Macroscale reactor

Microreactors provide a safe means by which reactions, including multistage schemes, can be undertaken where, otherwise, products involving unstable intermediates may be formed. This is exemplified by Fortt who showed that for a serial diazonium salt formation and chlorination reaction performed in a microreactor under hydrodynamic pumping, significant yield enhancements (15-20%) could be observed and attributed them to enhanced heat and mass transfer [77]. This demonstrates the advantage of microreactor-based synthesis where diazonium salts are sensitive to electromagnetic radiation and static electricity, which in turn can lead to rapid decomposition. Microreactors facilitate the ability to achieve continuous-flow synthesis, which is often not possible with conventional macroscale reactors and batch production. 

In their pioneering work, Jensen et al. demonstrated that photochemical transformation can be carried out in a microfabricated reactor [37]. The photomicroreactor had a single serpentine-shaped microchannel (having a width of 500 pm and a depth of 250 or 500 pm, and etched on a silicon chip) covered by a transparent window (Pyrex or quartz) (Scheme 4.25). A miniature UV light source and an online UV analysis probe were integrated to the device. Jensen et al. studied the radical photopinacolization of benzophenone in isopropanol. Substantial conversion of benzophenone was observed for a 0.5 M benzophenone solution in this microflow system. Such a high concentration of benzophenone would present a challenge in macroscale reactors. This microreaction device provided an opportunity for fast process optimization by online analysis of the reaction mixture. 

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. 

These characteristics make the cross-flow microreactor a useful experimental tool for investigating kinetics and optimizing reaction conditions. Experiments regarding CO oxidation confirmed the ability of the micro-packed bed reactor to deliver valuable information about kinetics and mechanism, which compares well with data previously obtained in macroscale reactors. 

For values of this Peclet number well below 1, as encountered in microreactors, a narrow molecular weight distribution can be achieved, while higher values, like those encountered in macroscale reactors, induce a drastic increase in the polydispersity index (Fig. 6.32) [37,48]. Therefore, microreactors can lead to better control over bulk or semi-dilute polymerization processes. 

The axial diffusion terms can be dropped for macroscale reactors for the reasons given in Section 8.2.4. Thus the equation to be solved in most of this chapter is... 

To the limited extent that mesoscale and smaller reactors are used for production, the obvious approach is to scale in parallel. More conventional scaleups, that is, increasing reactor dimensions, could be needed when the small reactor is used to find the perfect molecule by means of combinatorial chemistry. With possible exceptions where the ultimate in mixing and heat transfer is required, scaleups to conventional macroscale reactors should be possible. 

Length scale Macroscale (reactor scale) Mesoscale (particle scale)... 

Nu and Sh correlations developed for laminar flow [19] are often used to obtain transverse transport in both micro- and macroscale reactors [20]. Since the older correlations were developed using simplifying assumptions, they are not applicable for highly exothermic reacting flows and new correlations have been developed since [21, 22]. 

As discussed above, the stabdity condition is expected to reach extremum in sufficiendy large space (e.g., cross section of a fluidized bed) instead of local cell. The energy to sustain mesoscale structures in a fluidized bed comes largely from the mean relative motion between gas and particles on the macroscale. Furthermore, the dynamic evolution of mesoscale structure and its energy transfer is subject to both macroscale operating conditions and the conservation laws in microscale computational cells. As a result, a two-step scheme was proposed to fulfill the coupHng between EMMS and hydrodynamic conservation equations, called EMMS/matrix (Lu et al, 2009 Wang and Li, 2007). At the macroscale (reactor), the bi-objective optimization method in terms of min was first used to resolve the mesoscale parameters, say, dc and gc. These mesoscale parameters were then incorporated... 

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