Chapter 4 Positron emission imaging in Chemical Engineering

Better understanding, design and operation of engineering processes demand visualisation of the material flows within them under realistic conditions. Methods based on radioactive tracers enable visualisation to be performed on real processes taking place within opaque walls. Positron emission methods rely on detecting the pairs of back-to-back gamma rays produced when a positron (emitted in radioactive decay) annihilates with an electron, and are variants of positron emission tomography (PET) which is widely used in medicine for determining the distribution in 3D of a labelled fluid. In chemical engineering applications, extensive use has been made of the alternative technique of positron emission particle tracking (PEPT), invented at the University of Birmingham, in which a single tracer particle is radioactively labelled and can be accurately tracked at high speed. This has now been developed to the point where it has the capability to track tracer particles down to approximately 60 μm in size, moving at up to 10 m/s, yielding locations to within ±1 mm at frequencies better than 100 Hz. Most importantly, gamma rays are sufficiently penetrating that good location data can be obtained within real process vessels. Applications have been extremely diverse, and include both gas-phase-continuous and liquid-phase-continuous systems. Particularly strong contributions have been made to the study of mixing processes and applications of fluidisation. Hitherto, the method has been confined to the laboratory. However, a modular transportable positron camera has now been developed and has been used for the first time on large-scale plant at an industrial site.