Solids motion in bubbling gas fluidised beds

Gas fluidised beds are widely employed in the chemical, petrochemical, metallurgical, and pharmaceutical industries. Solids mixing in such devices plays a central role in controlling product quality and productivity. This paper 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 and quantify particle trajectory, solids flow pattern, solids velocity, and solids circulation frequency. It is shown that for relatively deep beds (cylindrical columns, group B particles), particles move upward in the central region, and downward near the wall. The average upward particle velocity is similar to 50% of the bubble velocity under the conditions of this study. Theoretical models are proposed to correlate the overall particle upward velocity and average particle velocity in the drift to the bubble velocity, and to estimate the duration of particle residence in the drift. Model predictions show reasonable agreement with experiments. It is suggested that solids motion in gas fluidised beds can be characterised by 'jump','idle' and 'relaxation' times. These times may also be linked to particle kinetic energy transfer in fluidised beds. (C) 2000 Elsevier Science Ltd. All rights reserved.