(270f) Time-Resolved X-Ray Tomography of Fluidized Beds

The dynamics of fluidized beds are challenging to study: the voidage distribution is changing quickly over time (i.e., a high temporal resolution is needed) and dense gas-solids flows are typically opaque to visible light. X-ray tomography can cope with these challenges. It is a non-intrusive technique that can be used for the reconstruction of the cross-sectional distribution of the different phases in fluidized beds. In Delft, we have an X-ray system consisting of three X-ray sources [1,2]. Originally, this system was equipped with 192 detectors, but we recently upgraded it to three vertical 30 x 30 cm detector plates with each 1524x1548 pixels, resulting in a resolution of about 1 mm.

We have applied our tomographic systems to a nanopowder fluidized bed. This is challenging, because of the much lower bulk density than for regular fluidized bed, as a result of the very high voidage [3]. We will show that the bulk density is a function of the height. Moreover, we will demonstrate that the history of the powder plays a strong role in nanopowders fluidization: stepwise increase of the gas velocity gives much poorer fluidization behaviour than starting with a high gas velocity, and stepwise decreasing it.

We also applied X-ray tomography to study the bubble hold up and its distribution in bubbling fluidized beds with and without vertical internals such as heat exchanger tubes. The goal is to understand the influence of vertical internals on the hold up. We show that with employed internals, the bubbles are more homogeneously distributed in the bed [4]. More, we have studied the influence of column diameter on bubble size and velocity in such a system [5].

[1] Van Ommen, J.R., Mudde, R.F., 2008, â??Measuring the Gas-Solids Distribution in Fluidized Beds - A Reviewâ?, International Journal of Chemical Reactor Engineering 6 (2008) R3.

[2] Mudde, R.F., Bubbles in a Fluidized Bed: A Fast X-Ray Scanner, AIChE Journal 57 (2011) 2684-2690.

[3] Quevedo, J.A., Omosebi, A., Pfeffer, R., â??Fluidization enhancement of agglomerates of metal oxide nanopowders by microjetsâ?, AIChE Journal 56 (2010) 1456-1468.

[4] Maurer, S., Wagner, E. C., Schildhauer, T. J., van Ommen, J. R., Biollaz, S. M., & Mudde, R. F. (2015). X-ray measurements of bubble hold-up in fluidized beds with and without vertical internals. International Journal of Multiphase Flow 74 (2015) 118-124.

[5] Maurer, S., Gschwend, D., Wagner, E.C., Schildhauer, T.J., van Ommen, J.R., Biollaz, S.M., & Mudde, R.F., â??Correlating bubble size and velocity distribution in bubbling fluidized bed based on X-ray tomographyâ?, Chemical Engineering Journal 298 (2016) 17-25.