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Modeling of agglomerate abrasion in fluidized beds Palanisamy, Karthikeyan


Agglomeration occurs in fluidized beds in many industrial processes. It can be a desirable phenomenon, for instance in the pharmaceutical industries, or undesirable, as when it occurs in the Fluid Cokers used in bitumen processing. Excessive agglomerate growth can lead to a reduced efficiency of the process and ultimately to defluidization. Mathematical modeling can provide insights into agglomerate formation and breakage. Modeling the physics of agglomerate breakage is complicated since the breakage model needs to account for a number of processes occurring inside the fluidized bed. Breakage of agglomerates can happen due to fragmentation (high energy based) or abrasion (low energy based), or a combination of both. Abrasion is the process of losing one or a few of the particles from the surface of the agglomerate. It occurs continuously inside the fluidized beds and cannot be neglected for Fluid Cokers. Fragmentation, on the other hand, is the fracture of an agglomerate into pieces due to the breakage of multiple bonds inside it. In this work, a physical model that describes abrasion is derived utilizing the concepts of Kinetic Theory of Granular Flows (KTGF). The model considers the collisions between the different solid phases—particles and agglomerates to obtain the frequency of abrasion and mass transfer between the phases. A Eulerian-Eulerian formulation along with KTGF is used to simulate abrasion of a large agglomerate inside a fluidized bed. All the simulations were performed with commercial CFD code ANSYS Fluent 16, which was augmented by user-defined functions for different experimental conditions and different bed materials using published experimental data. The comparison between the modeling predictions and experimental results demonstrated a good qualitative agreement with some quantitative variations. It was found that the model tends to underestimate the amount of abrasion for capillary dominant system possibly due to neglecting the fragmentation phenomenon in the model. Also the model over predicts abrasion for viscous dominant set up. At the end, a mechanistic approach is suggested for future modeling of agglomerate fragmentation.

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