Dissertation/Thesis Abstract

Influence of drag laws on segregation and bubbling behavior in gas-fluidized beds
by Leboreiro Hernandez, Jose, Ph.D., University of Colorado at Boulder, 2008, 457; 3303878
Abstract (Summary)

Gas-fluidized bed reactors have found a wide variety of industrial applications. However, their hydrodynamics behavior is extremely complex, making the design, scale-up, and optimization a challenging task. Mathematical models have provided valuable insight on the contributing physical mechanisms in gas-solid systems. In the present work, three mathematical approaches are used to study the effect of various drag laws and friction on bubbles and species segregation in low-velocity, gas-fluidized beds. Specifically, a soft-sphere discrete element method (DEM) model, a continuum (two-fluid) model, and a multi-phase particle-in-cell (MP-PIC) model are considered.

In the first portion of the work, the effect of the form of the “standard” drag law for gas-fluidized beds on the bubbling behavior of monodisperse systems is studied. The effect of the volume fraction chosen for transition and the form of the “stitching” function is considered. Overall, the bubble properties are affected by chosen form of the drag law.

In the second portion of the work, gas-fluidized beds with binary mixtures are targeted. Both the ad hoc drag treatment, which does not account for the composition of the mixture, and a lattice-Boltzmann-based drag law targeted specifically at polydisperse systems are utilized. Specifically, a MP-PIC model is used to evaluate the impact of various drag laws on segregation predictions. The drag law treatment is found to play a crucial role in the qualitative and quantitative nature of both segregation and bubbling predictions. In the third portion of the work, an extensive comparison between experimental, axial segregation profiles and predictions obtained from both continuum and MP-PIC models is presented. The results indicate that no one particular form of the drag law currently compares well throughout experimental data sets covering a wide range of conditions nor does it outperforms the others.

A possible cause of the observed mismatch may be due to the omission of friction in the previous simulations. Specifically, the lattice-Boltzmann drag treatment always overpredicts segregation. In the final portion of the work, the effect of friction on segregation is studied via a soft-sphere DEM. Preliminary results indicate that the incorporation of friction reduces segregation.

Indexing (document details)
Advisor: Hrenya, Christine M.
Commitee: Banerjee, Sibashis, Clough, David E., Sture, Stein, Weimer, Alan W.
School: University of Colorado at Boulder
Department: Chemical Engineering
School Location: United States -- Colorado
Source: DAI-B 69/03, Dissertation Abstracts International
Subjects: Chemical engineering
Keywords: Bubbling beds, Drag laws, Friction, Gas-fluidized beds, Polydisperse, Segregation
Publication Number: 3303878
ISBN: 9780549508656
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