The ninth lecture in the module Particle Technology, delivered to second year students who have already studied basic fluid mechanics. The different mechanisms for the removal of dust from gases are covered and the design equations used for control, modelling and understanding of the equipment are presented and derived. Examples of industrial equipment for gas cleaning are included.

1. Gas Cleaning Chapter 13 in Fundamentals Watch this lecture at www.vimeo.com Also visit; http://www.midlandit.co.uk/particletechnology.htm for further resources. Course details: Particle Technology, module code: CGB019 and CGB919, 2nd year of study. Professor Richard Holdich R.G.Holdich@Lboro.ac.uk

3. Diffusional collection

4. Target efficiency

5. Material balance - e.g. fibrous filter

7. Dyson vacuum cleaner The animated images shown above are reproduced by permission of Dyson Limited.

8. Inertia - rate of change of momentum How long does it take to reach the terminal settling velocity (gas or liquid)? Inertial collecting devices Stokes’ law and STOKES NUMBER - note the difference!

9. Inertia - rate of change of momentum Section 5.3

11. Where m is actual mass of particle - not buoyed mass.

13. Acceleration & Inertia Particles reach 99% of their terminal settling velocity very quickly. Can use similar approach to characterise the inertia within a system. Inertia can be used in gas cleaning systems.

14. Inertial collection Gas streamlines/flow bend easily round target. Flow Target Dust Inertia carries heavier particles onto target - if they stick this is inertial collection.

17. It has both particle and collection device properties in its definition.

19. Collection mechanisms Diffusion Inertia Bounce Sieving Target collection efficiency ~0.1 to 1 Particle diameter, microns.

20. Diffusional collection Small particles move randomly across flow. Flow Target Dust Diffusion means that particles can be captured even behind the target.

21. Material Balance Applicable to any device with a concentration gradient within the collection device. Example quoted is for a fibrous filter of the HEPA (high efficiency particulate air) type - this has a packing density of 2% (ish) fibres, 98% porosity.

22. Accumulation -1 ): Accumulation is (SI units of kg s Collection Interstitial . Projected . Mass . concentration efficiency target area velocity of the dust

23. Projected target area a AdL a AdL 2 p ( / ) d 4 f mass input - mass output = accumulation volume of fibres in height dL is The length of fibres in dL is fibre volume over fibre area, i.e.

24. Projected target area a AdL 4 p d f a AdL 2 p ( / ) d 4 f Projected area to the gas flow is the product of the length and diameter of the fibre d = f

25. Accumulation -1 ): Accumulation is (SI units of kg s Collection Interstitial . Projected . Mass . concentration efficiency target area velocity of the dust U a AdL 4 g r h . . . C s s - a p d 1 f

26. Mass Balance r CU A g s ¶ C é ù + r CU U dL A ê ú g g s ¶ L ë û - r U dC A g s rate of dust input into layer is rate of dust output from layer is hence accumulation is

27. Mass Balance U a AdL 4 g r h . . . C s s - a p d 1 f accumulation U dC A r g s =

28. Mass Balance & Accumulation é ù h a L 4 C s h = - = - - exp 1 1 ê ú p a - C d ( ) 1 ë û o f Hence, dL h a 4 dC s - = - p a C d ( ) 1 f at L=0 to C=C at L=L to give OVERALL C=C o efficiency of m Single target efficiency minimum at approx 0.4 m.

29. In turbulent flow: Critical trajectory within a boundary layer Particle Collection Efficiency

30. Hence, and Thus, equating the times Particle Collection Efficiency

31. Model based on fraction particles removed = fraction volume particles are being removed from: Particle Collection Efficiency Negative sign as removal

32. Particle Collection Efficiency Integrate over full length, and we want fractioncollected – not fraction remaining, hence: Deutch Equation – forelectrostatic precipitators, where Upis function of electric field strength

33. Scrubber and Venturi Scrubber Image located at http://en.wikipedia.org/wiki/File:Adjthroatplunger.jpg

34. Spray Tower Efficiency

36. Electrostatic Precipitator

37. Electrostatic Precipitator Image located at http://www.arb.ca.gov/training/images/281.jpg

38. Electrostatic Precipitator Image removed for copyright reasons. For a suitable example see http://www.alentecinc.com/company_profile.htm#Electrostatic%20precipitation.

39. Equipment Combined - Flowsheet Image located at http://www.tfhrc.gov/hnr20/recycle/waste/images/cfa.gif

40. Industrial SME NotesThe gas cyclone uses INERTIAL collection of dust whereas the hydrocyclone uses a centrifugal field force - it operates in a much higher viscosity medium. The two have very different operating principles.

41. 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. The animated images shown on slide 4 are reproduced by permission of Dyson Limited. Slide 33. Image of an adjustable throat venturi scrubber located on http://en.wikipedia.org/wiki/File:Adjthroatplunger.jpg. Slide 37. Image of an electrostatic precipitator reproduced with permission from http://www.arb.ca.gov/training/images/281.jpg. Slide 39. Public domain image located at http://www.tfhrc.gov/hnr20/recycle/waste/images/cfa.gif © 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.