Limitations and application scope of microchannel reactors

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Microchannel reactors have many advantages, but at the same time we must also be aware of their limitations. Such limitations can stem from the device itself, from factors related to the manufacturer, or from certain inherent ways of thinking in process development. In actual production, it is precisely because of these constraints that a considerable number of manufacturing units are unable to quickly transform this technology into an actual manufacturing process after introducing it. Here we will discuss the application scope of microchannel reactors. Firstly, strictly speaking, it is currently difficult to determine which reactions are applicable to microchannel reactors, as the characteristics of each reaction vary, and there are also a great many types of microchannel reactor devices. However, it is generally believed that 20-30% of the existing synthesis reactions can be upgraded using microchannel reactors. At the same time, by using microchannel reactors, we can implement approximately 20%-30% of the process flows that were previously considered dangerous. In other words, it appears that currently around 30–50% of chemical processes can be upgraded using microchannel reactors. In terms of structural characteristics, the current applications and limitations of microchannel reactors are as follows: 1. The reaction itself occurs at a high speed, but due to constraints related to mass transfer, the overall reaction rate remains low. Such reactions mainly involve liquid-liquid multiphase reactions, as well as physical processes such as liquid-liquid extraction. The characteristic of this process is that the reaction itself occurs rapidly, but the overall reaction rate is low due to the diffusion of the substrate within the liquid phase. In traditional reaction vessels, stirrers are generally used for mixing during reactions; this approach is inefficient as it fails to ensure thorough mixing between the two liquid phases, resulting in low reaction efficiency. In microchannel reactors, the reduced diffusion scale due to the small size of the channels enables such reactions to proceed rapidly. 2. Reactions that are fast in themselves, but involve intense reactions, strong exothermic effects, and result in products that are prone to degradation. Such reactions include nitration, diazotization, as well as certain hydrolysis and alkylation reactions. The nitration and diazotization reactions themselves are very rapid and intense, but in actual industrial operations, the reaction time is often measured in hours. This is because the heat transfer capacity of the reactor is limited; to prevent the temperature inside the system from rising uncontrollably, the reagents need to be added gradually. It can be said that the reaction speed is entirely determined by the heat transfer capacity. If a microchannel reactor with strong heat transfer capacity is used, reagents can be introduced quickly and the reaction can proceed smoothly. It can be said that this type of reaction has the greatest potential for industrialization and should be given priority as a process to consider. 3. Reactions that require strict control of the flow pattern inside the reactor. Such reactions mainly involve the synthesis of nanoparticles; these processes have been described previously. They utilize the regular flow patterns within microchannels to produce materials with a narrow particle distribution, thereby increasing the added value of the products. For such reactions, the yield of products is generally low, but the added value is high; sometimes a combination of several experimental setups can serve as a production facility, and they also have broad application prospects. 4. Some gas-liquid reactions can, in principle, be carried out using microchannel reactors; however, no optimal design for such reactors has yet been developed. Hydrogenation is the most prominent example here. There are many types of hydrogenation reactions, and while some of them have high reaction rates, they are limited by the diffusion of hydrogen into the liquid phase, which results in a lower overall reaction rate. Under such conditions, it is of course possible to utilize the mixing properties of microchannel reactors for reactions, similar to type 1 reactions; however, what is emphasized here is the gas-liquid mass transfer process. However, gas-liquid processes have their particularities, mainly in terms of fluid distribution and control, which results in the absence of suitable scaled-up gas-liquid microchannel reactors. Therefore, experimental research in this area is very active; in industrial applications, it is not feasible unless the production volume is low enough to allow the use of experimental equipment. 5. Microchannel reactors are not used for reactions containing solids whose particle size is 10% or more of the characteristic size of the microchannels and whose solid content exceeds 5%. Due to the tendency of microchannel reactors to clog, most reactors containing heterogeneous catalysts are not suitable for use with microchannel reactors. Furthermore, reaction systems that tend to produce larger particles are also unsuitable for use in microchannel reactors. Since most reactions are catalytic, this imposes significant limitations on the application of microreactors. For heterogeneous systems, catalysts are limited to a few types such as Pd/C. The current trend in scientific research is focused on how to fix catalysts in reactors. There are many solutions, but there are even more problems in implementing them. Industrial application is not possible in the short term; therefore, special caution must be exercised when using microchannel reactors for heterogeneous catalytic systems. 6. The amplification effect in research and development. Theoretically, microchannel reactors use a quantity amplification approach, so no amplification effect occurs. However, this is not the case in practice, as simply increasing the quantity leads to extremely high equipment and control costs. This means that during the actual amplification process, it may be necessary to adjust the channel dimensions and the arrangement of the reactors; such adjustments are very likely to cause changes in the specific surface area, transfer characteristic length, and residence time distribution of the microchannel reactor, leading to discrepancies between the actual production process and the pilot-scale experiments. 7. Issues related to the suitability of the process flow. Due to flux issues, microreactors face problems in selecting pumps and integrating with other subsequent processes. For example, if the flux in a microchannel reactor is low, it may prevent the selection of appropriate equipment for subsequent processes such as distillation. As a result, the subsequent process has to be made intermittent, or buffer units need to be added between the two processes. 8. The corrosion problem in microchannel reactors cannot be ignored; the requirements for corrosion resistance are higher than those for conventional reactors. We know that the channel size of microchannel reactors is extremely small. The corrosion standards for conventional vessels are still too low for microchannel reactors. For example, the corrosion allowance in our conventional containers can take a relatively large value of 0.1 mm/a. As long as the structural strength is ensured, it can be used normally. But the same is true inside microchannel reactors; the channels are already small, and if corrosion is severe, the characteristic dimensions of these channels will change, and internal leakage may even occur. Therefore, microchannel reactors have more stringent requirements regarding corrosion, especially for metal reactors; corrosion testing must be taken into consideration before they are put into operation. However, at present, in terms of process development, many organizations still apply the corrosion control standards for traditional reactors to microreactors, which is not sufficient. The existing microchannel reactor units have not been in operation for long, so corrosion issues have not yet become apparent. However, if there are issues with the material selection for reactors, some industrial facilities may encounter problems in the coming years.