Fluidization of nanoagglomerates in a rotating fluidized bed



Agglomerates of nanoparticles were fluidized in a rotating fluidized bed (RFB) system at different rotating speeds corresponding to 10, 20, 30 and 40 times the gravity force (9.8 m/s2). The powders, fumed silica Aerosil® R974, Aerosil® R972 and Aeroxide® TiO2 P25, with a primary particle size of 12, 16 and 21 nm, respectively, form micron sized nanoagglomerates having a very low bulk density of around 30 kg/m3 for the fumed silicas, and a somewhat higher bulk density of about 90 kg/m3 for Aeroxide® TiO2 P25. Their fluidization behaviors are described by the fluidized bed expansion, pressure drop and minimum fluidization velocity (Umf). It was found that the fumed silica agglomerates expanded considerably while the TiO2 agglomerates showed very little bed expansion. The minimum fluidization velocities for Aerosil® R974 and R972 ranged from 0.02 to 0.07 m/s and from 0.13 to 0.20 m/s for Aeroxide® TiO2 P25; Umf increased at higher rotating speeds for all powders. At gas velocities above Umf, the fluidized bed pressure drop of fumed silica agglomerates was higher than theoretically estimated by mathematical models found in the literature. Thus, it is believed that for these light particles additional tangential momentum effects may increase the bed pressure drop and a revised model is proposed. In addition, the agglomerate size and external void fraction of the bed are predicted by using a fractal analysis coupled with a modified Richardson-Zaki equation for data obtained with Aerosil® R972. © 2006 American Institute of Chemical Engineers AIChE J, 2006