(215d) Long-Term Transients in Fluidization of Oxide Nanoparticle Agglomerates

Fluidization of nanopowders obtained increasing interest from researchers in the past two decades. Interestingly, it was believed for a long time that nanopowders cannot be fluidized since they are classified as extreme-group-C, very cohesive powders, in the Geldart diagram. For these powders, the acting adhesion forces are too strong to allow fluidization. However, many studies showed that nanoparticles can be fluidized as micron-sized fractal agglomerates with very low densities and very high porosities. The high porosities of the agglomerates are very attractive because most of the particle surface is accessible for mass transfer and reaction.

The formation of the agglomerates in the fluidized bed is a dynamic process which includes events such as collision, unfolding, breaking, and reagglomeration. This makes it likely that the agglomerate size distribution will change over time. On the other hand, long-duration fluidization of nanopowders in the time scale of many minutes to hours is required for industrial applications such as the synthesis of carbon nanotubes, Atomic Layer Deposition (ALD), or photocatalysis. Therefore, a better understanding of, the influence of industrially relevant time scales on the properties of a fluidized bed is mandatory.

Here we present a detailed analysis of the agglomerate size distribution over time during long-time fluidization of oxide nanoparticles. A settling tube set-up was used to investigate the agglomerate size distributions and agglomerate density for multiple hours. We show that agglomerate sizes as well as the bed height decreases while the agglomerate density and the bed density increases. This was verified for different materials. Furthermore, we analysed the influence of the gas velocity and adhesion forces between the nanoparticles on the transient. We explain the long-term transient by two major effects. First: over the time stratification of the nanoparticle agglomerates occurs leading to the fact that larger agglomerates are more likely present in the lower part of the bed. Second: the agglomerate sizes change based on the dynamic events within the fluidized bed which is associated with the decrease in bed height.

Our experimental results clearly indicate that fluidization of nanopowders shows a long-term transient instead of steady state conditions which is highly relevant for industrial scale-up of nanoparticle fluidized-bed reactors.