Microreactor Or Micro Structured Reactor Or Microchannel Reactor

Brief Instruction of Microreactor Technology

Microreactor technology is a new technology mainly through process intensification to achieve green chemical synthesis, especially for the preparation of fine chemicals such as chemicals, pesticides, medicines, micro-materials and nano-materials. Microreactor technology is a micro-chemical technology that emerged in the 1990s. It is not a simple reduction of the existing reactor, but one of the new comprehensive systems that integrates various element such as material technology, micro-fabrication technology, sensor technology and control technology. Microreactor is a new type of miniaturized continuous flow pipeline reactor with micron-scale structural channels as the core to realize reaction, mixing, separation and other function. The "micro" of a microchannel reactor does not refer to the small size of the microreactor device or the small product yield, but rather to mean that the fluidic channels are on the micrometer or millimeter scale. The microreaction process refers to the reaction system process constructed by these devices. The microreaction system is also called the micro-chemical system, which is a micro-system for the chemical process. The design and development of the microchemical system is based on the idea of ​​miniaturizing chemical equipment without reducing the processing capacity of the equipment, by strengthening the rate and controllability of the flow, mixing and transfer processes in the system, the completion time of reaction and separation is shortened, reduce the stagnation of materials in the process, reduce the formation of by-products, for realize the green, safe and efficient chemical process. Microreactor technology has become one of the important development directions in the field of chemical process intensification widely used in the fields of medicine, pesticides, fine chemical products and intermediate synthesis.

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. Microreactors involve microfabricated structures to allow flow chemistry to be performed at micron scale. 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.

Technical Characteristics of Microreactor

• Large specific surface area and good heat transfer------Due to the microstructure inside the micro reactor, its specific surface area is very large, which can be hundreds or even thousands of times of the specific surface area of the agitating vessel.Therefore, it has good heat and mass transfer capacity. The heat exchange coefficient is usually at least 25kW m − 2K − 1, which can realize the instantaneous uniform mixing and efficient heat transfer of materials.
• High mass transfer efficiency and fast reaction speed-------The microchannel reactor can contain many microchannels, and the gas and liquid fluids in the microchannels can pass flow with 10-4 to 10-6m/s, The mixing reaction process of fluids can realize the instantaneous and uniform mixing reaction of materials, and make the reaction of ordinary chemical reactions Time is reduced from hours - tens of hours to seconds - minutes and high throughput is achieved.
• Accurate control of reaction temperature and time to improve yield-------The excellent heat transfer characteristics of the microreactor allow the reaction temperature to be precisely controlled within a certain range, which is of great significance for some reactions involving intermediate products and thermally unstable products in fine chemicals. The reactants flow continuously in the microchannel reactor The residence time of the raw materials under the reaction conditions can be precisely controlled. Once the optimal reaction time is reached, it is immediately passed to the next step or the reaction is terminated, which can effectively eliminate the by-products generated due to the long reaction time and improve the yield.
• Less fluid holding capacity, safer and more environmentally friendly for continuous production-------Due to the good heat transfer of the microreactor, the reaction temperature will not accumulate excessively and is accurately controlled within a certain range, which can reduce the potential risk of explosion. Microreactors are normally operated continuously, the amount of reactants in the microreactor is very small. This allows the subsequent processing of unstable intermediates and avoids typical batch workup delays. The residence time of unstable reaction medium is very short and will not decay, even if the reaction intermediate or final product is toxic and harmful substances, Also because the amount of products produced per unit time is small and the residence time is short, the harmfulness of safety accidents is reduced to a considerable extent. Many vigorous exothermic reactions that cannot be realized in batch process but can be realized in microreactors, even key reactions such as nitrification can be carried out safely at high temperature.
• Although a microreactor can synthesize chemicals only in small quantities, scale-up to industrial volumes is simply a process of multiplying the number of microreactors. In contrast, batch processes too often perform well in lab but fail at batch pilot plant level.
• Microreactor technology can also flexibly increase or decrease the number of micro-reactor modules in the system according to market changes, so as to realize on-time, on-site and bespoke production.
• Micro-processing technology can be used to integrate micro-mixing, micro-reaction, micro-heat exchange, micro-separation, micro-analysis and other unit operations and matching micro-sensors, micro-valves and other devices into a platform or even a control chip to realize platformization Or chip-based multi-functional operation, so as to achieve real-time monitoring and intelligent control of the micro-reaction system, improve the reaction speed, and save the reaction cost at the same time.

Type of Reactions Suitable for Microreactors

So far, the multi-purpose microreactor is still limited to homogeneous reaction, and to a certain extent, it is limited to gas-liquid phase and liquid-liquid phase reaction. According to statistics, about 30% of fine chemical reactions can be carried out in microreactors to improve the yield, selectivity or safety. The advantages of microreactor are mainly reflected in the following types of reactions: 

• Violently exothermic reactions-------For violently exothermic reactions, conventional reactor process generally adopt the method of gradual dropwise addition or batch addition. Even so, there will still be local overheating and a certain amount of by-products will be produced. Due to the good heat transfer characteristics of the microreactor, the reaction temperature can be precisely controlled, local overheating can be eliminated, and can be significantly improved product yield and selectivity. 
• Reaction with unstable reactants or products------Some reactants or products are very unstable and will decompose and reduce the yield of products after a residence time. The microreactor system is a continuous flow system, and the residence time of the reaction can be accurately controlled. Therefore, the decomposition due to the instability of reactants or products in the conventional reactor can be avoided. 
• Rapid reaction with strict requirements on the ratio of reactants------Some reactions have strict requirements on the ratio of reactants, and an excess of one of the reactants will cause side reactions, such as reactions requiring mono-substitution, there will be two-substituted and trisubstituted products are formed. The microreactor system can achieve homogeneous mixing instantaneously, which avoids the problem of local excess of each reactant and minimizes by-products. 
• Dangerous chemical reactions and high temperature & high pressure reactions------Some chemical reactions that are easily out of control, once out of control, will cause the reaction temperature and pressure to increase sharply, the reactants will punching even explosion. Because the reaction heat can be quickly exported, and the microreactor is a continuous flow reaction, the amount of online chemicals is very small, so the safety of the reaction is guaranteed in microreactors. 

Of course, not all reactions are suitable for microreactor systems, such as very slow liquid-solid reactions, reactions without exothermic and endothermic phenomena and traditional processes have already high selectivity and yield reactions.

Application Pitfalls of Microreactors

• 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 microreactors[8]being widely accepted as a beneficial alternative to batch reactors.
• 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). 
• 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. 
• Although microreactor sysytem can scale-up to industrial volumes by multiplying the number of microreactors and can reduce the cost, but the processing capacity of microreaction system is also greatly limited. During the number of microreactors is greatly increased, the complexity of microreaction monitoring and control system is also greatly increased, and the investment and operation cost for actual production are also greatly increased.

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