Fluidization Behavior of Elongated Particles - CFD-DEM Simulations and X-Ray Tomography Experiments

Gas-solid fluidized beds are widely used in process and energy industries. The performance of a fluidized bed is majorly governed by the formation of gas bubbles and their distribution, facilitating rapid solids mixing and high heat and mass transfer rates. On the other hand, very large bubbles (slugs) reduce gas-solid contact, limiting the particle mixing, mass and heat transfer characteristics in the gas-solid bed. Therefore, the study of bubbles in fluidized beds is a key factor for its scale-up. Even though bubble formation in gas-solid fluidized beds has been the subject of a large number of investigations, most of these investigations considered only particles that are close to spherical. In recent years, there is an increased interest to use fluidized beds for biomass processing. Biomass particles which are dried and milled, and possibly pelletized, are characterized by an elongated shape and behave considerably different from spheres when fluidized. So far, the literature is quite limited when it comes to bubble dynamics of fluidized beds with non-spherical particles.

In this work, we investigate bubble dynamics in 3D fluidized beds with Geldart D spherocylindrical particles using both experimental and numerical approach. For numerical simulations, we use an open source CFDEM code which uses coupled OpenFOAM computational fluid dynamics (CFD) solver to describe the fluid phase and LIGGGHTS, the discrete element method (DEM) to account for particle-particle interactions. This CFD-DEM code was adapted to take in to account spherocylindrical particles [1, 2]. Hydrodynamic interactions between fluid and spherocylindrical particles are modeled using correlations developed by Sanjeevi et al. [3]. Results from the CFD-DEM simulations are compared with experimental results obtained with X-ray tomography. This non-invasive experimental method gives us insight into properties of bubble formation in a 3D fluidized bed [4-5]. Comparison is made on the basis of bubble size and shape, bubble rise velocity, and bubbles and void fraction distributions.

References

^[1] V. V. Mahajan, T. M. J. Nijssen, J. A. M. Kuipers, J. T. Padding, (2018), “Non-spherical particles in a pseudo-2D fluidised bed: Modelling study”, Chem. Eng. Science, 192, 1105-1123

^[2] I. Mema, V. V. Mahajan, B. W. Fitzgerald, J. T. Padding, (2018), “Effect of lift force and hydrodynamic torque on fluidization of non-spherical particles”, Chem. Eng. Science, In Press, doi: 10.1016/j.ces.2018.10.009

^[3] S. K. P. Sanjeevi, J. A. M. Kuipers, J. T. Padding, (2018), “Drag, lift and torque correlations for non-spherical particles from Stokes limit to high Reynolds numbers”, Int. J. Multiphase Flow, 106, 325-337

^[4] F. Schillinger, T.J. Schildhauer, S. Maurer, E. Wagner, R.F. Mudde, J.R. van Ommen, (2018), “Generation and evaluation of an artificial optical signal based on X-ray measurements for bubble characterization in fluidized beds with vertical internals”. Int. J. Multiphase Flow, 107, 16-32

^[5] S. Maurer, D. Gschwend, E. C. Wagner, T. J. Schildhauer, J. R. Van Ommen, S. M. A. Biollaz, R. F. Mudde, (2016), “Correlating bubble size and velocity distribution in bubbling fluidized bed based on X-ray tomography”, Chem. Eng. J., 298, 17-25