STUDIES ON THE FLOW OF SOLIDS DOWN CYCLONE DIPLEGS

Experiments have been carried out on a 100 mm diameter cyclone dipleg 2.38 m long through which solids fluxes up to 500 kg (m2s)-1 were circulated. The pressure profiles show that the apparent density in the dipleg increased progressively from top to bottom. It was found that large quantities of gas were carried downwards by the solids. Hydrodynamically, the system resembles downward-flowing dense phase pneumatic transport, and predictions of the overall pressure drop based on this type of model show reasonable agreement with the experiment; however, the pressure drops can be predicted more accurately using a correlation based on the solids-gas volumetric ratio. The correlation predicts that for a suspended cyclone dipleg operating at constant pressure drop the downward gas flow increases as the length of the dipleg is increased, but that when the end of the dipleg is extended below the surface of a fluidized bed the percentage of gas recycled falls significantly. A calculation procedure is presented which is suitable for the design of diplegs for fluid bed catalytic reactors or for return lines in circulating fluidized bed combustors.