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Solids motion and mixing in high-density circulating fluidized beds Kirbaş, Görkem

Abstract

This study was directed to the hydrodynamics of vertical risers, such as those used in catalytic cracking and other chemical reactors. The work was carried out in risers of 0.2 m and 0.076 m diameters with FCC particles of mean diameter 70 μm, primarily using optical fibre probes and solid tracer response measurements. The flow characteristics of high-density gas-solids systems were investigated in detail by measuring the local transient particle velocity, solids concentration and solids fluxes at different radial locations and axial positions in the 0.2 m diameter riser operating over a wide range of net solids circulation fluxes (120 kg/m²s < Gs < 350 kg/m²s) and superficial gas velocities (5 m/s < Ug < 8 m/s). These data were used to generate radial and axial profiles of solids hold-up, particle velocity and solids flux and to show how the profiles were influenced by the operating conditions. The flow behaviour in the riser was observed to be a function of height as well as the operating conditions. With a change in operating conditions, the local particle velocity and solid fluxes in the relatively dilute core region of the riser changed more significantly than those in the wall region. The opposite trend was observed with respect to the local solid hold-up distributions. The effect of riser diameter on the local solids flow structure in high-density circulating fluidized bed systems was investigated by comparing the findings of this study with those reported by previous researchers in 76 mm and 104 mm diameter CFB risers. The increase in riser diameter in this study was observed to result in lower cross-sectional average solids holdup at the same superficial gas velocity and net circulation flux under high-density conditions. The local particle velocity in the central region was higher for the larger diameter riser, while the difference in the annulus was insignificant. The time-mean solids fluxes were found to be nearly equal in the central top section of the riser. The difference between the two sets of measurements increased closer to the wall. In the top section of the riser, in the wall region, the time-mean solids fluxes were larger for the smaller diameter riser. The opposite trend was observed for the bottom section of the riser in the wall region. In order to investigate the flow structure in the wall region, an annular downflow layer thickness δ (= 1- rc/R) was introduced, where rc was defined as the radial position where the time mean net solids flow switches from being upwards to downwards. The extent of correlation between particle velocity and solids hold-up fluctuations was examined by investigating their covariance. The increase in covariance under high-density conditions indicated the importance of utilizing flux measurements, rather than particle velocity measurements, in the detection of flow direction in the wall region. When solids circulation flux was increased together with the superficial gas velocity, annular downflow was observed to decrease. Solids mixing and motion were investigated in the 0.076 m diameter high-density circulating fluidized bed riser utilizing phosphorescent-coated FCC particles as the tracer. Special tracer coating, injection and detection techniques were developed for this part of the work. An axial dispersion model was utilized to determine axial solids dispersion coefficients, and the results are interpreted with the help of hydrodynamic data obtained in the same column. This study confirms that axial solids dispersion decreases when the dense suspension upflow regime is reached, i.e. when net downward flow of particles disappears at the wall.

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