| Summary: | Moving beds are devices where a slow flow of particulate solids moving under the influence of gravity is contacted with a flow of gas. Heat transfer rates, mass transfer rates, and consequently reaction rates, for these devices are often modelled using information from well-packed fixed beds. It has been found, however, that moving beds behave differently from well-packed fixed beds (Paterson <I>et al</I>. 1991). The cause of this different behaviour is a bed structure, which is a consequence of the movement of the particles, that yields gas flow maldistribution (Crawshaw <I>et al</I>., 1993). Gas flow maldistribution in moving beds was extensively studied by Moppett (1996) through determinations of gas residence time distributions (RTDs) and radial profiles of axial interstitial gas velocity. These results were obtained in beds of uniform sized particles. The present work extends the study of moving beds to cases where the bed is formed with mixtures of particles of different sizes. The observation of particle segregation by means of particle distribution determinations was carried out with the implementation of two novel techniques, the particle sampling device and the setting of the bed with gelatine techniques. The moving bed structures were further probed using gas flows, specially by the determination of gas RTDs and radial profiles of axial interstitial gas velocity. The significance of the mass fraction of the different sized particles in the mixture forming the beds and the conditions of the particle inlet were both analysed. It was found that the particles of different sizes segregate and the amount of the segregation is strongly dependent on the conditions used for the inlet of the particles into the bed, namely the diameter of the particle inlet tube and the free-fall gap between the top of the column and the top surface of the bed. Particle segregation causes moving bed structures which yield extreme gas flow maldistribution.
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