(216e) Investigation of Fluid Dynamic Parameters in Packed Distillation Columns Using X-Ray Computed Tomography

The flow behavior of two 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 three-phase distillation, 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 (70 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 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. 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 accumulation of the liquid between two packing segments and the liquid distribution along the packing height was 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. 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.

For the study on the flow behavior of the two immiscible liquids, water and n‑heptane were fed into the column with a baffle type distributor with two separate feeding points for each liquid to ensure that the two phases are evenly distributed on the packing. The flow behavior of the two liquids on the packing was analyzed by varying the total liquid load between 5 and 70 m³ / m² h. The feed ratio of water and n-heptane was 1:1, 2:1 and 3:1. Furthermore the flow morphology of the interacting liquids was investigated using a three dimensional visualization of short packing sections along the column.