Visualizing cold-flow fluidized beds with X-rays

Glass beads or sand particles are typically used as bed materials in fluidized beds due to their high sphericity, uniform properties, and resistance to breaking. X-ray imaging can be used to visualize these complex flows. Glass attenuates X-rays much more than the surrounding air and, consequently, the images may be nearly saturated in order to resolve the internal flow of a large diameter bed. This paper focuses on the use of alternative bed materials to increase X-ray penetration and resolution to enhance flow visualization in a 9.5 cm diameter fluidized bed. Melamine plastic, ground walnut shell, and ground corncob particles are qualitatively compared to glass beads using X-ray computer tomography (CT) imaging and X-ray radiography. The various beds are compared at three different flow rates and the ratio of superficial gas velocity to minimum fluidization velocity is constant for each bed material. X-ray CT imaging is used to provide a qualitative view of the local time-averaged solids concentration, and clearly shows differences in fluidization between the materials. Channeling is shown in melamine, walnut shells and corncob at low flow rates, however, the beds fluidize more uniformly as gas flow rate increases. In all cases, glass beads fluidize most uniformly and flow rate does not significantly affect fluidization uniformity. Radiographic movies confirm that visualizing internal flow structures of the glass bed is much more difficult than for other materials.