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. The Nusselt and Sherwood numbers are calculated as a function of position along the monolith channel.
For substrates of equal gas/solid contact area and equal thermal mass, the parallel plate monolith reactor lights off faster and achieves higher conversions than the cylindrical passage reactor at low flow rates. As the flow rate increases, however, the cylindrical passage monolith reactor lights off faster. The reaction zone in the parallel plate channels is closer to the reactor outlet than in the cylindrical channels. Our calculations show that the properties which give good light-off behavior also lead to more efficient cooling at the steady state and lead to reaction zone blow-out early in the life of the catalyst. Poorer heat transfer in cylindrical channels delays reaction light-off, but retains heat better. Radiation is not important in the temperature range encountered in automobiles. At high temperatures, however, it has a significant effect on the temperature profile.