particle technology- centrifugal separation

52
Centrifugal Separation Chapter 8 in Fundamentals Professor Richard Holdich [email protected] Course details: Particle Technology, module code: CGB019 and CGB919, 2 nd year of study. Watch this lecture at http://www.vimeo.com/10203052 Visit http://www.midlandit.co.uk/particle technology.htm for further resources.

Upload: the-engineering-centre-for-excellence-in-teaching-and-learning

Post on 19-Jan-2015

7.604 views

Category:

Education


7 download

DESCRIPTION

The seventh lecture in the module Particle Technology, delivered to second year students who have already studied basic fluid mechanics. Centrifugal Separation covers both sedimenting and filtering centrifuges as well as hydrocyclones. Adaptation of the gravity settling and conventional filtration models, to account for the conceptual centrifugal acceleration, is included. Examples of industrial equipment for centrifugal separation are included.

TRANSCRIPT

Page 1: Particle Technology- Centrifugal Separation

Centrifugal Separation

Chapter 8 in Fundamentals

Professor Richard Holdich

[email protected] 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.

Page 2: Particle Technology- Centrifugal Separation

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

Page 3: Particle Technology- Centrifugal Separation

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

Page 4: Particle Technology- Centrifugal Separation

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

Page 5: Particle Technology- Centrifugal Separation

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

.

Page 6: Particle Technology- Centrifugal Separation

Disc stack centrifuge

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

Page 7: Particle Technology- Centrifugal Separation

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

Page 8: Particle Technology- Centrifugal Separation

Gravity settling

Field force (weight) is:

Drag force is:

gx

s )(6

3

xU3

Giving:

18

)(2 gxU s

t

Page 9: Particle Technology- Centrifugal Separation

Centrifugal settling

Field force (weight) is:

Drag force is:

23

)(6

r

xs

xU3

Giving:

18

)(

d

d 22 rx

t

r s

Page 10: Particle Technology- Centrifugal Separation

Centrifugal settling

i.e. U = f(r)

r

18

)(

d

d 22 rx

t

r s

i.e. U = dr/dt

Page 11: Particle Technology- Centrifugal Separation

Sedimenting Centrifuges

Page 12: Particle Technology- Centrifugal Separation

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

Page 13: Particle Technology- Centrifugal Separation

Horizontal/axial residence time

where

Qt

Volume

LrrV )( 21

22

Page 14: Particle Technology- Centrifugal Separation

Sedimenting Centrifuges

Page 15: Particle Technology- Centrifugal Separation

Critical trajectory model

Residence time axially and radially is the same.

Q

V

x

rrt

s

22

12

)(

)/ln(18

Page 16: Particle Technology- Centrifugal Separation

Critical trajectory model

Multiply through by ‘g’:

)/ln(18

)(

12

22

rrg

gx

V

Q s

Page 17: Particle Technology- Centrifugal Separation

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.

Page 18: Particle Technology- Centrifugal Separation

Critical trajectory model- Eq 8.10 & 5.28!

Rearrange:

)/ln( 12

2

rrg

V

U

Q

t

c.f. a gravity settling basin

m2

Page 19: Particle Technology- Centrifugal Separation

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

Page 20: Particle Technology- Centrifugal Separation

Process parameters

tU

Q

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

m2

Page 21: Particle Technology- Centrifugal Separation

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.

Page 22: Particle Technology- Centrifugal Separation

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!

Page 23: Particle Technology- Centrifugal Separation

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

Page 24: Particle Technology- Centrifugal Separation

Hydrocyclone

Single unit and array:

Defined by diameterof cylindrical section

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

Page 25: Particle Technology- Centrifugal Separation

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)

Page 26: Particle Technology- Centrifugal Separation

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)

Page 27: Particle Technology- Centrifugal Separation

Principal features

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

Page 28: Particle Technology- Centrifugal Separation

Tangential velocity

Page 29: Particle Technology- Centrifugal Separation

Radial velocity

Page 30: Particle Technology- Centrifugal Separation

Axial velocity

Page 31: Particle Technology- Centrifugal Separation

Grade efficiency – Cut Point

Feed distribution is split into two fractions: Overflow

Underflow

Page 32: Particle Technology- Centrifugal Separation

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.

Page 33: Particle Technology- Centrifugal Separation

Grade efficiency

Equation:

feedin gradein mass

underflowin gradein massE

Page 34: Particle Technology- Centrifugal Separation

Grade efficiency

What is the grade efficiency of the following?

Overflow50 kg/h

Underflow50 kg/h

Page 35: Particle Technology- Centrifugal Separation

Grade efficiency

Equation:

feedin gradein mass

underflowin gradein massE

100%

Rf

0%

Page 36: Particle Technology- Centrifugal Separation

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.

Page 37: Particle Technology- Centrifugal Separation

Grade efficiency

Reduced grade efficiency:

fREE '

Normalised reduced grade efficiency:

f

f

R

REE

1

''

<100%

100%

Page 38: Particle Technology- Centrifugal Separation

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%.

Page 39: Particle Technology- Centrifugal Separation

Equilibrium Orbit Theory

Force balance: centrifugal

23 )(6

is rx

Tangential velocity:i

i

r

v

Liquid drag: Ux3 FD FC

Page 40: Particle Technology- Centrifugal Separation

Hydrocyclones - types and configurations

Oil/water separation - often offshore

Page 41: Particle Technology- Centrifugal Separation

Filtering Centrifuges

A perforated bowl - similar to a spin dryer

See box on page 83 for descriptions

Page 42: Particle Technology- Centrifugal Separation

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

Page 43: Particle Technology- Centrifugal Separation

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.

Page 44: Particle Technology- Centrifugal Separation

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.

Page 45: Particle Technology- Centrifugal Separation

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

Page 46: Particle Technology- Centrifugal Separation

Centrifuge - simple analysis – Fig 8.9

Definitions:

Ptotal = Pcake + Pmedium

Page 47: Particle Technology- Centrifugal Separation

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

Page 48: Particle Technology- Centrifugal Separation

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

Page 49: Particle Technology- Centrifugal Separation

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

Page 50: Particle Technology- Centrifugal Separation

Centrifuge - washing

Typical washing performance:

Wash volumes

Solute concn.Initial concn.

0

1

0.5

1 2 3

Flooded cake

Dewatered cake

Page 51: Particle Technology- Centrifugal Separation

Centrifuge - drainage

Time or dimensionless drainage time

Relative saturation

0

1

0.5

0.2 0.4

0.6

Irreducible saturation

SS* = S

Sinitial

Page 52: Particle Technology- Centrifugal Separation

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.