Microreactor Research Articles

CO2 Methanation on Hydrotalcite‐Derived Catalysts and Structured Reactors: A Review

CO2 methanation reaction has attracted renewed interest since the power‐to‐gas (PtG) concept emerged as a promising alternative for CO2 emission abatement using surplus renewable electricity. Although the reaction has been reported for more than a century, improvements in the catalytic system and reactor design remain challenging. Recently, hydrotalcite (HT) materials known for their facile synthesis and high performance are extensively used as precursors for supported catalysts in a wide range of reactions, including CO2 hydrogenation to CH4. Herein, a comprehensive overview on HT‐derived catalysts applied for CO2 methanation is provided. More importantly, new reactor concepts are extensively investigated, such as honeycomb and microchannel reactors, to overcome issues related to the high exothermic nature of the reaction. The latest achievements with respect to structured reactors are also comprehensively reviewed and thoroughly discussed.

Direction-Specific Release from Capsules with Homogeneous or Janus Shells Using an Ultrasound Approach.

A variety of approaches have been developed to release contents from capsules, including techniques that use electric or magnetic fields, light, or ultrasound as a stimulus. However, in the majority of the known approaches, capsules are disintegrated in violent way and the liberation of the encapsulated material is often in a random direction. Thus, the controllable and direction-specific release from microcapsules in a simple and effective way is still a great challenge. This greatly limits the use of microcapsules in applications where targeted and directional release is desirable. Here, we present a convenient ultrasonic method for controllable and unidirectional release of an encapsulated substance. The release is achieved by using MHz-frequency ultrasound that enables the inner liquid stretching, which imposes mechanical stress on the capsule's shell. This leads to the puncturing of the shell and enables smooth liberation of the liquid payload in one direction. We demonstrate that 1-4.3 MHz acoustic waves with the intensity of a few W/cm2 are capable of puncturing of particle capsules with diameters ranging from around 300 μm to 5 mm and the release of the encapsulated liquid in a controlled manner. Various aspects of our route, including the role of the capsule size, ultrasound wavelength, and intensity in the performance of the method, are studied in detail. We also show that the additional control of the release can be achieved by using capsules having patchy shells. The presented method can be used to facilitate chemical reactions in micro- and nanolitre droplets and various small-scale laboratory operations carried in bulk liquids in microenvironment. Our results may also serve as an entry point for testing other uses of the method and formulation of theoretical modeling of the presented ultrasound mechanism.

An effective hybrid solvent of MEA/DEEA for CO2 absorption and its mass transfer performance in microreactor

The gas-liquid two-phase flow and the mass transfer characteristics for CO2 absorption into the binary amine mixture of monoethanolamine (MEA) and N,N-diethylethanolamine (DEEA) aqueous solution were investigated in T-junction microchannel reactor with a square cross-section. The effects of gas-liquid flow rate and absorbent concentration on bubble length, velocity and mass transfer coefficient were studied. Three flow regimes including slug-bubbly flow, slug flow and slug-annular flow were observed under experimental condition, and the gas-liquid two-phase flow pattern map was depicted. It is found that the transitions between three flow regimes depend primarily on the competition of forces exerted on bubble in the T-junction. The kinetics of CO2 absorption into blended amine aqueous solutions was analyzed to understand the mass transfer enhancement mechanism of new hybrid solvent. Furthermore, the liquid-side overall mass transfer coefficient kL and the liquid-side overall volumetric mass transfer coefficient kLa were determined through processing experimental images recorded by a high-speed camera. The results show that kLa is in the range of (3.04–20.07) s−1 in this work, which is 2–3 orders of magnitude higher than that in the conventional equipment. The dimensionless Damköhler number (Da) and Peclet number (Pe) are introduced to correlate the liquid-side volumetric mass transfer coefficient, new correlation shows good prediction accuracy with the average deviation of 3.64%.

Electrochemical Conversion of CO2 to CO into a Microchannel Reactor System in the Case of Aqueous Electrolyte

A microchannel electrochemical reactor system was designed to facilitate the reduction of CO2 to CO, which is composed of a premixing section in the upstream consisting of a circular microchannel f...

Explosive synthesis: novel intrinsically safe method and application with micro-channel reactor

Novel Micro-channel method was introduced into explosive synthesis process. The method was discussed hereby with its intrinsic safety including marvelous inner-structure, high heat-transfer efficiency, and high mass-transfer efficiency compared to traditional kettle-based method. Micro-channel reactor for explosive synthesis was established with high precision control, the temperature difference of oil flow between import and export of micro-channel reactors was measured to be no more than 3°C, and the flux fluctuation of reactant flow were measured to be no more than 0.1ml/min. Micro-channel reactor was experimented with TATB explosive synthesis process. Nitration reaction showed no residue of raw material PG and 91.62% HPLC area of target material TNPG, alkoxylation reaction showed no residue of raw material TNPG and 98.01% HPLC area of target material TETNB, amination reaction showed 5.28% HPLC area of raw material and output of target material TATB. It was concluded to be verifiable including micro-channel method, the apparatus, and application in TATB synthesis process

Production of CO2-neutral liquid fuels by integrating Fischer-Tropsch synthesis and hydrocracking in a single micro-structured reactor: Performance evaluation of different configurations by factorial design experiments

Abstract Humankind must drastically reduce CO2 emissions within the next three decades to limit global climate change. Defossilization of the transport sector must contribute to the achievement of this ambitious aim. One promising option is the production of synthetic liquid fuels from CO2, water, and renewable electrical energy. Liquid fuels will also be used in the future, especially for heavy-duty vehicles and aviation. We studied the production of liquid fuels by integration of Fischer-Tropsch Synthesis (FTS) and Hydrocracking (HC) in a single micro-structured reactor. Thereby, we addressed the aspects of process simplification and process intensification. Both are particularly important for flexible small-scale plants for decentralized application in context of the energy transition. In this study, we systematically investigated the performance of the integrated FTS/HC process depending on different operating conditions (pressure, temperature, weight hourly space velocity, H2/CO synthesis gas ratio) and integration configurations by applying a factorial design approach. Beyond the selectivity of the process towards liquid fuels, we especially focus on the product quality affected by the type of produced hydrocarbons. We identified significant influence parameters on process performance, which is essential to know for practical application. We showed that the crude product quality was significantly influenced by the integration pattern. Therefore, we suggested more elaborate integration configurations to overcome the challenge of suboptimal operation conditions for the FT and HC catalyst and improve the product quality.

Fatty Acid Methyl Ester Synthesis through Transesterification of Palm Oil with Methanol in Microchannels: Flow Pattern and Reaction Kinetics

This work studied the flow pattern and reaction kinetics for fatty acid methyl ester (FAME) synthesis through transesterification of palm oil with methanol in a microchannel reactor. It was observe...

Rapid production of biodiesel in a microchannel reactor at room temperature by enhancement of mixing behaviour in methanol phase using volume of fluid model

The demand for biodiesel to reduce consumption of fossil fuels has motivated engineers to design a rapid and safe production process. However, the slow mass transfer of multiphase transesterification reaction hinders the overall reaction time. A continuous process using a microchannel reactor was proven to achieve complete reaction time in 40 s with high oil conversion of 98.6% due to the enhancement of mixing inside the methanol slug phase. The reaction was successfully performed at ambient room environment, without heating element in the reactor that reduces the design complexity and makes the operation energy efficient and safer. The enhancement of reaction was investigated based on the hydrodynamic factors such as interfacial area and mixing. Three-dimensional Computational Fluid Dynamics (CFD) simulation shows a torus-shaped recirculation structure inside methanol slug that enhances mixing. Microchannel reactor has been proven to enhance the mixing and capable in having faster, highly efficient, and safer process.

Influence of liquid composition on diffusive mass transfer in the lubricating film of Taylor flow—A study related to the hydrogenation of nitrobenzene

Micro-structured reactors operated in Taylor flow are promising for intensifying heterogeneously catalyzed gas-liquid reactions. By reaction progress or dilution, physical properties may change with liquid composition. Here, the effect of composition on physical mass transfer in the liquid film of Taylor flow is studied with regard to the hydrogenation of nitrobenzene to aniline. For the liquid mixtures nitrobenzene-aniline and nitrobenzene-ethanol, composition-dependent physical properties are determined using information from literature. By numerical solution of the unsteady one-dimensional diffusion equation with composition-dependent physical properties, the instantaneous hydrogen concentration across the liquid film is obtained for different nitrobenzene mole fractions. The results indicate that in a wide composition range of reactant (nitrobenzene) and product (aniline), hydrogen diffuses slower in the mixture than in pure liquids. On the other hand, dilution of nitrobenzene by ethanol increases hydrogen mass transfer in the liquid film. A theoretical relation is provided which allows estimating for given capillary number the necessary bubble length to achieve a certain hydrogen saturation in the liquid film during the passage of one Taylor bubble. This non-dimensional relation can be applied to other physical systems and may be useful to guide optimal hydrodynamic operation conditions for applications with Taylor flow in chemical engineering.

CFD analysis of CO2 absorption in a microporous tube-in-tube microchannel reactor with a novel gas-liquid mass transfer model

Due to the greenhouse effect, new technologies of CO2 capture attract wide attention. In this paper, computational fluid dynamics (CFD) was employed to simulate the absorption of CO2 in a microporous tube-in-tube microchannel reactor (MTMCR). Based on the eddy cell model, a novel gas-liquid mass transfer model considering the surface work was proposed and the simulated results of volumetric mass transfer coefficient agreed well with the published experimental data. In the lower turbulence range, the deviations between experimental data and predictions of kla using eddy cell model are over ± 40%. Meanwhile, results from the modified eddy cell model agree well with experimental data within ±10% deviation in a wide range of operating conditions. The novel gas-liquid mass transfer model could benefit for understanding the mass transfer mechanism and improving the mass transfer performance in reactors.

The use of a microchannel reactor for optimizing the production of 1-alkyl-3-methylimidazolium chlorides

The possibility to use microchannel flow reactors for obtaining kinetic and technological parameters of the synthesis of 1-butyl-3-methylimidazolium chloride (BMIMCl) ionic liquid was demonstrated for the reaction of 1-methylimidazole (MIm) with 1-chlorobutane in the absence of solvents. BMIMCl was produced with high selectivity and specific output in a microchannel flow reactor at temperatures 120–180 °C and contact times 2-45 min at a pressure of 20 bar. The positive result is obtained due to the laminar profile of the flow and a uniform distribution of the reagents’ concentration over the microchannel cross-section. Investigation of the process kinetics in a microchannel flow reactor made it possible to reveal that the reaction moves to the diffusion resistance mode at a temperature above 150 °C. The kinetic data obtained for the BMIMCl synthesis were used to develop methods for the production of 1-ethyl-3-methylimidazolium and 1-hexyl-3-methylimidazolium chlorides (EMIMCl and HMIMCl, respectively) under the conditions of a microchannel flow reactor. The approach suggested in this paper is of interest for the development of flow and batch setups for a small-tonnage production of dialkylimidazolium, ammonium and pyridinium salts by quaternization of the corresponding alkyl chlorides and nitrogen-containing bases.

Continuous 2000 K droplet-to-particle synthesis

Aerosol spray coupled with high-temperature pyrolysis is an emerging technique for continuous manufacturing of nanomaterials at large scale that demonstrates extremely high production efficiency. Current aerosol spray techniques using a tube furnace can only attain a low temperature range (generally <1500 K), rendering limited products as well as inhomogeneous heating within the comparatively bulky furnace chamber (typical dimensions of ≥2 cm in diameter and ≥40 cm in length), which leads to difficulties in product quality control. Here we report a “droplet-to-particle” aerosol technique coupled with a high-temperature (∼2000 K) micro-channel reactor, which provides ∼100-times smaller dimensions compared to conventional tube furnaces, enabling homogeneous and high-temperature nanomaterial manufacturing. To demonstrate the unique capability of this carbonized wood micro-channel reactor, we successfully synthesized multielement high entropy alloy/oxide nanoparticles (which typically require a high temperature (2000 K) to achieve uniform elemental mixing) in a continuous and support-free manner. Droplets atomized from the multielement precursors fly through the micro-channels heated to 2000 K by Joule heating with a residence time of only tens of milliseconds with a high energy conversion efficiency (>95%), during which salt decomposition and particle nucleation/growth occur. The high temperature critically enables homogeneous mixing of elements in the resultant nanoparticles and the short residence time is key to suppress particle growth and agglomeration. Compared with the traditional aerosol spray pyrolysis, the carbonized wood reactor can achieve a record high temperature (≥2000 K), a much shorter residence time (∼tens of milliseconds), highly efficient, uniform heating, and provide a platform for continuous nanomaterial manufacturing for a broad range of applications.

A critical analysis of electrospray techniques for the determination of accelerated rates and mechanisms of chemical reactions in droplets.

Electrospray and Electrosonic Spray Ionization Mass Spectrometry (ESI-MS and ESSI-MS) have been widely used to report evidence that many chemical reactions in micro- and nano-droplets are dramatically accelerated by factors of ∼102 to 106 relative to macroscale bulk solutions. Despite electrospray's relative simplicity to both generate and detect reaction products in charged droplets using mass spectrometry, substantial complexity exists in how the electrospray process itself impacts the interpretation of the mechanism of these observed accelerated rates. ESI and ESSI are both coupled multi-phase processes, in which analytes in small charged droplets are transferred and detected as gas-phase ions with a mass spectrometer. As such, quantitative examination is needed to evaluate the impact of multiple experimental factors on the magnitude and mechanisms of reaction acceleration. These include: (1) evaporative concentration of reactants as a function of droplet size and initial concentration, (2) competition from gas-phase chemistry and reactions on experimental surfaces, (3) differences in ionization efficiency and ion transmission and (4) droplet charge. We examine (1–4) using numerical models, new ESI/ESSI-MS experimental data, and prior literature to assess the limitations of these approaches and the experimental best practices required to robustly interpret acceleration factors in micro- and nano-droplets produced by ESI and ESSI.

Non-Dimensional Feature Engineering and Data-Driven Modeling for Microchannel Reactor Control

Catalytic plate microchannel reactors (CPRs) are a promising means for modular hydrogen/fuels production from distributed natural gas resources. However, the equipment miniaturization presents challenges for process control, including spatially-distributed models, limited availability of measurements, and fast process time constants. In the present paper, we investigate the use of data-driven models—specifically, artificial neural networks (ANNs)—to estimate temperature “hotspots” within CPRs. We prescribe nonlinear transformations of the model inputs in the form of well-known dimensionless quantities (e.g., Reynolds number), and we show that these engineered features can improve the prediction capability of computationally parsimonious ANNs using a first-principles reactor model. Finally, we present a simulation case study that demonstrates the use of a trained ANN for inferential model predictive control.

Modular microreactor with integrated reflection element for online reaction monitoring using infrared spectroscopy

We report on the fabrication of an internal reflection element (IRE) combined with a modular polymer microfluidic chip that can be used for attenuated total reflection (ATR) infrared spectroscopy. The IRE is fabricated from a silicon wafer. Two different polymers are used for the fabrication of the two types of modular microfluidic chips, namely polydimethylsiloxane (PDMS) and cyclic olefin copolymer (COC). The microfluidic chip is modular in the sense that several layers of mixing channels, using the herringbone mixer principle, and reactions chambers, can be stacked to facilitate the study of the desired reaction. A model Paal-Knorr reaction is carried out to prove that the chip works as intended. Furthermore, we highlight the strength of IR spectroscopy as a tool for reaction monitoring by identifying the peaks and showing the different reaction orders at the different steps of the Paal-Knorr reaction. The reduction of the aldehyde groups indicates a (pseudo) first order reaction whereas the vibrational modes associated with the ring formation indicate a zero order reaction. This zero order reaction can be explained with literature, where it is suggested that water acts as a catalyst during the dehydration step, which is the final step in the pyrrole ring formation.

Application of a clustered countercurrent-flow micro-channel reactor in the preparation of KMnF3 perovskite for asymmetric supercapacitors.

A clustered countercurrent-flow micro-channel reactor (C-CFMCR) with adjustable magnification times was constructed for the preparation of KMnF3 perovskite fluoride by a co-precipitation process, in which the concentrations and feed rates of reactants were precisely controlled. Benefitting from the enhanced micromixing efficiency of the microreactor, the KMnF3 particles prepared in C-CFMCR were smaller and less aggregated than those produced with traditional stirred reactors (STR). The prepared KMnF3 was applied as the electrode material in supercapacitors, and the electrochemical measurements showed that the KMnF3 obtained under optimal conditions had a discharge specific capacitance of ∼442 F g−1 at a current density of 1 A g−1, with a decline of ∼5.4% after 5000 charge–discharge cycles in an aqueous electrolyte of 2 M KOH. It was also found that the morphologies and electrochemical performances of the prepared KMnF3 particles changed accordingly with the micromixing efficiencies of C-CFMCR, which can be adjusted by the reactor structure and operating conditions. An asymmetric supercapacitor assembled with the KMnF3 and activated carbon exhibited an energy density of 13.1 W h kg−1 at a power density of 386.3 W kg−1, with eminent capacitance retention of ∼81.2% after 5000 cycles. In addition, only a slight amplification effect of C-CFMCR on the co-precipitation process was noticed, indicating that the C-CFMCR is a promising technology for the massive and controllable production of KMnF3 particles as well as other ultrafine particles.

Catalytic exchange of hydrogen isotopes intensified by two-phase stratified flow in wettability designable microchannels.

This work develops a reliable method to achieve stratified flow in squared cross-section microchannels by endowing the four channel walls with very different wettabilities. The gas-liquid flows are studied experimentally in six kinds of microchannels with different wetting partitions on the channel walls, where occurrence of the stratified flow is found to strongly depend on the wetting partition and the individual flow rate of gas or liquid. Among them, the microchannel possessing three hydrophobic and one hydrophilic walls (3S1H) shows great potential in facilitating liquid phase catalytic exchange (LPCE) of hydrogen isotopes. Subsequently, a 3S1H microchannel reactor coated with hydrophobic Pt/activated carbon/polydimethylsiloxane (Pt/AC/PDMS) is fabricated to perform the LPCE at 30-80 °C and in a wide range of flow rates of hydrogen and water. The experimental outcomes not only unveil the relationship of the isotopic exchange performance with the temperature and flow pattern but also exhibit an outstanding isotopic exchange performance under the stratified flow.

Application of a micro-channel reactor for process intensification in high purity syngas production via H2O/CO2 co-splitting

A stainless steel micro-channel reactor was tailor-made to an in house-design for process intensification propose. The reactor was used for a two-step thermochemical cycles of H2O and CO2 co-splitting reaction, in the presence of La0.3Sr0.7Co0.7Fe0.3O3 (LSCF). LSCF was coated inside the reactor using wash-coat technique. Oxygen storage capacity of LSCF was determined at 4465 μmol/g, using H2-TPR technique. H2O-TPSR and CO2-TPSR results suggested that a formation of surface hydroxyl group was the cause of H2O splitting favorable behavior of LSCF. Optimal operating reduction/oxidation temperature was found at 700 °C, giving 2266 μmol/g of H2, 705 μmol/g of CO, and 67% of solid conversion, when the H2O and CO2 ratio was 1 to 1, and WSHV was 186,000 mL/g.h. Activation energy of H2O spitting and CO2 splitting was estimated at 87.33 kJ/mol, and 102.85 kJ/mol The pre-exponential factor of H2O splitting and CO2 splitting was 595.24 s−1 and 698.79 s−1, respectively.

Thermal management of methanol reforming reactors for the portable production of hydrogen

Three-dimensional numerical simulations were performed to address the thermal management issues associated with the design of a methanol reforming microchannel reactor for the portable production of hydrogen. The design of the reactor was fundamentally related to the direct coupling of reforming and combustion reactions by performing them on opposite sides of dividing walls in a parallel flow configuration. Effective autothermal operation was achieved through a combination of microchannel reactor technology with heat exchange in a direction perpendicular to the reacting fluid flow. Computational fluid dynamics simulations and thermodynamic analysis were carried out to investigate the effect of various design parameters on the characteristics of the generation, consumption, and exchange of thermal energy within the system. The results indicated that the ability to control temperature and temperature uniformity is of great importance to the performance of the system. The degree of temperature uniformity favorably affects the autothermal operation of the reactor. Temperature uniformity of the reactor can be improved by controlling the rate of heat transfer through a variety of factors such as wall thermal conductivity, fluid velocities, and dimensions. High wall thermal conductivity would be greatly beneficial to the performance of the system and the temperature uniformity of the reactor.

Carbon dioxide desorption from aqueous solutions of monoethanolamine and diethanolamine in a microchannel reactor

In this study, the process of carbon dioxide (CO2) desorption from two saturated solutions of monoethanolamine (MEA) and diethanolamine (DEA) was performed in a microchannel made of stainless-steel grade 316 with a circular cross-section (diameter: 800 μm, length: 35 cm). The operating variables in this study were temperature (55, 75 and 95 °C), rich solvent flow rate (0.3, 0.9 and 1.5 ml/min), and inlet solvent concentration (10, 20 and 30 wt% of the amine). The mass transfer efficiency was determined based on the desorption percentage, volumetric liquid-side mass transfer coefficient (kLaV), volumetric overall mass transfer flux (NCO2aV), and energy consumption per unit mass CO2 (R). The results showed that the use of a microchannel significantly increased the mass transfer rate and decreased energy consumption per removed CO2. The results also showed that the amount of kLaV for the two solvents of MEA and DEA was 1.91 and 3.48 1/s, respectively. Moreover, the R-value for the two solvents of MEA and DEA was 1.3 and 1.63 MJ/kg CO2, respectively, which is at least three times lower than that of other mass transfer devices.