(156d) Measuring Cells As a Design Tool for Columns with Structured Packings.

Established methods for the design of packed columns for distillation, absorption and desorption are based on simplified and extensive model assumptions. Due to the high complexity of the phase distribution within the column and insufficient knowledge of the mass transfer behavior of real multicomponent systems, predictions on the actual separation performance of industrial columns can bear a high level of uncertainty.

A widely used parameter for characterizing the separation performance of structured packings is the HETP value, which, among other things, depends strongly on the physical properties of the components to be separated. Therefore, in many cases basic tests in calibrated lab columns are required for an unknown non-ideal system to separate. Scale-up of the plant to production scale is then done using the HETP value determined with standard test systems that are usually chosen from the list of recommended test systems for distillation columns compiled by Onken and Arlt[1].

The disadvantage of this column-based methodology is the comparatively large amount of investigated system required, which often cannot be provided and the high costs for conducting the experiments. For a fast and cost-efficient determination of the HETP value, respectively a weakening factor, which translates the HETP from an ideal to a real system, an apparatus as small as possible while maintaining scale-up capabilities is therefore desirable.

Pöschmann et al.[2] developed a measuring cell to fulfill these requirements and provided the proof of concept through experiments with the recommended test system cyclohexane – n-heptane (C6-C7) by comparing the results of the separation performance as well as the dry and wet pressure drop to columns of diameters ranging from DN 50 up to DN 250.

It was found, that on the one hand the measuring cell, with a channel of unperforated packing sheets, showed good agreement with separation performance as it would be expected for a perforated Mellapakâ„¢ 500.Y. While on the other hand, the DN 250 column showed a poorer performance with an unperforated packing compared to the perforated one. The hypothesis was, due to the relatively short packing bed and the missing perforation of the packing used within their experiments, the phase maldistribution caused the deterioration in separation performance for the bigger columns. To verify this hypothesis, it is necessary to eliminate one of the described effects. Therefore, further experiments within the DN 250 column were conducted with the same unperforated packing, but with a packing bed height of 2.5 m. To eliminate any effects of the used test system, the new experiments were conducted with the recommended test system usually used by Sulzer, Chlorobenzene - Ethylbenzene (CB-EB).

As the measuring cell showed consistent results for the system C6-C7 and good transferability to the expected performance of the perforated Mellapak 500.Y, experiments with CB-EB were also conducted in the measuring cell to compare these to the data of the DN 250 column.

The measuring cell shows good agreement with the expected performance of the Mellapak 500.Y (perforated) but the DN 250 column with an unperforated packing still underperforms for the new setup with CB-EB. Thus, the perforation of the packing can be identified as an important parameter for reducing the phase maldistribution. Missing perforation negatively effects the mass transfer efficiency. Furthermore, the results suggest that the measuring cell represents an ideally wetted channel of perforated packing sheets even though the packing installed within the measuring cell was not perforated.

To review this even further, a rate-based model with the mass transfer model from Olujic[3] was set up to describe the experiments. The model by Olujic was developed for the prediction of mass transfer in perforated packings. The modeling results show good agreement to data the measuring cell (unperforated), whereas the data from the DN 250 column (unperforated) are not matched. To examine the possible limits of the measuring cell, an unperforated Mellapak 250.Y packing was installed and tested with C6-C7 and CB-EB, where the measuring cell could also show consistent results as well as good alignment with the expected performance of a perforated Mellapak 250.Y.

All the findings described support the hypothesis, that the perforation of the packing is important for the liquid distribution and therefore the mass transfer efficiency. Furthermore, the findings indicate that the measuring cell represents an ideally wetted channel of perforated packing sheets and therefore has great potential as a tool in the design process of columns with structured packings for the separation of non-ideal mixtures.

The authors gratefully acknowledge financial support from the German Federal Ministry of Education and Research (BMBF) and project supervision by the DLR Project Management Agency for Environment and Sustainability for the project "ReProvAP" (Reduzierung der klimarelevanten Prozessemissionen durch die verbesserte Auslegung von strukturierten Packungskolonnen) with the funding codes 01LJ2002F.

[1] Onken, U., Arlt, W., 1990. Recommended Test Mixtures for Distillation Columns. The Institution of Chemical Engineers ISBN 0852952481

[2] Pöschmann et al., 2023. Will laboratory and pilot plant columns soon become superfluous? – A concept for the determination of structured packing characteristics in a measuring cell under distillation conditions. Sep.Purif. Technol. 325, 124617, DOI: 10.1016/j.seppur.2023.124617

[3] Olujic et al., 2004. Predicting the Efficiency of Corrugated Sheet Structured Packings with Large Specific Surface Area.; Chem. Biochem. Eng. Q. 18 (2) 89–96.