Solids residence time distribution in the secondary reactor of a dual circulating fluidized bed system

The present work reports on the characterization of solids residence time distribution (RTD) in the secondary reactor of a dual circulating fluidized bed (DCFB) system for chemical looping processes. This reactor takes part in the global solids loop but performs an internal recirculation of material as well. The study was carried out in a cold flow model especially designed for the fluid dynamical analysis of such system and provided with a novel residence time measurement device based on inductance measurements and developed in a previous work; ferromagnetic steel particles were used as tracer. Three main variables were studied regarding their influence on the solids residence time distribution, these are: global circulation rate, reactor inventory and reactor fluidization velocity. The use of a modular model allowed a reasonable description of the residence time distribution and a satisfactory fitting for all the experiments. The mean residence time in the global loop decreases inversely proportional to the global circulation rate while the mean residence time in the internal loop appears inversely proportional to the reactor fluidization velocity. The RTD in the internal loop is unexpectedly affected by the global circulation. No important changes are observed in the global loop mean residence time due to changes in the fluidization velocity. The diminution of inventory causes diminution in the mean residence times of both loops, but the ratio between up- and down-wards flows in the reactor appeared independent of the reactor inventory.