GAS MIXING IN GAS SOLIDS RISERS

On the basis of the assumption of a core-annulus structure, a model for gas mixing in the riser of a circulating fluidized bed was derived by considering a radially dispersed plug flow in the core and a well-mixed side-stream in the annulus. In order to determine the two fitting parameters in the model, experiments were conducted in a 30.5 cm i.d. riser. The ranges of superficial gas velocity and mean solids flux used are 2.6-5.0 m/s and 20-120 kg/m2 s, respectively, and the solids had a mean diameter of 71 mum and a particle density of 2450 kg/m3. Sulphur hexafluoride was continuously injected from a point source as tracer. Apparent Peclet numbers were found to be correlated with an apparent suspension Reynolds number by an equation of the type Pe = k1Re(k2). The annulus effect, evaluated as a Biot number, indicates that the side-stream is a recycling flow whose contribution to the overall mixing exhibits a maximum. These results are in agreement with the recent hydrodynamic studies and can explain some of the inconsistencies in the previous findings.