Temperature distribution in two different fluidization technologies applied to directly irradiated fluidized beds

Abstract

This work aims to compare two different fluidization technologies (bubbling and spouted beds) applied to directly irradiated fluidized beds, when both operate at similar conditions (mass of solid particles, airflow rates, radiation fluxes and medium bed particle heights). In both cases, the fluidized bed is irradiated from the top of the bed with a beam-down reflector with a 4 kW Xenon lamp working at 2 kW. There are different solid particles that can be used in fluidized beds. However, according to previous works [1], the most suitable material due to their optical properties is Silicon Carbide. Therefore, 7 kg and 10 kg of this material was necessary for spouted and bubbling beds, respectively. These masses of particles were exposed to similar radiation conditions from an optical point of view, using the same focal length between the top of the bed in both cases (Lfocal=1.29 m). The bubbling fluidized bed consists of a cylindrical geometry with an inner diameter of 31.5 cm. In this technology, the airflow passes homogeneously though the cross-sectional area of the bed and is supplied into the bed through a distribution plate with 89 holes at the lowest part, which separates the particles from the plenum. By contrary, spouted bed has a conical geometry with a bottom diameter of 10.8 cm, which corresponds to the inlet air diameter, and a top diameter of 31.5 cm. The movement of particles in each case is completely different due to the internal geometry. Bubbling fluidization presents particles agitation in the whole of the bed while in spouted bed case two clear regions are distinguished: the central or core region of the bed, where the voidage is very high, and the annular region around the jet. On the top of the spouted bed a form similar to a “fountain” appears, where the particles conveyed from the central jet are projected onto the top of the annular region. In this annular region, the particles move down slowly, while part of the gas percolates through the particles in a countercurrent configuration [2]. The results show how the spouted bed gets a similar behavior to the bubbling fluidized bed but only requiring one-third of its pumping costs, additionally, the thermal energy distribution in the center and periphery of the bed surface presented a behavior completely different. Furthermore, in both cases, higher airflow rates increase the mean temperature in the bed surface.