Post on 15-Apr-2017
CentrifugationDr. N. Banu, Associate Professor
Vels University
• use of the centrifugal force for the separation of mixtures
• More-dense components migrate away from the axis of the centrifuge
• less-dense components migrate towards the axis.
Centrifugation
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CentrifugationCentrifugation is used to separate materials of different density when a force greater than gravity is desiredThe type of industrial centrifugation unit:• Tubular bowl centrifuge (Narrow tubular bowl centrifuge or ultracentrifuge, decanter centrifuge, etc). Simple and widely applied in food and pharmaceutical industry. Operates at 13000-16000 G, 105-106 G for ultracentrifuge• Disc-stack bowl centrifuge. This type is common in bioprocess. The developed forces is 5000-15000 G with minimal density difference between solid and liquid is 0.01-0.03 kg/m3. The minimum particle diameter is 5 µm
Industrial centrifuges
Tubular bowl Chamber Disc
Tube◦ High centrifugal force◦ Good dewatering◦ Easy to clean
Chamber◦ Large solids capacity◦ Good dewatering◦ Bowl cooling possible
Disc type◦ Solids discharge◦ No foaming◦ Bowl cooling possible
◦ Limited solids capacity◦ Foams◦ Difficult to recover
protein
◦ No solids discharge◦ Cleaning difficult◦ Solids recovery difficult
◦ Poor dewatering◦ Difficult to clean
Properties of industrial centrifuges
Bacteria ◦ Small cell size◦ Resilient
Yeast cells◦ Large cells◦ Resilient
Filamentous fungi◦ Mycelial◦ Resilient
Cultured animal cells◦ Large cells◦ Very fragile
◦ High speed required◦ Low cell damage
◦ Lower speed required◦ Low cell damage
◦ Lower speed required◦ High water retention in
pellet
◦ Very susceptible to damage
Centrifugation properties of different cell types
meeting the challenge
a key role in many industrial processes, including the production of insulin, is to separate liquid phases and solids from each other
centrifugation can be utilized to separate the cellular debris from the released protein
purpose of the centrifugation
Types of centrifuges
decanter centrifuge
disk stack centrifuge
one of a kindproperties of the disk stack centrifugation
ideal for a wide range of separation tasks that involve
particle (biomass) size – 1.5 microns biomass percentage – 0.6%total solid percentage - 5.54%pr
oces
s!
• lower solids concentrations
•smaller particle sizes
working principleshow a disk stack centrifuge works
• separates solids and liquid phases in a continuous process• uses extremely high centrifugal forces• denser solids are forced outwards against the solid bowl wall• less dense liquid phases form concentric inner layers• inserting special plates provides additional surface settling area
working principleshow a disk stack centrifuge works
the whole is the sum of partsparts of the disk stack centrifuge
inlet zone• reduces shear forces and amount of
foaming• increases and avoids disturbances of the
separation processes occurring in the bowlliquid discharge section
• important that oxygen pick-up is kept to a minimum
• temperature increases in the liquid must be avoided to prevent problems later in the process
solids discharge section• remove solids by continuous solids discharge, intermittent solids
discharge or manual removal
the whole is the sum of partsparts of the disk stack centrifuge
disk stack area• heart of the centrifuge• key to good separation
performance lies in the efficiency of the disk stack
• layout and design of the distribution holes ensure that the process flow is evenly spread among all the disks
keys to exceptional performancedesign considerations
efficiency depends on• solids volume fraction• sedimentation area• rotational speed
efficiency can be improved if • particle diameter is increased (coagulation,
flocculation)• residence time • distance for sedimentation
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gDu pfp
g2
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rDu pfp
c22
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The centrifugation theoryThe terminal velocity during gravity settling of a small spherical particle in dilute suspension is given by Stoke’s law:
Where ug is sedimentation velocity under gravity, ρp is particle density, ρf is liquid density, µ is liquid viscosity, Dp is diameter of the particle, and g is gravitational acceleration.In the centrifuge:
uc is particle velocity in the centrifuge, ω is angular velocity in rad/s, and r is radius of the centrifuge drum.
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3132
2
tan312 rr
gN
2122
2
32
rrgb
The centrifugation theoryDisc-stack bowl centrifuge
N is number of disc, θ is half-cone angle of the disc. The r1 and r2 are inner and outer radius of the disc, respectively.
Tubular-bowl centrifuge
b is length of the bowl, r1 and r2 are inner and outer radius of the wall of the bowl.