(62b) Detection of Fluidization with Real-Time Magnetic Resonance Imaging

Unfluidized areas (dead zones) within gas-solid fluidized beds can deteriorate the mixing and heat transfer between the gas phase and the solid phase, and thus lower the performance, homogeneity and efficiency of the desired process. A thorough understanding of the conditions that lead to the formation of dead zones is therefore key in order to design, operate and scale fluidized bed reactors reliably. The experimental detection of dead zones inside three-dimensional (3D) fluidized beds can provide valuable information to validate theories and numerical models. However, due to the opacity of most granular materials, such measurements are challenging and require the use of intrusive probes^1,2 or non-intrusive tomographic techniques, e.g. x-ray tomography³, electric capacitance tomography (ECT), positron emission particle tracking (PEPT)⁴ or magnetic resonance imaging (MRI)⁵. While intrusive probes perturb the granular dynamics, the tomographic techniques suffer from limitations in either their ability to measure particle motion (x-ray CT, ECT, PEPT) or from poor temporal resolution (MRI)⁶.

Here we present a MRI methodology that can discriminate between fluidized and unfluidized areas in 3D beds of considerable size (cylindrical bed; diameter 190 mm and height 300 mm) at a frame rate of 20 Hz. The high temporal resolution of this technique allows studying not only the presence of dead zones but also their formation and disintegration. The presented measurement technique bases on a recent development in real-time probing of granular dynamics with MRI which uses array detection of the magnetic resonance and engineered granular signal sources⁷. It exploits both spin saturation effects and diffusion attenuation to create a strong dependency of the signal on particle motion.

We anticipate the presented technique to be a starting point for more sophisticated studies on the formation of dead zones in fluidized beds, such as determining the influence of internals, the distributor or bed geometry. Moreover, we envisage that the technique is suited to provide fundamental physical insights on the nature of fluidization, its relations to jamming and the glass transition⁸.

References

1. Geldart, D. & Kelsey, J. R. The use of capacitance probes in gas fluidised beds. Powder Technol. 6, 45–50 (1972).

2. Werther, J. & Molerus, O. The local structure of gas fluidized beds -II. The spatial distribution of bubbles. Int. J. Multiph. Flow 1, 123–138 (1973).

3. Mudde, R. F. Double X-ray Tomography of a Bubbling Fluidized Bed. Ind. Eng. Chem. Res. 49, 5061–5065 (2010).

4. Wildman, R. D., Huntley, J. M., Hansen, J. P., Parker, D. J. & Allen, D. A. Single-particle motion in three-dimensional vibrofluidized granular beds. Phys. Rev. E 62, 3826–3835 (2000).

5. Dennis, J. S. Magnetic Resonance Studies of Dead-Zones in Gas-Solid Fluidised Beds. (2013).

6. Pore, M. et al. Magnetic resonance studies of a gas-solids fluidised bed: Jet-jet and jet-wall interactions. Particuology 8, 617–622 (2010).

7. Penn, A. et al. Real-time probing of granular dynamics with magnetic resonance. Sci. Adv. Accepted, (2017).

8. Goldman, D. I. & Swinney, H. L. Signatures of glass formation in a fluidized bed of hard spheres. Phys. Rev. Lett. 96, 145702 (2006).