Wall-Flow Filters with Wall-Integrated Oxidation Catalyst
Martin Votsmeier, Umicore
This work focuses on a comparison of the reactor performance of the wall-flow filter and the conventional flow-through monolith. To this end a two-dimensional numerical model is set up for each of the two reactor configurations.
Concentration profiles in the wall-flow filter systematically change as a function of flow velocity.
At high flow velocities transport from the inlet channel into the porous wall is nearly entirely dominated by convection. This leads to uniform axial concentration profiles in the inlet and outlet channel and a steep gradient in the porous wall.
At low velocities radial transport into the porous wall is dominated by diffusive transport. This leads to a negligible radial concentration gradient between the inlet and the outlet channel.
Under most operating conditions relevant for an automotive exhaust catalyst the flow velocity is in an intermediate range with contributions of diffusive and connective transport.
The transition from entirely convection dominated transport at high space velocities to increasingly diffusion dominated transport at lower flow velocities is similarly found for first order kinetics and a generalized LHHW rate law.
Wall-flow filters show systematic conversion advantages over the conventional monolith for a first order reaction. For a reaction with LHHW-type kinetics this effect is not generally observed. It is one major result of this work that the relative performance of the two reactor configurations depends on the kinetics of the catalyzed reaction.