Transient operation of monolith catalytic converters: a two-dimensional reactor model and the effects of radially nonuniform flow distributions

Abstract

The transient operation of monolithic catalytic converters was stimulated using a two-dimensional equivalent continuum model that allowed us to systematically evaluate the effects of nonuniform flow disributions, of ambient heat losses, and of radially varying catalytic activity profiles on reactor performance. Non-uniform flow distributions characterized by high fluid velocities in an inner core of the reactor may substantially degrade its light-off performance, but such adverse effects can be alleviated by using “flow-tailoring” devices. The simulation results show that a step decrease in the feedstream temperature can lead to “wrong-way” behavior that here takes the form of large and highly localized overtemperature excursions. Ambient heat losses may substantially decrease the steady-state conversions obtained in a converter, although they only slightly retard the light-off process. Finally, the effects of radially nonuniform catalytic activity profiles are investigated. The presented results demonstrate that flow nonuniformities must be considered in comprehensive converter models together with other important design parameters that include the thermal mass of the reactor, exhaust gas temperature and stoichiometry, catalyst loading and gas—solid contact area.