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Gas residence time distribution and related flow patterns in spouted beds Lim, Choon Jim

Abstract

A two-region model of a spouted bed which takes into account the actual path followed by the gas in the annulus, has been developed to predict the residence time distribution of gas in the bed. The model is based on the assumptions of plug flow of gas in the spout and dispersed plug flow along the flow path in the annulus. The flow path in the annulus is described by postulating that all the gas entering the annulus from the spout at a given level travels radially and vertically along a particular flow path without radial dispersion or mixing. This picture is consistent with visual observations made using NO₂ gas as tracer. The hydrodynamic data needed as input to the model are gas velocities in spout and annulus, spout shape and spout voidage. The residence time distribution of gas together with the above-mentioned hydrodynamic features were measured experimentally for a wide range of spouting conditions. The RTD data were obtained from stimulus-response experiments using helium gas injected as a negative step into the spouting gas downstream of the gas inlet. The gas velocity in the spout was determined by pitot tube, and in the annulus by static pressure measurements. High speed cine-photography was employed to measure spout particle velocities (in half-sectional beds) and spout voidage distribution was determined from spout and annulus particle velocities by solids mass balance. The values of the axial dispersion coefficient for the annulus gas which is an adjustable parameter of the model, were estimated by comparing predicted and experimental RTD curves. The coefficients for spouted beds were found to be generally higher than those reported for packed beds, but at least an order of magnitude smaller than those for fluidized beds. The hydrodynamic data obtained were analyzed to test published theories and correlations and to improve upon these wherever possible. The Mamuro-Hattori equation was found to give good prediction of annulus longitudinal gas velocity profiles for 15.2 cm diameter beds but to under-estimate velocities for larger columns. The equation of Yokogawa et al. proved to be unsatisfactory for predictive purposes and was modified. The modified version can predict the gas velocity profile in the annulus correctly provided that one such profile for the particular solid material is known. The data from the present study showed good agreement with the equation of Grbavcic et al. for gas velocity at the top of the annulus. A simple model was formulated, based on the observed solids flow pattern in the annulus, which enables the calculation of solids flow path and retention time in the annulus from average particle velocity data. For particle velocity in the spout, the force balance model of Thorley et at. as amended by Mathur and Epstein was further improved by introducing the theoretical relationship between spout voidage and number of particles in the spout. The resulting equation was found to give good agreement with experimental values of not only spout particle velocity but also of spout voidage.

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