Particle Technology- Centrifugal Separation


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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.

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Particle Technology- Centrifugal Separation

  1. 1. Centrifugal Separation<br />Chapter 8 in Fundamentals<br />Watch this lecture at<br />Visit for further resources.<br />Course details: <br />Particle Technology, <br />module code: CGB019 and CGB919, <br />2nd year of study.<br />Professor Richard Holdich<br /><br />
  2. 2. Centrifugal separation<br /><ul><li>Sedimenting centrifuges
  3. 3. Particle motion in a centrifugal field
  4. 4. Sigma theory
  5. 5. Hydrocyclones
  6. 6. Grade efficiency & cut size
  7. 7. Filtering centrifuges
  8. 8. Adaptation of filtration equations
  9. 9. Washing (ratio) & Drying</li></li></ul><li>Scroll Discharge Decanter <br />Archimedian screw to convey solids out of the centrifuge<br />Imperforate bowl, i.e. sedimenting not filtering<br />Image courtesy of Thomas Broadbent & Sons Limited<br />Image courtesy of Siebtechnik GmbH <br />
  10. 10. Scroll Discharge Decanter <br />Screw rotates at only slight differential speed to the centrifuge - solids leave at one end, centrate at the other.<br />Image courtesy of Siebtechnik GmbH <br />
  11. 11. Tubular bowl centrifuge <br />This one is vertical axis - simple design with no internals for clarification or liquid/liquid separation - a more complicated design is the chamber bowl.<br />Image removed for copyright reasons. For an example product please see<br />
  12. 12. Disc stack centrifuge <br />Like a lamella clarifier: internal surfaces to encourage settling - usually used in oil/water separation and cream<br />
  13. 13. Sedimenting Centrifuges – <br />Let’s confine our analysis to a simple geometry - ignoring the complicated internal structures required to remove deposited solids and oil concentrates.<br />Liquid flow out<br />Inner radius<br />Air core<br />Outer radius<br />
  14. 14. Gravity settling<br />Field force (weight) is:<br /><ul><li>Drag force is:
  15. 15. Giving:</li></li></ul><li>Centrifugal settling <br />Field force (weight) is:<br /><ul><li>Drag force is:
  16. 16. Giving:</li></li></ul><li>Centrifugal settling<br />i.e. U = f(r)<br />r<br /><ul><li>i.e. U = dr/dt</li></li></ul><li>Sedimenting Centrifuges <br />
  17. 17. Centrifugal settling<br /><ul><li>limits: r=r1 at t=0 to r=r2 at t=t
  18. 18. Giving:</li></ul>i.e. the radial residence time in the machine<br />
  19. 19. Horizontal/axial residence time<br />where<br />
  20. 20. Sedimenting Centrifuges <br />
  21. 21. Critical trajectory model<br /><ul><li>Residence time axially and radially is the same.</li></li></ul><li>Critical trajectory model<br /><ul><li>Multiply through by ‘g’:</li></li></ul><li>Critical trajectory model<br /><ul><li>Multiply through by ‘g’:
  22. 22. Square bracketed term is the terminal settling velocity of a particle of size x.</li></li></ul><li>Critical trajectory model- Eq 8.10 & 5.28!<br /><ul><li>Rearrange:</li></ul>m2<br /><ul><li>c.f. a gravity settling basin</li></li></ul><li>Machine parameters<br />m2<br /><ul><li>The theoretical settling basin equivalent PLAN area given the dimensions of the machine in question and its operating conditions.</li></li></ul><li>Process parameters<br />m2<br /><ul><li>The measured value given the process flow rate and operating performance for the 100% cut-off.</li></li></ul><li>Sigma values<br /><ul><li>Sigma machine</li></ul>m2<br /><ul><li>Sigma process</li></ul>m2<br /><ul><li>The two sigma values are equal for 100% efficient machines - normally 40 to 60% may be achieved.</li></li></ul><li>Uses of sigma values<br />To compare between different machines of same geometry<br />Attempts to compare between different types of machines<br />Estimate of machine size required to replace gravity settling clarifier<br />You need a density difference!<br />
  23. 23. Flue gas desulphurisation<br />Feed:CaSO4 - 35water - 65 100%<br />Cake:CaSO4 - 70water - 30 100%<br />Centrate:CaSO4 - 2.7water - 97.3 100%<br />All concentrations as mass percent<br />
  24. 24. Hydrocyclone<br />Single unit and array:<br />Defined by diameterof cylindrical section<br />Image showing "Krebs gMAX® Hydrocyclones" courtesy of FLSmidth Krebs Inc.<br />
  25. 25. Means of separation<br />Centrifugal:<br />800 g in 300 mm hydrocyclone<br />50000 g in 10 mm hydrocyclone<br />Type of separator:<br />a classifier (i.e. splits into sizes)<br />a thickener (i.e. concentrates suspensions)<br />
  26. 26. Operating data<br />Diameters: 0.01 to 1 metre<br />Solid (cut) sizes: 2 to 250 microns<br />Flow rates (single unit): 0.1 - 5000 m3 h-1<br />Pressure drop: 6 to 0.4 bar<br />U/F solid content: up to 50% v/v (claimed)<br />
  27. 27. Principal features<br />Note: primary & secondary vortex, air core, U/F, O/F, tangential feed<br />
  28. 28. Tangential velocity<br />
  29. 29. Radial velocity<br />
  30. 30. Axial velocity<br />
  31. 31. Grade efficiency – Cut Point<br /><ul><li>Feed distribution is split into two fractions:</li></ul>Overflow<br />Underflow<br />
  32. 32. Grade efficiency<br /><ul><li>Fraction by mass of each grade entering the U/F of the hydrocyclone.
  33. 33. Recovery is the overall fraction entering the U/F - usually by volume.</li></li></ul><li>Grade efficiency<br /><ul><li>Equation:</li></li></ul><li>Grade efficiency<br /><ul><li>What is the grade efficiency of the following?</li></ul>Overflow50 kg/h<br />Underflow50 kg/h<br />
  34. 34. Grade efficiency<br /><ul><li>Equation:</li></ul>100%<br />Rf<br />0%<br />
  35. 35. Grade efficiency<br /><ul><li>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.
  36. 36. The reduced grade efficiency.</li></li></ul><li>Grade efficiency<br /><100%<br /><ul><li>Reduced grade efficiency:
  37. 37. Normalised reduced grade efficiency:</li></ul>100%<br />
  38. 38. Equilibrium Orbit Theory<br />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).<br />It must be equal to the cut size x50%.<br />
  39. 39. Equilibrium Orbit Theory<br />Force balance:<br />centrifugal<br /><ul><li>Tangential velocity:
  40. 40. Liquid drag:</li></ul>FD<br />FC<br />
  41. 41. Hydrocyclones - types and configurations<br />Oil/water separation - often offshore<br />
  42. 42. Filtering Centrifuges <br />A perforated bowl - similar to a spin dryer<br />See box on page 83 for descriptions<br />
  43. 43. Filtering Centrifuge – Section 8.3<br />Pusher<br />generally coarse solids > 50 microns<br />(semi)-continuous solids output<br />careful balance of slurry in<br />Image courtesy of Siebtechnik GmbH<br />
  44. 44. Filtering Centrifuge <br />Peeler<br />generally solids > 5 microns<br />usually intermittent solids output - slow to 50 rpm<br />Image removed for copyright reasons.<br />Please search online for an image of a peeler centrifuge.<br />
  45. 45. Filtering Centrifuge <br />Inverting Bag<br />generally solids > 5 microns<br />intermittent solids output<br />Image removed for copyright reasons.<br />Please search online for an image of an inverting bag centrifuge.<br />
  46. 46. Filtering centrifuge - full cycle<br /> Function Time(s) Time(%) Accelerate from 50 to 500 rpm 40 5 <br />Load/Filter at 500 rpm 277 32<br />Accelerate to 1050 rpm 90 10 <br />Spin dry at 1050 rpm 119 14<br />Wash at 1050 rpm 10 1 <br />Spin dry at 1050 rpm 236 27<br />Slow down to 50 rpm 90 10 <br />Unload at 50 rpm 15 2 <br />Total cycle time 877 100 <br />Basket load per cycle of solids 140 kg <br />Productivity 575 kg/hour <br />
  47. 47. Centrifuge - simple analysis – Fig 8.9<br />Ptotal<br />= Pcake<br />+ Pmedium<br />Definitions:<br />
  48. 48. Centrifuge - simple analysis<br />- same as for conventional filtration<br />However, the radius at which the cake forms is continually moving inwards and the geometry is not planar.<br />where:<br />
  49. 49. Centrifuge - simple analysis<br />Centrifugal head - the driving pressure:<br />where omega is in seconds-1 = (2 pi/60)RPM<br />Density is that of the slurry or liquid depending upon the operation: filtering or washing<br />
  50. 50. Centrifuge - washing<br />but rc remains constant during the washing stage. The time to wash with Vw m3 of solvent is:<br />
  51. 51. Centrifuge - washing<br />Typical washing performance:<br />1<br />Solute concn.<br />Initial concn.<br />0.5<br />Flooded cake<br />Dewatered cake<br />0<br />1<br />2<br />3<br />Wash volumes<br />
  52. 52. Centrifuge - drainage<br />1<br />Relative saturation<br />0.5<br />S* = S<br />Irreducible saturation<br />Sinitial<br />0<br />0.2<br />0.4<br />0.6<br />Time or dimensionless drainage time<br />
  53. 53. 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.<br />Slide 3 (Left). Image of a decanter centrifuge provided courtesy of Thomas Broadbent and Sons Ltd. See for details.<br />Slides 3 (right), 4, and 42. Images courtesy of Siebtechnik GmbH. See for details.<br />Slide 24. Image of"Krebs gMAX® Hydrocyclones" photo courtesy of FLSmidth Krebs Inc. See for details. <br />© 2009 Loughborough University<br />This work is licensed under a Creative Commons Attribution 2.0 License. <br />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.<br />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.<br />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.<br />