Cellular Structures as Custom-Made Catalyst Supports
Project Leader: Hannsjörg Freund
Researchers: Marion Börnhorst, Lena Bierhaus, Elodia Morales, Dominik Rudolf
Since heterogeneous catalysts are used in more than 80 percent of industrial chemical processes, research aimed at optimizing catalyst support structures is of particular importance. At the same time, this offers a tremendous opportunity for a wide variety of industrial sectors and application areas to improve process efficiency, safety, and sustainability. Therefore, our goal in this research area is to enable optimal reaction control through an ideal, tailor-made catalyst support structure within the reactor.
Based on a set of requirements regarding the reaction as well as the mass and heat transfer processes within the reactor, an (initially hypothetical) optimal distribution of catalytic activity and mass and heat transfer characteristics can be determined. A precise analysis can be performed using detailed numerical simulations of flow (Computational Fluid Dynamics, CFD), transport processes, and chemical reactions within the three-dimensional geometry.
To experimentally realize the determined optimal catalyst support structure, we rely on innovative manufacturing processes from the field of materials science. Of particular note here are various methods from the field of additive manufacturing (“3D printing”), which enable even highly complex geometric structures to be fabricated with high precision and reproducibility. The materials used include both polymers (for so-called cold-flow experiments) and metals (for investigations under reaction conditions).
In principle, the catalyst can be introduced into the reactor in various ways and in different forms: as a (randomly arranged) bed of particles, so-called pellets, or as a catalytically active coating on a catalyst support structure through which the flow passes and which consists of a continuous solid matrix. Catalyst pellets are typically on the order of a few millimeters to centimeters, whereas a catalyst support structure can completely fill the reaction chamber, meaning it has dimensions ranging from a few centimeters to meters. The following figure compares four catalyst support concepts in terms of heat and mass transfer, pressure drop, and design flexibility: catalyst bed, open-cell foams, monolithic honeycomb structures, and periodically open cellular structures (POCS). “periodic open cellular structures,” POCS).
More than ten years ago, Prof. Freund’s research group, in collaboration with Prof. Schwieger’s research group at FAU Erlangen-Nuremberg, pioneered the development of the POCS concept as a novel catalyst support and has since been working on its further development, characterization, and optimization of POCS as tailor-made catalyst support systems for both single-phase and multiphase reaction systems. POCS are support structures consisting of a unit cell that repeats periodically in all spatial directions. This makes POCS particularly well-suited for systematic investigations. Depending on the application, different types of unit cells may be advantageous: cubic cells, Kelvin cells, diamond cells, combinations of different cell types, etc.
The use of additive manufacturing (for metallic structures, e.g., via Selective Laser Melting (SLM) or Selective Electron Beam Melting (SEBM)) provides a very high degree of design freedom. This allows POCS to be manufactured with cell geometries of any complexity and optimized for the performance metrics relevant to the specific application.
The geometry of the unit cell influences the flow pattern through the support structure and, consequently, the pressure drop and mass transfer within the structure. It also affects heat transfer. Based on systematic data collection, semi-empirical models are typically developed for pressure loss, mass transport, and heat transfer. On the one hand, limited experimental access imposes constraints on the level of detail in such models; on the other hand, “simple” models are often preferred so they can be used in the context of numerical optimization. Nevertheless, a detailed understanding of the local transport processes occurring in POCS is essential for subsequent optimization, and research in this area is therefore a key component of current work.
A detailed investigation of local processes in complex catalyst support structures is experimentally very complex and feasible only to a limited extent; for this reason, numerical simulations are primarily used for this purpose. CFD simulations allow for a spatially resolved investigation of flow and transport processes within the complex support structures for any given unit cell type. This not only allows for an investigation of the influence of the unit cell type itself on flow and heat and mass transport within the POCS through which the fluid flows, but also enables the determination of the influence of geometric parameters such as web diameter and unit cell size. This provides a fundamental understanding of the relationship between the structure’s geometry and its effects on heat and mass transfer properties—and thus also on the reaction process. On the one hand, this allows for targeted influence on the reaction process, enabling the optimization of the structures to meet specific process requirements. On the other hand, it enables the derivation of relationships and correlations, which can then be incorporated into “simpler” semi-empirical models.
The detailed investigation of POCS using CFD simulations is supplemented by targeted experimental studies. In these studies, the additively manufactured structures are examined in laboratory reactors. In order to use the additively manufactured POCS in a reactor, catalytic functionalization is first required. The catalytically active material can be applied using typical, reproducible methods such as dip coating, spray coating, and electrophoretic deposition. Depending on the reaction system, impregnation is also used here, as is the case with conventional catalysts in heterogeneous catalysis.
For example, experimental studies on the partial oxidation of methanol to formaldehyde in a tubular reactor at a laboratory facility demonstrated that POCS exhibits significantly better heat transfer properties compared to a conventional catalyst bed. The formation of a hotspot in the tubular reactor during this exothermic reaction was significantly reduced by using POCS instead of a catalyst bed under otherwise identical conditions. This makes it possible to significantly reduce the selectivity toward the undesirable byproduct carbon monoxide (CO).
