Download Centrifugation

Centrifugation
Downstream Processing
Short Course
Kevin Street
May 2007
Gavin Duffy
Learning Outcomes
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After this lecture you should be able to…
Explain centrifugation
Compare centrifugal and gravitational forces
Describe different types of centrifuge
Define the sigma factor
Write the sigma factor for a bowl and disc stack
centrifuges
Determine the sigma factor for a given specification
General Idea
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Feed added to spinning bowl
Sedimentation of particles occurs
in centrifugal field
Flow is upwards at a particular
rate which determines residence
time in device
Separation happens if
sedimentation velocity is high
enough for particle to reach side
of bowl within residence time
Large particles have higher
settling velocities than small
particles
Both large and small are still
particles, have small Reynolds
no.s (<1) and obey Stokes’ Law
The Centrifuge
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Separation of milk into skimmed milk and cream is done
with a centrifuge
http://www.nzifst.org.nz/unitoperations/mechseparation4.htm
Centrifugal Motion
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Centrifugal acceleration = rω2
ω is the angular velocity in
rad/s
r is the radius of rotation
Centrifugal force = mrω2
m is the mass of the particle
Compared to gravity
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mg
Acceleration constant
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In direction of earth
Equilibrium velocity reached
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Terminal velocity given by:
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uT 
d 2  p   f g
18
d is particle diameter (m)
ρp is the particle density (kg/m3)
ρf is the fluid density (kg/m3)
g is acceleration due to gravity (m/s2)
 is the fluid viscosity (Pa.s)
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mrω2
Acceleration increases with r
Acceleration increases with ω
Away from axis of rotation
Equilibrium velocity never
reached
Instantaneous velocity:
r 2
u  uT
g
uT is the terminal velocity of the particle
r is the distance from axis of rotation
ω is the angular velocity
Sigma Factor
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The capacity of a centrifuge is defined by Σ
Q  uT 
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Q is the throughput (m3/s) at which all particles with a
terminal velocity ≥ uT (m/s) are retained
Σ has units of m2 and is equivalent to the cross
sectional area of a thickener with the same capacity
Activity – Determine Σ
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The contents of a fermenter are discharged to a
centrifuge
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Volume of material is 100 m3
Centrifugation time is 5 hrs
Particle size is 3 m – all particles of this size are separated
Density of solid phase 1090 kg/m3
Cell free liquid density 1025 kg/m3
Cell free liquid viscosity 0.005 Pa.s
Calculate the capacity factor, Σ
Bowl Centrifuge
Sigma Factor – Bowl Centrifuge
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For the bowl centrifuge:
 R


 rc H
g ln R r 
2
2
2
ω is the angular velocity (rad/s)
R is the outer radius of the bowl (m)
rc is the radius of the clarified discharge weir (m)
H is the height of the bowl (m)
r is the inner radius of the liquid in the bowl (m)
g is the acceleration due to gravity (m/s2)
A long thin bowl gives good separation
The Disc Stack Centrifuge
http://drugtopics.mediwire.com/main/Default.aspx?P=Content&ArticleID=172950
Large particles have higher settling velocities than small particles
Cellular debris ends up at the outer edge of the bowl
Soluble intracellular material passes through with the clarified liquid
Discs give a higher sigma factor
Benefit of Discs
The discs split the stream
into a large number of very
thin layers thereby
improving separation
Solids flow downwards on
bottom face of disc
Liquid flows upwards on
top face of disc
Sigma factor  no. of discs
Disc Stack Centrifuge Capacity
For the disc stack centrifuge:

2  n R  r

3
g tan 
2
3
ω is the angular velocity (rad/s)
n is the number of discs
R is the outer radius of the discs (m)
r is the inner radius of the discs (m)
θ is the angle between disc and vertical (rad)
g is the acceleration due to gravity (m/s2)
3
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Activity
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What is the capacity of the following disc stack
centrifuge?
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No. of discs = 400
Space between discs is 0.01 m
Inner radius = 0.05 m
Outer radius = 0.20 m
Half vertical angle = 45°
Rotational speed = 4500 rpm
What effect does an increase in diameter have on the
capacity?
What diameter is needed for the previous example?
Decanter Centrifuge
Rotating scroll
Rotating Bowl
http://www.sgconsulting.co.za/Products/Decanters/Decanters.htm
Clarified liquid