(467d) Ignition-Extinction Analysis of Methane Oxidative Coupling in Packed Bed Reactors

In laboratory experiments for catalytic oxidative coupling of methane (OCM), a small amount of catalyst powder is placed in a quartz tube that is placed inside a furnace, whose temperature is varied. For typical values of space times used (1 millisecond to 1 second based on flow rate at STP), most OCM catalysts are active only at temperatures above 750°C. Further, due to the highly exothermic nature of the coupling and deep oxidation reactions, the catalyst temperature can be much higher (and can exceed 1000°C in many cases). In such cases, it is well known that homogeneous gas phase reactions that occur in the interstitial space of the packing (and also in the inert regions before and after the catalyst packing) can be important in determining the selectivity to various products. In this work, we develop and investigate a coupled homogeneous-heterogeneous OCM system that describes the behavior of laboratory scale packed-bed reactors. The main focus is on the contribution of gas phase reactions on the observed ignition-extinction behavior of the system, i.e. the impact of gas phase reactions on the relative position of homogeneous and catalytic ignition and extinction points, the temperature and various species concentration profiles on the ignited branch and the product distribution through different reaction pathways. We also study the impact of heat loss (exchange) on the location of the ignition and extinction points in laboratory scale reactors. Further, as the operation of large scale catalytic reactors is closer to adiabatic, we determine the region of autothermal operation for adiabatic reactors in the feed temperature versus space time plane. The impact of gas phase reactions, inter and intra-particle temperature and concentration and temperature gradients on the region of autothermal operation is also studied and summarized.