Fluidization of fine and ultrafine particles using nitrogen and neon as fluidizing gases

Abstract

AbstractWhen fluidized by a gas, some agglomerated fine and ultrafine particles display a regime of uniform, nonbubbling fluidization known as agglomerate particulate fluidization (APF). The agglomeration of micrometric sized particles, or simple pre‐existing agglomerates in the case of nanoparticles, is governed by the balance between hydrodynamic shear forces and interparticle attractive forces. From this balance the theoretical scaling law Bog ∼ kD+2 has been derived, where Bog, the granular Bond number, is the ratio of the interparticle attractive force to particle weight, k is the ratio of agglomerate to particle size, and D is the fractal dimension. In the experimental program the behavior of gas‐fluidized beds of fine and ultrafine particles as affected by the use of neon and nitrogen as fluidizing gas is studied. The experimental results indicate that there is no relevant distinction between the sizes of agglomerates fluidized with the different gases as theoretically predicted. However, it is seen that the relatively small increment of gas viscosity opens up a new window of highly expanded agglomerate particulate fluidization (APF) behavior with a delayed onset of bubbling. For a sufficiently high‐gas viscosity, and/or smaller particle size, full suppression of the bubbling regime is observed. For nanoparticles exhibiting agglomerate bubbling fluidization (ABF) behavior, where bed expansion is small, and bubbling occurs soon after minimum fluidization, we also observe a delayed onset of bubbling when fluidizing with a gas of higher viscosity. © 2007 American Institute of Chemical Engineers AIChE J, 54: 86–103, 2008