(384c) Discrete Particle Simulations of Micro-Jet Assisted Fluidization of Nanoparticulate Agglomerates

In recent years, the fluidization behaviour of nanoparticles has received increased attention. It has been shown that nanoparticles can be fluidized, because they form agglomerates, with typical sizes of a few hundred micrometers. There are several assistance methods to impart external energy for overcoming interparticle forces within and between agglomerates of nanoparticles during fluidization, e.g., vibrating the column, using sound waves, or stirring the bed. When a secondary flow, using a downward facing micro-nozzle, is introduced into the fluidization system in the form of a high velocity (sonic) micro-jet, the fluidization quality of agglomerates of nanoparticles is greatly improved. The use of such a micro-jet has been shown to be a very effective assistance method [1]. Nanoparticulate agglomerates can show as much as a 50-fold increase in bed height (as compared to the stationary bed height) when processed by the high velocity micro-jet. The larger bed expansion signifies a much better dispersion of the agglomerates in the dilute phase, while maintaining particulate fluidization behavior and showing a well defined interface with the absence of bubbles. As such it is a useful technique to incorporate in fluidization studies in the presence of surface reactions, such as coating of nanoparticles using atomic layer deposition.

To obtain a better insight into the working of the micro-jet, we have simulated the system using a discrete particle model (DPM). We have chosen to represent each agglomerate by a single particle having the average agglomerate size and density to reach reasonable computation times. In the DPM, the individual trajectory of each particle is determined by approximating Newton's second law of motion. The forces acting on each agglomerate are gravity, the traction force of the fluid phase, and the force resulting from the interaction with other agglomerates. The motion of the fluid phase is determined from the volume averaged governing equations in an Eulerian framework. A qualitative comparison has been made between the simulation results and previous experimental results. The simulations indicate that the largest contribution to the agglomerate size reduction seems to come from agglomerate-agglomerate collisions: the collisional stress in the zone below the jet is one to two orders of magnitude larger than for the case without a jet.

[1] Quevedo, J.A., Omosebi, A., Pfeffer, R., 2010. Fluidization enhancement of agglomerates of metal oxide nanopowders by microjets, AIChE Journal, in press.