Modeling the axial and lateral mixing of solids in fluidized beds

The solid flow patterns in fluidized beds are often depicted as a number of convective currents induced by rising bubbles. This description is revisited in this work to develop a mathematical model for both the axial and the lateral mixing of solids in fluidized beds. The model uses concepts similar to those used in the countercurrent backmixing model, which is widely used for axial mixing only. Mixing experiments using coal and PVC (as a white tracer) were carried out to obtain experimental concentration maps for model validation. The model reproduces well the general features observed during the mixing experiments, as well as the effects of gas velocity, particle size, and the presence of internals. The selection of most model parameters can be justified with observations of bed and bubble properties. The choice of the exchange rate of solids between countercurrent phases is discussed in light of the new data derived in this work, previously published data, and a sensitivity analysis of the model predictions for this parameter.