Solids motion in bubbling gas fluidised beds

Gas fluidised beds are widely employed in the chemical, petrochemical, meta llurgical, and pharmaceutical industries. Solids mixing in such devices pla ys a central role in controlling product quality and productivity. This pap er presents some observations of particle motion in 3-D gas fluidised beds operated in the bubbling mode and at atmospheric pressure. The non-invasive positron emission particle tracking (PEPT) technique was used to observe a nd quantify particle trajectory, solids flow pattern, solids velocity, and solids circulation frequency. It is shown that for relatively deep beds (cy lindrical columns, group B particles), particles move upward in the central region, and downward near the wall. The average upward particle velocity i s similar to 50% of the bubble velocity under the conditions of this study. Theoretical models are proposed to correlate the overall particle upward v elocity and average particle velocity in the drift to the bubble velocity, and to estimate the duration of particle residence in the drift. Model pred ictions show reasonable agreement with experiments. It is suggested that so lids motion in gas fluidised beds can be characterised by 'jump','idle' and 'relaxation' times. These times may also be linked to particle kinetic ene rgy transfer in fluidised beds. (C) 2000 Elsevier Science Ltd. All rights r eserved.