In recent years, the use of consolidated structures (e.g. ceramic foams or monoliths) as catalyst support has been investigated as a promising alternative to conventional tubular fixed-bed reactors filled with catalyst particles (unconsolidated structure). It is generally assumed that ceramic foam structures feature enhanced mass and heat transfer characteristics and a lower pressure drop compared to conventional fixed-bed configurations. These features are particularly favorable in situations which involve high flow rates and/or strongly exothermic or endothermic reactions. However, as of today there is no quantitative understanding of the structural influence of the local foam structure on the flow field and the heat and mass transport (and thus on the reactor performance), hence further research in this direction is needed which is the motivation for our work.

In our present contribution, we combine experimental and numerical methods in order to obtain a better insight into the local transport phenomena. This approach allows for a more fundamental understanding of the relationship between the support structure, the local flow field, the local and global transport characteristics, and finally the reactor performance.

Integral quantities such as the global porosity and the integral pressure drop can easily be investigated using conventional experimental methods. For a more detailed analysis of local flow field quantities, we have to apply advanced numerical methods. In this work, the details of the 3-D geometrical structure are either generated numerically or supplied by an X-ray scan of the foam structure. For the latter case this allows us to perform numerical flow simulations and experimental investigations in the identical geometry. For the 3-D flow simulations on the pore-scale level we apply a lattice Boltzmann flow solver [1-2].

As an unexpected result of the investigations, we found that the pressure drop of the studied foams can feature values up to four times higher than that of a randomly packed fixed-bed – despite the significantly higher global porosity of the foam structure (0.88 vs. 0.38). Thus, global pressure drop correlations fail and give an extremely falsified prediction of the global flow characteristics of the foam. This clearly illustrates the need for a local investigation of the flow field. The results of the numerical flow simulation show that the high pressure drop can be attributed to the local cellular foam structure. Because the open windows between the cells are just a few µm in size, a very inhomogeneous flow distribution with more than 20 times the superficial velocity can be observed at these positions. These first very promising results are currently complemented by a more detailed comparative study of different geometrical structures.

[1] H. Freund, T. Zeiser, F. Huber, E. Klemm, G. Brenner, F. Durst, G. Emig, Chem. Eng. Sci., 58(3-6), 903-910 (2003).

[2] H. Freund, J. Bauer, T. Zeiser, G. Emig, Ind. Eng. Chem. Res., 44(16), 6423-6434 (2005).