M.H.J.M. de Croon

Design of adiabatic fixed-bed reactors for the partial oxidation of methane to synthesis gas. Application to production of methanol and hydrogen-for-fuel-cells

Abstract Adiabatic fixed-bed reactors for the catalytic partial oxidation (CPO) of methane to synthesis gas were designed at conditions suitable for the production of methanol and hydrogen-for-fuel-cells. A steady-state, one-dimensional heterogeneous reactor model was applied in the simulations. Intra-particle concentration gradients were taken into account explicitly, by solving the continuity equations in the catalyst pellet at each position along the fixed-bed reactor co-ordinate. The reactor designs are based on supported Ni catalysts, which catalyse the indirect formation of synthesis gas via total oxidation followed by steam reforming and water–gas shift. In both cases water was added as a reactant. Atmospheric, air-based fixed-bed CPO reactors for fuel-cell applications are feasible due to low catalyst temperatures. At high-pressure methanol conditions, however, catalyst deactivation will be very important as a result of the calculated high catalyst temperatures. The influence of the steam-reforming rate was investigated separately by performing simulations with the kinetic reforming models proposed by Numaguchi and Kikuchi (Chem. Eng. Sci. 43 (1988) 2295) and Xu and Froment (AIChEJ. 35 (1989) 88). The influence of the oxidation kinetics was studied as well. Application of different reforming models leads to significantly different maximum catalyst temperatures. Also, the possible occurrence of gas-phase reactions was investigated: homogeneous reactions will be very important at conditions suitable for methanol production.

Design of a microstructured reactor with integrated heat-exchanger for optimum performance of a highly exothermic reaction

The activity and the heat transfer characteristics of several microstructured reactors have been compared in the ammonia oxidation on Pt catalyst. The main parameters which influence reactor performance are catalyst loading, temperature, and the intrinsic conductivity of the reactor material. In case of aluminum as a reactor material, hot spot temperatures were within 5°C at full conversion of 6 vol.% NH3. Temperature gradients were considerably larger when the microreactor was made from pure platinum due to the smaller intrinsic material conductivity. As a result, the maximum N2O selectivity was by 20% lower than in the case of the aluminum-based reactor due to considerable differences in the selectivities between the central and wall channels. Experimental data obtained on the above microreactors were used to design an externally cooled cross flow microreactor/heat-exchanger operating at almost isothermal conditions even with a reaction mixture corresponding to an adiabatic temperature rise of about 1400°C. Such system can provide new opportunities for improvement of existing gas/solid catalytic processes with strongly exothermic reactions.

Development of the kinetic model of platinum catalyzed ammonia oxidation in a microreactor

The ammonia oxidation reaction on supported polycrystalline platinum catalyst was investigated in an aluminum-based microreactor. An extensive set of reactions was included in the chemical reactor modeling to facilitate the construction of a kinetic model capable of satisfactory predictions for a wide range of conditions (NH3 partial pressure, 0.01–0.12 atm; O2 partial pressure, 0.10–0.88 atm; temperature, 523–673 K; contact time, 0.3–0.7 ms). The elementary surface reactions used in developing the mechanism were chosen based on the literature data concerning ammonia oxidation on a Pt catalyst. Parameter estimates for the kinetic model were obtained using multi-response least squares regression analysis using the isothermal plug-flow reactor approximation. To evaluate the model, the behavior of a microstructured reactor was simulated by means of a complete Navier–Stokes model accounting for the reactions on the catalyst surface and the effect of temperature on the physico–chemical properties of the reacting mixture. In this way, the effect of the catalytic wall temperature non-uniformity and the effect of a boundary layer on the ammonia conversion and selectivity were examined. After further optimization of appropriate kinetic parameters, the calculated selectivities and product yields agree very well with the values actually measured in the microreactor.

Heat and mass transfer in a square microchannel with asymmetric heating

This paper describes the heat and mass transfer in a square microchannel that is heated from one side. This microchannel represents a reaction channel in a microreactor that is used to study the kinetics of the catalytic partial oxidation of methane. The microchannel is contained in a silicon wafer and is covered by a thin silicon sheet. At the top side of this sheet, heating elements are present which mimic the heat that is produced as a result of the exothermic chemical reaction. Correlations for Nusselt and Sherwood numbers as a function of the Graetz number are derived for laminar and plug flow conditions. These correlations describe the heat and mass transport at the covering top sheet of the microchannel as well as at its side and bottom walls. By means of computational fluid dynamic simulations, the laminar flow is studied. To determine an approximate laminar flow Nusselt correlation, the heat transport was solved analytically for plug flow conditions to describe the influence of changes in the thermal boundaries of the system. The laminar flow case is experimentally validated by measuring the actual temperature distribution in a 500 μm square, 3 cm long, microchannel that is covered by a 1 μm and by a 1.9 μm thick silicon sheet with heating elements and temperature sensors on top. The Nusselt and Sherwood correlations can be used to readily quantify the heat and mass transport to support kinetic studies of catalytic reactions in this type of microreactor.

Optimization of heat transfer characteristics, flow distribution, and reaction processing for a microstructured reactor/heat-exchanger for optimal performance in platinum catalyzed ammonia oxidation

The present work is focused on the demonstration of the advantages of miniaturized reactor systems which are essential for processes where potential for considerable heat transfer intensification exists as well as for kinetic studies of highly exothermic reactions at near-isothermal conditions. The heat transfer characteristics of four different cross-flow designs of a microstructured reactor/heat-exchanger (MRHE) were studied by CFD simulation using ammonia oxidation on a platinum catalyst as a model reaction. An appropriate distribution of the nitrogen flow used as a coolant can decrease drastically the axial temperature gradient in the reaction channels. In case of a microreactor made of a highly conductive material, the temperature non-uniformity in the reactor is strongly dependent on the distance between the reaction and cooling channels. Appropriate design of a single periodic reactor/heat-exchanger unit, combined with a non-uniform inlet coolant distribution, reduces the temperature gradients in the complete reactor to less than 4 °C, even at conditions corresponding to an adiabatic temperature rise of about 1400 °C, which are generally not accessible in conventional reactors because of the danger of runaway reactions. To obtain the required coolant flow distribution, an optimization study was performed to acquire the particular geometry of the inlet and outlet chambers in the microreactor/heat-exchanger. The predicted temperature profiles are in good agreement with experimental data from temperature sensors located along the reactant and coolant flows. The results demonstrate the clear potential of microstructured devices as reliable instruments for kinetic research as well as for proper heat management in the case of highly exothermic reactions.

Microchannel Plate Geometry Optimization for Even Flow Distribution at High Flow Rates

Microreactors generally consist of microstructured plates containing a large number of equal channels. The small diameter of the channels enables high heat and mass transfer rates. To exploit this feature and realize a high throughput within a small volume, it is necessary to use high flow rates. However, at these high flow rates it is not straightforward to obtain an even distribution of fluid flow over the individual microchannels. A three-dimensional computational fluid dynamics (CFD) model was used to calculate the flow distribution on a microstructured plate. Calculation time was reduced by introducing an artificial viscosity in the channel region. The calculations show that a transitional velocity exists, below which the flow distribution is independent of velocity and above which inertia effects start to influence the distribution. To optimize the flow distribution, nine different plate geometries were studied at flow rates between 0.1 and 100 m s–1, or 4 × 10–4 to 0.4 m3 h–1 per plate. By optimizing the plate geometry, the relative standard deviation of the flow distribution was reduced from 19 to 3%. Furthermore, it is shown that the optimal geometry depends on the flow rate, which thus needs to be taken into account in the design of microchannel plates.

A Kinetic Study of Ammonia Oxidation on a Pt Catalyst in the Explosive Region in a Microstructured Reactor/Heat-Exchanger

The application of an aluminum-based micro structured reactor/heat-exchanger for measuring reaction kinetics in the explosive region is presented. Platinum-catalyzed ammonia oxidation was chosen as a test reaction to demonstrate the feasibility of the method. The reaction kinetics was investigated in a wide range of conditions [NH 3 partial pressure: 0.03–0.20 atm, O 2 partial pressure: 0.10–0.88 atm; reactant flow 2000–3000cm 3 min –1 (STP); temperature 240–360°C] over a supported Pt/Al 2 O 3 catalyst (mass of Al 2 O 3 layer in the reactor, 1.95 mg; Pt/Al molar ratio, 0.71; Pt dispersion, 20%). The maximum temperature non-uniformity in the microstructured reactor was ca. 5°C, even at conditions corresponding to an adiabatic temperature rise of 1400°C. Based on the data obtained, a previous kinetic model for ammonia oxidation was extended. The modified 13-step model describes the data in a considerably wider range of conditions including those with high ammonia loadings and high reaction temperatures. The results indicate the large potential of microstructured devices as reliable tools for kinetic research of highly exothermic reactions.

A surface and a gas-phase mechanism for the description of growth on the diamond(100) surface in an oxy-acetylene torch reactor

A gas-phase and a surface mechanism were developed, suitable for multidimensional simulations of diamond oxy-acetylene torch reactors. The gas-phase mechanism was obtained by reducing a 48 species combustion chemistry mechanism to a 27 species mechanism with the aid of sensitivity analysis. The surface mechanism for growth on monocrystalline (100) surfaces developed, was based on literature quantum-mechanical calculations by Skokov et al. It consists of 67 elementary reaction steps and 41 species, and contains CH3 and C2H2 as gas-phase growth precursors and atomic hydrogen and oxygen to etch carbon from the surface. The gas-phase and surface chemistry models were tested in one-dimensional simulations, yielding dependencies of the growth rate on feed composition and surface temperature that are in qualitative agreement with the experiments. A more detailed study of the surface chemistry showed that, compared to CH3, acetylene contributes very little to diamond growth. Furthermore, molecular and atomic oxyg...

Pressure and temperature dependence of silicon doping of GaAs using Si2H6 in metalorganic chemical vapour deposition

The doping of GaAs with disilane in a metalorganic chemical vapour deposition (MOCVD) process has been investigated at various input Concentrations of disilane, total pressures and temperatures. The carrier concentration is linearly dependent on the input concentration of disilane. The temperature dependence of the incorporation process of silicon using disilane as a precursor changes as the total pressure varies. At a total pressure of 100 mbar the process is temperature independent in Contrast to the behaviour at total pressures of 20 and 1000 mbar. So the doping process with disilane appears to depend on the total pressure. Using the concept of chemical boundary layer, the results are explained in terms of the different rate determining steps in the doping process as a function of different total pressures.

Low-Pressure Chemical Vapor Deposition of LiCoO2 Thin Films: A Systematic Investigation of the Deposition Parameters

The feasibility of volatile precursor low-pressure chemical vapor deposition (LPCVD) for the production of LiCoO2 cathodes for all solid-state microbatteries was examined. To test this feasibility, and gain insight into the deposition behavior, the influence of the deposition parameters on the properties of LPCVD grown thin-film LiCoO2 cathodes was systematically investigated. The deposition temperature, concentration of the various reactants, and duration of thin-film growth were varied. The resulting LiCoO2 layers were subjected to X-ray diffraction, inductively coupled plasma-atomic emission spectrometry, Rutherford backscattering spectroscopy, and electrochemical analyses. Stoichiometry of the films could be controlled by varying the precursor flows. Samples deposited at high temperatures with the optimum stoichiometry showed a high crystallinity and a high electrochemical activity; a storage capacity corresponding to a reversible Li-content around the theoretical value of 0.55 per Co was reached, and a good cycling stability was obtained when using this electrode in combination with a solid-state electrolyte.