A microreactor or microstructured reactor or microchannel reactor is a device in which chemical reactions take place in a confinement with typical lateral dimensions below 1 mm; the most typical form of such confinement are microchannels. Microreactors are studied in the field of micro process engineering, together with other devices (such as micro heat exchangers) in which physical processes occur. The microreactor is usually a continuous flow reactor (contrast with/to a batch reactor). Microreactors offer many advantages over conventional scale reactors, including vast improvements in energy efficiency, reaction speed and yield, safety, reliability, scalability, on-site/on-demand production, and a much finer degree of process control.
Gas-phase microreactors have a long history but those involving liquids started to appear in the late 1990s. One of the first microreactors with embedded high performance heat exchangers were made in the early 1990s by the Central Experimentation Department (Hauptabteilung Versuchstechnik, HVT) of Forschungszentrum Karlsruhe in Germany, using mechanical micromachining techniques that were a spinoff from the manufacture of separation nozzles for uraniumenrichment. As research on nuclear technology was drastically reduced in Germany, microstructured heat exchangers were investigated for their application in handling highly exothermic and dangerous chemical reactions. This new concept, known by names as microreaction technology or micro process engineering, was further developed by various research institutions. An early example from 1997 involved that of azo couplings in a pyrex reactor with channel dimensions 90 micrometres deep and 190 micrometres wide.
Using microreactors is somewhat different from using a glass vessel. These reactors may be a valuable tool in the hands of an experienced chemist or reaction engineer:
1. Microreactors typically have heat exchange coefficients of at least 1 megawatt per cubic meter per kelvin, up to 500 MW m−3 K−1 vs. a few kilowatts in conventional glassware (1 l flask ~10 kW m−3 K−1). Thus, microreactors can remove heat much more efficiently than vessels and even critical reactions such as nitrations can be performed safely at high temperatures. Hot spot temperatures as well as the duration of high temperature exposition due to exothermicity decreases remarkably. Thus, microreactors may allow better kinetic investigations, because local temperature gradients affecting reaction rates are much smaller than in any batch vessel. Heating and cooling a microreactor is also much quicker and operating temperatures can be as low as −100 °C. As a result of the superior heat transfer, reaction temperatures may be much higher than in conventional batch-reactors. Many low temperature reactions as organo-metal chemistry can be performed in microreactors at temperatures of −10 °C rather than −50 °C to −78 °C as in laboratory glassware equipment.
1.微反应器的典型特征就是换热效率至少1MW m-3 K-1，高一些的可达到500 MW m-3 K-1。二普通的玻璃仪器只有1 l烧瓶〜10 kW m-3 K-1 1。因此，微反应器能够比反应釜更高效的散热，这也使像硝化这种的反应反应都能够在高温状态下安全进行。热区温度和高温持续时间传递散热而显着下降。微反应器给更佳的动力研究提供了可能，因为反应器温度梯度对反应速率的影响比任何传统反应釜都要小。同样对微反应器的制热和制冷都减少，甚至操作温度可以低至100℃。伴随着这种超级换热器的产生，反应温度可以比传统反应釜高上很多。很多的低温反应，如有机金属化学实验能够通过微反应器在-10°C反应条件下进行，而不必像实验室的玻璃装置那样温度需要低至−50°C到−78°C。
2.Microreactors are normally operated continuously. This allows the subsequent processing of unstable intermediates and avoids typical batch workup delays. Especially low temperature chemistry with reaction times in the millisecond to second range are no longer stored for hours until dosing of reagents is finished and the next reaction step may be performed. This rapid work up avoids decay of precious intermediates and often allows better selectivities.
3.Microreactors are normally operated continuously. This allows the subsequent processing of unstable intermediates and avoids typical batch workup delays. Especially low temperature chemistry with reaction times in the millisecond to second range are no longer stored for hours until dosing of reagents is finished and the next reaction step may be performed. This rapid work up avoids decay of precious intermediates and often allows better selectivities.
4.Although a bench-top microreactor can synthesize chemicals only in small quantities, scale-up to industrial volumes is simply a process of multiplying the number of microchannels. In contrast, batch processes too often perform well on R&D bench-top level but fail at batch pilot plant level.
5.Pressurisation of materials within microreactors (and associated components) is generally easier than with traditional batch reactors. This allows reactions to be increased in rate by raising the temperature beyond the boiling point of the solvent. This, although typical Arrhenius behaviour, is more easily facilitated in microreactors and should be considered a key advantage. Pressurisation may also allow dissolution of reactant gasses within the flow stream.
Although there have been reactors made for handling particles, microreactors generally do not tolerate particulates well, often clogging.Clogging has been identified by a number of researchers as the biggest hurdle for microreactorsbeing widely accepted as a beneficial alternative to batch reactors.So far, the so-called microjetreactor is free of clogging by precipitating products. Gas evolved may also shorten the residence time of reagents as volume is not constant during the reaction. This may be prevented by application of pressure.
Mechanical pumping may generate a pulsating flow which can be disadvantageous. Much work has been devoted to development of pumps with low pulsation.A continuous flow solution is electroosmotic flow(EOF).Typically, reactions performing very well in a microreactor encounter many problems in vessels, especially when scaling up. Often, the high area to volume ratio and the uniform residence time cannot easily be scaled.Corrosion imposes a bigger issue in microreactors because area to volume ratio is high. Degradation of few µm may go unnoticed in conventional vessels. As typical inner dimensions of channels are inthe same order of magnitude, characteristics may be altered significantly.
One of the simplest forms of a microreactoris a 'T' reactor. A 'T' shape is etched into a plate with a depth that may be 40 micrometres and a width of 100 micrometres: the etched path is turned into a tube by sealing a flat plate over the top of the etched groove. The cover plate has three holes that align to the top-left, top-right, and bottom of the 'T' so that fluids can be added and removed. A solution of reagent 'A' is pumped into the top left of the 'T' and solution 'B' is pumped into the top right of the 'T'. If the pumping rate is the same, the components meet at the top of the vertical part of the 'T' and begin to mix and react as they go down the trunk of the 'T'. A solution of product is removed at the base of the 'T'.
1.Microreactors can be used to synthesise material more effectively than current batch techniques allow. The benefits here are primarily enabled by the mass transfer, thermodynamics, and high surface area to volume ratio environment as well as engineering advantages in handling unstable intermediates. Microreactors are applied in combination with photo chemistry, electrosynthesis, multicomponent reactions and polymerization (for example that of butyl acrylate). It can involve liquid-liquid systems but also solid-liquid systems with for example the channel walls coated with a heterogeneous catalyst. Synthesis is also combined with online purification oftheproduct. Following Green Chemistry principles, microreactors can be used to synthesize and purify extremely reactive Organometallic Compounds for ALD and CVD applications, with improved safety in operations and higher purity products. In microreactor studies aKnoevenagel condensation was performed with the channel coated with a zeolite catalyst layer which also serves to remove water generated in the reaction. The same reaction was performed in a microreactor covered by polymer brushes.
2.A Suzuki reaction was examined in another study with a palladium catalyst confinedin a polymer network of polyacrylamide and a triarylphosphine formed by interfacialpolymerization:The combustion of propane was demonstrated to occur at temperatures as low as 300 °C in a microchannel setup filled up with an aluminum oxidelattice coated with aplatinum / molybdenumcatalyst:
3.Enzymes immobilized on solid supports are increasingly used for greener, more sustainable chemical transformation processes.Microreactors are used to study enzyme-catalyzed ring-opening polymerization of ε-caprolactone to polycaprolactone. A novel microreactordesign developed by Bhangale et al. enabled to perform hetero geneous reactions in continuous mode, in organic media, and at elevated temperatures. Using microreactors, enabled faster polymerization and higher molecular mass compared to using batch reactors. It is evident that similar microreactor based platforms can readily be extended to other enzyme-based systems,for example, high-throughput screening of new enzymes and to precision measurements of new processes where continuous flow mode is preferred. This is the first reported demonstration of a solid supported enzyme-catalyzed polymerization reaction in continuous mode.
Microreactors can also enable experiments to be performed at a far lower scale and far higher experimental rates than currently possible in batch production, while not collecting the physical experimental output. The benefits here are primarily derived from the low operating scale, and the integration of the required sensor technologies to allow high quality understanding of an experiment. The integration of the required synthesis, purification and analytical capabilities is impractical when operating outside of a microfluidic context.
Researchers at the Radboud University Nijmegen and Twente University, the Netherlands, have developed a microfluidic high-resolution NMR flow probe. They have shown a model reaction being followed in real-time. The combination of the uncompromised (sub-Hz) resolution and a low sample volume can prove to be a valuable tool for flow chemistry. Infrared spectroscopy Mettler Toledo and Bruker Optics offer dedicated equipment for monitoring, with attenuated total reflectance spectrometry (ATR spectrometry) in microreaction setups. The former has been demonstrated for reaction monitoring. The latter has been successfully used for reaction monitoring and determining dispersion characteristics of a microreactor.
Microreactors, and more generally, micro process engineering, are the subject of worldwide academic research. A prominent recurring conference is IMRET, the International Conference on Microreaction Technology. Microreactors and micro process engineering have also been featured in dedicated sessions of other conferences,such as the Annual Meeting of the American Institute of Chemical Engineers (AIChE), or the International Symposia on Chemical Reaction Engineering (ISCRE). Research is now also
conducted at various academic institutions around the world, e.g. at the Massachusetts Institute of Technology (MIT) in Cambridge/MA, University of Illinois Urbana-Champaign, Oregon State University in Corvallis/OR, at University of California,
Berkeley in Berkeley/CA in the United States, at the EPFL in Lausanne, Switzerland, at Eindhoven University of Technology in Eindhoven, at Radboud University Nijmegen in Nijmegen, Netherlands and at the LIPHT  of Université de Strasbourg in Strasbourg and  of the University of Lyon, CPE Lyon, France.
Depending on the application focus, there are various hardware suppliers and commercial development entities to service the evolving market. One view to technically segment market, offering and market clearing stems from the scientific and technological objective of market agents:
a. Ready to Run (turnkey) systems are being used where the application environment stands to benefit from new chemical synthesis schemes, enhanced investigational throughput of up to approximately 10 - 100 experiments per day (depends on reaction time) and reaction subsystem, and actual synthesis conduct at scales ranging from 10 milligrams per experiment to triple digit tons per year (continuous operation of a reactor battery).
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