Catalytic combustion of methane in a monolith washcoat: Effect of water inhibition on the effectiveness factor

Abstract

This paper reports the results of a numerical investigation of the diffusion and reaction of methane in the washcoat of a catalytic monolith reactor. The kinetic rate expression used is an empirical equation determined experimentally for a palladium oxide catalyst. The effect of water inhibition on the reaction rate is included in the model. A multi-species diffusion and reaction model is used to simulate the process. The model is solved in a 2-D space using a finite element method. It is observed that the inhibiting effect of water tends to lower the observed reaction rate and that a higher surface water concentration results in an increase in the observed effectiveness factor. The effectiveness factor depends on three dimensionless parameters. Strong diffusion limitation can lead to high water concentrations at the interior of the catalytic washcoat.On presente dans cet article les rksultats d'une etude numerique de la diffusion et de la reaction de methane dans la enduit d'un reacteur monolithique catalytique. L'expression de vitesse cinetique utilisee est une equation empirique determinee experimentalement pour un catalyseur d'oxyde de palladium. L'effet de l'inhibition de l'eau sur la vitesse de reaction est i n c h dans le modele. Pour simuler le procede, on utilise un modele de diffusion et de reaction a especes multiples. Ce modele est resolu dans un espace bidimensionnel a I'aide d'une methode d'elements finis. On observe que I'effet inhibiteur de I'eau tend a abaisser le vitesse de reaction observee et qu'une plus forte concentration d'eau de surface entraine une augmentation du facteur d'efficacite observe. Le facteur d'efficacite depend de trois parametres adimensionnels. Une forte limitation de diffusion peut conduire a des concentrations d'eau elevees a l'interieur de la couche de lavage catalytique.

One + One‐Dimensional Modeling of Monolithic Catalytic Converters

“…As discussed in a later section, the monolith energy equation assumes uniformity in the y-direction (substrate and washcoat are the same temperature because of their small relative thickness); hence, there is not any need to model the gaseous energy equation through the washcoat as T s = T m and it is isothermal in nature. This is confirmed by other researchers who state that because of the low reactant concentration and the thickness of the washcoat, it is valid to assume the washcoat is isothermal [36,37]. As a result, the diffusion velocity through a washcoat becomes:…”

“…The figure demonstrates a significant difference between the three types of models with the Liu et al simplification for parallel pore diffusion demonstrating similar results as the more in-depth calculation. If a choice had to be made between diffusion models, the parallel pore model is widely utilized and has been proven to provide good accuracy in comparison to experiments [23,30,34,36,37,54,61].…”

Effectiveness in catalytic washcoats with multi-step mechanisms for catalytic combustion of hydrogen

“…Thus, for the = 0.05 cases, the behavior of vs. T ext for the different wc in Fig. 5 follows trends much more similar to those derived from simple reaction rate expressions (Hayes et al, 1999).…”

“…In the previous numerical models using single-step reactions, fell off sharply from 1.0 even for washcoat thicknesses wc as small as 10 m in some cases (Hayes et al, 2001). The predicted drop in with increasing temperature for a given oxidation reactor largely depends on the modeled diffusivity of reactants in the porous washcoat, and results show that inaccurate kinetic rate expressions can be derived in these cases if intraphase diffusion is neglected (Groppi et al, 2001, Hayes et al, 1999.…”

One‐Dimensional Pseudo‐Homogeneous Packed‐Bed Reactor Modeling: I. Chemical Species Equation and Effective Diffusivity

“…since the parallel pore model has been widely utilized and proven to provide good accuracy in comparison to experiments [70,[76][77][78][79][80][81]. In this expression, the variables k s and k represent the respective thermal conductivities of the gas and packed-bed particles.…”