Fluidizing ultrafine powders with circulating fluidized bed

The hydrodynamic behavior of a number of ultrafine powders in a circulating fluidized bed was investigated. Different ultrafine powders displayed different fluidization behaviors in the dipleg and the powder return system, but their hydrodynamic behaviors in the riser were almost the same. The addition of coarse particles could improve the quality of fluidization, showing, in addition, that for the powder return system the V-valve was much better than the inclined butterfly-valve, because blockage might occur in the latter. Ultrafine powders could be easily recovered from the exhaust vent by cyclones due to the formation of agglomerates. Fan-shaped baffles in the dipleg could suppress slugging and ensure stable operation. Experimental voidage profiles in the riser showed that the voidage was low at the bottom and high at the upper part. There was, however, a relatively dense zone at the top because of exit effect. Such axial voidage profile in a riser was successfully calculated by using a two-phase model. The reason for non-uniform distribution of voidage was also discussed. (C) 1999 Elsevier Science Ltd. All rights reserved.