Microreactors short residence time

This microreactor system thus allows fast mixing, good temperature control and changing process parameters at short residence times - the overall time to pass all... 

This generates opportunities to use new reaction pathways not feasible in conventional macroscale reactors. For example, Sadykov et al. utilize rapid thermal quenching in conjunction with short residence times in a microreactor to suppress undesired side reactions in propylene production by the oxidative dehydrogenation of propane. 

The high selectivity to styrene during PS pyrolysis does not appear to depend on the type of reactor used. Thus, Sinn et a/.33 reported styrene yields of 79.8 and 71.6% when the pyrolysis was carried out in a sand fluidized bed reactor at 640 and 740 °C, respectively. Styrene yields of up to 92% have recently been reported by Lovett et al.30 during PS pyrolysis in a microreactor at high temperature (965 °C) and very short residence times (500 ms). 

Due to short residence times inside the micromixer almost no heat was released there. In the residence time tube, temperatures up to 150 °C have been observed. In these experiments, we were able to show that it is possible to finish the first reaction step on the continuous microreactor laboratory-scale plant in less than 60 s. The same reaction step in the cooled batch vessel of the production plant took about 4 h. With these results, we came to the conclusion that it should be possible to realize the first exothermic step of the process in a microreactor. After finishing this step continuously in a closed system, the reaction solution could be transferred into the existing batch vessel and be heated there to finish the second reaction step. As the time for the first step is reduced from several hours to a few minutes for the same amount of product, it should be possible nearly to double the capacity just by installing a microreactor right before the existing batch vessel to mix the first two educts. 

Burkle-Vitzthum, V., Moulis, F., Zhang, J., Commenge, J.-M., Schaer, E., and Marquaire, P.-M. (2014) Annular flow microreactor an efficient tool for kinetic studies in gas phase at very short residence times. Chem. Eng. Res. Des. doi 10.1016/j. cherd.2014.10.003. 

Because of the small reactor volume, accumulation of the labile trifluoroacetox-ydimethylsulfonium salt (3) and alkoxydimethylsulfonium salt (5) is minimized in a microreactor. Further, because of the short residence times, which can be applied in the microreactor, the exothermic Pummerer rearrangement of the unstable intermediate 3 is limited. HPLC pumps and the Ehrfeld microreactor system were employed in the system, allowing for a real continuous process. Operation at much higher temperatures was possible, up to 30 °C [49]. 

This microreactor system thus allows fast mixing, good temperature control, and changing process parameters at short residence times - the overall time to pass all reactors is between 8 and 11s [71,73], The timescale between end and start of a new operation is 0.01 s in order to avoid significant decomposition. Thereby, conversions and yields were determined for primary, secondary, cyclic, and benzylic alcohols in the temperature range from —20 to 20 °C, which were equal to or better than those of lower temperature batch reactions. The oxidation of cyclohexanol was run for 3 h at 20 °C and a stable process in terms of conversion and selectivity was observed. 

In this manner, a flow reaction can have a short residence time, allowing the oxiranyllithium species 2, which is an unstable reactive species with a short lifetime, to react with methyl iodide before the species decomposes. In contrast, a batch reaction cannot be completed in a short reaction time due to the time for its conduction needed for the reaction. A batch reaction on an extremely small scale may be completed within a reaction time of 1 min or less, but a batch reaction on a large scale cannot be completed in a short time because the dropping of s-BuLi to a solution of 1 requires an additional time. Fast dropping in a batch reactor may cause rapid increase in the temperature which accelerates the decomposition of 2. Thus, flow microreactor synthesis can offer crucial advantages. 

However, this is difficult to get fast mixing in a conventional batch reactor and also in a large flow reactor, raising the need for a flow microreactor. You may remember that the generation and the reaction of oxiranyllithium species described in Chap. 2 would fail if the residence time cannot be controlled in the order of seconds. This is why a flow microreactor equipped with a micromixer is needed for this reaction. This chapter focuses on extremely fast mixing in a micromixer, which is needed to set a short residence time. 

Abstract A system integrating multiple flow microreactors that are connected to one another allows multiple reactions to be carried out continuously, or allows space integration of reactions. Flow microreactors that can set short residence times enable integration of reactions involving unstable short-lived reactive species as intermediates. Space integration of not only reactions of the same type but also of reactions of different types can be performed. 

Polymerization of vinyl ethers initiated by an A -acyUminium ion pool has demonstrated that the molecular weight and the molecular weight distribution can be controlled by using the flow microreactor system (Fig. 10.5) [4, 5]. An iV-acyh-minium ion generated and accumulated by the cation pool method was used as an initiator, which was mixed with a vinyl ether at high speed using a micromixer. The polymerization proceeded in a flow microreactor and was complete within a short residence time. An amine was then introduced using a micromixer to terminate the polymerization. Thus, this polymerization can be called flash polymerization. 

Axial Molecular Diffusion. Molecular diffusion can be an important cause of a spread in residence time, particularly at low velocities (low space velocities and short bed lengths) and with fluids of high diffusivity. Therefore, it is important in microreactors with gas flow. 

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