(29a) Effects Of Vibration And Electric Field On The Agglomerate Particulate Fluidization Of Nanoparticles

Sieved silica agglomerates (< 500µm) and of primary particle size 12nm (Aerosil R974) are uniformly fluidized by nitrogen in a fluidlike and bubbleless highly expanded state. The bubbling regime, commonly observed for micron-size and larger particles above a critical gas velocity, is fully suppressed and the fluidized bed transits progressively to elutriation as the gas velocity is increased. The condition of nonbubbling fluidization has been related to the formation of highly porous, fractal structured agglomerates of several hundred microns in size, thus this type of fluidization has been termed as Agglomerate Particulate Fluidization (APF). Several studies have suggested that fluidized agglomerates of nanoparticles have a spherical shape with a fractal dimension D close to 2.5, in agreement with the diffusion-limited-agglomeration (DLA) model. The size and fractal structure of the agglomerates determines bed expansion as described by the modified Richardson-Zaki (R-Z) empirical equation. We have studied the effect on bed expansion of externally applied fields such as vertical sinusoidal vibrations and electric field. As seen in vibrofluidized beds (VFB) of both micron-sized and nano-sized particles, the agglomerate size is decreased and the bed expansion enhanced by the action of external vibration, without a relevant change of the fractal dimension and shape of the agglomerates. On the other hand, a recent study has shown a collapse of the fluidized bed upon application of an electric field, suggesting an increase of the agglomerate size. In this case, further agglomeration due to the attractive interaction between the induced electric dipoles in the direction of the field is expected, giving rise to anisotropic larger agglomerates. In our experimental work we apply vibration to the fluidized bed under the action of the electric field. Again vibration is expected to decrease agglomerate size, but the effect on the fractal dimension and shape of the agglomerates would depend on the relative strength between the van der Waals and dipole-dipole attractive forces, which is determined by the strength of the electric field. In the light of this simple view we will describe how bed expansion is affected by varying the relative intensity of the coupled vibrations and electric field.