Fluidization of nano-powders: Effect of flow pulsation

Abstract

The pulsation of the inlet air flow was used here to improve the fluidization behavior of nano-powders that show a strong agglomeration behavior. Some nano-powders, such as hydrophilic fumed silica (Aerosil 200) widely used in the paint and pharmaceutical industries, mostly exist as large agglomerates. Sizes of these agglomerates are often several orders of magnitude larger than their primary particle size. In order to investigate the effect of the flow pulsation on the fluidization of the nano-powder, two different sets of experiments were carried out. The first set consisted of the conventional fluidization while the second set comprised of three flow pulsation experiments of different frequencies. Pulsations in the inlet air flow were introduced using square waves of pulse durations of 2s, 5s and 10s, which were generated using a solenoid valve connected to a data acquisition system. In order to gain an insight into the fluidization dynamics, the pressure-drop transients were recorded at a sampling frequency of 100Hz using a fast response pressure transducer. Minimum fluidization velocities were evaluated from the experimental pressure-drop versus velocity data. Using the Ergun equation, the effective average diameter of agglomerates was evaluated from the experimental minimum fluidization velocity. The flow pulsation led to a greater homogeneity of the bed, a lowering of the minimum fluidization velocity and a substantial reduction in the agglomerate size. However, no clear effect of the frequency or the pulse duration was seen on the fluidization behavior.