Instantaneous auto-ignition temperatures for fuel-air admixtures on catalytically-active surfaces

Honeycomb catalysts offer, as an advantage, a lower pressure drop than that observed with a bed of catalytically-active particles, particularly when employing velocities of the reactants which would enable mass transfer control of the reaction. Mass transfer-controlled reactions are those reactions which are controlled by the velocity of the reactants passing through the catalyst body and by the type of flow of the reactants, namely degree of turbulence, through the catalyst body. These reactions take place essentially at a catalytic rate equal to the rate of mass transfer of reactants to the catalyst surface. Thus, the reaction rate is controlled or limited primarily by the extent of reactant-catalytic surface contact. The reaction rate is often expressed in terms of either the concentration of a product that is formed in a unit of time or the concentration of a reactant that is consumed in a unit of time. Alternatively, it may be defined in terms of the amounts of the reactants consumed or products formed in a unit of time. By increasing the velocity of the reactants, the period of contact of a turbulent flow of fluid with the catalytically-active metal component on the walls of the honeycomb structure is reduced, but the rate of mass transfer is increased since the distance for diffusion of the reactants in the gas phase to contact the catalyst is less. Hence the degree of completion of the reaction is not lessened in proportion to the reduction in catalytic time. On the other hand, if the velocity of the reactant gases through the flowthrough paths of the catalyst is such that the flow is laminar in nature, there is no offsetting compensation for the reduction in contact time due to an increase in velocity since the distance required for the reactants to diffuse through the vapor phase reactant stream and contact the catalytically-active surface is essentially unchanged. In practice, when employing gas velocities which provide laminar flow, the pattern of flow of the entering gases at the initial portion of the flowthrough paths will resemble turbulent flow until the flow pattern becomes settled. Velocities of the reactants which fall between laminar and turbulent flow, for example, transitional flow, may also be employed. However, due to the generally unpredictable nature of transitional flow and the possibility of significant variations in the pattern of flow with small variations in velocity, velocities in this range may not find as convenient an application to provide stable conversion rates in mass transfer-controlled reactions as velocities which provide laminar or turbulent flow. The fuel molecules entering this layer spontaneously burn without transport to the catalyst surface. As combustion progresses, it is believed that the layer becomes deeper. The total gas is ultimately raised to a temperature at which thermal reactions occur in the entire gas stream rather than only near the surface of the catalyst. Once this stage is reached within the catalyst, the thermal reactions continue even without further contact of the gas with the catalyst as the gas passes through the combustion zone. The term "instantaneous auto-ignition temperature" for a fuel-air admixture as used herein is defined to mean that the temperature at which the ignition lag of the fuel-air mixture entering the catalyst is negligible relative to the residence time in the combustion zone of the mixture undergoing combustion.