(33f) Residence Time Distribution Studies in a Fluidized Bed of Fumed Silica Nanopowder

Nanoparticles cannot be fluidized in a gas suspension individually but under the form of large porous agglomerates. There are some nanopowders that fluidized smoothly, showing a large bed expansion and without bubbles. These nanopowders are known to present an ?agglomerate particulate fluidization? (APF) behavior. The APF behavior is characterized by a large bed expansion of several times initial bed height and the absence of bubbles meaning that the powder is well dispersed in the gas phase. The excellent dispersion that these powders can achieve in gas phase may be of particular interest because of their potential use as support for catalyst or in processes involving adsorption/desorption of gases or volatile compounds. Little is known about the flow regime and the mixing in these largely expanded fluidized beds. Some preliminary studies indicate that these beds behave like a CSTR in which mixing is achieved in a very short time (Nam et al. 2004); however, preliminary results obtained with bubbleless largely expanded fluidized beds and a tracer in the gas phase indicate that they may behave like a PFR and CSTR in series. In the present work, agglomerates of hydrophobic nanoparticles (Aerosil® R974) were fluidized showing an APF behavior. Moisture was used as tracer which was fed at the bottom of the column as a step function. The moisture level was monitored after the bed with a sensor located right above the surface of the fluidized bed. The transient moisture level from the sensor at the top is fitted to a model that gives information on the space time of the PFR and the CSTR regimes that compose the largely expanded fluidized bed. In addition, fluffy agglomerates of carbon black are added to an already fluidized bed of Aerosil® R974 in order to study the transient mixing of solids. The modeling of the mixing of the gas and solid phases will be enhanced by understanding the mixing of two solid phases in the largely expanded fluidized bed of APF nanopowder.