Abstract
The processing of fine and ultrafine particles using a fluidized bed is challenging in view of their unpredictable hydrodynamic behavior due to interparticle forces. The use of assisted fluidization techniques in such cases can be effective in improving the bed hydrodynamics. This work investigates the dynamics of pulsed fluidized bed of ultrafine nanosilica subjected to square-wave flow pulsations. The pulse duration used in this study is sufficient to allow the complete collapse of the pulsed fluidized bed between two consecutive flow pulsations. The proposed pulsation strategy is carefully implemented using electronic mass flow controllers with the help of analog output signals from data acquisition system. Given that the different regions of the fluidized bed exhibit varying dynamics, which together contribute to overall bed dynamics, the bed transients in the upper, central, and lower regions of the fluidized bed are monitored using several sensitive pressure transducers located along the height of the bed. The effect of the flow pulsation on the hydrodynamics of the fluidized bed is rigorously characterized. A significant reduction in the minimum fluidization velocity was obtained and an increase in the bed homogeneity was observed due to flow pulsations. The frequency domain analysis of the signals clearly delineated the frequency of the various events occurring during the fluidization.
Highlights
Fluidized beds are widely used in particle processing operations owing to intimate contact between phases and efficient mixing that lead to high rates of heat and mass transfer
Besides analyzing the frequency response of bed transients, the experimental data are processed to evaluate the effect of pulsation on the mean pressure drop characteristics of different regions of the fluidized bed and the minimum fluidization velocity
The pressure drops for the pulsed fluidized bed are consistently higher than those of the unassisted fluidized bed
Summary
Fluidized beds are widely used in particle processing operations owing to intimate contact between phases and efficient mixing that lead to high rates of heat and mass transfer. Unlike the inverse square dependence of the pressure drop on the particle size for fixed beds at low velocities, the fluidized bed mode of operation limits the pressure drop to the effective weight of the bed This feature of the of fluidized bed allows the use of small particles with high effectiveness factor as catalytic support, thereby ensuring improved catalyst utilization that results in enhanced process efficiency. A close look on the experimental setup used for assisted fluidization technique of flow pulsation reveals that most studies have relied on the solenoid valve to introduce the square-wave flow pulsations [20,24,35] This configuration leads to pressure buildup in the inlet line when the flow is stopped during the “off” position of the solenoid valve. Besides analyzing the frequency response of bed transients, the experimental data are processed to evaluate the effect of pulsation on the mean pressure drop characteristics of different regions of the fluidized bed and the minimum fluidization velocity
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