AXIAL GAS MIXING IN A CIRCULATING FLUIDIZED BED

ABSTRACT Axial dispersion has been measured and characterized for a 0.152 m diameter by 9.3 m tall cold model circulating bed unit. Mixing was measured by making step changes in the inlet concentration of a tracer gas, helium, and monitoring the response at the riser exit. The results are interpreted both with the aid of a pseudo-dispersion coefficient and by rigorous Fourier transform techniques. Axial dispersion increases rapidly with an increase in solids hold-up in the riser. The geometry of the exit has a substantial influence on the dispersion at identical total solids hold-ups. Mixing can differ greatly from simple plug flow and from predictions of single phase axially dispersed plug flow models. A simple two-zone model is shown to provide a starting point for describing axial dispersion.