Evgeny V. Rebrov

Dissolved gas and ultrasonic cavitation : a review

The physics and chemistry of nonlinearly oscillating acoustic cavitation bubbles are strongly influenced by the dissolved gas in the surrounding liquid. Changing the gas alters among others the luminescence spectrum, and the radical production of the collapsing bubbles. An overview of experiments with various gas types and concentration described in literature is given and is compared to mechanisms that lead to the observed changes in luminescence spectra and radical production. The dissolved gas type changes the bubble adiabatic ratio, thermal conductivity, and the liquid surface tension, and consequently the hot spot temperature. The gas can also participate in chemical reactions, which can enhance radical production or luminescence of a cavitation bubble. With this knowledge, the gas content in cavitation can be tailored to obtain the desired output.

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.

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.

Two-phase flow regimes in microchannels

Capillary hydrodynamics has three considerable distinctions from macrosystems: first, there is an increase in the ratio of the surface area of the phases to the volume that they occupy; second, a flow is characterized by small Reynolds numbers at which viscous forces predominate over inertial forces; and third, the microroughness and wettability of the wall of the channel exert a considerable influence on the flow pattern. In view of these differences, the correlations used for tubes with a larger diameter cannot be used to calculate the boundaries of the transitions between different flow regimes in microchannels. In the present review, an analysis of published data on a gas-liquid two-phase flow in capillaries of various shapes is given, which makes it possible to systematize the collected body of information. The specific features of the geometry of a mixer and an inlet section, the hydraulic diameter of a capillary, and the surface tension of a liquid exert the strongest influence on the position of the boundaries of two-phase flow regimes. Under conditions of the constant geometry of the mixer, the best agreement in the position of the boundaries of the transitions between different hydrodynamic regimes in capillaries is observed during the construction of maps of the regimes with the use of the Weber numbers for a gas and a liquid as coordinate axes.

Structural investigations and magnetic properties of sol-gel Ni0.5Zn0.5Fe2O4 thin films for microwave heating

Nanocrystalline Ni0.5Zn0.5Fe2O4 thin films have been synthesized with various grain sizes by a sol-gel method on polycrystalline silicon substrates. The morphology, magnetic, and microwave absorption properties of the films calcined in the 673–1073 K range were studied with x-ray diffraction, scanning electron microscopy, x-ray photoelectron spectroscopy, atomic force microscopy, vibrating sample magnetometry, and evanescent microwave microscopy. All films were uniform without microcracks. Increasing the calcination temperature from 873 to 1073 K and time from 1 to 3 h resulted in an increase of the grain size from 12 to 27 nm. The saturation and remnant magnetization increased with increasing the grain size, while the coercivity demonstrated a maximum near a critical grain size of 21 nm due to the transition from monodomain to multidomain behavior. The complex permittivity of the Ni–Zn ferrite films was measured in the frequency range of 2–15 GHz. The heating behavior was studied in a multimode microwave cavity at 2.4 GHz. The highest microwave heating rate in the temperature range of 315–355 K was observed in the film close to the critical grain size.

Effect of resonance frequency, power input, and saturation gas type on the oxidation efficiency of an ultrasound horn

The sonochemical oxidation efficiency (η(ox)) of a commercial titanium alloy ultrasound horn has been measured using potassium iodide as a dosimeter at its main resonance frequency (20 kHz) and two higher resonance frequencies (41 and 62 kHz). Narrow power and frequency ranges have been chosen to minimise secondary effects such as changing bubble stability, and time available for radical diffusion from the bubble to the liquid. The oxidation efficiency, η(ox), is proportional to the frequency and to the power transmitted to the liquid (275 mL) in the applied power range (1-6 W) under argon. Luminol radical visualisation measurements show that the radical generation rate increases and a redistribution of radical producing zones is achieved at increasing frequency. Argon, helium, air, nitrogen, oxygen, and carbon dioxide have been used as saturation gases in potassium iodide oxidation experiments. The highest η(ox) has been observed at 5 W under air at 62 kHz. The presence of carbon dioxide in air gives enhanced nucleation at 41 and 62 kHz and has a strong influence on η(ox). This is supported by the luminol images, the measured dependence of η(ox) on input power, and bubble images recorded under carbon dioxide. The results give insight into the interplay between saturation gas and frequency, nucleation, and their effect on η(ox).

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.

Sol-gel synthesis of zeolite coatings and their application in catalytic microstructured reactors

Sol-gel hydrothermal synthesis is one of the most promising methods for the obtaining of zeolitic coatings (films, membranes) on the internal surface of channels of catalytic microstructured reactors. In this review, we discuss the basic methods for the synthesis of zeolite coatings, the processes that influence the rate of crystallization and crystal growth on a substrate, and the methods for modification of the substrate surface before the hydrothermal synthesis. By the example of the synthesis of β, A, and ZSM-5 zeolite coatings, it is shown that the hydrophilic behavior of the substrate and the presence of nano- and microroughness on it have a significant effect on the rate of nucleation of zeolite crystals and the homogeneity of obtained zeolite films. Depending on zeolite type and desired Si/Al ratio in the coating, by several examples. There exists a sufficiently narrow range of conditions (temperature, mixture heating rate, and ionic strength of solution) leading to zeolite coating formation on the substrate rather than to homogeneous crystallization in the authoclave volume. The fundamental mechanisms mechanisms responsible for the formation of zeolite coatings are presented. The acceleration of the hydrothermal synthesis under the action of microwave radiation is shown. The influence of different factors that should be taken into account to scale-up the hydrothermal synthesis is presented. Potential applications fields of microreactors and microadsorbers with zeolite coatings are discussed. Most industrial companies assign microtechnologies to the “high risk-high impact” group. The high risk is attributed, first of all, to the necessity of a cardinal change in the procedure sheet and to the application of new catalysts that allow an increasing rate of processes. Meanwhile, advantages of introduction of the new technologies—the basic ones being the reduction of energy consumption and significant decrease in the formation of by-products—allow companies to reduce operation costs.

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.