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

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• 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.