1. Downstream Processing Short Course May 2007 Kevin Street Gavin Duffy
Sedimentation
Downstream Processing
Short Course
May 2007
Kevin Street
Gavin Duffy

2. Bioprocess Overview Upstream Processing Intra-Cellular Product
Extra-Cellular
Product
Chemical/Enzymatic/
Mechanical/Physical
Cell Disruption
Solid-liquid
Separation
Centrifugation/Sedimentation,
Extraction, Filtration
Evaporation, Ultrafiltration,
Adsorption, Precipitation
Concentration
Purification
Chromatography
Crystallisation, freeze drying,
Spray drying, sterile filtration
Formulation
Final Product
Basic Biotechnology, 2nd Ed, Ch 9

3. Learning Outcomes After this lecture you should be able to…
Describe the sedimentation process and equipment
Describe the motion of particles in free fall
Calculate the terminal velocity of a particle

4. Sedimentation
This is the separation of a liquid from particles suspended in the liquid
A particle, falling from rest, accelerates under the force of gravity
The drag force increases so the acceleration decreases (liquid viscosity is important here)
Acceleration eventually becomes zero – the terminal velocity is reached
Terminal velocity is reached quickly, e.g. a 100 m particle in water reaches 2 mm/s in 1.5 ms
Upward velocity of liquid must be less than terminal velocity for sedimentation to work
We must know the terminal velocity!

5. Sedimentation Tank

6. Single Particle Terminal velocity
For low Particle Reynolds number:
Creeping flow
Drag coefficient increases with velocity
Stokes law region
For high Particle Reynolds number:
Inertial flow (fluid must accelerate out of path)
Drag coefficient constant

7. Drag coefficient The drag coefficient is defined as:
R’ is the drag force per unit projected area (N)
u is the velocity (m/s)
ρf is the fluid density (kg/m3)
(What are the units of CD?)
Stokes’ law region:
Intermediate region:
Newton’s law region:

8. Drag curve for motion of a particle in fluid
Stokes’
Newton’s
BL separation
Introduction to Particle Technology, Martin Rhodes, Ch 1

9. Sphericity Sphericity = surface area of equivalent sphere
surface area of particle
Equivalent sphere = sphere of same volume as particle
Deviation from sphere does not matter in Stokes’ law region as much as in Newton’s law region
Particles fall with their small surface pointed downwards in Stokes’ law region
The largest surface is pointed downwards in Newton’s law region

10. Activity – Calculate Terminal Velocity
What are the particle Reynolds number and terminal velocity for the following system?
Diameter 3 m
Density of solid phase 1090 kg/m3
Cell free liquid density 1025 kg/m3
Cell free liquid viscosity Pa.s
Data taken from a case study of r-HSA production with recombinant Pichia Pastoris prepared by L Van der Wielen, European Federation on Biotechnology

11. If you don’t know which region…
Calculate CDRe2 from the following eqn:
Use result to draw a line on the drag curve
For example, suppose CDRe2 = 8
Then, for Re = 10 Re2 = 100 CD = for Re = 1 Re2 = 1 CD = 8 for Re = 0.1 Re2 = 0.01 CD = 800
Use these points to draw the line and read the Particle Reynolds number. The velocity is then obtained

12. …..use the Re v Drag coefficient chartx x x

13. The Thickener Feed added gently just below surface
Upward velocity of liquid must be less than uT
Capacity depends on area: big area = low velocity (Q = va)
Degree of thickening depends on residence time which depends on height
Can heat tank to reduce viscosity and increase uT
Limit to solids flux
20/4/07

14. Batch Settling Test

15. Thickener Area Calculation
where A = area (m2) Q0 = feed rate of suspension (m3/s) Y = mass ratio liquid to solid in feed U = mass ratio liquid to solid in underflow C = particle volume fraction (1-ε) ρs = density of solid (kg/m3) uT = terminal velocity at conc. C (m/s) ρf = density of liquid (kg/m3)

16. Activity – Calculate Terminal Velocity
based on worked example 2.1 from Rhodes.