Steffen Tischer

Experimental and numerical study on the transient behavior of partial oxidation of methane in a catalytic monolith

The objective of this investigation is a better understanding of transient processes in catalytic monoliths. As an example, the light-off of the partial oxidation of methane to synthesis gas (H2 and CO) on a rhodium/alumina catalyst is studied experimentally and numerically. Methane/oxygen/argon mixtures are fed at room temperature and atmospheric pressure into a honeycomb monolith, which is preheated until ignition occurs. The exit gas-phase temperature and species concentrations are monitored by a thermocouple and mass spectroscopy, respectively. In the numerical study, the time-dependent temperature distribution of the entire solid monolith structure and the two-dimensional laminar reactive flow fields in the single monolith channels are simulated. A multi-step heterogeneous reaction mechanism is used, and the surface coverage with adsorbed species is calculated as function of the position in the monolith. During light-off, complete oxidation of methane to water and carbon dioxide occurs initially. Then, synthesis gas selectivity slowly increases with rising temperature.

Experimental and numerical investigation of the ignition of methane combustion in a platinum-coated honeycomb monolith

This paper represents an experimental and numerical study of the ignition of catalytic combustion of methane in a cylindrically shaped honeycomb monolith coated with platinum. The objective is the achievement of a better understanding of transient processes in catalytic combustion monoliths. In the experiment, cold methane/oxygen/argon mixtures are fed into the monolith, which is placed in a furnace used to heat up the monolith until ignition occurs. The ignition process is monitored by thermocouples and mass spectroscopy. In the numerical study, the time-dependent temperature distribution of the entire catalytic solid structure and the two-dimensional laminar flow fields of the single monolith channels are simulated. The latter predict the gaseous velocity, species concentrations, and temperature based on a boundary-layer approximation. A multistep heterogeneous reaction mechanism is used, and the surface coverage with adsorbed species is calculated as function of the position in the monolith. The heat balance for the solid structure is coupled with the single channel simulations by axial wall temperature profiles, representing the temperature boundary condition in the single channel simulation, and by heat source terms, derived from the gaseous heat convection and chemical heat release in the single channels. The procedure employs the difference in timescales of the temperature variation of the solid, which is on the order of seconds, and of the flow, which is on the order of miliseconds. Experimentally determined and numerically predicted ignition temperatures, as well as time-varying monolith exit temperatures, and fuel conversion during ignition are compared for several CH 4 /O 2 ratios. At the conditions applied, ignition starts at the rear end in the outmost channels.

Impact of the Inlet Flow Distribution on the Light-Off Behavior of a 3-Way Catalytic Converter

Numerical simulations are increasingly assisting research and development in the field of emission control of automotive vehicles. Our work focuses on the prediction of the tail-pipe emissions, based on a numerical simulation of the automotive catalytic converter. Besides the prediction of the tail-pipe emissions, an understanding of the processes occurring inside a monolithic catalytic converter implies new opportunities for the design of the optimum exhaust gas system. In this paper, we present a three-dimensional transient numerical study of the influence of the velocity distribution in front of the inlet face on the thermal behavior of the monolith during the light-off of a 3-way catalytic converter. The differences in the thermal and chemical behavior due to the shape of the velocity distribution are discussed. The recently developed code DETCHEM M O N O L I T H /1/ is used for the numerical simulation. This code, for the first time, combines two-dimensional simulations of the reactive flow inside a large number of single monolith channels including a heterogeneous multi-step reaction mechanism with a transient simulation of the three-dimensional temperature field of the entire converter.

Three-Dimensional Simulation of the Transient Behavior of a Three-Way Catalytic Converter

The ultimate goal in the numerical simulation of automotive catalytic converters is the prediction of exhaust gas emissions as function of time for varying inlet conditions, i.e. the simulation of a driving cycle. Such a simulation must include the calculation of the transient three-dimensional temperature-field of the monolithic solid structure of the converter, which results from a complex interaction between a variety of physical and chemical processes such as the gaseous flow field through the monolith channels, the catalytic reactions, gaseous and solid heat transport, and heat transfer to the ambience. This paper will discuss the application of the newly developed CFD-code DETCHEM MONOLITH for the numerical simulation of the transient behavior of three-way catalytic converters that have a monolithic structure. The code combines the two-dimensional simulations of the reactive flows in a representative number of monolith channels with a transient simulation of the three-dimensional temperature field of the solid structure of the converter including insulation and canning. The chemical reactions are modeled by a multi-step heterogeneous reaction mechanism, which is based on the elementary processes on the platinum and rhodium catalysts used. The integration over the chemical conversion in the single channels leads to the total conversion in the converter as function of time. This paper presents a numerical simulation of the startup phase of an automotive catalytic converter for temporally varying inlet conditions. The variation of the temperature distribution in the solid structure and in the single channels as well as the species profiles are described. The numerically predicted time-dependent conversion of the combustion pollutants is compared with experimental data. The potentials and limitations of the models and computational tools are discussed.

Mass Transfer Effects in Stagnation Flows on a Porous Catalyst: Water-Gas-Shift Reaction Over Rh/Al 2 O 3

Abstract Water-gas-shift (WGS) and reverse water-gas-shift (RWGS) reactions are numerically investigated in a stagnation-flow on a porous Rh/Al2O3 catalyst. External and internal mass transfer effects are studied using three different models for the mass transport and chemical conversion inside the porous catalyst: the dusty-gas model, a set of reaction-diffusion equations, and the effectiveness factor approach. All three models are coupled with the boundary layer equations to describe the potential flow on the stagnation disc, and a multi-step surface reaction mechanism is implemented. The numerically predicted species profiles in the external boundary layer are compared with recently measured profiles. Internal mass transfer limitations are more significant than external ones in case of the 100 μm thick catalyst layer. The effects of catalyst structure (thickness, mean pore diameter, porosity, tortuosity) as well as flow rate and pressure on chemical conversion are discussed.

Fast Solution for Large-Scale 2-D Convection-Diffusion, Reacting Flows

2-D convection-diffusion, reacting flows in a single channel of catalytic monoliths are investigated. The fluid dynamics are modelled by a steady state, boundary-layer equations, which is a large system of parabolic partial differential equations (PDEs) with nonlinear boundary conditions arising from the coupling between the gas-phase and surface processes. The chemical processes are modelled using detailed chemistry. The PDEs are semi-discretized by a method of lines leading to a large-scale, structured differential algebraic equations (DAEs). The DAEs are solved using a tailored BDF code. We exploit the structure of the Jacobian and freeze the diffusion coefficients during approximation of Jacobian by the finite difference. By applying our approach, the computation times have been reduced by a factor of 4 to 10 and more depending on the particular problem.

Three-Way-Catalyst Modeling - A Comparison of 1D and 2D simulations

In this paper we present a comparison of two different approaches to model three-way catalyst. First, a numerical sample case simulating light -off is used to compare the 1D and the 2D models. The advantages of each code are discussed with respect to required input data, detail level of the output, comparability, and computation time. Thus, the 2D model reveals significant radial temperature gradients inside the monolith during light-off. In a second step, the 2D model is compared with experimental data. One set of data consists of an air/fuel ratio varying sweep at isothermal conditions. Another set was gained by emission measurements during a real driving MVEG tests with varying substrate cell density & inlet conditions. From these experiments the applicability of the model to numerical parameter studies is discussed.

Numerical Simulation of Catalytic Reactors by Molecular-Based Models

Investigations in the field of high-temperature catalysis often reveal complex interactions of heterogeneous, homogeneous, and radical chemistry coupled with mass and heat transfer. The fundamental aspects as well as several applications of high-temperature catalysis are covered in the light of these interactions. Benefits of molecular-based numerical simulations are discussed. Furthermore, this chapter looks at challenges associated with parameter estimation.

Optimization of Reactive Flows in a Single Channel of a Catalytic Monolith: Conversion of Ethane to Ethylene

We discuss the modeling, simulation, and, for the first time, optimization of the reactive flow in a channel of a catalytic monolith with detailed chemistry. We use boundary layer approximation to model the process and obtain a high dimensional PDE. We discuss numerical methods based on the efficient solution of high dimensional stiff DAEs arising from spatial semi-discretization and SQP method for the optimal control problem parameterized by the direct approach. We have investigated the application of conversion of ethane to ethylene which involves a complex reaction scheme for gas phase and surface chemistry. Our optimization results show that the maximum yield, an improvement of a factor of two, is achieved for temperatures around 1300 K.

Towards a better understanding of transient processes in catalytic oxidation reactors

Abstract Light-off of the partial oxidation of methane to synthesis gas on a rhodium-coated cordierite honeycomb monolith at short contact times is studied experimentally and numerically. The objective of this investigation is a better understanding of transient processes in catalytic oxidation reactors. The numerical simulation predicts the time-dependent solid-temperature distribution of the entire reactor and the two-dimensional flow fields, species concentrations, and gas-phase temperature profiles in the single channels. A detailed reaction mechanism is applied, and the surface coverage with adsorbed species is calculated as function of the position in the monolith. During light-off, complete oxidation of methane to water and carbon dioxide occurs initially. Then, synthesisgas selectivity slowly increases with rising temperature.The numerically predicted exit temperature, conversion, and selectivity agree well with the experimentally derived data.