Nanofluidization electrostatics

J. M. Valverde, M. A. S. Quintanilla, M. J. Espin, and A. Castellanos

Phys. Rev. E 77, 031301 – Published 12 March 2008

Abstract

Electrostatic charging of powders is a relevant phenomenon for a number of industrial applications. The design of new processes and the use of high resistivity materials and ultrafine powders may lead to higher charging rates and to higher levels of charge accumulation that can become a serious problem. In this work we investigate experimentally electrostatic charging in nanofluidization. The behavior of a fluidized bed of silica nanoparticles under the influence of an electrostatic field is studied. The electric field is applied in the horizontal direction and perpendicular to the gas flow. On one hand, we observe the influence of the electric field on the bulk behavior of the fluidized bed, which suffers a collapse when the electric field is turned on. For strong electric fields the stationary state of the fluidized bed reminds one of that of a spouted bed, with a solid layer adhered to the wall and a low density core region of local high gas velocity. On the other hand, and in order to gain additional insight, we look at the trajectories of nanoparticle agglomerates as affected by the electric field. This images analysis reveals that these agglomerates are horizontally deflected towards the wall as a consequence of being charged. From the analysis of agglomerate trajectories the charge per agglomerate is estimated. Using these measurements the electrostatic forces between agglomerates are calculated and compared to van der Waals attractive forces.

Received 21 December 2007

DOI:https://doi.org/10.1103/PhysRevE.77.031301

Authors & Affiliations

J. M. Valverde¹, M. A. S. Quintanilla¹, M. J. Espin^1,2, and A. Castellanos¹

¹Faculty of Physics, University of Seville, Avenida Reina Mercedes s/n, 41012 Seville, Spain
²Department of Applied Physics II. EUAT.University of Seville, Avenida Reina Mercedes 4A, 41012 Seville, Spain

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Vol. 77, Iss. 3 — March 2008

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Images

Figure 1
Ratio of interparticle attractive force to particle weight as a function of particle size. As indicated interparticle forces are calculated according to the van der Waals interaction (solid lines) and electrostatic interaction (dashed lines). Results are shown for silicon dioxide ( $ϵ = 3.8 ϵ_{0}$ , $α ≃ 0.25$ , $β ≃ 1.3$ , $ρ_{p} = 2500 kg ∕ m^{3}$ ). In (a) the electrostatic force is calculated from the typical charge-to-mass ratio measured in pouring processes. In (b) the electrostatic force is calculated from the typical charge-to-mass ratio measured in pneumatic transfer processes. Only the attractive electrostatic force is calculated. There will be also a repulsive electrostatic force of the same order of magnitude between charges of the same sign.Reuse & Permissions
Figure 2
Average particle volume fraction of the fluidized bed $ϕ$ as a function of the superficial gas velocity $v_{g}$ for different strengths of the electric field applied.Reuse & Permissions
Figure 3
Series of photographs showing the irreversibility of the effect of the electric field on fluidized bed expansion. The superficial gas velocity is $2.6 cm ∕ s$ . The strength of the electric field $E$ is slowly increased from (a) to (c). (a) $E = 0 V ∕ m$ . (b) $E = 0.66 \times 10^{5} V ∕ m$ . (c) $E = 1.32 \times 10^{5} V ∕ m$ . In (d) the field is turned off. The arrows indicate the evolution of the free surface. In (d) the free surface cannot be neatly distinguished.Reuse & Permissions
Figure 4
Snapshots of the fluidized bed near the free surface showing the main qualitative features of the effect of the electric field on the flow field at different times since it was applied. In (a) there is no applied field. From (b) to (g) the strength of the applied horizontal field is $E = 2.5 \times 10^{5} V ∕ m$ . The arrows show the main agglomerate flow lines. Video of the experiment is available on request.Reuse & Permissions
Figure 5
Snapshots of the fluidized bed near the free surface showing the deflection of an agglomerate towards the wall. The time elapsed since application of the electric field is indicated. The strength of the applied horizontal field is $E = 2.5 \times 10^{5} V ∕ m$ . Video of the experiment is available on request.Reuse & Permissions
Figure 6
Snapshots of the fluidized bed near the free surface showing the individual trajectories of two complex agglomerates (squared and circled, respectively). The time elapsed since application of the electric field is indicated. The strength of the applied horizontal field is $E = 2.5 \times 10^{5} V ∕ m$ . Video of the experiment is available on request.Reuse & Permissions

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