Spatio-Temporal Dynamics of Reaction Zones in Catalytic Monolith Reactors
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Catalytic monolith reactors are widely used in exhaust after-treatment systems. Unlike traditional chemical reactors which are commonly operated around certain steady-states, monolith reactors are almost always in transient states due to the nature of their applications. In this work, we studied the spatio-temporal dynamics of monolith reactors in three important cases, in order to provide guidance for improving the design and control of after-treatment systems. In the first part, a physics-based one-dimensional transient model is proposed to study the spatio-temporal oxygen storage and release as well as the cold-start behavior of a three-way catalytic converter (TWC). As shown in this work, axial gradients in the temperature and stored oxygen profile cannot be neglected during cold-start, fast lean/rich cycling or sudden and significant changes in the inlet conditions to the TWC. By comparing the results of this model with the full washcoat diffusion–reaction model, we have also shown that the internal mass transfer coefficient approximation is valid for all practical purposes. In the second part, we investigated in detail three main reasons for failure of scale-up in monolith reactors: mass dispersion, heat conduction and heat loss. Monolith reactors are often studied in laboratory experiments using samples of the same properties except for a smaller length compared to the full-scale reactor. The common practice of matching the space time in lab- and full-scale systems results in unmatched axial mass and heat Péclet numbers and heat loss coefficient. It is shown that the unmatched dimensionless groups can lead to qualitatively different ignition/extinction behaviors in different scales. In the third part, the upstream creeping reaction zone in monolith reactors is studied. When front-end ignition is technologically difficult or economically impractical, back-end ignition followed by a fast upstream creeping reaction zone provides a feasible way to reduce cold-start emissions. In this work, the influence of various design parameters (e.g., solid thermal conductivity, heat capacity) and operating conditions (e.g., gas velocity, inlet temperature and concentrations) on the creep velocity is determined and summarized. In the pseudo-homogeneous limit, analytical criteria for reaction zones to creep upstream and correlations for the creep velocity are also presented.