Flowrate and heat transfer considerations for oxidation catalysts

Abstract

This paper presents the results of a study into the effects of flowrate and heat transfer in oxidation catalysts. The analysis was performed using a validated two-dimensional catalyst model, which has been designed to predict the behaviour of an oxidation catalyst during transient warm-up and light-off conditions. It is only during the simulation of transient conditions that the true effects of changes to the flowrate and heat loss to the substrate and surrounding materials can be observed. In addition, the two-dimensional design of the model used allows the reaction intensities to be viewed throughout the catalyst and gives the opportunity to examine these effects in both the radial and axial directions. The study has shown that the heat loss from the upstream section of exhaust pipe not only reduces the rate of increase of the feed gas temperature but also removes heat from the outer edges of the catalyst and delays the onset of reactions in these areas. Consequently, the importance of insulating the upstream exhaust pipe and the catalyst housing is demonstrated. It is also shown that increasing the thermal conductivity of the substrate reduces the light-off temperature by transporting heat to the cooler regions more rapidly. Increasing the void fraction of the substrate also has a signiflcant effect on the light-off temperature as less heat is absorbed from the feed gas. Finally, it is shown that increasing the flowrate, or space velocity, of the gas increases the resulting light-off temperature. For the conditions simulated, it was seen that increasing the flowrate by one order of magnitude increased the light-off temperature by only 13 °C. However, emissions breakthrough was seen to occur at a space velocity around 164 000 h 1.