Effect of surface energy on the transition from fixed to bubbling gas-fluidised beds

• Published in: Chemical engineering science Vol. 90; pp. 119 - 129
• Main Authors: Yang, Fang, Thornton, Colin, Seville, Jonathan
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.03.2013
• Subjects: Agglomerates; Bed expansions; Bonding strength; bubbles; Bubbling; Chemical engineering; Discrete element method; energy; Fluidization; fluidized beds; gases; Granular materials; Homogeneous expansion; Powder technology; Simulation; Surface energy; Two dimensional; Homogeneous expansion; Powder technology; Fluidization; Discrete element method; Simulation; Granular materials
• ISSN: 0009-2509; 1873-4405
• DOI: 10.1016/j.ces.2012.12.034

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. ▸ Umf is an isostatic state that is the transition to fluid-like behaviour. ▸ The so-called ‘homogeneous expansion’ regime is not in fact homogeneous. ▸ The idea that bed expansion is due to the ‘elasticity’ of the bed is not tenable.