Particle Technology Gas Cleaning
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Particle Technology Gas Cleaning

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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 ...

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.

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Particle Technology Gas Cleaning Presentation Transcript

  • 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
  • 2. Gas Cleaning
    • Inertia
    • 3. Diffusional collection
    • 4. Target efficiency
    • 5. Material balance - e.g. fibrous filter
    • 6. Types of equipment
  • Collection mechanisms
    Diffusion
    Inertia
    Bounce
    Sieving
    Target collection efficiency
    ~0.1 to 1
    Particle diameter, microns.
  • 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
  • 10. Force Balance
    Apparent (buoyed) mass, drag & inertia:
    • Apparent mass is density x volume:
  • Force Balance
    • Therefore:
    • 11. Where m is actual mass of particle - not buoyed mass.
    • 12. Validity depends on Stokes’ law being applicable.
  • Force Balance
  • 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.
  • 15. Force Balance - Inertial Collection of Particles
    Consider only drag & inertia:
    • mass is density x volume & rearranging:
    • 16. where:
  • Force Balance - Inertial Collection of Particles
    • Make dimensionless as follows:
  • Force Balance - Inertial Collection of Particles
    • The solution to the above equation is the same under all conditions so long as the parameters making up the term on the left may be allowed to vary individually but in such a way as to keep the overall value the same.
  • Force Balance - Inertial Collection of Particles
    • Based on radius or diameter:
  • Stop Distance
    • Integrating using Ug= 0 and Up=U0, at the start, provides the distance taken for a particle injected into still air to come to a halt - The Stop Distance.
  • The Stokes Number
    • N.B. a dimensionless number and a measure of the SYSTEM inertia.
    • 17. It has both particle and collection device properties in its definition.
    • 18. Hint - high inertia given by terms on the top & vice versa for those underneath.
  • The Stokes Number
    = Stk =
    Particle collectionefficiency
    Stokes number
  • 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
  • 35.
  • 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.