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Particulate Scrubbers

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  1. Particulate Scrubbers • Types of scrubbers: spray chamber and venturi scrubber • Theory and design consideration • Pressure drop • Contacting power Reading: Chap. 7 Aerosol & Particulate Research Lab

  2. Spray Chamber • Collecting medium: • Liquid drops • Wetted surface Recirculated water Q: What parameters will affect the collection efficiency? Q: Any other arrangement of air & water? Water to settling basin and recycle pump Vertical spray chamber (countercurrent flow) Aerosol & Particulate Research Lab

  3. Q: Is the gas velocity of any concern? Is droplet size important? Aerosol & Particulate Research Lab

  4. Cyclone Spray Chamber & Impingement Scrubber Q: Is used water recirculated? Flagan & Seinfeld, Fundamental of Air Pollution Engineering, 1988 Aerosol & Particulate Research Lab

  5. Venturi Scrubber High efficiency even for small particles VG: 60 - 120 m/s QL/QG: 0.001 - 0.003 Q: ESP for sticky, flammable or highly corrosive materials? Handbook of Air Pollution Control Engineering & Technology, Mycock, McKenna & Theodore, CRC Inc., 1995. Aerosol & Particulate Research Lab

  6. 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) Aerosol & Particulate Research Lab

  7. 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 Aerosol & Particulate Research Lab

  8. 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 Aerosol & Particulate Research Lab

  9. 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)? Aerosol & Particulate Research Lab

  10. Aerosol & Particulate Research Lab

  11. 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) Aerosol & Particulate Research Lab

  12. (Impaction parameter Kp is used in textbook; Kp= 2 St) Impaction only Q: Why is there an optimal size? rp = 2 g/cm3 Q: The operating condition of a vertical countercurrent spray chamber are: QL/QG = 1 L/m3, VG = 20 cm/s, dd = 300 mm and z = 1 m. Calculate the collection efficiency of 8 mm particles through this chamber. Assume atmospheric pressure, 25 oC and rp of 1 g/cm3. Aerosol & Particulate Research Lab

  13. 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 Sauter mean droplet diameter s, rL and m should be in cgs QL and QG should be of the same unit k1 = 58600 if VG is in cm/s = 1920 if VG is in ft/s Aerosol & Particulate Research Lab

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

  15. Venturi scrubber collecting a metallurgical fume 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 Contacting power, hp/1000 cfm (PT in hp / 1000 acfm) Nt: Number of transfer unit (unitless) (1 inch of water = 0.1575 hp/1000 cfm) Q: Tests of a venturi scrubber show the results listed on the right. Estimate the contacting power required to attain 97% efficiency. Aerosol & Particulate Research Lab

  16. Aerosol & Particulate Research Lab

  17. Quick Reflection Aerosol & Particulate Research Lab