Bubble distribution and behaviour in bubbling fluidised beds

Bubble spatial distribution is a prime determinant of the process properties within a fluidised bed, and is coupled to and governed by the flow supply. Bubbles induce heterogeneity and the gas within them rises through the bed without much interaction with other phases. As more flow generates more bubbles, process efficiency can be severely affected. A planar gas-solid fluidised bed was used for observation of the bubbling phase directly and explicitly using real-time vision instrumentation. Frequency domain and statistical analyses of the bed bubble void fraction led to several descriptions of the bed dynamics. Spatial and temporal imaging analysis was used to process image data of the bed and has produced a visual indication of the bubble residency time history patterns for a given state. The results of the analysis further supported and consolidated the relationship between bubble spatial distribution and bed behaviour. The frequency and standard deviation of local changes in the state of the bed are demonstrated to be useful for inferring the bed fluidisation quality. It was also shown that the interaction between the bubbles and the vessel walls affected bubble placements in the bed immediately after introduction until their departure at the top. Subsequently, it was observed that at a given time the spatial distribution of bubbles governed the behaviour of successive bubbles, their properties as well as their positions further up the bed. Also, it is proposed that the introduction of bubbles completely alters the bed dynamics compared with those for smooth fluidisation, and this dominates the whole process. (c) 2006 Elsevier Ltd. All rights reserved.