(452g) Coupled Numerical and Experimental Analysis of Transport Properties in 3D-Printed Periodic Cellular Media to Enable Catalytic Reactors Design

Process Intensification is a key topic in academic and industrial research aiming at the development of more compact and energy-efficient processes [1]. In this context, structured catalysts have been considered as a promising solution to overcome the limitations of conventional catalytic supports. Currently, cellular materials (foams, Periodic Open Cellular Structures – POCS) are envisioned as potential candidates for enhanced catalyst substrates. Thanks to the possibility of advanced manufacturing and design offered by 3D printing, POCS allow for extreme flexibility in the geometrical optimization (cell shape, strut size, solid fraction) to tailor the design to the process needs [2]. In this view, the fundamental understanding of the coupling between the geometrical properties and the transport phenomena is essential for the optimization of the supports and their industrial application.

In this work, we propose a combined experimental and numerical investigation of the transport properties of POCS aimed at the derivation of engineering correlations to enable the development and optimization of enhanced catalytic reactors. Computational Fluid Dynamics (CFD) allows for the analysis of the transport properties as a function of the geometrical parameters, enabling the understanding of the effect of single morphological parameters (e.g. cell shape, porosity, cell diameter) on the overall performances and providing deep insight in the transport mechanisms.

The significant computational burden of CFD simulations hinders their direct application to the industrial reactor scale. Hence, a hierarchical approach following the procedure proposed by Rebughini et al. [3,4] is employed to investigate representative structure volumes by means of CFD.

While numerical simulations allow to evaluate the transport properties of structures regardless of the limits imposed by their manufacturability, they are unable to provide a broad range of data capable of completely define the performances of a specific structure. Contextually, experimental analysis performed on 3D-printed replicas, while requiring specialized manufacturing infrastructures, can provide enough data for the complete characterization of the most promising geometries and cross-validation with CFD simulations.

In this view, high precision 3D printing technology enables the fabrication of highly accurate structures. The precision and accuracy of currently available SLA 3D printers, combined with the wide range of available materials, allows for their effective deployment in the experimental analysis of fluid-solid interactions. The possibility to create faithful physical replicas of digital structures allows for the direct comparison between simulations and experiments hence providing a reliable tool for the cross-validation of experimental and computational results.

As a result, the numerical and experimental analyses can be combined and exploited to formulate engineering correlations able to predict the structures behaviour as a function of the geometrical and physical parameters [5].

As an example of this approach, an analysis of pressure drops and mass transfer over TKKD cellular structures is presented. Experimental investigation is performed both in cold flow (pressure drops) and reactive conditions (mass transfer) over 3D printed resin samples with different geometrical characteristics. Pressure drops are evaluated by flowing air and measuring the differential pressure across 3D printed resin samples. Transport properties are assessed by reactive runs performed on samples realized with a High Temperature 3D printed resin (HDT_@0.45MPa = 289°C). The oxidation of H₂ over Pd/CeO₂ in rich conditions is used as the test reaction to allow for the onset of external mass transfer limited regime at temperatures compatible with the resin. The conversion of O₂ (the limiting reactant) in external mass transfer limited regime is employed to evaluate the mass transfer coefficients due to the similarity between its diffusivity and CO while the large excess of H₂ prevents back-diffusion problems.

In parallel, CFD simulations are performed on the same geometry. The working fluid is CO diluted in air at 298K, 1 atm, the test reaction is the total oxidation of CO. Hence, the mass transfer properties are assessed in steady state conditions by evaluating CO conversion (limiting reactant). Pressure drops and conversion are together estimated for a representative elementary volume (REV, [6]) made of a set of unitary cells.

Figure 1 (left) shows the result of the numerical analysis (lines) along with experimental results (dots) for the TKKD geometry, showing a very good agreement between CFD and experiments. The analysis shows the effects of the different geometrical parameters (cell size, porosity) on the pressure drops. In this view, the envisioned approach offers a reliable tool for assessing the pressure drop functional dependency on the structures geometrical properties. Figure 1 (right) shows the outcome of the analysis of the gas-to-solid transport properties of the same geometry. Experimental and numerical data (dots) are exploited to develop a gas/solid mass transfer correlation for the TKKD unit cell able to predict its behaviour as a function of the geometrical and the physical parameters. The proposed dimensionless correlation (line) shows a good agreement with respect to the available data.

In conclusion, we propose a methodology for the fundamental investigation of transport properties in periodic cellular substrates based on coupled numerical and experimental analysis. The combined use of CFD and experiments allows for the fast screening of a wide variety of different structures while enabling the in-depth characterization of the most promising geometries without the limitations of either numerical or empirical independent approaches The illustrated case study highlights the potential of such approach in the development of engineering correlations able to describe substrates transport properties regardless of the specific geometries. Thanks to the gathered knowledge and to commercial 3D printing solutions, it is then possible the manufacture catalytic substrates with tailored properties for each specific applications with materials such as metals (Steel FeCrAlloy, Copper, Titanium, and others) or ceramics, with details as small as 200μm. In this view, the proposed methodology allows for the proficient exploitation of the extreme design flexibility of advanced 3d printing, thus enabling effective process intensification.

References

[1] A.I. Stankiewicz at al. Chem. Eng. Prog. 96 (2000) 22–33.

[2] C. Busse at al. Chem. Eng. Process. Process Intensif. 124 (2018) 199–214.

[3] S. Rebughini, et al., React. Chem. Eng. 3 (2018) 25–33.

[4] S. Rebughini,et al., Chem. Eng. J. 289 (2016) 471–478.

[5] M. Bracconi, et al., Chem. Eng. J. 377 (2019.)

[6] M. Bracconi et al., Chem. Eng. J. 352, pp. 558-571 (2018).

Acknowledgements

This project has received funding from the European Research Council under Grant Agreement no. 694910 (INTENT).