(195d) Tomographic Determination of Fluid Dynamic Parameters for Three‑Phase Systems 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, poorly understood and rarely described in literature. Therefore and due to the frequently occurring negative influence of the additional liquid phase on the mass transfer the objective of this work is to determine the fluid dynamics in a three-phase system.

The used X-ray computed tomography scanner (CT), which has been customized designed for process technology application, enables the determination of local fluid dynamic parameters under process conditions. X-ray source and detector rotate around the vertical axis and are height-adjustable what enables the investigation of the whole unmoved column without influencing the flow. The high spatial (80 μm) and temporal (1 fps) resolution of the generated CTâ??images and the selection of suitable materials enable, in combination with subsequent image processing steps, a precise determination of the individual phases in the entire column.

All experiments were performed in a glass column with an inner diameter of 100 mm and a packing height of 1 m. Different surface-treated Mellapak 500.Y of the Sulzer Chemtech AG were used to investigate the influence of the surface modification on the results. Water/dichlorobenzene/air was used as three-phase system, because preliminary CT tests showed distinguishable gray values of all phases. The liquid load, both the ratio (1:3, 1:1, 3:1) and the total amount (2 to 40 m² m^-3 h^-1), and also the supply order of the liquids 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 longâ??term experiments were conducted to investigate temporal changes of the flow behavior. For a quantitative evaluation of the experimental data a new image processing routine was developed to receive reproducible and meaningful results. It is building on an evaluation method for twoâ??phase systems, developed at the same institute, and includes a new segmentation algorithm to assign every phase its own gray value. This allowed the quantification of amount and distribution of the phase interfaces and the hold-ups as a function of place, time and packing parameters.

As a result, for example, a significant reduction of the interfacial area between water and air was proven, because the water forms preferable rivulets in the packing corrugations and the 1,2-dichlorobenzene forms films, which overflow the packing and the water rivulets.