Modeling species transport inside mesopores of catalytic washcoated monoliths for emission control

The industrial reduction of pollutants and greenhouse gases is mainly carried out in catalytic washcoated monolith reactors. The reaction at the catalytic active site is strongly influenced by heat and species transport limitations. Therefore, optimization of the monolith geometry (e.g., channel dimension and shape) and the washcoat morphology (e.g., porosity profile, pore diameter, location of catalytic active sites) has great potential to improve the performance of emission control reactors. Spatially resolved modeling of the system can significantly contribute to a better process understanding and is a necessary step towards computer-aided reactor and catalyst design. However, it is challenging due to the multiscale nature of such reactors, ranging from macroscopic flow in the monolith channel (mm) to pore diffusion coupled with reaction in the mesoporous regions (nm) within the washcoat.
Pore-resolved simulations require pore geometries of real catalyst systems. Recent developments in high resolution X-ray and electron tomography allow to obtain pore geometries not only on the macropore scale but also on the mesopore scale. In addition to the pore structure, such tomograms also contain information about the location of catalytic active regions. ... mehrFor the description of transport phenomena in the monolith and mesopore scale, the Navier-Stokes equation can be solved with well-established computational fluid dynamics (CFD) methods, since the continuum assumption is valid for a Knudsen number Kn smaller than 0.01. In contrast, at the mesopore scale, the continuum assumption breaks (Kn < 0.01) and the Navier-Stokes equation is no longer valid. Since molecular dynamic simulations are too computationally expensive for such large and complex geometries, a coarse-grained molecular dynamic method is essential to model the transport coupled with the catalytic reaction within the mesoporous regions of the washcoat.
In this contribution a novel approach to model multicomponent species transport in mesoporous systems for arbitrary Knudsen numbers is presented. The Boltzmann equation is solved to describe species fluxes using the quadrature-based moment method (QBMM). The molecules of the multicomponent gas mixtures are treated as hard spheres with different diameters. Molecule-molecule interactions are described as inelastic binary hard sphere collisions using the generalized Boltzmann-Enskog collision operator. Spatial species fluxes and concentrations are reconstructed from the velocity distribution function used in the Boltzmann equation.
The presented model is the basis for pore-resolved simulations of mesoporous catalysts. For the first time, the QBMM method is used to describe species transport within the mesopores of a catalytic system. In future work, we will implement chemical reactions as boundary conditions at the pore wall. These simulations are an essential part of the hierarchical multiscale modeling framework of catalytic washcoated reactors. The different scales are coupled by using effective transport parameters derived from spatially resolved simulations of representative regions of smaller scales.