5. Centrifugation Processes

Course Instructor : Dr. A. Margaritis, Ph.D., F.C.I.C., P. Eng.

Professor of Biochemical Engineering

Fall 2007

Centrifugation

• Separation of solids by centrifugal force

- Solid (particles) in liquids

- Immiscible liquids of different densities (emulsion droplets) dispersion behave like solid particles

Principles of centrifugal separation and filtration

(a)Bowl stationary(b)Sedimentation in rotating imperforate bowl(c)Filtration in rotating perforate basket

Perry’s Chemical Engineers’Handbook. 6thed. McGraw Hill, NY 1984

Physics of centrifugation

Physics of centrifugation

Centrifugal force on unconstrained particles in suspension

• Accelerates to speed of rotating fluid at every radial position due to viscous drag by fluid

• Impulses on particle tangential to series of orbits

• Net effect –particle moves radially outward from center of rotation

Centrifugal force on unconstrained particles in suspension

Relative centrifugal factor (RCF); g factor

2centrifugal accelerationRCF = acceleration due to gravity

rg

ω=

Centrifugal force on unconstrained particles in suspension

2 3 2( ) ( )6

3 (Particle velocity)=3

P C P P P P

D P P

Fc m a V r D r

drF D Ddt

πρ ρ ω ρ ρ ω

πμ πμ

= = − = −

=

FC= centrifugal force

FD= viscous (Stokes law)drag forcem

P= effective particle mass(bouyancy)

VP= particle volume

ρP= particle density

ρ= fluid density

DP = particle diameter

μ= fluid viscosity

Centrifugal force on unconstrained particles in suspension

At steady state radial particle velocity : FC = FD

mP= effective particle mass

vg= terminal velocity in the gravitational field

vg, g are constant

Centrifugal sedimentationSedimenting centrifuges:

Separate phases (solid –liquid, liquid –liquid) by forcing

migration of denser phase away from axis of rotation.

Examples:

1. Decanter

2. Disk stack

3. Tubular bowl

1. Decanter centrifuge: two liquid phases + one solid phase

2. Disk stack

3. Tubular bowl Centrifuge

Sharples (Alfa Laval) A26 Tubular Bowl Centrifuge

Cylindrical bowl centrifuge: Basic Sigma (Σ) theory

• Used to characterize continuous centrifuges

• Valid for cylindrical bowl centrifuges (tubular bowl)

and dilute suspension of solids (≤20 g/L)

• Time required for centrifugal sedimentation = time

for fluid element to travel from point of entry to

discharge

Basic Sigma (Σ) theoryAssumptions:

- Plug flow of liquid parallel to axis of rotation

- particle / drop reaches radial velocity (dr/dt) immediately upon entering the centrifuge (pond)

- Stokes law applies to centrifugal settling

- particle “captured”if it reaches the centrifuge wall before fluid exits, otherwise it leaves with the liquid steam (centrate)

Basic Sigma (Σ) theory

Basic Sigma (Σ) theory• Critical particle trajectory

Trajectory of particle of critical diameter entering at r1,sufficiently large for particle to be captured

V = centrifuge (bowl) volume

Basic Sigma (Σ) theory

QC/vg (unit of area) represents the required plan area of a settling tank,

operating under ideal conditions, needed to perform the same

clarification as the centrifuge

LHS is called the process parameters

RHS is called the machine parameters –theoretical value for centrifuge

at 100% particle capture efficiency

Both sides of equation represented by (Σ) sigma

Liquid–liquid separation by centrifugation

Liquid–liquid separation by centrifugation

Liquid–liquid separation by centrifugation

Liquid–liquid separation by centrifugation

Cylindrical bowl centrifuges: concept of 50% capture

Cylindrical bowl centrifuges: concept of 50% capture

Cylindrical bowl centrifuges: concept of 50% capture

For D >DPC particles are eliminated from the liquid stream

For D <DPC particles stay in the effluent or centrate

For D = DPC they are split between the two streams(D = particle diameter)

Cylindrical bowl centrifuges: concept of 50% capture

Tubular –bowl centrifuge

• Feed enters bottom of bowl through nozzle under pressure

• Incoming liquid is accelerated and moves upwards

• Solids travel upward and receive radial velocity based on size and

weight in centrifugal force field

• If the particle trajectory intersects bowl wall it is removed

• Used for low solids loading in feed

Tubular –bowl centrifuge• Capture efficiency (Σ(1/2) )

Working principles of solid settling in settling tank, settling tank with disks, disk stack centrifuge.

Photo of Disk stack centrifuge

Alfa Laval – disc stack centrifuge technology

Design Equation for Disk Stack centrifuge

• Feed enters to the center of bowl near floor and rises through a

series of disks or cones spaced 0.4 –3 mm apart

• Angled disks (35o–50o half angle with vertical)

• Two types of disks are available-those with holes which provide

channels through which liquid rises-those with solid disks

Design Equation for Disk Stack centrifuge

Stacked cones in Disk stack centrifuge

Design Equation for Disk Stack centrifuge

Purpose of disks is to reduce sedimentation distance, and also reduce chance of retrainment of particles in liquid.

Centrifugal extraction

• Counter-current extraction performed in a centrifugal force field Recovery of antibiotics (penicillin, erythromycin, bacitracin)

• AdvantageVery short residence time –seconds minimizes product degradation (acid hydrolysis of penicillin)

Centrifugal extraction

Example: Podbelniak extractor

Bailey and Ollis, Biochemical Engineering Fundamentals, 2nded., McGraw Hill, NY 1986

Scale-up of centrifuge, Batch centrifugation

Lab/pilot scale experiments;

Step 1. Spin broth in 50 mL tubes in swing-out rotor centrifuge at various RCF (g

forces)

Step 2. Measure clarity of supernatant, sediment volume and moisture content of

sediment

Step 3. If RCF x t > 2x107 s for desired clarity or solids thickening, then consider

broth conditioning to enhance flocculation of particles.

Step 4. If RCF x t < 2x107 s, for desired clarity or solids thickening, then there is no

need to condition the broth.

For batch scale-up of centrifuge operate at the RCF x t value determined in step 1 .

2centrifugal acceleration ω rRCF = =acceleration due to gravity g

Remember:

Swing-out rotor centrifuge

Hettich ROTINA 35 Tabletop Centrifuge

Scale-up of centrifuge, Continuous centrifugation

The following steps are recommended for scale-up of a Continuous centrifuge:

Step 1. Use small scale continuous centrifuge of same type (disc stack or tubular).

Step 2. Process broth under varying conditions until desired liquid clarification or solid thickening is obtained.

Step 3. Calculate machine parameters (Σ1) and flow rate (Q1)under these desired conditions for the small centrifuge.

Step 4. If Q1/Σ1< 10-8 m/s for 80% solids recovery, consider broth conditioning to enhanceflocculation of solids to larger particles.

Step 5. If Q1/Σ1>10-8 m/s for 80% solids recovery, there is no need for broth conditioning.

Step 6. Calculate required machine parameters (Σ2) for production scale centrifuge, and process flow rate (Q2)using the following scale-up equation:

Where: 1 = small scale centrifuge, 2 = large commercial scale centrifuge

Perry’s Chemical Engineers’Handbook, 6thed., McGraw Hill. NY 1984

Perry’s Chemical Engineers’Handbook, 6thed., McGraw Hill. NY 1984

Bailey and Ollis, Biochemical Engineering Fundamentals, 2nd ed., McGraw Hill, NY1986

6.3.7. Microbial Biomass Centrifugal Sedimentation Summary

Bailey and Ollis, Biochemical Engineering Fundamentals, 2nd ed., McGraw Hill, NY1986

Perry’s Chemical Engineers’Handbook, 6thed., McGraw Hill. NY 1984

Choice of centrifuges