Experimental and theoretical investigation of fluid dynamics and mass transfer in sandwich packings
Motivation:
Sandwich packings are a new kind of internals in separation columns. Similar to random or conventional structured packings, they are used to provide a large contact area between the gaseous and the liquid phase. Sandwich packings consist of two layers of corrugated sheet structured packings with different specific surface area which are alternatingly arranged in the column. As a result of this combination, bubbly flow can evolve in the hold-up layer. Compared to conventional packed columns, the interactions between the phases are much higher, especially in the bubbly flow area and the froth regime evolving above. This leads to a higher separation efficiency of the column and therefore to the saving of energy.
![Schematic of a column and the flow regimes which occur during the operation with sandwich packings](https://tu-dresden.de/ing/maschinenwesen/iet/pbm/ressourcen/bilder/bilder-projekte/p32.png/@@images/84d11bac-ca6d-4918-9877-b637a5bf88bd.png)
Schematic of a column and the flow regimes which occur during the operation with sandwich packings
Objectives:
In the first funding period, a rate-based modeling approach was developed based on the experimental characterization of segment-specific fluid dynamics using fast X-ray tomography and was validated with CO2 absorption measurements.
In the second funding period, the project aim is to further improve the model for application to rectifications and thus finalize the development of the overall model for sandwich packed columns. In this phase of research, the regime-specific dispersion coefficients, liquid phase residence times and liquid-side mass transfer coefficients, the phenomenological description of packing flooding for the correct determination of loading and flooding points and the extension of fluid dynamic correlations to account for changes in material properties will be considered. For the overall evaluation of sandwich packed columns in comparison to conventional packing internals, the finalized model will be used to theoretically investigate both chemical absorption processes and rectification processes.
Methods and results:
The investigations on fluid dynamics planned at TU Dresden will be carried out in a column (Ø100 mm) with the packing combination B1-750/B1-250, which will initially be operated in countercurrent for model substance systems.
For the comprehensive characterization of the flow processes with changes in the material properties (viscosity and surface tension), two independent measurement techniques are essentially used: Ultrafast X-ray tomography as a non-invasive measurement method, offers the possibility of mapping phase fractions and phase interfaces with high temporal and spatial resolution. In addition, high-precision pressure sensors along the column are used to record axial pressure profiles, which can be used to draw conclusions about the transition regions of the evolving flow configurations. From these measurements, the hydrodynamic parameters liquid holdup, gas-liquid phase interface, pressure drop and froth height are determined.
Fast X-ray tomography and a macroscopic model based on a stability analysis of the volume-averaged Navier-Stockes equations are used for the phenomenological description of loading and flooding points of sandwich packings. The topographically determined loading points are used to validate the fit of the model.
To develop correlations for the sandwich packing, the residence times and dispersion coefficients of the individual packing regions are first determined taking into account the backmixing effects. This is determined by the axial preparation of a tracer between two measuring points (distance L) for individual areas.