(262a) A Study of the Effect of Fines Size Distribution on the Fluidization and Rheological Behaviour of Alumina Powders

The industrial employment of fluidized bed technology involves processes that are strongly dependent on the operating conditions, such as temperature and particle size distribution of the material. In particular, the presence of fines represents a major issue for fluidized bed industrial applications, which typically employ between 10-50% by weight of fine material in order to maximize reactor performance. However, the relative importance of the various fine sub-cuts on the fluidization behaviour has not been fully understood as yet.

This paper investigates the effect of adding different fines cuts on the fluidization and the rheological behaviour of an alumina powder by adding small fines (i.e. 0-25 micron) and big fines (i.e. 26-45 micron) to the alumina material previously deprived of fines.

A mechanically stirred Fluidized Bed Rheometer (msFBR) developed at UCL is used to characterize the flow properties of the alumina samples differing in fines size distribution, at rates of aeration ranging from the fixed bed to the incipiently fluidized bed. The effect of changing the impeller depth on the torque measurements is also systematically analysed. Using the University of Salerno modified Peschl Shear Cell, the failure properties of the materials have been determined and a rheological model has been developed to determine the stress distribution in the msFBR at any given rate of aeration.

The model allowed comparing the rheological behaviour of the powders investigated and determining the effect of fines size distribution on powder rheology. The interpretation of the failure properties of the materials provided an indication of the capability of the powders to flow and thus to attain fluidization.