Investigation of the bubble behaviour at the free surface of a large three-dimensional gas fluidized bed

Abstract

Gas fluidization is generally associated with the formation of bubbles that critically influence the performance of fluidized bed processes (FBPs). Therefore, in the design, simulation and operation of FBPs, it is very essential to know the behaviour of the bubbles at the free surface. The size and growth of bubbles play an important role for determining properties such as bed expansion, solids entraiment, in-bed heat transfer and solid mixing. This paper presents a study on the behaviour of bubbles at the free surface of a large three dimensional gas-fluidized bed with square section of 61×61 cm2. Measurements were carried out to determine the effects of bed height and excess air velocity on the bubble eruption diameter, frequency and bubble fraction. All experiments were performed at freely bubbling mode and the flow characteristics of bubbles were recorded by a video camera. Bed materials used were 593 μm raw perlite and 1233 μm sand falling within the categories of Geldarts Groups B and D, respectively. The fixed bed height ranged from about 8–18 cm for raw perlite and 9–26 cm for sand. The excess air velocity was varied between 0·5 and 1·75 cm s−1 for raw perlite and 13 and 25 cm s−1 for sand. Equations related to the bubble count, frequency, flow area shape factor and through-flow coefficient were given using a modified form of two-phase theory of fluidisation. Observations were made to validate the two-phase theory for two different particles. The flow area shape factor was in the range of 0·47–0·81 for raw perlite and 0·20 to 0·57 for sand, with mean values of 0·6 and 0·4, respectively. The through-flow coefficient was found to be between −0·68 and 2·82 for raw perlite and between 3·27 to 15·87 for sand, and was larger than predicted values of classical bubble models. © 1998 John Wiley & Sons, Ltd.