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Electrostatics of Pneumatic Conveying of Granular Solids Using Electrical Capacitance Tomography

This study investigates the electrostatics of pneumatic conveying of granular solids using electrical capacitance tomography (ECT). The effects of operation conditions on wall charge accumulation and special pattern formation are examined. The influence of conveying velocity and the addition of antistatic powder are also studied.

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Electrostatics of Pneumatic Conveying of Granular Solids Using Electrical Capacitance Tomography

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  1. Week #7Dynamics of Particulate systems (Part 2) Chi-Hwa Wang Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore, 117585, Singapore. E-mail: chewch@nus.edu.sg.

  2. On the Electrostatics of Pneumatic Conveying of Granular Solids Using Electrical Capacitance Tomography (ECT) Kewu Zhu1, Chi-Hwa Wang1,2, Shuji Matsusaka3, Hiroaki Masuda3 1 Singapore-MIT Alliance, 4 Engineering Drive 3, Singapore 117576 2 Department of Chemical & Environmental Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore, 117576 3 Department of Chemical Engineering, Kyoto University, Kyoto 615-8510, Japan

  3. Recycle Feed Hopper Air from compressor mains CV2 Solid Feed Hopper CV1 CV3 0.85 m Rotameter Rotary air lock feeder 0.71 m Weight Indicator PC ECT Experimental P

  4. Physical properties of particles used in this work Present study considered coarse PP particles compared to 500m glass powder, and anti-static powders

  5. Pneumatic Conveying Electrostatic Analysis • Characterize wall charge accumulation. • Investigate concentration measurement under static charge effect. • Examine the effects of operation conditions on the wall charge accumulation. • Study the mechanism for special pattern formation.

  6. h = 43 mm h1 = 4 mm • = 450  1 6 4 2 h 5 3 h1 U 1 – sensor 4 – voltmeter 2 – pipe segment 5 – support 3 – copper plane 6 – clamp Schematic diagram of voltmeter measurement

  7. Power voltage supply Thermohygrostat Electromagnetic shield Vibrating upper electrode Lower electrode Powder Computer Electrometer Contact potential measurement

  8. Contact potential measurement for particles and inner wall of PVC pipe

  9. Variation of time averaged solids concentration distribution –influence of electrostatic charges Gs = 0.08 kg/s horizontal pipe1.2 m from downstream bend

  10. Charge accumulation at the wall during pneumatic conveying system – effect of conveying velocity Vertical pipeZ =2.05 m Gs = 0.08 kg/s

  11. Charge accumulation at the wall during pneumatic conveying system – by ECT & electrostatic voltmeter Pneumatic conveying of polypropylene granules. Solids mass flow rate Gs = 0.08 kg/s. A: surface potential, U = 18.9 m/s, horizontal conveying; B: surface potential, U = 17.3 m/s, vertical conveying; C: solid concentration , U = 18.9 m/s, horizontal conveying; D: solid concentration, U = 17.3 m/s, vertical conveying.

  12. (a) (b) t (d) N W E (c ) S Distribution of polypropylene particles in a vertical riser flow – annular capsule flow Slugging flow Ug = 13.0 m/s Gs = 7.0 kg/(m2.s) Z = 2.05 m K. Zhu, S.M. Rao , C.H. Wang, and S. Sundaresan “Electrical Capacitance Tomography Measurements on the Vertical and Inclined Pneumatic Conveying of Granular Solids“, Chem. Eng. Sci. 58(18) 4225-4245 (2003).

  13. Polymer film Pipe wall Sections A & C Test station B electrometer Aluminum foil Induced current measurement

  14. (a) (b) (a) MPCT measurement (b) ECT Measurement U = 14.3 m/s, Gs = 0.08 kg/sMoving capsule flow

  15. Influence of Larostat-519 antistatic powder U = 23.4 m/s, Gs = 0.08 kg/s(a) w/o larostat(b) with larostat K. Zhu, S.M. Rao , Q.H. Huang, C.H. Wang, S Matsusaka, and H. Masuda, “On the Electrostatics of Pneumatic Conveying of Granular Materials Using Electrical Capacitance Tomography“, Chem. Eng. Sci., 59(15) 3201-3213(2004).

  16. Scanning electron micrographs of polypropylene particles – after conveying (a) (b) (a) – with addition of larostat 519 powder (b) – without larostat 519 powder

  17. Glass Powder Tests • Experiments are repeated for glass powder (mean diameter = 500 mm, density = 2980 kg/m3). Similar trends in the induced current are found with complicated polarity changes. • It is noted that the induced currents detected for a fresh pipe segment (as high as several nA) are significantly larger than that for a used pipe (less than 0.1nA). • The surface potential measured using the electrostatic voltmeter increases slowly from -0.2kV only to around ~3 kV for a used pipe, while the surface potential increases drastically to a value higher than 20 kV for a fresh pipe. • This observation confirms the induced current measurement results and suggests that attrition of conveying pipe may considerably alter the pipe properties for static electricity.

  18. Summary (5) • ECT measurement drifting is due to electrostatic charge • Reliable solid volume fraction can be obtained if the electrostatic charge influence is considered • Wall charge accumulation are characterized by ECT measurement and this is consistent with electrostatic voltmeter observation • Wall electrostatic charges are enhanced with lower conveying velocities at the same solids flow rate • Current due to charged particles detected by MPCT were in good agreement with ECT measurement • Addition of antistatic powder (larostat 519) can reduce the charge generation

  19. Introduction CFD studies on the pneumatic conveying • Gas-solid flow system-complicate-difficult to scale-up • Predictive, design, and troubleshooting tool • Success of CFD – accuracy and reliability of the model • Horizontal pneumatic conveying Focus: • to numerically study horizontal dilute-phase pneumatic conveying • to investigate the formation of solids clusters in a horizontal pneumatic conveying line

  20. Scope • Experimental validation • Mean velocity of gas and solid phases • Solids volume fraction • Gas-phase turbulence • Model parameters sensitivity analysis • Effect of operation condition • Superficial air velocity (U) • Solids mass loading (m) • Cluster formation • Grid independent study • Effect of es and ew • Origin of cluster • Effect of feeding condition

  21. Model equations Y: k,  (Launder & Spalding, 1974) (Mallo, 1997)

  22. Gas-phase (Sinclair and Jackson, 1989) Solid-phase

  23. (Mallo, 1997) Viscous dissipation turbulence interaction Interface drag coefficient (Schuh et al., 1989) = 24(1+0.15ReS0.687)/ReS 0<ReS<=200 24(0.914 ReS0.282+0.0135 ReS)/ReS 200< ReS <=2500 0.4008 ReS >2500

  24. Boundary conditions • Inlet • Specify ug, us, as, s, k,  • Outlet • Specify p • Wall • Gas phase: no-slip • Solid phase • V: partial slip (Johnson & Jackson, 1987, J. Fluid Mech.) •  (Johnson & Jackson, 1987, J. Fluid Mech.) where k, : standard wall function (Launder & Spalding, 1974, Comput. Method in Appl. Mech. & Eng.)

  25. Schematic diagram for the simulation geometry Inlet ug, us, as Outlet P0 Station A 3.5m away from inlet Tsuji and Morikawa (1982, J. Fluid Mech.) • Particles: polystyrene • Density = 1020kg/m3 • Particle diameter = 200m • Conveying velocities U: 6 -20m/s • Mass loading (m): ~6 • Method: Laser Doppler Velocimetry Model parameters: es= 0.90 ew = 0.7 (Bolio et al., 1995, AIChE J.) = 0.002 as,max=0.65 Geometry parameters: Length = 4 m Diameter = 3.05 cm

  26. A Zp ZL Non-uniform gas + solid flow at outlet y z A  g Uniform gas and solid flow at inlet Y (r) A x A Center Line L Schematic diagram of simulation geometry. The inclination angle of the conveying pipe varies from 0 deg to 90 deg. The diameters of the pipe used in the current studies are 3.05 cm, 4.0 cm and 8.0 cm. The flow quantities predicted normally taken at fully developed region along the central line L at the cut section A, ZL meter away from the feeding end. The length of the conveying pipe is ZP.

  27. s s s s Grid-independent study a. 200x6x6 b. 300x8x8 c. 400x10x10 d. 800x10x10 U=15.6 m/s Gs=49.7 kg/(m2.s)

  28. Comparison of the model predictions with the measurements by Tsuji and Morikawa (1982) of pneumatic conveying of 200 mm solids in a pipe with a diameter of 3.05 cm. (a) (b) (1) ug/U; (2) us/U; (c) along the line L at U=10 m/s. m = 2.2. (, ZL, ZP) = (00, 3.56m, 4 m). (a) laminar flow; (b) gas turbulence; (c) Tsuji and Morikawa measurements (1982).

  29. s s s Contour plots of the solids volume fraction a. es=0.5 b. es=0.9 c. es=1.0 U=15.6 m/s Gs=49.7 kg/(m2.s) ew = 0.7

  30. s s s Contour plots of the solids volume fraction a. ew=0.2 b. ew=0.7 c. ew=0.99 U=15.6 m/s Gs=49.7 kg/(m2.s) es = 0.7

  31. (i) (iii) (ii) Distribution of as, ug, us (a) es = 0.9, ew = 0.7; (b) es = 1.0, ew = 0.7; (c) es = 0.9, ew = 0.99. U=15.6 m/s Gs=49.7 kg/(m2.s)

  32. Comparison of the model predictions with the measurements by Tsuji and Morikawa (1982) of pneumatic conveying of 200 mm solids in a pipe with a diameter of 3.05 cm. Q (b) m =2.8 (a) m=2.8 m =2.2 m=2.2 m =0.9 m=0.9 (a) ug/U; (b) us/U; (c) (u’2)1/2/U along the line L at U=15m/s. m = 2.8.(, ZL, ZP) = (00, 3.56m, 4 m).

  33. (a)  = 00  = 450  = 600  = 750  = 900  = 00  = 450  = 600  = 750  = 900 (b) Inclination angle influence on the flow quantities along the line L of pneumatic conveying of 3.0 mm particles in a pipe with a diameter of 8.0cm. (a) as; (b) ug/U; (c)us/U; m = 3.85. es=0.9, ew=0.7,  = 0.02. (ZL, ZP) = (7.5 m, 10 m).

  34. b a c (i) (ii) Time evolution of Ug and as • ug’/Ug (inlet) • as’/ as (8.5 m) • ug’/Ug (8.5 m) (i) Fluctuations Amplitude: 10% Ug Frequency: 3.3 Hz (ii) – psd (as)

  35. (a) a inlet 3.3 m 6.0 m 8.5 m inlet (b) 3.3 6.0  8.5 Development of as along the conveying pipe Fluctuations Amplitude: 50% as Duration: 0.25 sec

  36. Development of as along the conveying pipe (10%, 3.3hz) K. Zhu, C.K. Wong, S.M. Rao, and C. H. Wang. “Pneumatic Conveying of Granular Solids in Horizontal and Inclined Pipes”, AIChE Journal, 50(8) (2004).

  37. Development of as along the conveying pipe (50%, 3.3hz) K. Zhu, C.K. Wong, S.M. Rao, and C. H. Wang. “Pneumatic Conveying of Granular Solids in Horizontal and Inclined Pipes”, AIChE Journal, 50(8) (2004).

  38. Model predictions agreed reasonably well with the experiments reported by Tsuji et al. (1982). The variation of model parameters had not significantly altered the simulation results. Asymmetric distribution of the velocity and solid volume fraction were predicted for the horizontal conveying. The extent of asymmetry increased with decreasing air superficial velocity and increasing solids mass loading. Clusters were predicted at very fine grid scheme. Particle-wall collision had a significant effect on the solids distribution. Particle-particle collision and interphase interaction had important effects in the formation of solids clusters. Fluctuations in air velocity and large solids volume fraction at the inlet could be the causes for the moving dunes observed experimentally SUMMARY (6)

  39. Discrete Element Method Simulation for Pneumatic Conveying Systems • Discrete Element Method (DEM) • Originally developed for describing the mechanical behavior of assemblies of discs and spheres Cundall, P. A. and O. D. L. Strack. A discrete numerical model for granular assemblies. Geotechnique, 29, 47–65. 1979.

  40. DISCRETE ELEMENT METHOD

  41. COMPUTATIONAL FLUID DYNAMICS • Coupling between CFD and DEM via fluid-particle drag force model • Additional source term in momentum equation to represent reaction force on fluid

  42. FLUID DRAG FORCE Di Felice, R. The voidage function for fluid-particle interaction systems. Int. Journal Multiphase Flow, 20, 153–159. 1994.

  43. GRANULAR FLOW SIMULATION Wang, C.-H., R. Jackson, S. Sundaresan. Instabilities of fully developed rapid flow of a granular material in a channel. Journal of Fluid Mechanics, 342, 179–197. 1997.

  44. f = 0.1 Kx = 0.1 f = 10.0 Kx = 3.5 f = 0.0 GRANULAR FLOW SIMULATION

  45. GAS FLUIDIZATION

  46. GAS FLUIDIZATION

  47. Plug Flow Dispersed Flow PNEUMATIC CONVEYING Zhu, K., S. M. Rao, C.-H. Wang, S. Sundaresan. Electrical capacitance tomography measurements on vertical and inclined pneumatic conveying of granular solids. Chemical Engineering Science, 58(18), 4225–4245. 2003.

  48. Material properties and system parameters

  49. VERTICAL PNEUMATIC CONVEYING

  50. HORIZONTAL PNEUMATIC CONVEYING

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