(11c) Bifurcation Analysis of Gas Phase Methane Oxidative Coupling

We present a detailed bifurcation analysis of methane oxidative coupling in the gas phase (without catalyst) using a global kinetic model with various oxidation, reforming and dehydrogenation reactions. The main goal is to determine the methane conversion (X) and C₂+ products yield (Y) in different types of large scale reactors under various feed and operating conditions. The global kinetic model used satisfies the thermodynamic constraints and is fitted to literature data as well as new data obtained under nearly isothermal conditions.

Hysteresis (ignition and extinction) behavior of the system is investigated as a function of the CH₄/O₂ ratio as well as feed temperature and space time. Hysteresis may exist at ambient feed temperature (300K), for low CH₄/O₂ ratios (3 or lower) with space times ~ 1s. When hysteresis exists, the best yield of ethylene can be obtained close to the extinction point on the ignited branches. Typically, reactions forming C₂+ products are dominating in the reactor when the temperature on the ignited branch is above 1200K and the space time is very short. Acetylene becomes the main C₂+ product, together with much excess of hydrogen and carbon monoxide than water and carbon dioxide, as reactor temperature or space time are increased beyond ignition.

The impact of axial backmixing of species and energy is also studied using axial dispersion model and single phase 2-mode model. It is found that when the thermal backmixing is highest and species backmixing is lowest, the extinction point can be pushed to lower values of feed temperatures and/or to lower space times with the yield of intermediate product increased.