Characterization of the upward motion of an object immersed in a bubbling fluidized bed of fine particles

Abstract

The axial motion of HDPE, Acetal, and PTFE spheres immersed in a 3-D bed of the conventional fluidization materials was studied using the non-invasive radioactive particle tracking (RPT) technique. Coarse and fine sand as well as FCC were chosen as the fluidization material. The experiments were carried out at two excess gas velocities, i.e. Ue=0.25m/s and Ue=0.50m/s.Rise of an object immersed in a bubbling fluidized bed can be descried by two characteristic velocities, i.e. rise and upward velocity. When circulation pattern is fully established, the mean upward and rise velocity to the mean bubble velocity ratios become comparable. It is shown that the object rise takes place principally in the drift of bubbles. The mean object rise velocity is primarily influenced by the excess gas velocity. Comparatively, the ratio between the physical properties of the fluidization medium and the object such as size and density has trivial impact on the rise velocity of the object. The mean object upward velocity can satisfactorily be predicted with a theoretical model with no adjustable parameter.The frequency of the object rise along the bed is independent of the object properties but influenced by the fluidization material. Comparing to coarse sand and FCC, the objects immersed in fine sand rise less frequently. This is attributed to the relatively longer residence time of the object in the emulsion phase of fine sand particles. The intermittent circulation of the medium and large size Acetal spheres in beds of FCC and fine sand is featured by the low upward and rise object velocities and the very limited height travelled by the object.