UBC Theses and Dissertations
Mathematical modeling of interaction of wet particles and application to fluidized beds Darabi, Pirooz
In many industrial operations, such as fluidized bed granulators, coaters, and fluid cokers, a binding or reacting liquid is introduced into the system. Due to the effects of liquid, the multi-phase transport phenomena of these systems are more complicated compared to conventional gas-solid fluidization systems. In this thesis, mathematical modeling is used to study the interaction of wet particles. First, a coalescence model is developed to describe the binary collision of wet particles. The model is in the form of a wet coefficient of restitution and is used to determine the critical velocity—the boundary between coalescence or rebound outcomes—for a range of capillary numbers. Model predictions are compared with the available experimental data and good agreement is found. The model accounts for both liquid viscosity and surface tension effects and is used to investigate the boundary between collisions with dominant capillary and respective viscous effects. Then, by incorporating time- and temperature-dependant variations of the viscosity and thickness of the liquid coating, the model is used to determine the agglomeration tendency of bitumen-coated coke particles in fluid cokers. A simplified mathematical model and numerical solution of the Navier-Stokes equations are used to study the rupture of stretching liquid bridges between two solid spherical particles. The simplified model considers the geometry of the problem in which the gas-liquid interface is represented with a parabola. The numerical simulations of the Navier Stokes equations are performed with FLUENT and are used to investigate the viscous, surface tension, inertial, gravitational, and contact angle effects on the rupture distance and liquid distribution. Finally, the interaction of multiple wet particles is addressed by implementing the wet coefficient of restitution proposed in this thesis, using MFIX, an open-source Discrete Element Method (DEM) tool. DEM simulations of a fluidized bed consisting of mono-sized solid spherical particles pre-coated with identical liquid coatings are performed, and the effect of coating viscosity and thickness on the fluidization behaviour is investigated. Snapshots of the instantaneous particle positions are presented, and time-averaged values of the bed centroid in the y-direction, wet coefficient of restitution, and relative normal collision velocity are analyzed.
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