Intermediate micromixing

Most models for "intermediate micromixing" can be compared by means of their "segregation" function s(A,a). Here are some examples taken from the literature ... 

Intermediate micromixing parameters have been shown to... 

Figure 5.15. Two examples of age-based micromixing models. In the top example, it is assumed that fluid particles remain segregated until the latest possible age. In the bottom example, the fluid particles mix at the earliest possible age. Numerous intermediate mixing schemes are possible, which would result in different predictions for micromixing-sensitive reactions. 

Finally, we note that Dudukovic has shown that, for isothermal reactors, the degree of micromixing can significantly influence the appearance of multiple steady-state operating conditions, this being particularly likely for kinetic mechanisms which display maximum reaction rates at intermediate concentration levels [38, 39]. 

A Si micromixer was constmcted with seven vertical pillars (10 pm dia.) arranged perpendicular to the flow in a staggered fashion within a 450-pL mixing chamber, as shown in Figure 3.42. Turbulent mixing of sodium azide and horse heart myoglobin was achieved in 20 ps. This provided the fast mixing essential for a downstream freeze-quench procedure to trap metastable intermediates [472]. 

Real flows in continuous polymerization vessels are always intermediate between the completely micromixed and completely segregated conditions. Most studies of CSTR reactions assume one or the other type. Despite this artificiality, there have been some successes in modelling actual polymerizations. 

As mentioned before, quantitative measures for the degree of micromixing intermediate to the extremes of maximum mixedness and complete segregation are difficult to estabhsh on an a priori basis. Thus, aside from the demonstration given here to demonstrate the potential effects of micromixing on reactions of differing kinetic order, a more quantitative treatment must await the development of various models that can describe the mixing process. 

As reported by Santos and Metzger [50], Ziegler-Natta polymerization can be carried out in a microflow system coupled directly to the ESI source of a Q-TOF mass spectrometer (Figure 14.12). In the first micromixer, catalyst (Cp2ZrCl2-MAO) and monomer solutions are mixed continuously to initiate the polymerization. The polymerization occurs in the microtube reactor. The solution thus obtained is introduced into the second micromixer M2, where the polymerization is quenched by acetonitrile. The quenched solution is fed directly into the ESI source. The transient cationic species can be characterized by mass spectrometry. This is the first case where an alkyl zirconium cation intermediate in the homogeneous Ziegler-Natta polymerization of ethylene is detected directly. 

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