20.04.2021

-  Maria Isabelle Maiwald - Institute of Laser and System Technologies -

The project is dealing with the development and optimization of unit cells, which are used as additive manufactured periodic open cell structures (POCS) in reactors e.g. as a catalyst support.  In particular, structures with a high specific surface are developed, characterized and compared on the basis of different parameters. This research is done within the I3-Lab “Smart Reactors” in close cooperation with the Institute of Multiphase Flows (IMS), the Institute of Chemical Reaction Engineering (ICRT), the Institute of Solids Process Engineering and Particle Technology (SPE) and the Institute of Technical Biocatalysis (ITB).

There are three scientific aspects that are examined in detail: 

1) How can unit cells be optimized with regard to surface enlargement?
2) Which manufacturing and design restrictions are relevant for selected materials?
3) How to achieve a further enlargement of the surface area?

The first aspect of the project deals with the development and optimization of unit cells for a catalyst support structure, which is to be produced additively. In addition to already known POCS, such as polyhedra or star tetrahedra, promising other structures with high specific surface area were developed and compared using different parameters. Star tetrahedrons and a modified form of the Schwarz Diamond P show the largest specific surface for strut-based cells. Surface-based cells still need to be optimized in terms of fluid mechanics. The unit cells 'cube on top' have proven to be suitable for uniform bubble formation [1],[2].

For the second aspect of the project, the process parameters for Laser Powder Bed Fusion (LPBF) of stainless steel were investigated and optimized to build thin structures and lattices. These process parameters were applied to different strut structures in order to determine the manufacturing limits. With the optimized parameter set, struts were successfully build in all orientation angles, even parallel to the build plate. So it was possible to print various POCS in different dimensions. The minimum strut diameter that could be printed safely in all orientations is 0.6 mm. In addition, reactors with a diameter from 3.5 mm up to 86 mm were produced. Figure 1 shows a selection of POCS for use in reactors.

Furthermore, the process parameters during the Additive Manufacturing (AM) process were also optimized to achieve surface enlargement due to high roughness. On the right side of Figure 1 are three structures, each of them has been printed with different parameters (standard, without and with contour) resulting in different roughness. The difference in surface quality is shown in Figure 2.

First tests in the reactor at the Institute of Multiphase Flows show large differences between the parameter sets e.g. the structures with the highest roughness shows a much smaller mean Sauter mean diameter than the other structures with lower roughness.

Another approach to increase surface identified in the project is a selective etching process. Here, the surface of a sample made of an aluminium alloy is etched which creates a very porous surface.

[1] Spille, C., Lyberis, A., Maiwald, M.I., Herzog, D., Hoffmann, M., Emmelmann, C., Schlüter, M. (2020), SMART-Reactors: Tailoring Gas Holdup Distribution by Additively Manufactured Lattice Structures, Chemical Engineering & Technology, 43 (10), 2053-2061. doi: 10.1002/ceat.202000211

[2] Büscher, N., Spille, C., Kracht, J., Sayogo, G., Dawood, A., Maiwald, M., Herzog, D., Schlüter, M., Liese, A. (2020), Counter-Currently Operated Reactive Extractor with Additively Manufactured Enzyme Carrier Structure, Org. Process Res. Dev. 24 (9) 1621–1628. doi: 10.1021/acs.oprd.0c00205

• Maria Isabelle Maiwald 
• I3 Lab Smart Reactors