(521bq) Process-Level Objectives for Reactor Design and Their Application to the Optimization of Adiabatic Multi-Zone Reactors for Oxidative Coupling of Methane

Oxidative coupling of methane (OCM) is a single direct reaction that converts methane into ethylene with the help of a catalyst using some amount of oxygen. The OCM takes place at high temperatures, usually between 700 to 900 °C, and generates a significant amount of heat; the control of which is a major challenge. The reaction is governed by a selectivity-conversion compromise that strongly depends on the used catalyst and operating conditions. This work explores the use of economic objective functions in reactor design to address the disconnect between design objectives at the reactor scale and those at the process level. The reactor design optimization is performed for a multi-zone reactor considering design decisions on the distribution of methane, oxygen, and inert feeds along the reactor, feed ratios of each zone inlets, cold shot temperatures, and zone sizes. The adiabatic, fixed-bed OCM reaction zones are modeled as one-dimensional (1-D) and pseudo-homogeneous. The reactor network simulation model comprises the zone models together with mass and energy balances of mixing nodes at each reaction zone inlet. The optimal settings for design decisions are identified with the use of Simulated Annealing (SA). The development of the simulator and the implementation of the SA search scheme will be presented in detail. OCM reactor design optimization is first performed to maximize C₂ yield. Then, the optimal relationship between methane conversion and C₂ selectivity is explored as another example of how reactor optimization is approached. Next, alternative objective functions are formulated incorporating process performance aspects such as product and raw material costs, energy costs, and power and energy requirements for separation. These objective functions are formulated such that only information that can be readily available or estimated during reactor design optimization is considered. Differences in observed OCM reactor designs across objectives will be presented and discussed.