Frank Behrendt

NUMERICAL MODELING OF CATALYTIC IGNITION

Catalytic ignition of CH4, CO, and H2 oxidation on platinum and palladium at atmospheric pressure is studied numerically. Two simple configurations are simulated: the stagnation flow field over a catalytically active foil and a chemical reactor with a catalytically active wire inside. The simulation includes detailed reaction mechanisms for the gas phase and for the surface. The gas-phase transport and its coupling to the surface is described using a simplified multicomponent model. The catalyst is characterized by its temperature and its coverage by adsorbed species. The dependence of the ignition temperature on the fuel/oxygen ratio is calculated and compared with experimental results. The ignition temperature of CH4 oxidation decreases with increasing CH4/O2 ratio, whereas the ignition temperature for the oxidation of H2 and CO increases with increasing fuel/oxygen ratio. The kinetic data for adsorption and desorption are found to be critical for the ignition process. They determine the dependence of the ignition temperature on the fuel/oxygen ratio. A sensitivity analysis leads to the rate-determining steps of the surface reaction mechanism. The bistable ignition behavior observed experimentally for lean H2/O2 mixtures on palladium is reproduced numerically. The abrupt transition from a kinetically controlled system before ignition to one controlled by mass transport after ignition is described by the time-dependent codes applied.

Modelling and simulation of heterogeneous oxidation of methane on a platinum foil

Abstract The heterogeneous oxidation of methane-air mixtures in a stagnation point flow onto a platinum foil is investigated numerically and results are compared with experiments. The analysis includes a detailed reaction mechanism in the gas phase as well as on the surface. The heterogeneous ignition occurring around 600°C, extinction, and autothermal behaviour are interpreted in terms of elementary steps at the gas-surface interface.

Development of a parallel direct simulation code to investigate reactive flows

Abstract Solving the Navier-Stokes equations with detailed modeling of the transport and reaction terms remains at the present time a very difficult challenge. Direct simulations of two-dimensional reactive flows using accurate models for the chemical reactions generally require days of computing time on todays most powerful serial vector supercomputers. Up to now, realistic three-dimensional simulations remain practically impossible. Working with parallel computers seems to be at the present time the only possible solution to investigate more complicated problems at acceptable costs, however, lack of standards on parallel architectures constitutes a real obstacle. In this paper, we describe the structure of a parallel two-dimensional direct simulation code using detailed transport, thermodynamic and reaction models. Separating the modules controlling the parallel work from the flow solver, it is possible to get a high compatibility degree between parallel computers using distributed memory and message-passing communication. A dynamic load-balancing procedure is implemented in order to optimize the distribution of the load among the different nodes. Efficiencies obtained with this code on many different architectures are given. First examples of application conceding the interaction between vortices and a diffusion flame are shown in order to illustrate the possibilities of the solver.

Turbulent non-premixed combustion in partially premixed diffusion flamelets with detailed chemistry

The formulation of an extended laminar flamelet model of non-premixed turbulent combustion is developed. The microscopic elements in the turbulent ensemble are taken as laminar diffusion flamelets that are distinguished by the extent of stretching to which they are exposed and, additionally, by the degree to which their reactants are premixed. Extensive laminar flamelet computations are performed using a detailed mechanism for the methane-air system consisting of more than 250 elementary reactions of 39 chemical species. The degree both of flow field stretching and of partial premixing of reactants is varied systematically. The agreement of the numerical results with experimental data available from the literature is found to be excellent. First numerical simulations of turbulent flames on the basis of the extended flamelet model are performed. The results show that partial premixing of laminar diffusion flamelets is essential for the prediction of turbulent flame structures.

Experimental investigation and computational modeling of hot filament diamond chemical vapor deposition

A joint investigation has been undertaken of the gas-phase chemistry taking place in a hot-filament chemical vapor-deposition (HFCVD) process for diamond synthesis on silica surfaces by a detailed comparison of numerical modeling and experimental results. Molecular beam sampling using quadrupole mass spectroscopy and resonance-enhanced multiphoton ionization time of flight mass spectroscopy (REMPI-TOF-MS) has been used to determine absolute concentrations of stable hydrocarbons and radicals. Resulting species of a CH4/H2, a CH4/D2 (both 0.5%/99.5%) and a C2H2/H2 (0.25%/99.75%) feedgas mixture were investigated for varying filament and substrate temperatures. Spatially resolved temperature profiles at various substrate temperatures, obtained from coherent anti-Stokes Raman spectroscopy (CARS) of hydrogen, are used as input parameters for the numerical code to reproduce hydrogen atom, methyl radical, methane, acetylene, and ethylene concentration profiles in the boundary layer of the substrate. In addition,...

Kinetic model of an oxygen‐free methane conversion on a platinum catalyst

In many metal‐catalyzed conversion processes of hydrocarbons at atmospheric pressure a carbonaceous overlayer quickly builds up at the catalyst covering nearly the whole surface. However, the metal still remains catalytically active. Several models have been proposed over the years to explain the crucial role of the carbonaceous overlayer during the conversion of hydrocarbons. The model presented here contemplates adsorbate effects, which means that surface carbon modifies the dehydrogenation activity of Pt. A hydrocarbon reaction mechanism on platinum, including C1 and C2 species, is established. The mechanism is based on elementary reactions offering the opportunity of using the same mechanism for a wide range of applications. It is also applied to extended simulations of higher pressures and smaller flow velocities revealing increased C2H6 yields under these conditions.

Formal treatment of catalytic combustion and catalytic conversion of methane

Abstract Catalytic combustion and conversion of methane is investigated numerically for transient one-dimensional flow configurations (catalytic foil and catalytic wire). The reactive flow is coupled with the processes at the gas–surface interface. The analyses include detailed reaction mechanisms in the gas phase and on the surface as well as a detailed transport model. Computational tools are applied to study the ignition of heterogeneous methane oxidation on a platinum foil and the transition from heterogeneous to homogeneous combustion in this system. Furthermore, the interaction of gas phase and surface reactions in catalytic conversion of methane is considered. Time-dependent selectivity and conversion are calculated.

Simulation of homoepitaxial growth on the diamond (100) surface using detailed reaction mechanisms

Abstract One-dimensional reactive-flow simulations of a hot-filament CVD-system including detailed surface reaction mechanisms for homoepitaxial diamond growth are carried out. A growth model for the diamond (100) surface based on elementary chemical reactions steps is introduced. This surface reaction scheme includes the incorporation of the CH 3 radical in the diamond lattice. Homoepitaxial growth on the (100) surface is modelled for a wide range of experimental reactor parameters. The experimental growth rates are compared with simulations for two different surface reaction schemes. It is found that the scheme based on growth at monoatomic steps on the reconstructed (100) surface is more realistic. It shows qualitative agreement with experimental data, whereas the more simple mechanism for an unrealistic unreconstructed (100) surface cannot explain the surface temperature dependence of the growth rate correctly.

Simulation of reactive flow in filament‐assisted diamond growth including hydrogen surface chemistry

One‐dimensional reactive‐flow simulations of a hot‐filament chemical vapor deposition‐system including surface reactions of H atoms and H2 molecules are reported. The corresponding governing equations of mass, momentum, chemical species, and energy are solved assuming a stagnation‐point flow. In the model, the filament is catalytically active to dissociate H2 molecules, and the net surface reaction is the recombination of H atoms to H2 molecules. It is shown that the surface chemistry strongly influences the H and CH4 concentration profiles, whereas only a minor influence on CH3 and C2H2 concentrations—possible candidates as precursors for diamond growth—is observed. The influence of the surface temperature on gas‐phase species concentration is discussed. At surface temperatures below 1000 K, the CH3‐concentration dependence on the substrate temperature can be characterized by an activation energy of 14 kJ/mol which is in good agreement with experiment. The simulations show that this activation energy is a pure gas‐phase effect due to recombination of methyl radicals and H atoms to methane in the cool gas layer near the substrate.

Experimental and theoretical investigation of CO oxidation on platinum: Bridging the pressure and materials gap

Optical IR-visible sum-frequency generation (SFG) surface vibrational spectroscopy was applied for in situ detection of chemisorbed CO during heterogeneous CO oxidation on a polycrystalline platinum catalyst. The substrate temperature was between 300 and 700 K at a total pressure of 20 mbar. Experiments were carried out under laminar flow conditions in a well-defined stagnation point flow geometry to allow for a detailed comparison with numerical reactive flow simulations. These were carried out to investigate the interaction among surface heterogeneity, catalytic surface reactions and gas-phase processes, and their coupling by molecular transport. To reproduce the experimental results, the model for the Pt foil is based on two different adsorption sites: � 80% A and 20% B sites. The activation energy for desorption of a CO molecule on a clean surface was found to be 183 kJ/mol with a pre-exponential factor of 3 • 10 19 s � 1 on the A sites and 220 kJ/mol with a pre-exponential factor of 5 • 10 21 s � 1 on the B sites. A strong dependency of the desorption energy on the CO coverage was found on the A sites, dropping to 71 kJ/mol on a CO covered surface. These values match the heat of adsorption on Pt(111) and Pt(311) reported by King et al., suggesting structural similarities of these surfaces to a Pt foil. B sites were found to be effectively blocked in the presence of oxygen. But since no significant CO2 production was observed at low temperatures, we conclude that B sites are not the active sites for the CO2 formation. This finding could be explained by the strong bonding of oxygen to those sites.