(530b) Application of X-Ray Computed Tomography in Extraction Columns: A Feasibility Study

Computed tomography has become more important for the non-invasive investigation of technical devices. This measuring technique allows the observation of fluid dynamics and liquid distribution over the entire object without influencing the fluid flow. With suitable components it is even possible to determine the mass transfer of one liquid or gas phase into a liquid.

The experiments are performed with a customized computer tomograph at our institute that enables measurements of flows in vertical objects. The detector and the X‑ray source are rotating around the column and the column is not moved, so that the flow inside is not influenced by centrifugal force. As a result of the high spatial (80 µm) and temporal (1000 projections per second) resolution, dynamic processes inside separation columns can be monitored with a high precision. Fluid dynamic parameters in distillation columns (like interfacial area, liquid hold-up and liquid film thickness on the packing or between two liquid phases) can be directly estimated from the recorded CT images of a packing cross section at different height positions along the column. For the evaluation of the recorded CT-images several steps of image treatment were applied to finally obtain the relevant data directly from the images.

In chromatographic columns we are able to describe the breakthrough behavior of a liquid tracer and implement the experimental data in a CFD-model to predict a stationary phase for a specific separation problem. In packed distillation columns we can determine fluid dynamic parameters like hold-up, interfacial area and the liquid film thickness on the packing over the entire packing height and the entire cross section area. Three dimensional visualizations show that there are especially rivulet flows in the gaps between two plates and windings of the packing.

A decisive point in the design of extraction columns is the mass transfer between the phases. With computed tomography it is possible to determine the in-situ mass transfer in a suitable system. In extraction columns the droplet size is several millimeters and the velocity about 0.1 m / s. With computed tomography we are able to determine the shape of the droplets and by averaging the CT-images over the time it is possible to determine the temporal progress of the mass transfer from the surrounding into the droplet or the other way round. The change of the gray values in a CT‑image results in a concentration profile in the direction of the mass transfer. With this information we can determine a mass transfer profile into the droplet at several height positions of the column over the entire cross-sectional area.