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Particle and gas dynamics of high density circulating fluidized beds Liu, Jinzhhong


High Density Circulating Fluidized Bed (HDCFB) reactors have found applications in a number of important industrial processes including fluid catalytic cracking and other catalytic reactors, but received little attention in academic research. This project is a continuation of a series of HDCFB studies at the University of British Columbia. The dual-loop CFB unit was modified to further increase the solids circulation flux to ~ 600 kg/m²s, compared with the maximum of ~ 400 kg/m²s in the original design. Experiments were conducted in a 0.076 m diameter, and 6.4 m tall riser at superficial air velocities between 4 and 9 m/s. FCC particles with mean diameter 70 pm and density 1600 kg/m³ are used as the bed material. An integrated dual-functional optical fiber probe was developed to provide simultaneous determination of local instantaneous solids volume concentration, particle velocity and solids flux in multi-phase suspensions. The probe has 3 fibers in parallel with the middle fiber projecting light into the riser and the other two fibers receiving reflected light from moving particles. The local instantaneous particle velocities are determined by cross-correlating the waveforms from the two receiving fibers over time periods of 10-40 ms, finding the time lag at which the cross-correlation function reaches a maximum value, and dividing into the effective distance, L[sub e] , which is calibrated experimentally. The solids concentration is obtained by applying a calibration equation to the average values over the same cross-correlation periods. Adding a Quartz glass window to the front of the probe tip can dramatically improve the linearity of the concentration calibration function and increase the accuracy of the measurements. The local instantaneous solids flux was calculated as the product of the simultaneously determined suspension density (volume concentration times particle density) and particle velocity. The probe was confirmed to provide reasonably accurate simultaneous measurements of local instantaneous particle velocities, solids concentrations and solids fluxes. Local particle velocity and concentration were measured simultaneously for high solids fluxes and high-density conditions using the optical fiber probe. Local solids flux profiles were compared with those obtained from a sampling method using a suction probe. The results show that the local time-mean velocity and flux profiles are both high in the central region and decrease toward the wall. Downward particle velocity and solids flux occur in the near-wall region for a range of operating conditions. The downward flow of particles is reduced or even eliminated, leading to Dense Suspension Upflow regime for high superficial gas velocities and high solids fluxes. Local time-mean solids fluxes from the optical fiber probe and suction probe are in reasonable agreement. However the product of local time-mean particle velocity and local time-mean suspension density differs significantly from the true time-mean solids flux, consistent with the analysis of Bi et al. (1996). For a certain range of conditions, there was net upflow at the wall in the lower part of the riser and downflow at higher levels, indicating that the onset of the dense suspension upflow regime is height-dependent and that fast fluidization and dense suspension upflow regime can coexist in a single riser. The transition from the fast fluidization flow regime to the dense suspension upflow regime can be characterized in terms of the cross-sectional solids hold up and the superficial gas velocity. A correlation was developed to predict the transition and a new regime map is obtained. Gas residence time distributions were obtained using helium tracer and a stepchange input. A radially non-uniform dispersion model was developed and compared with the axially dispersed plug flow model. The resulting axial dispersion coefficients are somewhat reduced when the effect of the non-uniform velocity and voidage profiles is included. For high solids density conditions, axial gas dispersion decreases as the riser is dominated by dense suspension upflow conditions, i.e. as downward flow of particles disappears at the wall. This is consistent with the hydrodynamic measurements.

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