A fully distributed model for the simulation of a catalytic combustor

Abstract

This paper presents a comprehensive two-dimensional model for the simulation of one single channel of a catalytic combustor. The main advantage of the presented model is that the effects occurring in the gas phase, in the washcoat layer and in the substrate are calculated simultaneously in a transient simulation. The non-isothermal reaction/diffusion problem in the washcoat is solved. When a chemical reaction occurs within a porous catalyst, the reaction rate may be affected by the effect of pore diffusion in the washcoat and by mass transfer in the gas phase. In the literature the Thiele approximation is most commonly used to take account of pore diffusion in computer models. The Thiele approximation is based on various assumptions, i.e. simplified geometry and isothermal conditions throughout the washcoat. Nearly all models in literature use predefined Nu- and Sh-number correlations in order to account for the velocity of heat and mass transfer. The model presented in this paper needs neither the Thiele approximation nor predefined Nu- or Sh-number correlations as a priori input. It therefore offers a possibility to review all assumptions concerning these correlations critically by interpreting the results of the fully coupled solution. An effectiveness factor profile for the thin annular shell geometry obtained from the numerical simulations is compared with an effectiveness factor profile for the flat plate geometry calculated using the Thiele approximation. It is found that the use of the Thiele approximation could lead to the reactor performance being overestimated at high temperatures, at low temperatures its use could lead to an underestimation. The shape of the Nu- and Sh-number profiles obtained from the simulation differs clearly from the profiles found in literature. The Nu number profile exhibits a discontinuity that moves through the channel during transient heat up.