(339b) Non-Invasive Analysis of Hydrodynamics and Mass-Transfer in Liquid-Liquid Extraction By Means of Computed Tomography

Flow phenomena inside packed liquid-liquid extraction columns are poorly understood due to numerous effects that influence droplet motion and restricted experimental access in such opaque systems. While single droplets were studied extensively with optical measurement techniques, these are not applicable in opaque packed columns or at high disperse hold-up. Advanced measurement techniques as X-ray computed tomography allow for non-invasive observations of fluid dynamics in those systems at high spatial resolution.

In this work fluid dynamic parameters such as drop size distribution and disperse hold‑up are investigated inside a packed liquid-liquid extraction column. Three different non-invasive measurement techniques are applied: X-ray radiography is used to measure disperse hold‑up at high temporal resolution (500 fps). The local hold-up is validated by integral hold-up values measured by the differential pressure method. Data on drop sizes, drop distribution and deformation is obtained from tomographic images with a high spatial resolution (80 µm) and a temporal resolution of 1 fps.

The experiments are conducted in a glass column with internal diameter of 50 mm and a packing height of 800 mm. The heavy organic phase (1,2-dichlorobenzene) is dispersed at the top of the column while the aqueous continuous phase flows in counter-current mode. In order to reduce the droplet velocity to the temporal resolution of the tomographic setup, viscosity is increased by adding polyethylene glycol.

The influence of a perforated and unperforated corrugated sheet packing on the above-mentioned hydrodynamic parameters is compared. In both packing geometries, droplets pass only a fraction of the cross-sectional area and preferred droplet paths can be identified. In contrast to absorption/distillation columns, disperse phase holdup is distributed uniformly inside the structured packing without accumulation at the packing intersections.

Beside the hydrodynamic investigation, we present a suitable test-system for non‑invasive tomographic measurements of mass-transfer in liquid-liquid systems. High atomic numbers and densities of metal ions allow the tomographic measurement of ion concentrations. Therefore, it is possible to measure non-invasively local concentration profiles in both phases during metal liquid-liquid extraction.