(149c) Enhanced Fluidization of Agglomerates of Nanoparticles

Nanoparticles produced by a pyrogenic process cannot be fluidized in a gas suspension individually but in the form of large porous agglomerates. Some nanopowders show agglomerate particulate fluidization (APF) behavior characterized by the absence of bubbles, low minimum fluidization velocity, and a large bed expansion. When conventionally fluidized, the bed expansion is several times (about 5) the initial bed height before reaching the minimum bubbling velocity. Nanopowders that show this behavior are Aerosil® 200, R974, 300, R972 among others. However some nanopowders such as Aeroxide® TiO2 P25, Aeroxide Alu C and Aerosil® 90 show agglomerate bubbling fluidization (ABF) behavior because of the presence of bubbles at gas velocities higher than the minimum fluidization velocity. These nanopowders show a limited bed expansion (less than double initial bed height) and a high minimum fluidization velocity when conventionally fluidized, and an abundance of bubbles that makes the fluidized bed interface unclear.

In the present work, a secondary flow using a jet is introduced which greatly improves the fluidization behavior of agglomerates of nanoparticles. Agglomerates that show an APF behavior show a bed expansion more than double that observed by conventional fluidization. The larger bed expansion signifies a much better dispersion of the agglomerates in the dilute phase. In some cases, a 10-fold increase in bed expansion is obtained while maintaining the particulate fluidization behavior, showing a well defined interface and the absence of bubbles. Furthermore, when agglomerates of ABF behavior are exposed to the jet, the fluidization behavior is changed to APF type showing a bed expansion much larger the initial bed height and the absence of bubbles.

In addition, average agglomerate sizes and agglomerate size distributions of Aerosil R974 (APF behavior) and Aerosil 90 (ABF behavior) nanopowders were measured in-situ in a gas fluidized bed by using a FBRM® probe, and images were obtained with a PVM® probe both from Lasentec. The measured average agglomerate sizes are relatively smaller than the values reported in the literature for Aerosil R974.