Microreactor
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A microreactor or microstructured 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 compared with batch reactors often used otherwise.
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[edit] History
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[citation needed] in Germany, using mechanical micromachining techniques that were a spinoff from the manufacture of separation nozzles for uranium enrichment[citation needed]. 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. Similar research is now also conducted at various academic insitutes around the world, e.g. at the Massachusetts Institute of Technology (MIT) in Cambridge/MA and at Oregon State University in Corvallis/OR in the United States, at the EPFL in Lausanne, Switzerland, or at Eindhoven University of Technology in Eindhoven, Netherlands.
[edit] Benefits
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:
- Microreactors typically have heat exchange coefficients of at least 1MW/(m³ K) up to 500 MW/(m³ K) comparing to a few kilowatt as standard glassware (1l Flask approx 10 kW/(m³ K)). 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.
- 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 allows often better selectivities.
- Continuous operation and mixing causes a very different concentration profile when compared with a batch process. In a batch, reagent A is filled in and reagent B is slowly added. Thus, B encounters initially a high excess of A. In a microreactor, A and B are mixed nearly instantly and B will be NOT exposed to a large excess of A. This may be an advantage or disadvantage depending on the reaction mechanism, but it is important to be aware of such different concentration profiles.
Typically, reactions performing very well in a microreactor encounter many problems in vessels, especially when scaling up. Microreactors are better for faster and more exothermic reactions. Unstable intermediates are also an argument to consider the application of a microreactor. An attempt at visualizing the benefits of microreactors is illustrated with application examples at Microreactor Chemistry.
[edit] Problems
Although there have been reactors made for handling particles, microreactors generally do not tolerate particles well, often clogging. Clogging has been identified by a number of researchers as the biggest hurdle for microreactors being widely accepted as a beneficial alternative to batch reactors. Gas evolved may also shorten the residence time of reagents by pushing out material much faster than anticipated.
[edit] T reactors
One of the simplest forms of a microreactor is 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'.
[edit] Applications
[edit] Synthesis
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.
[edit] Analysis
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.
[edit] Academic research
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).
[edit] Market structure
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 30 - 100 experiments per day and reaction subsystem, and actual synthesis conduct at scales ranging from 10 milligrams to several tens of megagrams per year.
(b) Modular (open) systems are serving the niche for investigations on continuous process engineering lay-outs, where a measurable process advantage over the use of standardized equipment is anticipated by chemical engineers. Multiple process lay-outs can be rapidly assembled and chemical process results obtained on a scale ranging from several grams per experiment up to approximately 100 kg at a moderate number of experiments per day (3-15). A secondary transfer of engineering findings in the context of a plant engineering exercise (scale-out) then provides target capacity of typically single product dedicated plants. This mimicks the success of engineering contractors for the petro-chemical process industry.
(c) Manufacturer of microstructured components are commercial development partners to scientists in search of novel contacting patterns or patterns to spatially arrange matter in a manner specific to the chemistry or physics system under investigation. Such development partners typically excel in the set-up of comprehensive investigation and supply schemes to model a desired contacting pattern or spatial arrangement of matter. To do so they predominantly offer information from proprietary integrated modelling systems that combine computational fluid dynamics with thermokinetic modelling or advanced enthropical modelling where materials are concerned. Moreover, as a rule, such development partners establish the overall application analytics to the point where the critical initial hypothesis can be validated and further confined.
