Transient three-dimensional simulations of a catalytic combustion monolith using detailed models for heterogeneous and homogeneous reactions and transport phenomena

Abstract

The application of a newly developed computational tool, DETCHEM^MONOLITH, for the transient two- and three-dimensional simulation of catalytic combustion monoliths is presented. The simulation is based on the coupling of a transient 2D/3D heat balance of the solid monolith structure with steady-state calculations of the reactive flow in a representative number of channels. The two-dimensional single-channel model uses a boundary-layer approximation including detailed models for heterogeneous and homogeneous reactions as well as transport phenomena. As an example, the computational tool is applied to study the hydrogen-assisted catalytic combustion of methane in a platinum-coated honeycomb monolith.

Introduction

Monolithic catalysts play an important role in various applications such as automotive catalytic converters [1], large-scale facilities for natural gas conversion [2] and catalytic combustion [3], [4]. These systems have received widespread experimental and theoretical attention due to environmental issues and the potential of producing useful chemicals with a reduced consumption of resources. In comparison with experiments, a detailed simulation of the underlying processes will help to verify the theoretical models. Moreover, the simulation becomes an efficient tool in the analysis of the transient flow and thermal phenomena within the catalyst. The results then can be used to design more efficient systems.

In recent years, several proposals have been made for the numerical simulation of monolithic catalysts. Koltsakis et al. [5] used a global model for the catalytic chemical reaction and a plug flow model for the single-channel flows in order to solve a two-dimensional transient heat conduction equation for the monolithic structure. Elementary-step reaction mechanisms for surface and gas phase chemistry and a detailed transport model have been applied for the simulation of flow fields inside single channels of monolithic catalysts used for catalytic partial oxidation of methane [6] and ethane [7]. However, the numerical solution of the full Navier–Stokes equations coupled with complex chemistry models is computationally very expensive because the chemistry contributes to the stiffness of the equation system. Therefore, a simulation of the transient behavior of the entire catalytic monolith using detailed models for chemistry and transport has not yet been realized.

Raja et al. [8] evaluated the application of a plug flow model, a boundary-layer model, and a Navier–Stokes model for catalytic combustion monoliths. In their study, it was shown that for a wide range of problems from moderate to high Reynolds numbers, the boundary-layer model is sufficient to describe a single channel, while the plug flow model fails [8]. Several applications of catalytic monoliths are designed for high space velocities, and particularly at those conditions the boundary-layer model appears to be appropriate. Furthermore, the residence time of the reactive mixture in the monolithic channel can often be assumed to be small in comparison with the variations in the thermal state of the solid monolithic structure. Therefore, the simulation of the reactive flow in the single channels and the thermal variations of the monolithic catalyst can partly be decoupled.

In this work, we focus on those spatially structured monolithic catalysts, where the time scales of variations in the gas phase are much smaller than those of the thermal changes in the monolithic structure. Then, the flow through the single monolith channels, which are assumed to have a cylindrical shape, is modeled by a two-dimensional boundary-layer approach with elementary-step gas phase and surface reaction mechanisms as well as a detailed description of the transport properties. The spatial structure of the monolith enables us to set up a three-dimensional model for the heat transport in the solid monolith, which is coupled to the reactive flow by enthalpy source terms derived from the simulation of a representative number of single channels. The developed computer code, named DETCHEM^MONOLITH, for the first time offers the possibility of performing transient 2D and 3D monolith calculations using such detailed models for transport and chemistry in the individual channels.

As an example, we present a numerical simulation of the hydrogen-assisted oxidation of methane in platinum-coated honeycomb monoliths. The predicted temperature and conversion are compared with experimental data presented in a previous study [9].