Three-dimensional CFD simulation of pattern formation in a shallow packed-bed reactor for oxidative coupling of methane

Abstract

Stationary or spatio-temporal patterns of catalytic reactions can cause operating challenges in shallow packed-bed reactors with large diameter to depth aspect ratios. Partial ignition patterns were observed in a pilot scale oxidative coupling of methane (OCM) reactor operating on the ignited branch near the extinction point. A three-dimensional (3D) computational fluid dynamics (CFD) model was developed for the down-flow shallow packed-bed reactor using porous media approach, with user-defined functions for heterogeneous heat transfer and Langmuir-Hinshelwood form reaction kinetics. The CFD simulation results show that, once the operating condition is nearing or beyond the extinction point (at lower feed temperature or shorter space time), 3D temperature patterns form due to either thermo-flow or thermo-kinetic steady-state multiplicity. The simulations provide insights into the pattern locking mechanism that stems from the non-linear hydrodynamic-kinetic-thermal coupling in the bed. Specific pattern observed in the CFD solution depends on perturbation sources such as unstructured space discretization, local heat loss through refractory walls, variations in catalyst activity or porosity of the packed-bed, as well as non-uniform initial conditions such as preheating, among others. The results indicate that the non-uniformity of bed physical and chemical properties (such as packing porosity, catalyst activity, or thermal properties) can dominate the pattern in practical reactor operation. Based on parametric studies, methods to mitigate pattern formation are recommended, such as more uniform incoming flow, uniform catalyst bed, uniformly preheated bed, higher catalyst activity (operating farther from extinction point), and/or higher bed thermal conductivity.