particle technology- centrifugal separation

Centrifugal Separation

Chapter 8 in Fundamentals

Professor Richard Holdich

R.G.Holdich@Lboro.ac.uk Course details: Particle Technology, module code: CGB019 and CGB919, 2nd year of study.

Watch this lecture at http://www.vimeo.com/10203052

Visit http://www.midlandit.co.uk/particletechnology.htm

for further resources.

Centrifugal separation

Sedimenting centrifuges Particle motion in a centrifugal field Sigma theory Hydrocyclones Grade efficiency & cut size Filtering centrifuges Adaptation of filtration equations Washing (ratio) & Drying

Scroll Discharge Decanter

Archimedian screw to convey solids out of the centrifuge

Imperforate bowl, i.e. sedimenting not filtering

Image courtesy of Thomas Broadbent & Sons Limited Image courtesy of Siebtechnik GmbH

Scroll Discharge Decanter

Screw rotates at only slight differential speed to the centrifuge - solids leave at one end, centrate at the other.

Image courtesy of Siebtechnik GmbH

Tubular bowl centrifuge

This one is vertical axis - simple design with no internals for clarification or liquid/liquid separation - a more complicated design is the chamber bowl.

Image removed for copyright reasons. For an example product please see

http://www.sharpenntechnologies.com/pcat-gifs/products-large2/high-speed-centrifuge1111-2.jpg

.

Disc stack centrifuge

Like a lamella clarifier: internal surfaces to encourage settling - usually used in oil/water separation and cream

Sedimenting Centrifuges –

Let’s confine our analysis to a simple geometry - ignoring the complicated internal structures required to remove deposited solids and oil concentrates.

Air core

Inner radius

Outer radius

Liquid flow out

Gravity settling

Field force (weight) is:

Drag force is:

gx

s )(6

3

xU3

Giving:

18

)(2 gxU s

t

Centrifugal settling

Field force (weight) is:

Drag force is:

23

)(6

r

xs

xU3

Giving:

18

)(

d

d 22 rx

t

r s

Centrifugal settling

i.e. U = f(r)

r

18

)(

d

d 22 rx

t

r s

i.e. U = dr/dt

Sedimenting Centrifuges

Centrifugal settling

i.e. the radial residence time in the machine

limits: r=r1 at t=0 to r=r2 at t=t

Giving:

18

)(

d

d 22 rx

t

r s

2212

)(

)/ln(18

sx

rrt

Horizontal/axial residence time

where

Qt

Volume

LrrV )( 21

22

Sedimenting Centrifuges

Critical trajectory model

Residence time axially and radially is the same.

Q

V

x

rrt

s

22

12

)(

)/ln(18

Critical trajectory model

Multiply through by ‘g’:

)/ln(18

)(

12

22

rrg

gx

V

Q s

Critical trajectory model

Multiply through by ‘g’:

)/ln(18

)(

12

22

rrg

gx

V

Q s

Square bracketed term is the terminal settling velocity of a particle of size x.

Critical trajectory model- Eq 8.10 & 5.28!

Rearrange:

)/ln( 12

2

rrg

V

U

Q

t

c.f. a gravity settling basin

m2

Machine parameters

)/ln( 12

2

rrg

V

The theoretical settling basin equivalent PLAN area given the dimensions of the machine in question and its operating conditions.

m2

Process parameters

tU

Q

The measured value given the process flow rate and operating performance for the 100% cut-off.

m2

Sigma values

)/ln( 12

2

rrg

VSigma machine m2

tU

QSigma process m2

The two sigma values are equal for 100% efficient machines - normally 40 to 60% may be achieved.

Uses of sigma values

To compare between different machines of same geometry

Attempts to compare between different types of machines

Estimate of machine size required to replace gravity settling clarifier

You need a density difference!

Flue gas desulphurisation

Feed:CaSO4 - 35water - 65 100%

Cake:CaSO4 - 70water - 30 100%

Centrate:CaSO4 - 2.7water - 97.3 100%

All concentrations as mass percent

Hydrocyclone

Single unit and array:

Defined by diameterof cylindrical section

Image showing "Krebs gMAX® Hydrocyclones" courtesy of FLSmidth Krebs Inc.

Means of separation

Centrifugal: 800 g in 300 mm hydrocyclone 50000 g in 10 mm hydrocyclone

Type of separator: a classifier (i.e. splits into sizes) a thickener (i.e. concentrates

suspensions)

Operating data

Diameters: 0.01 to 1 metre

Solid (cut) sizes: 2 to 250 microns

Flow rates (single unit):0.1 - 5000 m3

h-1

Pressure drop: 6 to 0.4 barU/F solid content: up to 50% v/v

(claimed)

Principal features

Note: primary & secondary vortex, air core, U/F, O/F, tangential feed

Tangential velocity

Radial velocity

Axial velocity

Grade efficiency – Cut Point

Feed distribution is split into two fractions: Overflow

Underflow

Grade efficiency

Fraction by mass of each grade entering the U/F of the hydrocyclone.

Recovery is the overall fraction entering the U/F - usually by volume.

Grade efficiency

Equation:

feedin gradein mass

underflowin gradein massE

Grade efficiency

What is the grade efficiency of the following?

Overflow50 kg/h

Underflow50 kg/h

Grade efficiency

Equation:

feedin gradein mass

underflowin gradein massE

100%

Rf

0%

Grade efficiency

i.e. we need to correct for effect due to flow split in order to reliably record the ability of the device to act as a classifier.

The reduced grade efficiency.

Grade efficiency

Reduced grade efficiency:

fREE '

Normalised reduced grade efficiency:

f

f

R

REE

1

''

<100%

100%

Equilibrium Orbit Theory

A particle orbiting on the LZVV has no net tendency to move into the primary vortex (then O/F) or secondary vortex (then U/F).

It must be equal to the cut size x50%.

Equilibrium Orbit Theory

Force balance: centrifugal

23 )(6

is rx

Tangential velocity:i

i

r

v

Liquid drag: Ux3 FD FC

Hydrocyclones - types and configurations

Oil/water separation - often offshore

Filtering Centrifuges

A perforated bowl - similar to a spin dryer

See box on page 83 for descriptions

Filtering Centrifuge – Section 8.3

Pusher generally coarse solids > 50 microns (semi)-continuous solids output careful balance of slurry in

Image courtesy of Siebtechnik GmbH

Filtering Centrifuge

Peeler generally solids > 5 microns usually intermittent solids output - slow to

50 rpm

Image removed for copyright reasons.

Please search online for an image of a peeler centrifuge.

Filtering Centrifuge

Inverting Bag generally solids > 5 microns intermittent solids output

Image removed for copyright reasons.

Please search online for an image of an inverting bag centrifuge.

Filtering centrifuge - full cycle

Function Time(s) Time(%) Accelerate from 50 to 500 rpm 40 5Load/Filter at 500 rpm 277 32Accelerate to 1050 rpm 90 10Spin dry at 1050 rpm 119 14Wash at 1050 rpm 10 1Spin dry at 1050 rpm 236 27Slow down to 50 rpm 90 10Unload at 50 rpm 15 2

Total cycle time 877 100Basket load per cycle of solids 140 kgProductivity 575 kg/hour

Centrifuge - simple analysis – Fig 8.9

Definitions:

Ptotal = Pcake + Pmedium

Centrifuge - simple analysis

- same as for conventional filtration

However, the radius at which the cake forms is continually moving inwards and the geometry is not planar.

hrA oo 2where:

AQRA

cVP m /)(

om A

QR

A

cVP

Centrifuge - simple analysis

Centrifugal head - the driving pressure:

2/)( 222Lo rrP

where omega is in seconds-1 = (2 pi/60)RPMDensity is that of the slurry or liquid depending upon the operation: filtering or washing

Centrifuge - washing

but rc remains constant during the washing stage. The time to wash with Vw m3 of solvent is:

hrR

rr

hC

rrQ

o

m

c

os

Lom

2ln

2

2/)( 222

wo

m

c

os V

r

R

r

rC

Pht

ln2

Centrifuge - washing

Typical washing performance:

Wash volumes

Solute concn.Initial concn.

0

1

0.5

1 2 3

Flooded cake

Dewatered cake

Centrifuge - drainage

Time or dimensionless drainage time

Relative saturation

0

1

0.5

0.2 0.4

0.6

Irreducible saturation

SS* = S

Sinitial

This resource was created by Loughborough University and released as an open educational resource through the Open Engineering Resources project of the HE Academy Engineering Subject Centre. The Open Engineering Resources project was funded by HEFCE and part of the JISC/HE Academy UKOER programme.

Slide 3 (Left). Image of a decanter centrifuge provided courtesy of Thomas Broadbent and Sons Ltd. See http://www.broadbent.co.uk/en/about for details.

Slides 3 (right), 4, and 42. Images courtesy of Siebtechnik GmbH. See http://www.tema.co.uk/images/products/7_1.jpg for details.

Slide 24. Image of"Krebs gMAX® Hydrocyclones" photo courtesy of FLSmidth Krebs Inc. See http://www.flsmidthminerals.com/Products/Classification/Hydrocyclones/Hydrocyclones.htm for details.

© 2009 Loughborough University

This work is licensed under a Creative Commons Attribution 2.0 License.

The name of Loughborough University, and the Loughborough University logo are the name and registered marks of Loughborough University. To the fullest extent permitted by law Loughborough University reserves all its rights in its name and marks which may not be used except with its written permission.

The JISC logo is licensed under the terms of the Creative Commons Attribution-Non-Commercial-No Derivative Works 2.0 UK: England & Wales Licence.  All reproductions must comply with the terms of that licence.

The HEA logo is owned by the Higher Education Academy Limited may be freely distributed and copied for educational purposes only, provided that appropriate acknowledgement is given to the Higher Education Academy as the copyright holder and original publisher.