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Radioactive tracing is often the only fine and non-intrusive technique for characterising the flow of phases in chemical reactors. Provided that methodological constraints have been respected, this tracing allows to study, from a chemical engineering point of view, the flow of a phase by the measurement of local residence-time distributions (RTD) or internal ages distributions. From these distributions one can deduce the nature of the marked flow, which is required for scale extrapolation; the example of a packed column is given to illustrate this point. These distributions may validate computational fluid dynamics (CFD) codes as it is pointed out in the tracing of a crystallisation process by desublimation of a hot gas by a cold one. In this case, computed aerodynamics is confirmed by the experimental build-up of a systemic model; the reaction zone is then numerically localised with a high degree of confidence. A specific property of the radioactive tracing is that the signal monitored by a nuclear probe is inversely proportional to the volumetric flow rate near the probe. Then, modelling the enthalpic balance leads in a simple way to the local corresponding temperature as it is featured in the example of the tracing of a quenching tower. These measurements, even if they can point out maldistributions of flows, are not suitable for their localisations; this is the purpose of static or dynamical imaging techniques. Among them, the single photon emission computed tomography (SPECT) enables to reconstruct the image of the repartition of a tracer in the cross-section of a cylindrical reactor from a limited number of nuclear detectors. Advantages and limitations of SPECT are presented and an example of localisation of maldistributions is given. Finally, the correct treatment of images given by a gamma camera constitutes an interesting way for the visualisation and estimation of the repartition of a radioactive tracer in a laboratory three-phase catalytic reactor of the Mahoney-Robinson type used for the desulfurization of gas oils. The influence of the agitation speed on the local efficiency of the gas and liquid mass transfers can be pointed out.

Radioactive Tracing As Aid for Diagnosing Chemical Reactors