(189g) Combined Real-Time Magnetic Resonance Imaging and Numerical Simulation of Gas Bubble Splitting and Coalescence in Fluidized Beds

Gas bubbles in fluidized bed reactors influence heat, mass and momentum transfer between the gas and solid phases and in turn the design of the reactor. In particular, the size of the bubbles is an important parameter, since an increasing bubble size enhances particle mixing, but reduces gas-solid contacting. A precise control of the bubble size distribution is therefore crucial to ensure process continuity and the desired performance of the fluidized reactor. This demands a profound understanding of the mechanisms that lead to bubble splitting and coalescence. However, real-time data of these processes are difficult to acquire in a three dimensional bed due to the opaque nature of granular matter. Magnetic resonance imaging (MRI) has been shown to be a suitable method for the non-invasive measurement of solid fraction and particle velocity.

We have recently implemented MRI acceleration techniques to speed up data acquisition and enable the real-time investigation of granular dynamics (1). In this work, we study the transient splitting and coalescence of gas bubbles in a variety of fluidized bed geometries (i.e. with and without inserts) by means of combined MRI measurements and numerical simulations. Single gas bubbles are injected through a central orifice at the bottom of the bed. These MRI measurements are complemented by numerical simulations using computational fluid dynamics simulation coupled with a discrete element method (CFD-DEM) (2).

Both the numerical simulations and the MRI measurements of the bubble dynamics show very similar dynamics during bubble splitting and coalescence. Additionally, the simulations give insight into the solid motion at the grain level and provide information on the gas flow in and around the bubbles. Combing our experimental and numerical results allowed us to draw conclusions concerning bubble stability and the mechanism that control bubble splitting and coalescence. We are confident that our findings contribute to obtaining a better understanding of the complex interplay between the gas and solid phase in fluidized beds.

1. Penn, A., et al., "Real-time probing of granular dynamics with magnetic resonance," Science Advances, 3 (9), pp. 1-7 (Sep 2017).
2. Kloss, C., et al., "Models, algorithms and validation for opensource DEM and CFD-DEM," Progress in Computational Fluid Dynamics, 12 (2-3), pp. 140-152 (2012).