Abstract
Two-dimensional DEM–CFD simulations have been performed in order to examine the effect of surface energy on the transitional behaviour from fixed bed to bubbling bed for Geldart Type A particles. The results of the simulations presented in the paper show that any effect of surface energy on the magnitude of Umf is not due to increasing bed resistance as a result of increasing the interparticle bond strength. It is demonstrated that Umf corresponds to a deterministic (isostatic) state that is in effect the initiation of the transition from solid-like to fluid-like behaviour. It is also shown that the so-called ‘homogeneous expansion' regime is not in fact homogeneous. This is because the system, when U>Umf, consists of agglomerates. Consequently, the idea that bed expansion is due to the ‘elasticity’ of the bed is not tenable. In order to break up the agglomerates and create a fully fluidised bed that will allow bubbling to occur, higher superficial gas velocities are required for higher values of surface energy. Once the bed is fully fluidised and bubbling occurs the effect of surface energy becomes insignificant.
Highlights
Many industrial chemical manufacturing processes rely on obtaining intimate contact between a gas and a solid
The significance of the results reported in this paper is that in reality, the value of Umf may depend on the value of surface energy this is solely due to the higher voidage of the initial bed and not to increased bed resistance resulting from stronger interparticle bonds
Visualisations, including video sequences, have been used to illustrate how the particle configuration, the particle velocity field, the fluid velocity field and the spatial distribution of interparticle contacts vary as the superficial gas velocity is increased
Summary
Many industrial chemical manufacturing processes rely on obtaining intimate contact between a gas and a solid. The most effective way of doing this is by fluidising the solids with a gas. In the manufacture of some ultrafine high technology materials such as silicon carbide and titania, it is attractive to use as small as possible a particle size, giving high surface area per unit volume. There is a limit to such reduction in size, caused by the increased effects of van der Waals forces at small particle diameters, which effectively make it impossible to fluidise particles below a few tens of micrometres in size. F. Yang et al / Chemical Engineering Science 90 (2013) 119–129 maximum expansion beyond which bubbling occurs remain empirical; for example the most commonly used correlation for minimum bubbling velocity remains that of Abrahamsen and Geldart (1980)
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