(155c) Investigation of Fluid Dynamics in Structured and Random Packings Using X-Ray Computed Tomography

The flow behavior of one or more liquid phases in packed distillation columns is hardly understood, since with common optical measuring techniques local parameters in a packing are not achievable.

In this work the X-ray computed tomography (CT) is used as non-invasive measuring technique. 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, fluid dynamic parameters (like liquid hold-up, wetted packing surface, interfacial area and liquid film thickness) inside separation columns can be monitored with a high precision. All parameters are directly determined from the recorded CT images at different height positions along the column without a comparison to reference images of dry or wetted packing sections. For the evaluation several steps of image processing were applied to finally obtain segmented images with specific gray values for each component. The accuracy of the measuring technique including the developed segmentation algorithm was verified on a dry and wetted packing through a comparison of the mass of the (wetted) packing and the calculated values out of the segmented images.

The experiments were performed in a glass column with an inner diameter of 100 mm and a packing height of approximately 1000 mm. For the investigation of the fluid dynamics in structured packings SULZER Mellapak 500.Y and Raschig SuperPak 350.Y packings and as random packings Raschig SuperRing and Raflux and RSMR rings from RVT were used. For the investigation of the influence of the substance properties on the fluid dynamics different liquids (water and organic compounds) were used. The liquid load was varied between 2 and 50 m² m^-³ h^-1 and the F-factor was increased to 2.5 Pa^0.5 or the flooding point.

For every packing type the integral liquid hold-up over the entire packing as well as the hold‑up profile along the packing height was determined. For an evaluation of the liquid distribution in radial direction the packing cross section was separated in three annuluses of equal area and the liquid hold-up was read out from the CT images for each annulus. For an analysis of the wetting efficiency the wetted packing surface area was determined and compared to the geometrical packing surface area. In a next step the interfacial area between liquid and gas phase was determined to obtain the real mass transfer area. The results show a not evenly wetted packing surface and a bigger liquid film thickness than expected through correlations from literature for each packing type.

Furthermore the flow morphology of the liquids was investigated using a three dimensional visualization of short packing sections along the column.