1. Particle Control Techniques David Leith Dept. of Environmental Sciences and Engineering University of North Carolina at Chapel Hill

2. Objectives of the Tutorial 1.Discuss particle control at an “intermediate” level 2.Provide some supplemental information and references 3.Provide some tools to help illustrate the concepts discussed

3. Materials Provided u Outline of the Tutorial u CD with the following files: –This Powerpoint Presentation Control.ppt –Excel Spreadsheet: Control.xls –Supplemental Reading: Control.pdf –Tutorial Outline Outline.pdf

4. Introductions u Who are you? u Is there a particular aspect of control technology that brings you here? We will try to shape the tutorial somewhat to reflect your interests

5. Outline of the Tutorial 1.Design Parameters Efficiency, pressure drop 2.Examples of Control Devices 3.Collection Mechanisms Impaction, diffusion, electrostatic attraction BREAK 4.Control Equipment Cyclones, scrubbers, filters, ESPs

6. Design Parameters Characterize collector performance 1.Collection Efficiency 2.Pressure Drop 3.Size and Initial Cost - not discussed here

7. Collection Efficiency,  u Efficiency: fraction of incoming particles collected  Fractional Efficiency,  (d): Efficiency vs. diameter  Overall Efficiency,   : –Depends on fractional efficiency,  (d) and –Size distribution, F(d) Effic’y,  d  Particle Diameter 0 1    0 o ) d ( d ) d ( F ) d ( See Spreadsheet

8. u Difference in static pressure upstream vs downstream of collector u Fan operating cost = constant x  P x Q Pressure Drop,  P CollectorTo Fan PP Air Flow, Q

9. Outline of the Tutorial u Design Parameters Efficiency, pressure drop u Examples of Control Devices u Collection Mechanisms Impaction, diffusion, electrostatic attraction u Control Equipment Cyclones, scrubbers, filters, ESPs

10. Inertial Collectors: Cyclones u High inlet loadings –Wet or dry particles  High , d > 10  m   P 1 kPa (4” w.g.) u Low initial cost u Moderate operating cost u Applications: –Sawdust –Rock dust

12. Low Energy Scrubbers u High inlet loadings –Wet or dry particles u Hot gases OK  High , d > 10  m   P 1 kPa (4” w.g.) u Moderate initial cost u Moderate operating cost –water and slurry disposal

14. High Energy Scrubbers u High inlet loadings u Hot gases OK  High , d > 0.5  m   P 10 kPa (40” w.g.) u Moderate initial cost u High operating cost u Applications: –Metallurgical processes

16. Cleanable Fabric Filters u Moderate inlet loadings  High , all particles   P 1.5 kPa (6” w.g.) u Moderate initial cost u Moderate operating cost u Applications: –Dry dusts –Power plants

18. Disposable Media Filters u Very low inlet loadings  High , particles of all sizes   P 0.3 kPa (1” w.g.) u High replacement cost u Moderate operating cost u Applications: –Cleanrooms –Nuclear, drugs

20. Electrostatic Precipitators: 2-Stage u Low inlet loadings –Good for mist  High , d > 0.5  m   P 0.1 kPa (0.4” w.g.) u Moderate initial cost u Moderate operating cost u Applications: –Indoor air quality

22. Electrostatic Precipitators: 1-Stage u Moderate inlet loadings  High , d > 0.2  m   P 0.5 kPa (2” w.g.) u High initial cost u Moderate operating cost u Applications: –Power plants –Cement plants

24. Outline of the Tutorial u Design Parameters Efficiency, pressure drop, size and cost u Examples of Control Devices u Collection Mechanisms Impaction, diffusion, electrostatic attraction u Control Equipment Cyclones, scrubbers, filters, ESPs

25. Particle Collection Basics u Term in brackets is dimensionless group u Particle distance depends on collection mechanism u Collector distance depends on collector type Collection = F Distance particle travels Distance characteristic of collector

26. Inertial Impaction Particle deviates from gas streamline due to its inertia Gas streamline Object Particle D d V Impaction depends on Stokes Number, Stk Stk = Particle stop distance Dimension of target

27. Collection by Inertial Impaction Stokes Number, Stk  0 1 D18 CVd Stk cp 2    Impaction is important for big particles that move fast

28. Diffusion Diffusion depends on inverse of Peclet Number, Pe Particle deviates from gas streamline due to its Brownian Motion Gas streamline Object Particle D d Pe -1 = Particle diffusion distance Dimension of target V

29. Collection by Diffusion DV D Pe -1  d3 TkC D c   Diffusion is important at high temperatures for small particles that move slowly Pe -1  0 1 TkC c VDd3  

30. Electrostatic Attraction Particle deviates from gas streamline due to Electrostatic Attraction Electrostatic collection depends on: Distance due to electrostatic force Dimension of the collector Gas Flow Electric Field Charged Particle V W X + - +

31. Collection by Electrostatics Electrostatics are important for charged particles in high electric fields  0 1 W t / X W t X n e E C c t 3  d X = n increases with E and d

32. Other Mechanisms: Less Important u Interception u Gravity u Radiometric forces –Thermophoresis –Diffusiophoresis –Stephan flow –Photophoresis

33. BREAK Take five minutes, then reconvene

34. Outline of the Tutorial u Design Parameters Efficiency, pressure drop, size and cost u Examples of Control Devices u Collection Mechanisms Impaction, diffusion, electrostatic attraction u Control Equipment Cyclones, scrubbers, filters, ESPs

35. Objective of Equipment Design  1. Determine efficiency,  (d)  2. Determine pressure drop,  P Effic’y,  d  Particle Diameter 0 1

36. Cyclone Operation Gas forms vortex, also flows toward axis Centrifugal force, F c, pushes out; Drag force, F D, pushes in Big particles go out toward wall Small particles go in toward axis

37. Cyclone Collection Mechanisms  (d) = f [ d 50, cyclone dimensions ] core inp 2 50 d18 Vd Stk   = = f [cyclone dimensions] Efficiency, d 50,  (d) Pressure Drop,  P []dimensionscyclonef 2 V 2 g  = P  x

38. Cyclone Equations u Collection u Pressure Drop See Handout and Spreadsheet

39. Other Issues with Cyclones u Wall erosion Sticky particles u Good operation under extreme conditions Sampling cyclones are different u Performance optimization is possible –For given gas flow and pressure drop, what size and shape gives maximum efficiency?

40. Venturi Scrubber Operation u Small particles impact on large droplets Relative velocity between drops and particles causes impaction Water in Large droplets with cargo of particles collect in entrainment separator

41. Venturi Collection Mechanism Need enough liquid droplets to provide good coverage Relative velocity causes particle impaction onto water droplets Particles embedded in air have high velocity Accelerating droplets have low velocity D18 Vd Stk C cp 2    p - d d

42. Venturi Equations 2 d 35.0Stk           2/3 G L diw/g d Q Q 918.0 VV 0050.0 d            )1XXX1(2*V 22 de  Ld gDit d16 CL3 1X                               2 w/g d/g w/g d/g de 2 w/g g L L V V V V 1*VV Q Q P Collection Pressure Drop See Handout and Spreadsheet

43. Other Issues with Scrubbers u Water supply and treatment expensive u Vapor plume can be a problem u Effective entrainment separation necessary u Performance optimization is possible –For given gas flow and pressure drop, what throat diameter and water use rate gives highest efficiency?

44. Filter Operation particle fiber Particle collection on fibers impaction, diffusion, electrostatics… Need enough fibers to provide good coverage gas

45. Filter Collection Mechanisms u Single fiber collection by: –Impaction, Stk –Diffusion, Pe -1 –Electrostatics, Wt/X u Fibers in filter combine through –Solidity (volume fraction of fibers) –Thickness –Fiber diameter

46. Filter Equations Collection Pressure Drop 44 3 2 ln Ku 2      2 I 2 StkJ  f Cp 2 d18 CVd Stk    D Vd Pe gf ... DRDIRT  3 2 D Pe Ku 1 58.2                               T f d L 1 4 exp1 2 f g dKu LV16 P   See Handout and Spreadsheet

47. Other Issues with Media Filters u No effective theory for dirty filters –Pressure drop increases with use –Efficiency increases (solids) or decreases (liquids) with use u Ineffective gaskets and holes in media occur u Pleated media provide optimum performance –Maximize filter surface; minimize filter thickness

48. Electrostatic Precipitators Particles acquire charge: field charging due to ions that follow electric field lines electric field diffusion charging due to molecular motion of ions random motion of ions Charged particles in electric field move toward collection plate

49. High voltage to electrodes causes electric field Electrodes spaced between grounded plates Operation - One Stage Precipitator Particles charge and collect at the same time Field charges particles and moves them toward plates Practical precipitators have many flow channels that operate in parallel

50. ESP Equations Particle ChargingParticle Collection           Tk tNecdK ln eK Tkd )t(n iiE E 2 1 2 2 2                    tNZeK tNZeK eK dE )t(n iiE iiE E 142 3 2            eK dE )t(n E 42 3 2 d CEen W c   3          Q AW exp 1 See Spreadsheet

51. Other Issues with ESPs u Electrodes can deteriorate with time u Electrical problems occur –Back corona –Dust resistivity problems; gas conditioning u Gas flow problems occur –Ineffective gas distribution –Gas flow through hopper (sneakage)

52. Summary u Mechanisms cause particle collection –Impaction, diffusion, electrostatics u Collector Performance depends on: –Mechanisms, –Configuration of the device

53. Summary, Cont’d u Collector performance described with models Based on physics of mechanisms and collectors u Models are inexact but can often provide insight into collector performance

54. Discussion