(248e) The Effect of Gas Viscosity on the Agglomerate Particulate Fluidization State of Fine and Ultrafine Particles

Fine and ultrafine particles agglomerate in gas-fluidized beds forming fractal and multifractal structures that produce a highly effective interaction with the fluidizing gas, giving rise to a state of bubbleless fluidization called Agglomerate Particulate Fluidization (APF). The agglomeration of fine particles in the case of micrometric sized grains 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 Bo≈k^(D+2) has been derived, where Bo, 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 our experimental program we analyze the role of gas viscosity on particle agglomeration in APF beds of fine and ultrafine particles using Neon and Nitrogen as fluidizing gases. We have fitted data of the fluidized bed settling velocity and gas velocity vs. particle volume fraction to the modified Richardson-Zaki law for fractal agglomerates and in-situ photographs of nanoparticle agglomerates have been taken. These measurements confirm that there is no relevant distinction between the clusters (agglomerates) fluidized with 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 APF state with delayed onset of bubbling. Measurements on the local state of fluidized beds of fine particles indicate that enhanced gas viscosity affects essentially the equilibrium between void splitting and void coalescence. Void splitting is favored in a high viscosity gas due to the large resistance opposed by coherent swarms of solids to be diluted.