Abstract
The use of sound vibration as well as mixing of particles is investigated in this paper for improving the fluidizability of nano-powders. Amorphous anhydrous silicon dioxide. Aerosil 200, widely used in paint and pharmaceutical industries, was fluidized with air at superficial gas velocities as high as 25 cm/s. In the first part of the study, the bed was subjected to sound at 125 dB at a frequency 200 Hz. In the second part, a carefully selected additive material of the Geldart group A classification, presently sand, was added to the bed in relatively small proportions of 3, 6 and 11 wt.%. For the in situ monitoring of the fluidization dynamics, pressure fluctuations were acquired at a sampling frequency of 200 Hz using a fast-response sensitive-pressure transducer along the test section of the column located 11 cm and 23 cm above the distributor. The data thus obtained were analyzed in both time and frequency domains, and used for the reconstruction of system attractors and the determination of its principal eigenvalues. The combination of quantitative tools and digital images of the bed showed that both sound vibration and particle mixing improved the fluidization. The sound-assisted fluidization resulted in the de-agglomeration of the nano-powder, resulting in an expansion of the bed that was reflected in smaller values of the average pressure-drop. The fluidization was characterized by a strong periodic behavior. The addition of small proportions of group A powder, on the other hand, initially increased the average pressure-drop due to the concomitant rise in the bulk density, and led to a bubbling-like behavior reflected in more turbulent and less periodic fluidization. While the widely used sound-assisted fluidization exhibited a limited range of bed operation owing to the high elutriation rates of the bed material, not much limitation was observed for the case of particle mixing. This study showed that the use of appropriate small proportions of inert group A particles may be advantageous in improving the fluidization of nano-powders compared to the use of the energy intensive sound vibration. (C) 2010 Elsevier B.V. All rights reserved.