Centrifugation Downstream Processing Short Course Kevin Street May 2007 Gavin Duffy.
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Transcript of Centrifugation Downstream Processing Short Course Kevin Street May 2007 Gavin Duffy.
Centrifugation
Downstream ProcessingShort CourseKevin StreetMay 2007Gavin Duffy
Learning Outcomes
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
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
Separation of milk into skimmed milk and cream is done with a centrifuge
http://www.nzifst.org.nz/unitoperations/mechseparation4.htm
Centrifugal Motion
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
mg Acceleration constant
In direction of earth Equilibrium velocity
reached
Terminal velocity given by:
mrω2
Acceleration increases with r
Acceleration increases with ω
Away from axis of rotation Equilibrium velocity never
reached Instantaneous velocity:
18
2 gdu fpT
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)
g
ruu T
2
uT is the terminal velocity of the particler is the distance from axis of rotationω is the angular velocity
Sigma Factor
The capacity of a centrifuge is defined by Σ
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
TuQ
Activity – Determine Σ
The contents of a fermenter are discharged to a centrifuge
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
For the bowl centrifuge:
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
rRg
HrR c
ln
222
The Disc Stack Centrifuge
http://drugtopics.mediwire.com/main/Default.aspx?P=Content&ArticleID=172950
Large particles have higher settling velocities than small particlesCellular debris ends up at the outer edge of the bowlSoluble intracellular material passes through with the clarified liquidDiscs 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:
ω is the angular velocity (rad/s) n is the number of discsR 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)
tan3
2 332
g
rRn
Activity
What is the capacity of the following disc stack centrifuge?
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
http://www.sgconsulting.co.za/Products/Decanters/Decanters.htm
Clarified liquidRotating Bowl
Rotating scroll