(156b) Microstructural Investigations of Nano Particle Fluidisation in Model 2-D and 3-D Beds Using High Speed X-Ray Imaging and Microtomography

ABSTRACT

The X-ray microtomography technique provides valuable in-situ, non-destructive structural information on the morphological changes that take place nano fluidisation of powder samples.  We can look into changes of agglomerates dynamically and examine the final microstructural features. Unlike other available imaging techniques which are usually surface techniques, X-ray microtomography imaging allows us to see through the sample and allow us to reconstruct 3-D internal structure of the sample.  It allows us to identify processing routes that would provide better preserved nanostructural features.  Changes in the agglomerate structure and density can clearly be seen with different fluidisation conditions.

The fluidisation of nano-particles is very important in the industrial preparation of coatings and surface films from nano-engineered particles. The handling of ultra-fine particles (below 20 microns) is very challenging due to their extremely cohesive nature and such systems are very difficult to fluidise using conventional industrial fluidised beds due to the presence of these strong inter-particle forces. The particles are extremely fine and characterised at being at the extreme end of the Geldart C type particles.  It is therefore presumed that the fluidisation of nano-particles is extremely difficult due to the very large surface active forces.

However, there is experimental evidence in the literature that nanoparticles can be fluidized and the fluidisation takes place in the form of fluidisation of ?pseudo- agglomerates? [1-3].  Indeed, agglomerates observed can also be rather big; some even on the millimetre size range. The current theoretical and modelling studies all require direct measurement of voidage, local solids concentration, particle size measurements, agglomerate size and distribution, agglomerate structure and porosity. In all the studies on ultrafine/nanoparticle fluidization reported in the literature, physical properties of the particles were either estimated by making some simple theoretical assumptions or using other tentative imaging techniques or used intrusive measures such as local probes which would interfere with the flow around them. There is a serious need to determine these parameters dynamically and non-intrusively.

Metallic oxide particles such as zinc and copper oxides with average sizes  from 30 -60 nm were used in 2-D and 3-D fluidisation experiments at different gas velocities and the resulting internal bed microstructures were compared before and after fluidisation.  The X-ray tomography equipment provides the 3-D agglomerate size distribution as shown in Figure 1. This is used for calibration of the bed contents prior to fluidisation.

We can also follow the nano-agglomerates dynamically, finding out whether they are breaking or reforming. Figure 2  shows snapshots of X-ray images during fluidisation at different gas velocities from a thin fluidised bed.  Figure 3 shows magnified images of the dynamic behaviour of large agglomerates during fluidisation.  The images presented in Figures 2 and 3 are taken from an X-ray projection video with a spatial resolution of 30 micrometres and a temporal resolution of 40 milliseconds.

The fluidised beds were also used to measure the interstitial pore pressure drop as a function of the fluidising velocity. From these graphs, it has been possible to obtain a mean value of the ?pseudo-agglomerate? size using the conventional Carman-Kozeny equation. The predicted values of the mean effective agglomerate sizes compared well with the size distribution of agglomerates observed during x-ray imaging of the static and fluidised beds. Further work is underway to characterise the dynamics of growth and decay of agglomerates above the distributor as a function of the fluidisation velocity, particle loading and fluidising gas density and viscosity; see figure 3.

REFERENCES

[1] Valverde J.M. et al., Physics Review, E , (2003a) 67, 051305.

[2] Valverde J.M. et al., Physics Review, E, (2003b) 67, 016303.

[3] Zhu C et al, Powder Technology, (2004), 141, pp 119-123.