Tracking single particles in process equipment or probing processes using positrons

Abstract

In Positron Emission Particle Tracking (PEPT), a single positron-emitting tracer particle is placed within the bulk of particles in the device of interest. The tracer will typically be of the same material as the bulk, or comparable with it in size and density. A positron emitted from the tracer annihilates with an election, leading to the production of two near collinear 'back-to-back' gamma rays. The detection of these and subsequent pairs of gamma rays enable the tracer to be located in three dimensions by simple triangulation. Typically, a particle moving at 1 ms −1 can be located to within 1 mm 500 times a second, whilst at 0.1 ms −1 the spatial resolution is improved to 0.5 mm for a location calculated 50 times a second. The tracer particles position data can then be processed to yield information such as particle velocity, the residence time distributions within specified zones, and the frequency of entering these zones. A real time visualisation of the tracer motion enables the technique to be used for rapid diagnostic and development studies. In the current work, PEPT is being applied in the investigation of a wide range of Particle Technology unit operations including mixing, gravity driven flows, rotary kilns and fluidisation. In each case, detailed trajectories have been obtained, enabling new insights into particle behaviour. In mixing, for example, it has been possible to obtain dispersion coefficients relating to each volume element of the bed, so enabling the separate contribution of each part of the mixer to be distinguished.