(325c) Investigation of Fluid Dynamic Parameters in Packed Distillation Columns Using X-Ray Computed Tomography

The flow behavior of one or more liquid phases in packed distillation columns is hardly understood. The final aim of this work is to develop a model for the design and process optimization of distillation processes, using reliable data for the flow behavior of two liquid phases on a packing.

In this work the X-ray computed tomography (CT) is used as non-invasive measuring technique. The experiments are made 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 several steps of image treatment were applied to finally obtain segmented images with specific gray values for each component. The accuracy of 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 1000 mm consisting of five packing segments. For the investigation of the fluid dynamics five different special designs of Mellapak 500.Y packing from Sulzer Chemtech AG were used, to see the influence of the surface modification on the fluid dynamics. The special designs are a smooth, fluted and gridded texture with holes and a smooth and gridded texture without holes. Additionally the influence of the rotation of the packing segments to each other by 0° and 90° on the liquid distribution along the packing height and the accumulation of the liquid between two packing segments were determined. For the investigation of the influence of the substance properties on the flow morphology and the fluid dynamics, different liquids with low and high viscosities were used. The liquids were fed with a baffle distributor and the liquid load was varied between 5 and 50 m² / m³ h and the F-factor was increased to 3 Pa^0.5.

In a former study we showed that the packing is not evenly wetted and the liquid film thickness is not constant over the entire packing. Hence we developed a method to determine the liquid distribution and the flow morphology on a packing cross section and the packing height. Due to the fact, that the packing surface is not evenly wetted, we determined the interfacial area between the packing and the liquid as well as between the liquid and the surrounding out of the CT-images. Thereby we were able to calculate a packing surface efficiency for different liquid and gas loads.

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