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