Sedimentation

1. 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 Centrifugation/Sedimentation, Extraction, Filtration Solid-liquid Separation Evaporation, Ultrafiltration, Adsorption, Precipitation Concentration Chromatography Purification 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 0.005 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 = 0.08 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 chart x 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 http://www.filtration-and-separation.com/thickener/sld004.htm 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.