425667 Defluidization of Cohesive Particles: Interaction Between Cohesion, Young's Modulus and Static Bed Height

Monday, November 9, 2015: 1:27 PM
254A (Salt Palace Convention Center)
Peiyuan Liu, Casey Q. LaMarche, Kevin M. Kellogg and Christine M. Hrenya, Chemical and Biological Engineering, University of Colorado at Boulder, Boulder, CO

Fluidized beds are widely used in industry due to high rates of mass and heat transfer. Handling fine particles (<100 micron) in fluidized beds is essential in many chemical and pharmaceutical processes. Compared with coarse particles, finer particles (Group C and A) show more complicated behavior in fluidized beds (e.g., channeling and non-bubbling fluidization), as a result of the pronounced effect of inter-particle cohesion. Though the continuum theory to predict the fluidization behaviors for non-cohesive particles is well-established, a corresponding theory for cohesive particles remains in its infancy. A promising tool to drive the development of continuum theory for cohesive particles is the coupled Computational Fluid Dynamics-Discrete Element Method (CFD-DEM), which solves the dynamics of every particle in the solid phase, providing a straightforward incorporation of cohesion. However, quantitative validation of numerical results by comparison with experiments is rarely conducted for cohesive particles. Due to computational limitations, CFD-DEM simulations often use artificially soft particles and systems much smaller than experiments, making the direct comparison between simulation and experimental results questionable. This work assesses the effects of material stiffness and system scale for cohesive particles based on CFD-DEM simulations for the defluidization of Group A glass particles. The simulation results were compared with experiment with the objective of finding appropriate conditions for model validation. Cohesion from van der Waals forces was incorporated in CFD-DEM accounting for the surface roughness of particles. Simulation results show that the behavior of cohesive particles is sensitive to the Young's modulus of particles and the static height of particles in the fluidized bed, while such sensitivity is absent if cohesion is removed. These observations are understood by the enhanced cohesive effect at low Young's modulus (i.e., artificially soft particles) and decreasing bed porosity with increasing static bed height. Comparison with experiments shows better agreement by using the true material Young's modulus and sufficiently large static bed height at which scale-independence is achieved, both of which are thus considered prerequisites in simulations for meaningful model validation. This study highlights the caution needed when interpreting CFD-DEM results obtained based on unrealistically small Young's modulus and small-scale systems, particularly particles where cohesive effects are non-negligible.

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