Design Of Microreactor Research Articles

Design, fabrication and characterization of microreactors for high temperature syntheses

Microfluidic reactors offer many potential advantages in several research and industrial fields such as processes intensification, which includes a better reaction control (kinetics and thermodynamics), a high throughput and a safer operational environment (reduced manipulation of dangerous reagents and low sub-products generation). Nevertheless, scaling-down limitations appear concerning the materials used in the fabrication of microreactors for most of the liquid-phase reactions, since they usually require high temperatures (up to 300°C), solvents and organic reagents. In this work, the development of a set of modular and monolithic microreactors based on the integration of microfluidics and a thermal platform (sensor/high-temperature heater) is proposed to perform high temperature reactions. The reliability and performance of both configurations were evaluated through an exhaustive characterization process regarding their thermal and microfluidic performance. Obtained results make the devices viable for their application in controlled and reproducible synthetic processes occurring at high temperatures such as the synthesis of quantum dots. The proposed microfluidic approach emerge as an engaging tool for processes intensification, since it provides better mass and temperature transfer than conventional methods with a reduction not only of the size and energy consumption, but also of by-products and reagents consumption.

A kinetic model for H2 production by plasmolysis of water vapours at atmospheric pressure in a dielectric barrier discharge microchannel reactor

Abstract A kinetic model has been developed for water vapour dissociation flowing at atmospheric pressure under nonthermal plasma conditions and analysed for different pathways of water dissociation, reaction time scales and key kinetic steps for H 2 production. A stream of pure water vapour was considered. Different pathways of decomposition such as, dissociation reactions, electron dissociative attachment, dissociative ionisation reactions, ionisation reactions and dissociative excitation reactions were included along with other pathways for different species produced and consumed with in plasmolysis. Dissociation reaction was found to dominate the water vapour plasmolysis followed by electron dissociative attachment reaction being the other important kinetic step. Reaction time scales and concentration were found to be in direct relation with electron density and reached their respective highest values (concentration of H 2 , C H 2 = 16 moles/m 3 and τ reaction ≅ 10 −2 s) at maximum electron density (10 21 m −3 ). The time scale for reaching 99% of hydrogen produced (steady state concentration of H 2 ) was found to be around 10 −2 s which showed H 2 could be produced utilising very little power in micro-plasma reactors, in agreement with published theory (Lozano-Parada JH, Zimmerman WB. The role of kinetics in the design of plasma microreactors. Chemical Engineering Science 2010; 65(17):4925–30). When the model was run as dissociative attachment pathway (Reaction (8)) being the primary reaction to break water vapour, concentration of H 2 was reduced from 16 moles/m 3 to 4 moles/m 3 . However, it additionally introduces the H − radicals into the system which subsequently control the production of H by electron detachment. A detailed analysis was done to find out the major steps influencing the overall kinetics of water vapour plasmolysis. Recombination reaction of H–H atoms was not found to be significant for the production of H 2 . The key step for the generation of H 2 was the reaction between H and HO 2 . Finally, a mechanism consistent with the kinetically important steps is proposed.

Design and optimization of electrochemical microreactors for continuous electrosynthesis

The study focuses on the design and construction, as well as the theoretical and experimental optimization of electrochemical filter press microreactors for the electrosynthesis of molecules with a high added value. The main characteristics of these devices are firstly a high-specific electrochemical area to increase conversion and selectivity, and secondly the shape and size of the microchannels designed for a uniform residence time distribution of the fluid. A heat exchanger is integrated into the microstructured electrode to rapidly remove (or supply) the heat required in exo- or endothermic reactions. The microreactors designed are used to perform-specific electrosynthesis reactions such as thermodynamically unfavorable reactions (continuous NADH regeneration), or reactions with high enthalpy changes.

Nanofiber‐Membrane‐Supported TiO2 as a Catalyst for Oxidation of Benzene to Phenol

We applied a simple, low‐cost design of glass capillary microreactor for the catalytic oxidation of benzene to phenol at ambient conditions. Polyvinylchloride‐nanofiber‐membrane‐supported titania nanoparticle (TiO2‐PVC) as catalyst and in situ production of hydroxyl radicals as oxidant. The reaction was monitored by gas chromatography‐mass spectrometry (GC‐MS). The reaction conditions were optimized and the performance of the microreactor was then compared with the conventional laboratory scale reaction which used hydrogen peroxide as oxidant. The microreactor gave a better yield of 14% for phenol compared to 0.14% in the conventional laboratory scale reaction. Reaction conditions such as reaction time, reaction pH, and applied potential were optimized. With optimized reaction conditions selectivity of >37% and >88% conversion of benzene were obtained.

Managing Solids in Microreactors for the Upstream Continuous Processing of Fine Chemicals

The management of solid compounds is a major challenge facing the upstream, continuous processing of pharmaceuticals and fine chemicals. Many reactions relevant to fine chemical production either react with or form insoluble materials, which become problematic in continuous flow microreactors. The deposition, growth, or bridging of compounds can limit fluid flow from the micro- to the mesoscale, and thereby render continuous reactors inoperable. A comprehensive approach for managing solids consists of solids identification, the development of the root failure mechanism(s), and the application of active and passive techniques for the prevention and remediation. This review examines the basic principles of microreactor design for reactions that involve solids, toward the goal of performing the continuous flow processing of fine chemicals.

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.

Orifice microreactor for the production of an organic peroxide – non-reactive and reactive characterization

In this article, the transfer of a two-step, biphasic, and exothermic peroxide synthesis into a microreactor assisted process is discussed as well as the non-reactive and reactive characterization of the developed orifice microreactor. Residence time distribution measurements showed nearly ideal plug-flow behaviour. The Bodenstein number at a flow rate of 21 mL min−1 is 180 and the corresponding cell number is 90, indicating a narrow residence time distribution. The determined residence times at two different flow rates are in good agreement with the theoretical values of 3.2 s and 1.5 s. The influence of flow rate on droplet size distribution is discussed as well as the influence of orifice geometry on the resulting energy density. These measurements showed a very small droplet size distribution over a wide range of flow rates applied. The smallest mean droplet size of 7 μm was obtained for a flow rate of 75 mL min−1. It is shown that a change from baffle type to conical orifices allows increasing of the throughput by keeping the homogenizing pressure similar to the baffle type system at a lower throughput. The measurement of the temperature profile on top of the thin reaction plate, covering the reaction channel is possible due to a special design of the orifice microreactor and enables monitoring the heat production under reactive conditions. A benchmark based on the product output of an industrial semi-continuous process points out the potential of micro process technology to intensify existing processes. On the examples of four fictive production scale microreactor assisted processes, it is shown that the footprint as well as the reaction volume can significantly be reduced. Using an orifice microreactor of the size of a shoebox the calculated space–time-yield for a product output of ca. 196 kg L−1 h−1 is 905 kg L−1 h−1. This is orders of magnitude higher than for the industrial semi-continuous process.

Environmentally optimized microreactor design through Life Cycle Assessment

Environmentally optimized microreactor design through Life Cycle Assessment

Nanostructure‐Driven Analyte–Interface Electron Transduction: A General Approach to Sensor and Microreactor Design

A concept describing the nanostructure-directed dynamics of acid/base interaction and the balance between physisorption and chemisorption on an extrinsic semiconductor interface is evaluated and compared for n- and p-type semiconductors. The inverse hard/soft acid/base (IHSAB) concept, as it complements the HSAB concept, defines the nature of a dominant physisorption behavior and enables the creation of a matrix of controllable interactions. The technology results in the coupling of Lewis acid/base chemistry with the extrinsic semiconductor majority carriers. Nanoporous silicon layers facilitate the application of nanostructured metal/metal oxides, which provide sensitivity and selectivity for the modified interface. Applied fractional depositions can produce a dominant reversible physisorptive (sensors) or chemisorptive (microreactors) interaction at the semiconductor interface as the nanostructures act as antennas to focus the interaction. The dynamic natures of n- and p-type silicon are evaluated and compared, by focusing on the controlled manipulation of these semiconductors as they are modified with nanostructures and interact with the gas-phase analytes. The observed semiconductor responses correlate well with the temperature dependence of the extrinsic semiconductor, the population of the donor or acceptor levels, and the inherent mobilities of electrons. The response of the modified n-type semiconductors is found to exceed that of comparable p-type systems. The IHSAB concept can be extended to assess the properties of several additional semiconductor interfaces including nanowires. The results obtained not only pertain to sensor and microreactor array design, but also suggest the importance of the dynamic changes created, as the majority charge-carrier concentrations are manipulated and the Fermi energies are modified through chemical interaction.

Design of Non-Isothermal T-Shaped Microreactors with Engulfment Flow by Using a Simplified Model

T-shaped microreactors (T-MRs) with engulfment flow can increase product yield but their high driving pressure is a considerable concern to engineers. Thus, it is important to design a T-MR that can give maximum product yield within a limitation on driving pressure. The use of computational fluid dynamics model is not practical for optimal design of T-MRs due to its heavy computational load. In this research, an extended pseudo three-dimensional lamellar model (eP3DLAM) is proposed as a simple model of non-isothermal T-MRs to efficiently solve the optimal design problems. The usefulness of eP3DLAM is evaluated through a case study on T-MR design. The design result shows that eP3DLAM can accurately predict the concentration and temperature distributions in T-MRs and that the design period of T-MRs can be drastically reduced by using eP3DLAM.

Influence of internal heat removal upon effectiveness, stability of an exothermic catalytic film: Implications for process intensification

Microreactors employing thin catalytic films present a unique situation in catalyst design owing to the removal of symmetric internal boundary condition (i.e. zero heat flux) employed in catalyst design analysis to-date. The allowance for substantial heat removal and/or addition rates at the internal boundary of the catalytic film via process intensification (e.g. heat-exchanger reactors, recuperative combustors) can have significant implications on catalyst effectiveness and ignition/extinction behavior. This manuscript presents a simplified one-dimensional model of two catalytic thin films exchanging heat through a common dividing wall to demonstrate the importance of catalyst design for the cases of (i) internal cooling or heating of a catalytic film, and (ii) coupling of exothermic and endothermic reactions via separate catalytic films. The former case is analogous to microreactor designs for heat removal from an exothermic process, while the latter addresses catalyst design for heat-exchanger reactors coupling endothermic reactions (e.g. steam reforming) with exothermic reaction (e.g. partial oxidation). For a single film catalyzing an exothermic reaction, internal cooling was found to expand the region of steady-state multiplicity and simultaneously decrease catalyst effectiveness, whereas internal heating resulted in stabilization. For the case of two films separately catalyzing exothermic and endothermic reactions and exchanging heat via a common supporting wall, a combination of high wall thermal conductance and intermediate Thiele modulus for the endothermic reaction was capable of preventing ignition/extinction behavior while maintaining individual film effectivenesses in excess of unity. These findings not only give practical insight into the intrinsic behavior of catalysts employed in recuperative and heat-exchanger reactors, but also illustrate the need for future development of rigorous stability criteria for such systems.

Parallel flow of immiscible liquids in a microreactor: modeling and experimental study

Two-phase parallel flow can be utilized in microreactor engineering for performing reactions and extractions, and also for achieving efficient phase separation at the exit of the microreactor. Typical laminar flow at the microscale allows two phases to flow in parallel without mixing, allowing highly controlled conditions for a specific chemical process. Since the liquid with the higher viscosity has a tendency to occupy a larger fraction of the microchannel, the position of the interface can be controlled through the adjustment of flow rates. The prediction of the position of the interface is important for microreactor design and operation and requires the solution of the governing equations of fluid mechanics. In this work, the theoretical description for two-phase parallel flow, based on the Navier–Stokes equations in three dimensions was expressed with a mathematical model and validated with experimental observations. The movement of the interface was achieved through the adjustment of fluid properties according to the position of the central streamlines. The predicted position of the interface was in good agreement with experimental data. A correlation for the flow rate ratio required for positioning the interface in the middle of the channel for various viscosity ratios was proposed, as well as a correlation for the prediction of the parallel to slug transition for a water/n-hexane system.

Electrochemical Microreactor Design for Alkoxylation Reactions—Experiments and Simulations

Two types of the bipolar electrochemical microreactor (BEMR) were investigated for methoxylation of 4-methylanisole: A block-type BEMR with a fixed number of parallel electrodes and a filter-press-type BEMR with a variable number of parallel electrodes. On the basis of a numerical parametric study, the current utilization factor η was presented for the two microreactors as a function of the dimensionless current I* for different system parameters. Conversion and selectivity of the alkoxylation reaction was studied experimentally and the performance of both BEMR was compared. Even though the optimization of the reactor design is still needed particularly with respect to liquid distribution into the individual parallel microchannels, heat management and gas removal, the presented results indicated that the bipolar arrangement of electrochemical microreactors certainly offers a great potential for the intensification of alkoxylation reactions.

Optimal design of multi-channel microreactor for uniform residence time distribution

Multi-channel microreactors can be used for various applications that require chemical or electrochemical reactions in either liquid, gaseous or multi phase. For an optimal control of the chemical reactions, one key parameter for the design of such microreactors is the residence time distribution of the fluid, which should be as uniform as possible in the series of microchannels that make up the core of the reactor. Based on simplifying assumptions, an analytical model is proposed for optimizing the design of the collecting and distributing channels which supply the series of rectangular microchannels of the reactor, in the case of liquid flows. The accuracy of this analytical approach is discussed after comparison with CFD simulations and hybrid analytical-CFD calculations that allow an improved refinement of the meshing in the most complex zones of the flow. The analytical model is then extended to the case of microchannels with other cross-sections (trapezoidal or circular segment) and to gaseous flows, in the continuum and slip flow regimes. In the latter case, the model is based on second-order slip flow boundary conditions, and takes into account the compressibility as well as the rarefaction of the gas flow.

Optimization of Pd Membrane Reactor for Direct Oxidation of Aromatic Compounds

Computational fluid dynamic has already become a widely used and indispensable design and optimization tool in many technical areas. In the present work, the CFD simulations have been coupled with complex chemical reactions to model a membrane tubular reactor which is used to produce phenol from benzene in the vapor phase. Hydrogen dissociates on the palladium layer and reacts with oxygen to give active oxygen species, which attack benzene to produce phenol. In principal, reaction occurs in the surface of palladium and conversion of benzene is increased by changing the length and diameter of the Pd coated PSS tubes. The reactor length and diameter are two geometrical factors which are concerned in the present study. Although increasing the reactor length increase the conversion of benzene to phenol but the concentration of the phenol start to decrease. Based on the data provided by the experiments, a mathematical model has been constructed to conduct a simulation which leads us to an optimum design of a new tubular membrane micro-reactor.

Quantitative prediction of flow patterns in liquid–liquid flow in micro-capillaries

Flow patterns in microstructured reactors (or microchannels) play an important role in dictating the mass transfer rates. In the present work, experiments were carried out to investigate the two phase (liquids) flow patterns in microchannels with different cross sections and contacting geometries. The pattern formation was analysed and conditions were classified in the three regions: surface tension dominated (slug flow), transition (slug-drop and deformed interface flow) and inertia dominated region (annular or parallel flow). A criterion for liquid–liquid flow pattern transition was developed using Capillary and Reynolds numbers based on the work of Dessimoz et al. [1] for gas–liquid systems. Finally, it was applied to the literature data and good agreement was obtained. The criterion is suitable for capillaries with hydraulic diameter up to 3 mm independently of cross section form and is an important predictive tool for the rational design of micro reactors for liquid–liquid reactions.

Guidelines for optimal design of coflow enzyme microreactors

Enzyme microreactors are used as research tools because of many advantages including low reagent consumption and the general notion that mass transfer restrictions are reduced. The use of microchannels can indeed shorten the characteristic mass transfer time but may also affect the productivity of the microreactor. To what extent mass transfer restrictions affect the reaction rate and the productivity is determined by parameters such as the enzyme properties, operating conditions, and dimensions of the microreactor. This article provides the correlations between these parameters for coflow enzyme microreactors obeying Michaelis–Menten kinetics. These correlations outline the design space based on reduced mass transfer restrictions and maximum productivity, respectively. The methodology that yields the design space provides a generic hands-on approach to optimally design coflow enzyme microreactors. We conclude that effectiveness is an important parameter for designing microreactors, but – depending on the reactor configuration – does not necessarily lead to optimized throughput. We demonstrate design windows to illustrate this effect and how to take advantage of it.

Liquid–liquid electro-organo-synthetic processes in a carbon nanofibre membrane microreactor: Triple phase boundary effects in the absence of intentionally added electrolyte

An amphiphilic carbon nanofibre membrane electrode (ca. 50 nm fibre diameter, 50–100 μm membrane thickness) is employed as an active working electrode and separator between an aqueous electrolyte phase (with reference and counter electrode) and an immiscible organic acetonitrile phase (containing only the redox active material). Potential control is achieved with a reference and counter electrode located in the aqueous electrolyte phase, but the electrolysis is conducted in the organic acetonitrile phase in the absence of intentionally added supporting electrolyte. For the one-electron oxidation of n-butylferrocene coupled to perchlorate anion transfer from aqueous to organic phase effective electrolysis is demonstrated with an apparent mass transfer coefficient of m = 4 × 10 −5 m s −1 and electrolysis of typically 1 mg n-butylferrocene in a 100 μL volume. For the two-electron reduction of tetraethyl-ethylenetetracarboxylate the apparent mass transfer coefficient m = 4 × 10 −6 m s −1 is lower due to a less extended triple phase boundary reaction zone in the carbon nanofibre membrane. Nevertheless, effective electrolysis of up to 6 mg tetraethyl-ethylenetetracarboxylate in a 100 μL volume is demonstrated. Deuterated products are formed in the presence of D 2O electrolyte media. The triple phase boundary dominated mechanism and future microreactor design improvements are discussed.

Analysis of chaos and FRET reaction in split-and-recombine microreactors

The mixing and reaction performance of a split-and-recombine (SAR) microreactor was enhanced on modification of the geometric configuration. A rotation of fluid is induced on shrinking the structure of the splitting and reorientation region in alternate directions, thus improving the pattern of multi-lamination and enhancing the chaos of the fluid. To design and to analyze systematically the performance of the reactors, an effective method involving chaotic analysis and fluorescent resonant-energy transfer (FRET) is proposed. The structural design of a passive microreactor to generate an effective contact between the reagents is of great practical significance. SAR microreactors of four types with various microstructures were designed to illustrate the effects of geometric patterns (i.e., arrangement and dimensions) on mixing and reaction. Through analysis of the chaos, we revealed numerically the dynamic mixing governed by multi-lamination and chaotic mechanisms in the devices. The results show that specific structural designs induce rotation and rearrangement of fluids, thus elongating their material interface; the mixing of the fluids consequently improved. We investigated the hybridization of two complementarily labeled oligonucleotides in the devices by means of FRET. How the devices affected the rate of hybridization was thereby assessed, verifying that FRET is a technique capable of estimating the practical applicability of these devices.

Methanol steam reforming in microreactor with constructal tree-shaped network

The construcal tree-shaped network is introduced into the design of a methanol steam microreactor in the context of optimization of the flow configuration. A three-dimensional model for methanol steam reaction in this designed microreactor is developed and numerically analyzed. The methanol conversion, CO concentration in the product and the total pressure drop of the gases in the microreactor with constructal tree-shaped network are evaluated and compared with those in the serpentine reactor. It is found that the reaction of methanol steam reforming is enhanced in the constructal tree-shaped microreactor, since the tree-shaped reactor configuration, which acts an optimizer for the reactant distribution, provides a reaction space with larger surface-to-volume ratio and the reduction of reactant velocities in the branches. Compared with the serpentine microreactor, the constructal reactor possesses a higher methanol conversion rate accompanied with a higher CO concentration. The conversion rate of the constructal microreactor is more than 10% over that of serpentine reactor. More particularly, the reduction of flow distance makes the constructal microreactor still possess almost the same pressure drop as the corresponding serpentine reactor, despite that the bifurcations induce extra local pressure loss, and the reduction of channel size in branches also causes pressure losses.