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Particulate Control-2 Fabric Filters Particulate Scrubbers

Particulate Control-2 Fabric Filters Particulate Scrubbers. Lecture notes adapted from Prof. Dr. Dentel Notes and Prof. Dr. Chang-Yu Wu. Fabric Filters. Well known and accepted method for separating dry particles from a gas stream

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Particulate Control-2 Fabric Filters Particulate Scrubbers

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  1. Particulate Control-2Fabric FiltersParticulate Scrubbers Lecture notes adapted from Prof. Dr. Dentel Notes and Prof. Dr. Chang-Yu Wu

  2. Fabric Filters • Well known and accepted method for separating dry particles from a gas stream • Many different types of fabrics, different ways of configuring bags in a baghouse and different ways of flowing the air through the bags. • There are 3 common types of baghouse based on cleaning method • Reverse-air • Shaker • Pulse-jet

  3. Fabric Filters

  4. Fabric FiltersA shaker baghouse Filter compartements

  5. Fabric Filters

  6. Fabric Filters

  7. Filtration Theory

  8. Filtration Theory

  9. Filtration Theory Figure 6.2 pp 186

  10. Filtration Theory

  11. Filtration Theory

  12. Design Considerations

  13. Cleaning Cycles • tf: time interval between two cleanings of the same compartment • tr: time interval between cleanings of any two compartment

  14. Variation of pressure drop with time DPm DP tr tc Time

  15. Cleaning Cycles

  16. Maximum Filtering Velocities in Shaker or Reverse Air Baghouses • Table 6.1

  17. Fabric Selection Table 6.2

  18. Pulse Jet Filters • Introduced 45 years ago captured one-half of the industrial air filtration market • Air is filtered through the bags from outside to the inside, a cage inside each bag prevents the bag from collapsing • The bags are cleaned by short blast of high pressure air (90-100 psi) • Each bag is pulsed every few minutes • On stream use

  19. Pulse Jet Filters • There are no compartments and thus no extra bags which reduces size and cost (for a large coal-fired power plant, the baghouse is so large that it is designed with separate compartments) • Since bags are placed from the top, no need to provide walkways between rows of bags (reducing the size) • Felted fabrics can be used at much higher air to cloth ratio (higher filtering velocities)

  20. Pulse Jet Filters • Table 6.5. Maximum Filtering Velocities for Various Dust or Fumes

  21. Advantages

  22. Disadvantages

  23. Example

  24. Example

  25. Example

  26. Other Considerations • Temperature and Humidity : Fabrics have different maximum allowable teperatures. Low T can cause condensation of acid and/or blinding of the fabric with wet dust • Chemical nature of gas: Different fabrics hav different resistance to acisd or alkalies • Fire/explosion: Some fabric are flammable; Some dust are explosive • Dust Handling: dust removal rate, conveyor system, and hopper slope should all be considered

  27. Wet Scrubbers

  28. Particulate Scrubbers Types of scrubbers: spray chamber and venturi scrubber Theory and design consideration Pressure drop Contacting power Reading: Chap. 7 2014/8/10 Aerosol & Particulate Research Lab 29

  29. Spray Chamber • Collecting medium: • Liquid drops • Wetted surface Recirculated water Water to settling basin and recycle pump Vertical spray chamber (countercurrent flow) 2014/8/10 Aerosol & Particulate Research Lab 30

  30. 2014/8/10 Aerosol & Particulate Research Lab 31

  31. Cyclone Spray Chamber & Impingement Scrubber Flagan & Seinfeld, Fundamental of Air Pollution Engineering, 1988 2014/8/10 Aerosol & Particulate Research Lab 32

  32. Venturi Scrubber High efficiency even for small particles VG: 60 - 120 m/s QL/QG: 0.001 - 0.003 Handbook of Air Pollution Control Engineering & Technology, Mycock, McKenna & Theodore, CRC Inc., 1995. 2014/8/10 Aerosol & Particulate Research Lab 33

  33. Theory: Spray Chamber Volume of each droplet Total number of droplets that pass the chamber per second VG QL: volumetric liquid flow rate Droplet concentration in the chamber Vd Vtd Vd: droplet falling velocity relative to a fixed coordinate Vtd: droplet terminal settling velocity in still air (i.e. relative to the gas flow) 2014/8/10 Aerosol & Particulate Research Lab 34

  34. At a given time dt, the distance a droplet falls is Volume of air that flows through the cross-section area of a single droplet during the time dt Total effective volume of gas swept clean per second by all droplets in dz Total number of particles swept clean per second by all droplets in dz 2014/8/10 Aerosol & Particulate Research Lab 35

  35. Total number of particles removed per second over dx QL Particle penetration in a countercurrent vertical spray chamber Cross-sectional area of all the droplets QG 2014/8/10 Aerosol & Particulate Research Lab 36

  36. If QL in gal/min and QG in cfm, z in ft and dd in mm Particle penetration in a cross-flow spray chamber Q: How do we have higher collection efficiency? Q: What are the collection mechanisms (we need it for hd)? 2014/8/10 Aerosol & Particulate Research Lab 37

  37. 2014/8/10 Aerosol & Particulate Research Lab 38

  38. Deposition of Particles on a Spherical Collector Particle Reynolds # Particle Stokes # Particle Schmidt # Diameter ratio Viscosity ratio Single droplet collection efficiency d (diffusion) (interception) (impaction) 2014/8/10 Aerosol & Particulate Research Lab 39

  39. (Impaction parameter Kp is used in textbook; Kp= 2 St) Impaction only rp = 2 g/cm3

  40. Venturi Scrubber • Use intertial impaction of suspended particles on water droplet formed by gas atomization

  41. Venturi Scrubbers: Calvert Design Particle penetration through a venturi scrubber Kpo=2St (aerodynamic diameter) using throat velocity f = 0.5 for hydrophilic materials, 0.25 for hydrophobic materials Atomization produces a wide distribution of droplet size. However using the Sauter mean droplet diameter (dd) equation can be solved with satisfactory results. sin dyne/cm, rL in g/cm3 and m should be in poise QL and QG should be of the same unit k1 = 58600 if VG is in cm/s = 1920 if VG is in ft/s 2014/8/10 42

  42. Pressure Drop Venturi Scrubber lt: venturi throat length X: dimensionless throat length Ex: 10” water, 2 mm, h = ? 2014/8/10 Aerosol & Particulate Research Lab 43

  43. Contacting Power Approach When compared at the same power consumption, all scrubbers give the same degree of collection of a given dispersed dust, regardless of the mechanisms involved and regardless of whether the pressure drop is obtained by high gas flow rate or high water flow rate Nt: Number of transfer unit (unitless) (PT:contacting powerin hp / 1000 cfm) a and b: coefficient and exponent of PT PT should be determined from the friciton loss across the wetted portion of the scrubber. 44

  44. Contacting Power Approach Venturi scrubber collecting a metallurgical fume Contacting power, hp/cfm

  45. Example Q: Tests of a venturi scrubber show the results listed on the right. Estimate the contacting power required to attain 97% efficiency. (PTcontacting powerin hp / 1000 acfm) Nt: Number of transfer unit (unitless) (1 inch of water = 0.1575 hp/1000 cfm)

  46. Example Convert friction loss to contacting power (hp/1000 cfm): 1 in H20 =0.1575 hp/1000cfm

  47. Example Substractin Eq A from Eq B: A B

  48. 2014/8/10 Aerosol & Particulate Research Lab 49

  49. Problem 7.1

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