Fixed bed catalytic reactors are widely used in the chemical industry. They are used for a variety of applications, such as steam reforming (SRM), dry reforming (DRM) and catalytic partial oxidation of methane (CPOX). The aforementioned processes play an important role in the production of hydrogen and synthesis gas from methane, the latter of which can also be provided from renewable sources and the processes can thus make an important contribution to climate protection.
Heterogeneous catalytic reactions are characterized by a high reaction enthalpy. For the safe and efficient operation of the reactors, it is therefore essential to efficiently remove or introduce heat into the system. Therefore, reactors with small tube diameters are chosen, which are interconnected to form so-called tube bundle reactors. On the other hand, the pressure loss should be kept low, which is why relatively large catalyst particles are used. This leads to reactors with a small tube-to-particle diameter ratio.
The assumption of a homogeneously distributed bed voidage and a fluid dynamic plug-flow profile is no longer justified in this type of reactor, since local wall effects dominate. This significantly effects the energy and mass transfer and thus also the reaction kinetics locally. The design of these reactors with simplified model approaches is problematic, since on the one hand they cannot represent significant effects (e.g. local hot spots) and on the other hand they require knowledge of effective transport parameters (e.g. dispersion coefficient, effective thermal conductivity, effective viscosity). Correlations for the calculation of these parameters are often of limited use and show considerable variations.
Computational Fluid Dynamics (CFD) provides us with a tool to calculate the fluid dynamics as well as the superimposed heat and mass transport in a spatially resolved manner. For this purpose, the complex flow space between the particles is completely resolved and the transport processes are described by numerically solving the Navier-Stokes equations. A coupled description of the heat transport within the particles is also possible. We thus have a tool to perform detailed investigations that would not be feasible experimentally in this flexibility and quality.
A DEM-CFD coupled workflow is used for the numerical simulation. First, a representative randomly packed bed is generated using the Discrete Element Method (DEM). Subsequently, the position and orientation of each particle is extracted and based on these data a CAD description of the bed morphology is generated. For the generation of the computational grid, a special meshing strategy is used, where, depending on the problem, either only the flow space or in addition to it also the particles are meshed.
In addition to the successive extension and experimental validation of the developed method, the simulation results are used to improve the phenomenological understanding of fluid dynamics and transport processes occurring in fixed-bed reactors. With respect to process intensification, new reactor concepts and particle shapes are numerically tested and evaluated with respect to reactor performance. The determination of effective transport parameters, which are necessary for simplified models, is also the focus of my work. The aim is to obtain more reliable results also with simplified models.