(329b) Tomographic Investigation of a Three?Phase System in Packed Columns

The fluid dynamic behavior of two poorly miscible liquids and a gas phase in packed columns, as it occurs in three-phase distillation, is very complex and poorly understood. Although structured packings provide significant separation improvements and less pressure losses at two-phase applications, trays are in industry the preferred internals for three-phase applications. The reason, among other aspects, is the unpredictable risk of a reduction of the separation performance by the second liquid in packed columns.

Therefore the objective of this work is to determine the fluid dynamics of a three-phase system in packed columns as the basis for understanding the phenomena on internals. For this purpose X-ray computed tomography (CT) is a suitable non‑invasive measuring technique, which allows the determination of local parameters in packed columns under process conditions. The high resolution of the CT‑images (80 μm) and the phase contrast of the system water/1,2‑Dichlorobenzene/air enable, in combination with an image processing routine, a precise determination of the individual phases.

All experiments were performed in a 100 mm column with a packing height of 1 m. Several structured and random packings were investigated. The ratio and the total amount of the liquid loads were varied. Besides the investigations over the entire column, detailed images in 1 mm and 150 μm intervals enable a closer look at local flow patterns. Moreover long‑term experiments over several hours were conducted to investigate temporal changes of the flow. As described in literature, it is important for the performance, which liquid phase is fed first into the column. Therefore and due to the influence of the initial distribution of the liquids on the flow morphology, different experiments were performed on this issue. Furthermore several fluid dynamic assumptions from literature and single sheet experiments were checked.

Whereas the focus was on the feasibility and qualitative results in the previous year, this year the focus is on quantitative results and validation. Segmented 3D visualizations of packing sections allow an insight from all perspectives and simplify the flow analysis for the first time. After identifying films, rivulets and bridges as the main flow pattern qualitatively, their shares on hold‑ups and interfacial areas were quantified. The results confirm the qualitative analysis. A combination of this flow morphology identification with residence time results should allow a better understanding of the influence of the single flow pattern on mass transfer. A determination of the variance in long-term measurements gives information about the dynamics of three-phase systems in packed columns. Furthermore the hold-ups and the interfacial areas of both liquids were quantified and a dependency on the position in the column and as a function of the liquid loads was proven.