Lubow Maier

Hydrogen assisted catalytic combustion of methane on platinum

The objective of this paper is to study hydrogen assisted catalytic combustion of methane on platinum experimentally and numerically. In the experiment, we measure the exit temperatures of methane/hydrogen/air mixtures flowing at atmospheric pressure through platinum coated honeycomb channels. A single channel of this monolith is investigated numerically by a two-dimensional Navier-Stokes simulation including an elementary-step surface reaction mechanism. Furthermore, a one-dimensional time-dependent simulation of a stagnation flow configuration is performed to elucidate the elementary processes occurring during catalytic ignition in the mixtures studied. The dependence of the hydrogen assisted light-off of methane on hydrogen and on methane concentrations is discussed. The light-off is primarily determined by the catalyst temperature that is a result of the heat release due to catalytic hydrogen oxidation. Increasing hydrogen addition ensures light-off, decreasing hydrogen addition requires an increasing methane feed for light-off.

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.

Coupling Complex Reformer Chemical Kinetics with Three-Dimensional Computational Fluid Dynamics

A new capability is developed that enables the modeling of certain logistics-fuel reformers. The system described in this paper considers a shell-and-tube configuration for which the catalytic reforming chemistry is confined within the tubes. The models are designed to accommodate detailed gas-phase and catalytic reaction kinetics, possibly including hundreds of species and thousands of reactions. The shell flow can be geometrically complex, but does not involve any complex chemistry. An iterative coupling algorithm is developed with which the geometrically complex flow is modeled with FLUENT and the chemically complex reforming is confined to straight tubes. The paper illustrates the model using propane partial oxidation and reforming as an example.

Thermodynamic Considerations on the Oxidation State of Co/γ-Al2O3 and Ni/γ-Al2O3 Catalysts under Dry and Steam Reforming Conditions

The oxidation state of the active metal is an important factor for catalyst stability under dry and steam reforming conditions. This work explores the correlation of the oxidation state of the active metal with the coking behavior of alumina‐supported cobalt and nickel catalysts from a thermodynamic point of view. To this end, the thermodynamics of the oxidation of Co/γ‐Al2O3 and Ni/γ‐Al2O3 were investigated by using calculations at both standard and technical reforming conditions. It is shown that oxidation of nickel by water or CO2 cannot occur spontaneously under reforming conditions regardless of participation of the alumina support material because of the positive Gibbs reaction energies. Cobalt, in contrast, is more easily oxidized and may form CoAl2O4 through interaction with the support. This phase may react with surface carbon to regenerate the catalyst after carbon formation through thermal cracking of methane. A Mars–van Krevelen type reaction scheme is proposed to explain the higher coking resistance of cobalt compared to nickel.

Surface reaction kinetics of the oxidation and reforming of propane over Rh/Al2O3 catalysts

A multi‐step surface reaction mechanism for partial oxidation and steam reforming of propane over Rh/Al2O3 catalysts is presented. The mechanism is also applicable to model reactions of the subsystems H2/CO/H2O/CO2/O2/CH4. A stagnation–flow reactor with a catalytically coated disk is used to determine the surface reaction rate and spatial concentration profiles on top of the catalytic plate using a micro‐probe sampling technique. The reactor configuration facilitates one‐dimensional modeling of coupled diffusive and convective transport within the gas‐phase boundary layer coupled with detailed heterogeneous chemistry models of the zero‐dimensional surface. The reaction system is studied at varying inlet concentrations and temperatures. The established reaction kinetics are furthermore tested by simulation of autothermal reforming of propane in an annular reactor previously described by Pagani.

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.