(543b) Effect of Electrostatics On Bubbles in Gas-Solid Fluidized Beds

The generation of electrical charges, reported in gas-solid fluidized beds for over sixty years, can cause serious problems like wall sheeting in polyolefin reactors, leading to costly shutdown, electrical shock hazards and even explosions. Understanding the associated phenomena plays an important role in learning how to avoid these problems. In this study an attempt has been made to broaden the understanding of electrostatics in gas-solid fluidized beds by adopting computational fluid dynamics (CFD), using the Two-Fluid-Model in MFIX (open-source Multiphase Flow with Interphase eXchange software originated by the U.S. Department of Energy). The Maxwell equations were incorporated in the MFIX code in this work. The resulting model is then used to investigate how electrostatic charges modify bubble shape, size, velocity and interaction for three different cases: (a) single bubbles, (b) bubble pairs in vertical and horizontal alignment, and (c) freely bubbling bed. In each of these cases, a two-dimensional column, partially filled with mono-sized particles, is simulated for uncharged and charged particles.

In case (a), it is predicted that single bubbles elongate and rise more quickly in charged particles than in uncharged ones. For Case (b) electrostatics cause asymmetry of coalescence for a pair of vertically-aligned bubbles, while it causes the migration of a side bubble towards the axis of the column and changes the leading-trailing role for a pair of horizontally-aligned bubbles. Finally in case (c), the simulation predicts that electrostatic charges decrease bubble size and frequency in the free bubbling regime, accompanied by a change in the spatial distribution of bubbles, causing them to rise more towards the axis of the column.

An attempt was also made to test experimentally the single bubble simulation results. To reach this goal, a two-dimensional fluidization column was built with a central jet to inject single bubbles. The setup is also equipped with a novel Faraday cup device to measure the charge density of particles accurately. The experimental results show a decrease in bubble size and an increase in bubble height-to-width ratio with increasing charge density, accompanied by an increase in particles raining from the bubble roof.