Centrifugal Separation Chapter 8 in Fundamentals Professor Richard Holdich Course details: Particle Technology, module code: CGB019 and CGB919, 2 nd year of study. Watch this lecture at Visit 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 LimitedImage 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 pcat-gifs/products-large2/high-speed- centrifuge jpg. pcat-gifs/products-large2/high-speed- centrifuge 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:  Giving:

Centrifugal settling  Field force (weight) is:  Drag force is:  Giving:

Centrifugal settling  i.e. U = f(r) r  i.e. U = dr/dt

Sedimenting Centrifuges

Centrifugal settling  i.e. the radial residence time in the machine  limits: r=r 1 at t=0 to r=r 2 at t=t  Giving:

Horizontal/axial residence time  where

Sedimenting Centrifuges

Critical trajectory model zResidence time axially and radially is the same.

Critical trajectory model zMultiply through by ‘g’:

Critical trajectory model zMultiply through by ‘g’:  Square bracketed term is the terminal settling velocity of a particle of size x.

Critical trajectory model - Eq 8.10 & 5.28! zRearrange: zc.f. a gravity settling basin m2m2

Machine parameters zThe theoretical settling basin equivalent PLAN area given the dimensions of the machine in question and its operating conditions. m2m2

Process parameters zThe measured value given the process flow rate and operating performance for the 100% cut-off. m2m2

Sigma values zSigma machinem2m2 zSigma processm2m2 zThe 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: CaSO water % Cake: CaSO water % Centrate: CaSO water % All concentrations as mass percent

Hydrocyclone  Single unit and array: Defined by diameter of cylindrical section Image showing "Krebs gMAX® Hydrocyclones" courtesy of FLSmidth Krebs Inc.

Means of separation  Centrifugal:  800 g in 300 mm hydrocyclone  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): m 3 h -1  Pressure drop:6 to 0.4 bar  U/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 zFeed distribution is split into two fractions: Overflow Underflow

Grade efficiency zFraction by mass of each grade entering the U/F of the hydrocyclone. zRecovery is the overall fraction entering the U/F - usually by volume.

Grade efficiency zEquation:

Grade efficiency zWhat is the grade efficiency of the following? Overflow 50 kg/h Underflow 50 kg/h

Grade efficiency zEquation: 100% RfRf 0%

Grade efficiency zi.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. zThe reduced grade efficiency.

Grade efficiency zReduced grade efficiency: zNormalised reduced grade efficiency: <100% 100%

Equilibrium Orbit Theory z 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 x 50%.

Equilibrium Orbit Theory z Force balance:  centrifugal zTangential velocity: zLiquid drag: FDFD FCFC

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 rpm40 5 Load/Filter at 500 rpm Accelerate to 1050 rpm90 10 Spin dry at 1050 rpm Wash at 1050 rpm10 1 Spin dry at 1050 rpm Slow down to 50 rpm90 10 Unload at 50 rpm15 2 Total cycle time Basket load per cycle of solids140kg Productivity575kg/hour

Centrifuge - simple analysis – Fig 8.9 Definitions:  P total =  P cake +  P medium

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. where:

Centrifuge - simple analysis Centrifugal head - the driving pressure: where omega is in seconds -1 = (2 pi/60)RPM Density is that of the slurry or liquid depending upon the operation: filtering or washing

Centrifuge - washing but r c remains constant during the washing stage. The time to wash with V w m 3 of solvent is:

Centrifuge - washing Typical washing performance: Wash volumes Solute concn. Initial concn Flooded cake Dewatered cake

Centrifuge - drainage Time or dimensionless drainage time Relative saturation Irreducible saturation S* = S S initial

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 for details. Slides 3 (right), 4, and 42. Images courtesy of Siebtechnik GmbH. See for details. Slide 24. Image of"Krebs gMAX® Hydrocyclones" photo courtesy of FLSmidth Krebs Inc. See for details. © 2009 Loughborough University This work is licensed under a Creative Commons Attribution 2.0 License.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.