor
The rotor is the large rotating element of a centrifuge into or
onto which samples are loaded. It is driven about a fixed axis
(or shaft) by the drive mechanism, with expenditure of large
amounts of energy.
Types of rotors
There are two fundamental types of rotors:
1. Fixed-angle rotor: In fixed angle rotors the tubes containing
samples are placed into shields or openings in the rotor at one
particular pre-set angle. The tubes are thus tilted with their
tops closer to the shaft than their bottoms, and remain in that
fixed position during the run, regardless of rotor speed.
2.
Swinging-bucket rotor: In swinging bucket rotors, the tubes
are initially vertical. The bottoms of the sample tubes then
swing outward freely as the shaft rotates, and the tubes are
actually horizontal during the run. By the time the centrifuge
stops, however, the tubes have returned to their starting
vertical position. Swinging-bucket rotors are particularly
useful for sedimenting a sample through a density gradient
A. Fixed Angle Rotor
B. Swinging Bucket Rotor
Application of Centrifuges
•
Production of bulk drugs:
In the bulk drug industry, whenever a crystalline material is to be separated
from a suspension, e.g., aspirin is separated from its mother liquor by
centrifugation.
•
Production of biological products:
a) Separation of blood cells.
b) Purification of insulin by selectively precipitating other fraction of
proteins.
c) Separation of most of the proteinaceous drugs and macromolecules.
•
Biopharmaceutical analysis of drugs:
Drugs present in the blood, tissue fluids and urine are normally present in the
form of colloidal dispersion. Centrifugation is used for separating the drugs
which is essential for the evaluation of pharmacokinetic parameters and
bioequivalence studies.
• Evaluation of suspension and emulsion:
Centrifugation method is used as a rapid empirical test parameter for
the evaluation of suspension and emulsion.
e.g., A stable emulsion should not show any signs of separation even
after centrifuging at 2000-3000 rpm at room temperature.
• Ultracentrifugation are used for determination of molecular weight
of serum albumin, insulin etc.
• Isolation of bacterial cells, fungal and actinomycete mycelium and
spores from liquid growth and fermentation media is facilitated by
laboratory centrifuge.
• Removal of finely suspended solid matter (clarification) from
aqueous or oily materials can be carried out by centrifuging at high
speeds without the necessity of a filter.
• Ultracentrifuge can be used for separation of virus particles which
has potential industrial applications.
Types of Centrifuges
Centrifuges can be classified on the basis of speed, temperature.
(A) SPEED
The "speed" of a centrifuge is measured in revolutions per
minute, or rpm. Centrifuges are generally divided into 3
categories based on their maximum attainable speed:
1. "Low-speed": to maximum of ~ 5 x 103 rpm.
2. "High-speed": to maximum of ~2 x 104 rpm.
3. "Ultracentrifuges": to maximum of ~105 rpm.
(B) TEMPERATURE
Centrifuges are either refrigerated or not refrigerated.
Refrigerated centrifuges have a built-in refrigeration unit
surrounding the rotor, with a temperature sensor and
thermostat permitting selection of a particular temperature or a
permissible temperature range that is maintained during
centrifugation. Many biological samples are temperature
sensitive, and centrifugation in the cold (say, 1-4oC) is
frequently required.
Centrifuges that are not refrigerated are normally used at
whatever temperature the room they are in happens to be. This
is typically described in research reports as "room temperature"
or "ambient temperature“.
Classification of Centrifuges On The Basis of Its Use
Centrifuges
Laboratory equipment
Commercial equipment
1. Swing-out arm type
2. Angle type
3. Ultracentrifuge
Batch
1. Top driven
2. Under driven
Perforated bowl types
Semicontinuous
Sedimentation centrifuges
Continuous or
Supercentrifuge
Vertical
1. Simple bowl
2. Bowl with plates
Horizontal
continuous
decanters
Industrial Centrifuges
Sedimentation Type Centrifuges / Centrifugal
Sedimenters
It produces sedimentation of solids based on the
difference in the densities of two or more phases of
the mixture.
Examples include:
a) Tubular bowl centrifuges
b) Conical disk centrifuges
Figure: Tubular bowl Centrifuge
Figure: Conical disk centrifuge
Advantages of Centrifugal Sedimenters
Centrifuges are compact and occupy a very small space.
• The separating efficiency is very high, so that the particles
are deposited very rapidly. It is suitable , if the sediment is
a fluid deposit or sludge.
• Two immiscible liquids are easily separated on a
continuous process .
• Centrifugation is the ideal technique for dealing with
difficult solids; for example , substances that are slimy or
compressible and would block a filter medium .e.g..
coagulated proteins.
• By controlling the speed of rotation and rate of flow coarse
particle are separated, while finer particles remain in the
suspension.
Disadvantages of Centrifugal Sedimenters
•
•
The construction of sedimentation centrifuge is complicated
due to high speed of rotation requires as special driving
mechanisms.
The capacity is limited.
Uses of Centrifugal Sedimenters
1.
2.
3.
4.
5.
Dirt and water are removed from oils. Example, in the
purification of olive oil or fish liver oil, this method is used.
Lanolin is recovered from the wool by scouring process.
The liquid /liquid phases are separated in the extraction of
drugs, for examples, manufacture of antibiotics.
Bacterial enzymes are prepared by removing bacteria.
Suspensions containing low concentration of solids can be
separated quickly.
6. Blood plasma is separated from whole blood.
7. Starch is collected after washing and purification
states.
8. In the manufacture of insulin, liquor is clarified so
as to remove the precipitated proteins.
9. Stability of emulsion is examined.
10. Different particle size fractions are separated by
using control of speed of rotation and rate of flow.
Perforated Bowl Centrifuge / Centrifugal Filters
• It’s a filtration centrifuge.
• Separation occurs through a
perforated wall based on the
difference in the densities of
solid and liquid phases.
• During centrifugation the
liquid phase passes through
the perforated wall, while
solid phase is retained in the
bowl.
• The solid is removed after
cutting the sediment by a
blade after stopping the
centrifuge.
Figure: Perforated bowl centrifuge
Advantages of Perforated Centrifuge
• The Centrifuge is very compact and it occupies very little floor
space.
• It can handle slurries with a high proportion of solids and even
those having paste like consistency.
• The final product has very low moisture content.
• In this method, the dissolved solids are separated from the cake.
• The process is rapid.
Disadvantages of Perforated Centrifuge
 The entire cycle is complicated resulting in considerable labor
costs.
 The process is intermittent, working on the batch principle.
 On prolonged operation, the solids may form hard cake, due to
the centrifugal force, which is difficult to remove simultaneously.
Uses of Perforated Bowl Centrifuge
• Used for removing unwanted solids from a liquid.
e. g precipitated proteins are removed from
insulin.
• Extensively used for separating crystalline drugs
(such as aspirin) from the mother liquor.
• Sugar crystals are separated using perforated basket
centrifuge.
THE END