J.C. Schouten

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.

Kinetic study of propylene epoxidation with H2 and O2 over Au/Ti–SiO2 in the explosive regime

A kinetic study of propene epoxidation with hydrogen and oxygen over a Au/Ti-SiO2 catalyst has been performed in a wide range of reactant concentrations including the explosive region in a micro reactor. The observed rate dependency on the reactants for the epoxidation and the competing direct water formation is discussed in relation to the current mechanistic insights in the literature. The formation rate of propene oxide is most dependent on the hydrogen concentration, in which the formation of an active peroxo species on the gold nanoparticles is the rate determining step. Deactivation is mainly caused by consecutive oxidation of propene oxide. Oxygen favours the regeneration of the deactivated catalytic sites. Water formation and propene epoxidation are strongly correlated. Water is formed via two routes: through the active peroxo intermediate responsible for epoxidation and from direct formation without involving this active intermediate. Improving the hydrogen efficiency should distinguish between these two routes of water formation. The active peroxo intermediate in epoxidation is competitively consumed by hydrogenation and epoxidation. The active gold site is blocked during deactivation.

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.

Miniaturization of heterogeneous catalytic reactors: Prospects for new developments in catalysis and process engineering

This paper gives an overview of the research done since 1999 at Eindhoven University of Technology in the Netherlands in the field of miniaturization of heterogeneous catalytic reactors. It is described that different incentives exist for the development of these microstructured reaction systems. These include the need for efficient research instruments in catalyst development and screening, the need for small-scale reactor devices for hydrogen production for low-power electricity generation with fuel cells, and the recent quest for intensified processing equipment and novel process architectures (as in the fine chemicals sector). It is demonstrated that also in microreaction engineering, catalytic engineering and reactor design go hand-in-hand. This is illustrated by the design of an integrated microreactor and heat-exchanger for optimum performance of a highly exothermic catalytic reaction, viz. ammonia oxidation. It is argued that future developments in catalytic microreaction technology will depend on the availability of very active catalysts (and catalyst coating techniques) for which microreactors may become the natural housing.

Vapor pressures of precursors for the CVD of titanium nitride and tin oxide

The vapor pressure curves for CVD precursors for TiN coatings and SnO2 layers are presented. The precursors were Ti(NMe2)4 and Me3CTi(NMe2)3 for TiN and (C4H9)SnCl3, SnCl4, MeSnCl3, Me2SnCl2, Me3SnCl, and SnMe4 for the SnO2 system. No significant decompn. was obsd. for 5 of the Sn precursors. Ti(NMe2)4 and Me3CTi(NMe2)3 had enthalpies of evapn. of 63 +- 6 J/mol and 56 +- 5 J/mol, resp. The values measured were in good agreement with previously reported values for the compds. [on SciFinder (R)]