Catalytic Microreactor Research Articles

Design of a Plate‐Type Catalytic Microreactor with CO2 Permeation Membrane for Water‐Gas Shift Reaction

AbstractA method for designing membrane reactors with microchannels is discussed. The design equations were simplified under various assumptions, and a simple model that only incorporates the material balance equations was developed. The activity of the catalyst and the membrane capacity are important properties of a membrane reactor. In the proposed design method, these two parameters are combined into the nondimensional number κ. The simple model was applied to the water‐gas shift reaction and the results were compared with those obtained from the detailed model by computational fluid dynamics calculations. Moreover, a reactor was designed that could achieve a CO concentration of 10 ppm or less. In this plate‐type reactor, the temperature distribution in the channel can be controlled by adjusting the wall temperatures.

Optimal geometry of catalytic microreactors: Maximal reaction rate density with fixed amount of catalyst and pressure drop

In this paper, the dimensionless reaction rate density of a two-dimensional channel with catalyst on its boundary was maximized by optimizing its aspect ratio H/L. Order of magnitude expressions in the limit of small H/L and in the limit of large H/L were developed and then intersected to predict the optimal aspect ratio and maximum reaction rate. Numerical simulations were performed and the results corroborated very well with the scale analysis. The results showed that the optimal design of microreactors depends on the Schmidt number (Sc), the Bejan number (Be) and the Catalyst number (Ct). Correlations based on those numbers are proposed. The analysis was performed for steady-state laminar flows, using the dilute species assumption and a first order kinetic expression.

Alumina-grafted macro-/mesoporous silica monoliths as continuous flow microreactors for the Diels–Alder reaction

Bimodal macro-/mesoporous silica monoliths (6mm diameter, 1–4cm length) were functionalized with aluminum oxide allowing the preparation of a new solid Lewis acid catalyst named Al-MonoSil. Al-MonoSil was used as catalytic microreactor for the in-flow implementation of the Diels–Alder reaction. Due to the interconnected and homogeneous system of macropores of 5μm within the monolith, the Al-MonoSil shows high conversion and productivity in continuous flow without pressure drop. The external mass transfer limitations and the productivity were controlled by adjusting monolith length and flow rate. Al-MonoSil is a stable microreactor for long time productivity for Lewis acid–catalyzed reactions.

On-chip fabrication of silver microflower arrays as a catalytic microreactor for allowing in situSERS monitoring

Silver microflower arrays constructed by upright nanoplates and attached nanoparticles were fabricated inside a microfluidic channel, thus a robust catalytic microreactor for allowing in situ SERS monitoring was proposed. On-chip catalytic reduction shows that the silver microflowers have high catalytic activity and SERS enhancement.

Gold nanoparticle synthesis in microfluidic systems and immobilisation in microreactors designed for the catalysis of fine organic reactions

Our work is focused on two applications of fine tunable microfluidic systems, first to optimize heterogeneous size nanoparticle synthesis and second to build catalytic microreactors for advanced organic reactions. The first part of our work consists in the use of an original microfluidic setup for gold nanoparticle synthesis, which allows a high control of the reaction parameters as the reactants flow, the concentration, the temperature, and the reaction time. We show that using such microfluidic systems permit a better control of the reaction parameters for producing homodispersed 1–2 nm gold nanoparticle. The second part of our work deals with the incorporation of gold nanoparticles into silica capillaries to build catalytic microreactors dedicated to fine chemical reactions. Our strategy consists in the immobilization of gold nanopadiegolirticles onto the inner surface (2D dispersion) or into the inner volume (3D dispersion) of functionalized silica microcapillaries. Characterizations show that by different functionalization procedures, those gold nanoparticles are well anchored inside the microcapillary.

Micro PEM fuel cell system with NaBH4 hydrogen generator

A micro PEM fuel cell system with a NaBH4 hydrogen generator is presented. The micro PEM fuel cell contained glass bipolar plates and a microreactor that initiated a catalytic hydrolysis reaction in the NaBH4 solution. The entire system was developed using MEMS technology. The total system comprised the micro PEM fuel cell, a catalytic microreactor, hydrogen separator, micropump, and a NaBH4 solution cartridge. The microreactor generated a hydrogen flow rate of 16.1ml/min from a NaBH4 feeding rate of 60μl/min. The conversion efficiency was 98.8% and the reactor temperature was 41°C during the operation. The maximum power output of micro fuel cell was 174.6mW for a current of 0.45A. Cyclic performance tests were conducted in order to assess the practical operation of the micro fuel cell system.

A simple kinetic model for oxidative coupling of methane over La 0.6Sr 0.4Co 0.8Fe 0.2O 3-δ nanocatalyst

Abstract A simplified kinetic model for the oxidative coupling of methane over a La 0.6Sr 0.4Co 0.8Fe 0.2O 3-δ nanocatalyst is presented. The kinetic model was developed by experimental data in a catalytic micro-reactor covering a wide range of reaction conditions (0.04< PO 2<0.15 atm, 0.2< P CH4<0.85 atm, 800< T <900 °C). Power law rate expressions were used for all reactions. The reaction scheme proposed in this work includes the most important reactions of oxidative coupling of methane and those involved in most of the available mechanisms in the literature. From the experimental data, kinetic parameters, i.e., pre-exponential factors, activation energies and power law exponents, were estimated. The compatibility of model results with experimental data was investigated and the accuracy of the model prediction was evaluated. Rates of methane consumption, C 2+ and CO x formation, methane conversion, and C 2+ selectivity and yield were compared with experimental data using presented kinetics. The kinetic model was also compared with four previous kinetic models in terms of methane conversion.

Numerical Investigation on the Hydrogen-Assisted Start-Up of Methane-Fueled, Catalytic Microreactors

The hydrogen-assisted start-up of methane-fueled, catalytic microreactors has been investigated numerically in a plane-channel configuration. Transient 2-D simulations have been performed in a platinum-coated microchannel made of either ceramic or metallic walls. Axial heat conduction in the solid wall and surface radiation heat transfer were accounted for. Simulations were performed by varying the inlet pressure, the solid wall thermal conductivity and heat capacity, and comparisons were made between fuel mixtures comprising 100% CH4 and 90% CH4–10% H2 by volume. A significant reduction in the ignition (tig) and steady-state (tst) times was evident for microreactors fed with hydrogen-containing mixtures in comparison to pure methane-fueled ones, for all pressures and reactor materials investigated, with hydrogen having a direct thermal rather than chemical impact on catalytic microreactor ignition. The positive impact of H2 addition was attenuated as the pressure (and the associated CH4 catalytic reactivity) increased. Reactors with low wall thermal conductivity (cordierite material) benefited more from hydrogen addition in the fuel stream and exhibited shorter ignition times compared to higher thermal conductivity ones (FeCr alloy) due to the creation of spatially localized hot spots that promoted catalytic ignition. At the same time, the cordierite material required shorter times to reach steady state. Microreactor emissions were impacted positively by the addition of hydrogen in the fuel stream, with a significant reduction in the cumulative methane emissions and no hydrogen breakthrough. Finally, gas-phase chemistry was found to elongate the steady-state times for both ceramic and metallic materials.

Measurement of reaction heats using a polysilicon-based microcalorimetric sensor

Abstract In this work we present a low-cost, low-power, small sample volume microcalorimetric sensor for the measurement of reaction heats. The polysilicon-based microcalorimetric sensor combines several advantages: (i) complementary metal oxide semiconductor technology (CMOS) for future integration; (ii) elements of silicon micromachining (MEMS) to control thermal performance; (iii) heterogeneous catalysts for selective detection and analysis of individual gas compounds; and (iv) microfluidics for optimized control over the reaction conditions. A comprehensive study on the electrical properties of polysilicon thin films as a potential material for temperature monitoring of highly exothermic reactions is presented. Resistive measurements were performed up to 800 °C and a temperature coefficient of resistance (TCR) of 3.24 × 10 −4/°C was derived in the quasi-linear resistive range between 300 and 500 °C. The polysilicon thin film temperature sensors show a good stability. Due to its excellent compatibility with silicon technology and chemical inertness, doped polysilicon thin films can be successfully applied in catalytic microreactors and sensor microsystems at high operating temperatures. We demonstrate the performance of polysilicon sensors by the detection of reaction heats for a model reaction – the catalytic oxidation of propane in air at concentrations in the range of 0.01–0.8 vol.% which is below the low explosion level (LEL) of propane of 2.1% [1]. The sensor exhibits immediate and reversible response upon exposure to propane in air. By choosing a selective catalyst the specificity of the sensor can be tuned to different gases.

Effect of structural, thermal and flow parameters on steam reforming of methane in a catalytic microreactor

Steam reforming of methane in microchannels, embedded in a monolith is numerically modelled. Horizontal heating layers at equal intervals within the monolith are maintained at constant temperature. The channels are coated internally with catalyst to enhance gas–solid heterogeneous reaction. The numerical method combines the analytical solution for heat transfer through a fin, extended to a stack of fins, and the reactive flow of gases through an iterative procedure. The method offers a tool for quick design of a micro-structure, without considering detailed CFD-based model. In addition, the method can be suitably modified to address thermal management in electronic chip. The temperature within a stack between two heating layers drops near the centre of the stack, in case of an endothermic reaction. This drop, signifying the deviation from isothermal behaviour is found more near the heating layer, and tapers off near the centre of the stack. When the feed temperature is significantly less than the temperature of the heating layer, the portion of the reactor, away from the heating layer remains at a substantially lower temperature, particularly when the number of channels between two heating layers is large. Accordingly, the conversions in the individual channels at the outlet are affected. If the channel wall becomes thicker, the drop in fluid temperature away from the heating layer is more. The increase in feed velocity leads to larger drop in temperature and overall conversion. The decrease in thermal conductivity and the increase in number of channels between two heating layers enhance the temperature drop. None of these functionalities appears to be linear.

Propane combustion in non-adiabatic microreactors: 1. Comparison of channel and posted catalytic inserts

The reduction in size of catalytic microreactors results in high heat and mass transfer rates and a significant increase in the surface area to volume ratio. A further increase in the catalytic surface area can be achieved in a scaled-down version of fixed bed reactors. Since micro-fixed bed reactors are often deemed impractical due to their large pressure drops, one could use precisely structured inserts to increase the surface area, enhance mixing and manipulate the flow distribution. Catalytic propane combustion in microreactors with multi-channel and posted inserts, which consist of multiple static structures (walls separating various channels and pillar-like structures, respectively) in the flow channel of a microreactor, is considered in this series of two papers. In this first paper, we present numerical comparison of multi-channel and posted catalytic inserts for non-adiabatic self-sustained propane combustion. The inserts are oriented axially along the flow direction. We show that channel and post microreactors have similar performance for low thermal conductivity of the inserts. The in-line arrangement of the posted structures is preferred over a staggered arrangement because the former provides higher propane conversion and more stable combustion. The role of thermal conductivity of the microreactor wall structure and the catalytic inserts is investigated. The thermal conductivity of the microreactor structure affects the performance of the posts but not the channels; this is contrary to the effect of catalyst insert thermal conductivity where it is vice-versa. The channel microreactor is more stable towards high flow-rate blowout limit, whereas the post microreactor is significantly more stable at the lower flow-rate extinction limit. This results in stable operation of the post microreactor under more fuel-lean mixtures than the channel microreactor.

ChemInform Abstract: Microstructured Catalytic Reactors

AbstractReview: 282 refs.

Functional silica monoliths with hierarchical uniform porosity as continuous flow catalytic reactors

Silica rod monoliths of 6 mm diameter and 1–5 cm length were obtained by phase separation between silica gel and water in presence of aqueous soluble polyethylene oxide (spinodal decomposition). They exhibit a three-dimensional network of uniform flow-through macropores with adjustable size between 5 and 25 μm. The skeleton of the monolith has a thickness comparable to the size of the macropores and is composed of amorphous silica with a mesoporosity adjustable between 7 and 35 nm. These monolith rods, called MonoSil, were cladded and used as continuous flow catalytic microreactors after introduction of the active sites, via grafting of organic functions, as sulfonic acid groups (HSO 3-MonoSil) and inorganic species, as aluminium oxide (Al-MonoSil); or via the transformation of the amorphous silica skeleton into MCM-41, ZSM-5 and zeolite A (LTA-MonoSil) by a pseudomorphic transformation process with the preservation of the macroporous network. LTA-MonoSil was used in catalysis after cationic exchange of the former Na + cations with K + cations to yield to K-LTA-MonoSil exhibiting strong basic sites. The catalytic behavior of these modified monoliths was evaluated as continuous flow microreactors for the triacetine transesterification, the Diels–Alder reaction and the Knoevenagel condensation. These monolithic reactors demonstrated higher productivities than batch or packed-bed reactors for the described reactions.

Stability of hetero-/homogeneous combustion in propane- and methane-fueled catalytic microreactors: Channel confinement and molecular transport effects

The steady hetero-/homogeneous combustion of lean propane/air and methane/air mixtures in a platinum-coated, plane channel-flow catalytic microreactor has been investigated at pressures of 1 and 5 bar, channel heights of 1.0 and 0.3 mm, and wall thermal conductivities of 2 and 16 W/mK. Stability limits were assessed as a function of fuel type, inlet velocity, and imposed external heat losses. Parametric studies were performed with a full-elliptic, two-dimensional numerical model employing elementary gas-phase (homogeneous) reaction schemes for both fuels, a detailed heterogeneous (catalytic) reaction scheme for methane and a recently developed global reaction step for the oxidation of propane on Pt. Comparisons between the stable combustion regimes of methane and propane revealed a strong impact of the fuel molecular transport properties on the stability and maximum allowable mass throughput. The higher diffusive transport of methane was critical in maintaining wider high inlet velocity stability limits (blowout) compared to those of propane, despite the higher catalytic and gas-phase reactivity of the latter. On the other hand, at the low velocity limits (extinction), propane exhibited a wider stability envelope. Gas-phase chemistry had a strong impact on the blowout limits, even at channel heights as low as 0.3 mm. For the same mass throughput, smaller channel heights tolerated higher heat losses at the extinction branch of the combustion stability envelope thanks to increased transverse fuel transport, while at the same time they exhibited narrower limits at the blowout stability branch due to insufficient residence times at higher inlet velocities. The stable combustion regime of propane increased substantially at 5 bar compared to the same mass throughput at 1 bar, owing to a positive p +0.75 dependence of the propane catalytic reactivity on pressure. Finally, the role of high wall thermal conductivity in widening the blowout limits for both fuels has been demonstrated.

A Microchemical System with Continuous Recovery and Recirculation of Catalyst‐Immobilized Magnetic Particles

A microchemical system for continuous flow catalytic reactions with magnetic catalyst is presented. The automatic separation of catalyst particles and recirculation by the microchemical system makes it possible to realize fully the advantages a catalytic microreactor can offer. It would be applicable to various catalytic reactions with aid of already reported magnetic catalyst or new magnetic catalyst.

Hollow polymer microspheres containing a gold nanocolloid core adsorbed on the inner surface as a catalytic microreactor

Hollow polymer microspheres with different polarity and functional group for the shell layer containing gold nanocolloid cores adsorbed on the inner surface were prepared by selective removal of sandwiched silica layer from the corresponding gold/silica/polydivinylbenzene (Au/SiO2/PDVB), Au/SiO2/poly(ethyleneglycol dimethacrylate) (Au/SiO2/PEGDMA), and Au/SiO2/poly(ethyleneglycol dimethacrylate-co-methacrylic acid) (Au/SiO2/P(EGDMA-co-MAA) tri-layer microspheres, respectively. The tri-layer microspheres were synthesized by distillation precipitation polymerizations of divinylbenzene (DVB), ethyleneglycol dimethacrylate (EGDMA), EGDMA together with methacrylic acid (MAA) in presence of 3-(methacryloxy)propyltrimethoxysilane (MPS)-modified gold/silica (Au/SiO2) core–shell particles as seeds, which were prepared by coating of a layer of silica onto the surface of Au nanocolloids with the aid of polyvinylpyrrolidone (PVP) via a modified Stober method. The catalytic property and stability as a microreactor of the hollow polymer microspheres with Au nanocolloid cores adsorbed on the inner surface were studied by the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AnP) with sodium borohydride (NaBH4) as reductant. Transmission electron microscopy (TEM) and Fourier transform infrared spectra (FT-IR) were used for characterizing the morphology and structure of the resultant microspheres.

Modelling of a Catalytic Micro-Reactor Coupling Endothermic Methane Reforming and Combustion

In this study the mathematical modelling of a catalytic microstructured plate reactor for the production of hydrogen was performed in 2D and 3D geometry. The proposed reacting system uses the heat generated by an exothermic reaction (combustion) to sustain endothermic reforming reactions. Therefore, it pertains to those devices useful for producing the feed for fuel cell system for the remote generation of electrical power. However, because of the compactness of the reacting system it can also be considered in the context of apparatus aiming at process intensification. Within this frame the catalytic contribution of both exothermic and endothermic reactions was modelled considering the classic Langmuir-Hinshelwood surface kinetic theory. The advantage of using a real 3D geometry configuration consists in the possibility of considering the importance of the entering and boundary effects with particular attention to fluid stagnation and heat hot spots. The trade off of such a choice is certainly the huge increase of computing time and/or of the power of the computing facility. With respect to other works performed with similar reactor geometry and reacting systems this does not use simplifying assumptions such as catalyst layers modeled by one-dimensional approach, fully developed laminar flow or transverse heat and mass transfer taken into account through lumped heat and mass transfer coefficients. Results of simulations presented here concentrates on the comparisons between results of: countercurrent (CTC) and concurrent (CNC) flow patterns of the reactant streams; of simulations carried out with 2D and 3D models and of the influence of the thickness of the catalytic layers on the reactor performance. Simulations indicates that CNC flow pattern of reactants streams allows a better performance of the reactor since positive temperature differences between the catalyst layers and the gas in the channels maintain along the whole reactor and, consequently, there are not heat flux inversions, which occur under CTC flow pattern. Results also showed that as concerns an adiabatic reactor, whatever the operating conditions, 2D and 3D models yield substantially the same results. Finally, modelling demonstrated that for a realistic catalyst layer configuration thicknesses larger than 50 µm are useless for enhancing the reactor performance. The feasibility of the model proposed may show its potential in fast and easy implementation of several combustion and reforming fuels so to significantly enhance the performance prediction of real processes.

Control of the thickness of mesoporous titania films for application in multiphase catalytic microreactors

A new method of sol–gel polymer template synthesis of mesoporous catalytic thin films has been proposed which allows controlling the chemical nature of the film, the porosity, thickness and loading with an active species. The mesoporous films with a long-order structure can be obtained in a narrow range of surfactant-to-metal precursor molar ratios from 0.006 to 0.009. The catalytic film thickness was varied from 300 to 1000 nm while providing a uniform catalyst distribution with a desired catalyst loading (1 wt. % Au nanoparticles) throughout the film. The films were characterized by TEM, SEM, ethanol adsorption and contact angle measurements. The calcination of the as-synthesized films at 573 K reduced Ti 4+ sites to Ti 3+. A 300 nm thick Au-containing film showed an initial TOF of 1.4 s −1 and a selectivity towards unsaturated alcohols as high as 90% in the hydrogenation of citral. Thicker films demonstrated a high selectivity towards the saturated aldehyde (above 55%) and a lower intrinsic catalytic activity (initial TOF of 0.7–0.9 s −1) in the absence of internal diffusion limitations.

Numerical investigation on the start-up of methane-fueled catalytic microreactors

Transient simulations have been performed in a plane-channel, methane-fueled microreactor made of either cordierite or FeCr alloy walls and coated with a platinum catalyst. A two-dimensional model for the flow domain was used, which included detailed catalytic and gas-phase chemical reaction mechanisms. In the solid wall, axial heat conduction and surface radiation heat transfer were accounted for. Simulations were performed by varying the inlet pressure, the solid wall thermal conductivity and heat capacity, the inlet velocity, and the equivalence ratio at fuel-lean stoichiometries. The effect of solid material properties as well as the impact of gas-phase chemistry and surface radiation on the ignition ( t ig ) and steady-state ( t st ) microreactor times has been assessed. An increase in inlet pressure from 1 to 5 bar induced a ∼50% reduction in both t ig and t st owing to the enhancement of the catalytic reactivity with rising pressure. A similar behavior was also attested when increasing the equivalence ratio from 0.4 to 0.6. Reactors with low wall thermal conductivity (cordierite material) exhibited shorter ignition times compared to higher thermal conductivity ones (FeCr alloy) due to the creation of spatially localized hot spots that promoted catalytic ignition. At the same time, the ceramic material required shorter times to reach steady-state. Higher inlet velocities reduced the time required for steady-state, however, at the cost of increased cumulative reactor emissions. Surface radiation heat transfer played a key dual role in the start-up process of low thermal conductivity channels. Radiation increased t ig by removing heat away from the initial hot spot, but from the other side it decreased t st due to a very efficient transfer of heat from the rear to the front of the reactor. Gas-phase chemistry elongated the steady-state times for both ceramic and metallic materials and impacted the emissions of catalytic microreactors.

Enhancing Reaction Selectivity by Intentional Control of Concentration Profile in Catalytic Microreactor

This paper reports the results of a computational fluid dynamics (CFD) simulation for a sequential heterogeneous catalytic reaction by comparing the reaction selectivity in a catalytic microreactor with that in a conventional catalytic packed-bed reactor. The comparative simulation revealed that the catalytic microreactor has an advantage over the conventional reactor in the diffusion control regime, and that the control of concentration distribution of reactants by molecular diffusion in the microspace can enhance the selectivity of the desired product. The concentration distribution was intentionally controlled by the shape of the microreactor. The yield of the desired product in the microreactor was 1.16 times that in the packed-bed reactor. CFD simulation of methanol decomposition was also performed by fitting the parameters with the experimental results. The results of the CFD simulation verified the advantage of the microreactor.