Bubble Dynamics in a Fluidized Bed

Abstract

Fluidized bed reactors are catalytic multiphase reactors capable of processing large volumes with relatively high mass and heat transfer, which is appealing to a variety of industries. The presence of voids, in this case gas bubbles, within these systems can be detrimental to the efficiency and are desired to be minimized. Further fundamental understanding of the bubbles is necessary to model, design, and control them. However, the study of bubbles is difficult, as these beds are typically opaque and 3D. Current studies of bubbles within fluidized beds are often limited to use of quasi-2D set ups, specialized particles, or internal measuring devices. This research utilizes a newly constructed experimental setup consisting of three x-ray source and flat panel detector pairs that produce 2D projections. This setup allows for investigation into the bubble dynamics inside a 3D cylindrical column with industrial particles. The possibilities and limitations of the use of 2D projections to obtain fully 3D time-resolved reconstructions were explored through simple 3D reconstructions of injections of single bubbles. These 3D reconstructions showed the potential of the setup for 3D time-resolved studies of the bubble dynamics and laid the groundwork for future studies. Apart from the ability to obtain 3D reconstructions, the 2D projections can be used to study the dynamics of the bubbles. In this research, 2D projections of an injection of a single bubble were used to develop a deeper fundamental understanding of the shape, motion, and dynamics of an individual bubble traversing through a bed. The bubble shape and size were obtained through a horizontal slicing technique, which assumes axis-symmetry around the vertical axis. Different background fluidization levels, injection volumes, injection velocities, particles, and column diameters were investigated. An analysis of the individual bubble trajectories and the statistical averages provided insight into the interconnection of these factors. The two previously studied bubble stages, formation and rising, were observed. The rising stage was observed to have two regions, stable and unstable. The stable region, following the formation stage, occurs when the motion of the bubble is essentially rectilinear. The unstable region, following the stable region, occurs when the bubble moves sideways while traversing the column in an irregular motion. A simple analogy to gas bubbles in a liquid was drawn, suggesting the bubble cloud, or particles surrounding the bubble, plays a vital role, especially in the formation of the bubble. Standard correlations for the bubble rise velocity based on a constant bubble Froude number were found to be insufficient at accurately describing the bubble velocity and dynamics. The Froude number for individual bubbles appears to be non constant and dependent on parameters that have not been previously considered, like the level of background fluidization.