Graduate Thesis Or Dissertation
 

Dynamic behavior of ferromagnetic particles in a liquid-solid magnetically assisted fluidized bed (MAFB) : theory, experiment, and CFD-DPM simulation

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  • Magnetically Assisted Fluidized Bed (MAFB) is a novel technology where an external magnetic field with constant gradient interacts with magnetically susceptible particles. The linear magnetic field creates two type of forces: external and interparticle magnetic forces. A theoretical mathematical model based on the interaction of ideal dipoles is proposed to account for the interparticle magnetic forces. This model is validated with data obtained from the specially designed repulsion force experiment. The required magnetic field (B= -∇B*z+B₀) decays linearly with the column height (z), with maximum field strength at the distributor plate where z=0. Alginate beads, containing ferromagnetic powder, are used as the dispersed phase and water as the fluidizing media. Three different experiments are performed: 1) Pressure drop and bed expansion are measured at different magnetic fields and compared with corresponding measurements in a conventional fluidized bed. The overall pressure drop increases and the bed height decreases because of the magnetic field applied. 2) Two types of particles of same diameter: magnetically susceptible (lighter) and non-susceptible (heavier), with density difference of 10%, are used to study the mixing-segregation behavior in the MAFB. Complete mixing is achieved when the apparent weight of the susceptible particle (due to magnetic field) equals the non-susceptible particle weight. 3) Selective magnetic separation of a binary mixture of paramagnetic and non-magnetic particles is studied to demonstrate the MAFB feasibility. The magnetic separation results show that approximately 88% of paramagnetic particles are retained into the MAFB. The fluid dynamics and particle motion are simulated in a 2-D model using Computational Fluid Dynamics (CFD) and Discrete Particle Method (DPM) approach. The CFD uses the SIMPLE method to integrate the volume-averaged Navier-Stokes equations, and the DPM is based on the Newton equation of particle motion. A Fortran code, AZTECA, is developed to perform the simulation of pressure drop, bed expansion, and mixing-segregation experiments. The number of simulated particles is between 2500-3500. The data postprocessing is done with a Visual Basic code, BOLITAS. The simulation results obtained with the AZTECA code show excellent agreement with the experimental data. Typical difference between experimental and simulated results is less than 10%.
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