A discussion on transport phenomena and three-way kinetics of monolithic converters

Abstract

A one-dimensional monolithic catalyst model is used to develop a global heterogeneous reaction mechanism for three-way applications. Separate conservation equations for the gas and the solid phase are coupled by the introduction of transport coefficients. Due to the relevance of transport phenomena on the overall conversion efficiency, the adequacy of the considered correlations for heat and mass transfer coefficients is elucidated, prior to the investigation of chemical kinetics. Resolved Nusselt ( Nu) and Sherwood ( Sh) number distributions in a monolith channel at steady-state and transient conditions, including catalytic reactions, are compared to local a priori correlations for constant and non-constant boundary conditions at the wall taken from literature. The reaction rate formulations are modified to achieve acceptable agreement with experimental data of palladium–rhodium catalysts for typical operating conditions of automobile applications. The conversion behavior of the lumped parameter model is validated against a wide range of air/fuel ratios and temperatures. Varying feed gas concentrations ( λ -sweep) and light-off experiments for stoichiometric and lean inlet conditions as well as a typical operating condition for secondary air injection are considered. The reaction mechanism is then used to predict the conversion performance of an aged catalyst. Instead of calibrating the complete kinetic parameter set, the adaption to the aged system is achieved by a physically meaningful reduction of the available reactive surface area only. Finally, the simulation results of an FTP75 drive cycle are compared to experimental data without further tuning.