A Novel System for Measuring Solids Dispersion In Circulating Fluidized Beds

A novel, simple and accurate experimental measurement system with fast response was developed to measure transient local solid tracer concentration distributions to obtain information on solids mixing and dispersion in risers of circulating fluidized beds operating at a wide range of operating conditions. Phosphorescent-coated FCC particles, used as the tracer, were activated by high intensity UV light and injected at the return leg of high-density circulating fluidized bed (HDCFB) unit. The tracer concentration was measured by detection systems at three radial positions and three axial locations in the riser of the HDCFB unit operating at superficial gas velocities of 4 to 8 m/s and solids fluxes up to 450 kg/m2s. An axial dispersion model was utilized to determine axial solids dispersion coefficients, and the results were interpreted with the help of hydrodynamic data obtained in the same column. Comparison of the axial dispersion coefficients at different operating conditions revealed that axial dispersion tended to decrease with increasing overall solids circulation flux and decreasing superficial gas velocity. Low solids dispersion coefficients at high suspension densities resulted from the disappearance of the net downward flow of particles at the riser walls signifying the existence of the dense suspension upflow regime