Prediction of solids residence time distribution in cross-flow bubbling fluidized bed

Abstract

Cross-flow bubbling fluidized beds (BFBs) have been widely used in dual fluidized bed systems such as chemical and heat looping. Understanding the residence characteristics of solids in such system is important for better design and optimization of reactors. Previously we experimentally measured the residence time distributions (RTDs) of sands in a cross-flow rectangular BFB by using coal particles as tracer. A computational investigation of solids RTD using multi-fluid Eulerian method combined with the species transport equation showed that the RTD of sands could be correctly represented by that of coal particles, and the simulation results well agreed with experimental data. Parametric studies demonstrated that, under the considered operation conditions, the influence of tracer injection time period on the predicted solid residence times was nearly ignorable. Simulation results revealed that in the investigated cross-flow BFB the solids RTD is closely related to solids inventory and solids flux. Through proper data processing, it was found that the descending part of solids RTD profile can be uniquely fitted by an empirical exponential function. A semi-empirical approach was thus developed and further validated, for the first time in the literature, to predict the entire profile of solids RTD, in which the ascending part of solids RTD profile is obtained through CFD simulation whereas the descending part is given by the fitted empirical exponential function.