(340b) Nuclear Techniques for Detailed Investigation of Cohesive Particle Flows

There is an increasing interest in fine particles, a few micron or less than a micron in diameter. Such particles are often difficult to process because of their cohesive nature. Similar problems are encountered for wetted particles. This means, for example, that a regular fluidized bed does not provide sufficient mixing for such cohesive particles, but that additional methods such as stirring or vibration should be applied. This makes the particle flow even more complex than the already complicated behavior of a fluidized bed. In recent years, we have developed and deployed X-ray imaging and single-photon emission radioactive particle tracking to obtain detailed information about vibrated fluidized beds, stirred fluidized beds and horizontal stirred beds. In this work, we will discuss some examples of results obtained with these techniques.

The X-ray setup consists of a triplet of cone-beam sources (X-ray tubes) and three 1548-by-1524 pixel detector panels. It can be used to study systems up to 20 cm in diameter. For imaging, we often use only a single source and detector panel. The X-ray imaging reveals that for a fluidized bed of micro-silica, vibration reduces channeling, but does not significantly change the agglomerate size. Vertical vibration shows to be much more effective than elliptical vibration. However, it cannot prevent stratification in the bed. When using a stirrer to enhance the powder mixing, the design of the stirrer is of large influence: it should maximize the sweeping coverage and avoid the creating of gas pathways. When required, the three sources and panels can be used to make a full 3D tomographic reconstruction of a system under study.

While the X-ray imaging gives precise information about the density distribution, single-photon emission radioactive particle tracking can be used to obtain the trajectory of a representative particle. We use a polystyrene bead with an activated gold core – emitting gamma-rays – as the tracer particle. Three identical γ-radiation slit collimator detectors are placed equidistantly around the system of interest. We demonstrate for a horizontal stirred bed that increasing the agitator rotation speed and the fill level of the bed both lead to an increase in the solids circulation.