Quantification of solids flow in a gas–solid riser: single radioactive particle tracking

Solids in risers of circulating fluidized beds (CFB) exhibit local backflow and recirculation. Measurement of the concentration-time response to an impulse injection of tracer, even at two elevations cannot determine the residence time distribution (RTD) of solids uniquely. Hence, evaluation of RTD in risers from conventional tracer responses is difficult and often not possible. In addition, estimating the solids circulation rate in these closed loop systems, is a non-trivial problem. In this work, a single radioactive particle in the CFB loop is tracked during its multiple visits to the riser and, by invoking ergodicity, solids circulation rate, accurate solids RTD and additional information on the solids flow pattern in the riser are estimated. A calibration curve was established for the overall solids mass flux as a function of superficial gas velocity. A second peak in the probability density function (PDF) of the solids RTD curve in the riser was observed for operating conditions in the fast-fluidization regime.