The hydrodynamics 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 and 192 detectors, allowing as to obtain the voidage distribution in two horizontal planes [1,2].
Recently, we have made several advances regarding X-ray tomography. Concerning the data analysis, we have compared the ‘traditional’ approach of algebraic reconstruction with an alternative genetic algorithm. Both methods have their specific advantages, and we will show that a hybrid method combines the best of both worlds . We also developed a new method for very fast analysis, enabling on-line reconstruction of the X-ray data.
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. We will show that the bulk density is a function of the height. Moreover, the strong improvements that can be obtained with a micro-jet  will be characterized using the X-ray tomography.
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 .
 Van Ommen, J.R., Mudde, R.F., 2008, “Measuring the Gas-Solids Distribution in Fluidized Beds - A Review”, International Journal of Chemical Reactor Engineering 6 (2008) R3.
 Mudde, R.F., Bubbles in a Fluidized Bed: A Fast X-Ray Scanner, AIChE Journal 57 (2011) 2684-2690.
 Yang, X., van Ommen, J.R., Mudde R.F., “Comparison of genetic algorithm and algebraic reconstruction for X-ray tomography in bubbling fluidized beds”, Powder Technology 253 (2014) 626-637.
 Quevedo, J.A., Omosebi, A., Pfeffer, R., “Fluidization enhancement of agglomerates of metal oxide nanopowders by microjets”, AIChE Journal 56 (2010) 1456-1468.
 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, in press.
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