Fluidized Beds with Internals: A Real-Time Magnetic Resonance Imaging Study of Gas Bubbles and Particle Motion

Conzelmann, N., Empa, Swiss Federal Laboratories for Materials Science and Technology
Boyce, C. M., Columbia University
Penn, A., ETH Zurich
Pruessmann, K. P., ETH Zurich and University of Zurich
Müller, C. R., ETH Zurich
Fluidized beds are frequently equipped with internals in order to modify the fluidization dynamics within the bed, such as reducing the size of gas bubbles, or enforcing specific particle circulation patterns[1,2]. Traditionally, the design of internals is often based on rules of thumb and empirical correlations derived from optical measurements in pseudo two-dimensional (2D) systems, or by inserting probes into 3D systems. However, such measurements are either influenced by wall effects (pseudo 2D systems) or do interfere with the particle flow (intrusive probe measurements). More recently, sophisticated tomographic techniques have been applied to study fluidization dynamics in freely bubbling fluidized beds, such as X-ray tomography[3], electrical capacitance tomography[4] and magnetic resonance imaging (MRI)[5–7]. In order to design and scale-up fluidized bed reactors with internals reliably and efficiently, we need fundamental insight into fluidization dynamics in large, 3D systems with internals.

In this work we used a recently developed real-time magnetic resonance imaging methodology[7], to quantify the motion of particles and the size and distribution of gas bubbles within a cylindrical model system of diameter 190 mm with internals of different sizes. Instantaneous (Taquisition = 4.7 ms) snapshots of particle position and velocity of the particle phase reveal strongly reduced bubbling and reduced particle mobility in the wake region of the horizontal obstacles with circular cross section. Interestingly, also the region below the internals showed changes in their fluidization dynamics, such as an increased number of gas bubble formation.

The measurements presented here, provide valuable fundamental insight into the effects of internals on the fluidization dynamics in fluidized bed reactors. We anticipate that they might prove helpful to design reactors and to test numerical simulations.

[1] D. Kunii, O. Levenspiel, Fluidization Engineering, 2nd ed., Butterworth-Heinemann, 1991.

[2] J.S. Dennis, Properties of stationary (bubbling) fluidized beds relevant to combustion and gasification systems, in: Fluid. Bed Technol. Near-Zero Emiss. Combust. Gasif., 1st ed., Woodhead Publishing, Cambridge, 2013: pp. 77–146.

[3] R.F. Mudde, Bubbles in a fluidized bed: A fast X-ray scanner, AIChE J. 57 (2011) 2684–2690. doi:10.1002/aic.12469.

[4] T.C.C. Chandrasekera, Y. Li, D. Moody, M.A.A. Schnellmann, J.S.S. Dennis, D.J.J. Holland, Measurement of bubble sizes in fluidised beds using electrical capacitance tomography, Chem. Eng. Sci. 126 (2015) 679–687. doi:10.1016/j.ces.2015.01.011.

[5] C.R. Müller, J.F. Davidson, J.S. Dennis, P.S. Fennell, L.F. Gladden, a N. Hayhurst, M.D. Mantle, A.C. Rees, A.J. Sederman, Real-time measurement of bubbling phenomena in a three-dimensional gas-fluidized bed using ultrafast magnetic resonance imaging., Phys. Rev. Lett. 96 (2006) 154504. doi:10.1103/PhysRevLett.96.154504.

[6] C.M. Boyce, N.P. Rice, J.F. Davidson, A.J. Sederman, J.S. Dennis, D.J. Holland, Magnetic resonance imaging of gas dynamics in the freeboard of fixed beds and bubbling fluidized beds, Chem. Eng. Sci. 147 (2016) 13–20. doi:10.1016/j.ces.2016.03.005.

[7] A. Penn, T. Tsuji, D.O. Brunner, C.M. Boyce, K.P. Pruessmann, C.R. Müller, Real-time probing of granular dynamics with magnetic resonance, Sci. Adv. 3 (2017) e1701879. doi:10.1126/sciadv.1701879.