## 3 Results and discussion
### Pressure Drop and Vertical Segregation of Fluidizing Solids  
The pressure drop over the bed was measured and compared to two different scenarios without packing: Equal mass of fluidizing solids and equal bed height. The pressure drop of the packing solids without addition of fluidizing solids was measured separately and found to be close to zero. Adding packing solids and fluidizing solids together contributed to a higher bed pressure drop than a conventional fluidized bed with equal fluidizing solids mass. [PERSON] et al. (2018) hypothesized that this increase is present due to abrasive interaction between the bed material and packing. Another hypothesis is that drag forces increase due to the collapse of the conventional bubble-emulsion flow structure--negating low pressure paths (bubbles) to form. It was also measured that a confined fluidized bed however has a lower total pressure drop than a conventional fluidized bed of the same height. As previously observed by e.g., [PERSON] (2018) the pressure drop decreased slightly with increased fluidization velocity, in the range tested. This effect can likely be attributed to segregation of packing and fluidizing solids, since the mentioned interaction decreases with the amount of fluidizing solids in the packing. A bed without packing, but with equal height as one with, exhibited a higher overall pressure drop than experiments when packing was used. In Figure 5(a) pressure drop normalized by the mass of fluidizing solids is shown. Since bed height and mass were varied, this paints a more accurate picture of the difference in pressure drop related to the packing material used than the total pressure drop. As expected, the experiments without packing showed similar pressure drops when normalized. From Figure 5(b), which shows the pressure drop from fluidizing solids inside the packing, it can be seen that the size of the packing solids affected the pressure drop: Increasing with packing size. The 6.35 mm packing was tested but it was observed that fluidizing solids gathered along one side of the bed, letting large amounts of gas bypass the bed. This led to a significantly lower pressure drop compared to packings of larger size. For this reason, it was omitted from Figures 6 and 7. With ECA packing, pressure drop did not increase significantly with superficial gas velocity. It was observed for this packing that the packing solids initially floated on top of the fluidizing solids. When the bed was fluidized at u\({}_{0}\) = 18 cm/s the packing started to move slightly. The effect became more apparent when the velocity was increased further, and the fluidizing solids and packing solids were co-fluidized. Figure 5: Comparison of mass transfer in fluidized and confined fluidized beds, with varying types of packing. (**a**) Mass transfer coefficient over time. (**b**) Average mass transfer coefficient as a function of pressure drop. Segregation of bed material and packing was expected to have an impact on the mass transfer performance of the confined fluidized bed since bubbles can grow freely if the fluidizing solids are not suspended within the packing solids. Figure 7 shows that the voidage of the fluidizing solids inside the packing increased with superficial gas velocity, indicating that fluidizing solids move from the voids in the packing solids to the space above the packing. The voidage of a bed without packing solids is shown for comparison. Voidage was very similar for the two larger ASB packings tested. The ECA packing did, as previously mentioned, act as flotsam rather than being stationary. This meant segregation could not be measured by the method used for the other packings. For ECA Figure 7 instead shows an estimation of the average voidage and how it expanded uniformly with increased gas flow. Figure 6: (**a**) Pressure drop of a fluidized bed with or without packing, normalized by total mass of fluidizing solids. (**b**) Pressure drop over the packing of a confined fluidized bed of a certain height, normalized by mass of fluidizing solids inside the packing.