(575af) Effect of Heterogeneities In Activity on Pattern Formation In Catalytic Reactors

Catalytic reactors are widely used to carry out many chemical reactions. It is well known that concentration and temperature patterns can develop in these reactors due to interaction of heat and mass transport processes and nonlinear chemical reactions. At present, there is sufficient experimental evidence indicating the formation of stable temperature patterns in packed-bed catalytic reactors. However, most existing models are unable to predict such stable temperature patterns unless unrealistic values of parameters (such as the effective solid phase mass diffusivity being higher than the heat diffusivity) are assumed. With uniform catalyst activity, we have found that only moving temperature fronts can exist for Le >= 1, where Le is the fluid phase Lewis number (ratio of heat to mass diffusivities in the gas phase). This disagreement between model predictions and experiments may be due to the inherent heterogeneity in catalyst activity that is present in most laboratory and industrial packed-bed catalytic reactors.

In this work, we investigate the effect of heterogeneity in catalyst activity on pattern formation in catalytic reactors. Since the reaction rate is high in the region of high catalytic activity, the local heat release leads to a positive feedback and enhances the formation of localized hot spots. Conduction within the solid phase and convection in the fluid phase inhibit this effect. The competition between these processes may lead to stable localized hot spots which may not exist for the case of uniform activity. Our preliminary calculations with non-uniformly active catalysts showed that stable stationary temperature patterns can exist for Le>=1. We analyze the effect of the magnitude and length scale of heterogeneity on the pattern formation in catalytic reactors.