Development of a Liquid Catalyst for Diesel Soot Oxidation - From powder to prototype

Applied Science

Tomography based simulation of reactive flow at the micro-scale: Particulate filters with wall integrated catalyst

Three-way catalyst material was deposited inside the pores of a ceramic particulate filter and the pore geometry as well as the distribution of the catalyst in the pores was determined by X-ray microtomography. On the resulting 3D geometry, the flow field through the pores was computed and the convection-diffusion-reaction equation in the open pores and the catalyst particles was solved assuming a first order model reaction taking place in the catalyst. The conversion in the filter wall was compared to a homogeneous model with the same dimensions and catalyst content and it was found that the conversion in the pore network is lower than predicted by the homogeneous model, indicating the presence of some kind of in-pore micro-scale transport limitation. Analysis of the flow field showed channeling of the flow through a few large pores and blocking of many other pores by the catalyst that leads to a broadening of the residence time distribution. While the broadened residence time distribution has some negative effect on the conversion, diffusion limitation in the catalyst particles was identified as the main reason for the reduced conversion compared to the homogeneous model. When diffusion in the washcoat was described by a standard effectiveness factor based on a generalized Thiele modulus with an effective diffusion length fitted to the results of the full pore-scale simulation, a very good agreement between the pore-scale simulation and the homogeneous model was found. The effective diffusion length obtained by this fit is surprisingly large, compared to the apparent size of the washcoat particles. This can be explained by the limited accessibility of the washcoat due to the confinement in the pore structure. Finally, the 3D micro-scale model was coupled to a channel-scale model representing one pair of inlet and outlet channels. It is shown that micro-scale diffusion limitations have a higher impact on the overall reactor performance than the channel-scale mass transport. This suggests that for filter design and for reactor modeling more emphasis should be placed on micro-scale transport effects