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The heat and mass transfer characteristics of a monolith reactor influence both its light-off performance and the steady-state temperature and concentration profiles. The transient behavior of an adiabatic monolith reactor is bounded by two limiting channel geometries: (a) cylindrical passages and (b) slits confined between parallel plates. These geometries represent extremes in heat/mass transfer characteristics for monolith channels and, therefore, allow the analysis of light-off behavior for the practical range of geometries and process conditions in automobile exhausts. Also, these two geometries allow an analytical solution for the velocity, concentration and temperature profiles in the gas phase. This study analyzes the light-off performance of monolith reactors with comparable voidage and surface area using automotive exhaust oxidation kinetics. The energy balance for the solid walls includes all three modes of heat transport: convection, conduction and radiation.

Further stringent emission legislation requires advanced technologies, such as sophisticated engine management and advanced catalyst and substrate to achieve high catalytic performance, especially during the light-off phase. This paper presents the results of calculations and measurements of hydrocarbon and carbon monoxide light-off performance for substrates of different wall thickness, cell density and cell shapes. The experimental data from catalyst light-off testing on an engine dynamometer are compared with theoretical results of computer modeling under different temperature ramps and flow rates. The reaction kinetics in the computer modeling are derived from the best fit for the performance of conventional ceramic substrate (6mil/400cpsi), by comparing the theoretical and experimental results on both HC and CO emissions. The calibrated computer model predicts the effects of different wall thickness, cell density and cell shape.