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.蜂窝催化剂（Honeycomb catalysts）的优势之一在于其压降低于催化活性颗粒填充床的压降，尤其当反应物流速使得反应处于传质控制状态时。 传质控制反应是指反应速率由流经催化剂本体的反应物流速，以及反应物在催化剂本体内的流动类型（即湍流程度）所控制的反应。 此类反应的催化速率本质上等于反应物向催化剂表面的传质速率。 因此，反应速率主要由反应物与催化表面的接触程度所控制或限制。 反应速率通常以单位时间内生成的产物浓度，或单位时间内消耗的反应物浓度来表示。 或者，也可通过单位时间内消耗的反应物总量或生成的产物总量来定义反应速率。 提高反应物流速时，湍流流体与蜂窝结构壁面上的催化活性金属组分的接触时长会缩短，但传质速率会提升，因为气相反应物扩散至催化剂表面的距离变短了。 因此，反应的转化率并不会随催化接触时间的缩短而成比例降低。 反之，若反应物气体流经催化剂流通通道时的流动为层流，则无法通过提高流速来抵消接触时间缩短带来的影响，因为反应物穿过气相反应物流并扩散至催化活性表面所需的距离基本保持不变。 实际应用中，当采用层流流速时，流入气体在流通通道初始段的流动模式会类似湍流，直至流动模式趋于稳定。 反应物流速介于层流与湍流之间的区间（如过渡流）同样可被采用。 但由于过渡流通常具有不可预测性，且流速小幅变化即可导致流动模式发生显著改变，因此在传质控制反应中，该流速区间难以像层流或湍流流速那样便于实现稳定的转化率。 进入该层的燃料分子无需迁移至催化剂表面即可自发燃烧。 随着燃烧进行，该层厚度被认为会逐渐增加。 最终，整体气流被加热至足以使整个气流发生热反应的温度，而非仅在催化剂表面附近发生反应。 当催化剂内部达到该阶段后，气流在通过燃烧区时，即便不再与催化剂接触，热反应仍会持续进行。 本文中所用的燃料-空气混合物的“瞬时自燃温度（instantaneous auto-ignition temperature）”一词，定义为：进入催化剂的燃料-空气混合物的点火延迟，相较于该混合物在燃烧区内的停留时间可忽略不计的温度。