Macromixing time

For a full understanding of the nature of the reactor, the flow and mixing status in an SCISR are studied first [16, 31]. 

The comparative results indicated by the data given in Fig. 10.3 are completely opposite to those of Brauer [14] who reported much shorter mixing times in the stagnation jet mixer than in STR. Although the stagnation jet mixer lacks some of the key elements needed for the reactor, it has a flow configuration very similar to that of the SCISR. This phenomenon is unexpected, but the multiply repeated experiments verify that the data are true. 

The results of a theoretical analysis indicate that the longer macromixing time in the SCISR results from its special flow configuration, because in the SCISR there are flow regions without mixing. 

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Much of the literature around mixing times has been developed around the mixing of two liquids in agitated stirred tanks. The macromixing time t can be defined as the time for the concentration to settle within, say, 2 percent of its final value (98 percent homogeneity). With a standard turbine in a baffled tank and Re (= nD2 p/ t) > 5000,... 

When local micromixing is slow compared to the reaction time scale and the macromixing time scale is smaller than the process time scale, the performance of a reactive flow process is controlled only by the micromixing. In such cases, though there is no macroscopic segregation, reactants are not mixed on a molecular scale (see the right bottom case of Fig. 5.5). Several micromixing models have been developed to simulate such reactive flow processes. Some of the widely used models are ... 

As regards scale-up it must therefore be noted that mixing efficiency in small devices is more favorable than in large ones. This must also be taken into account in baffled tanks, although here fx.y.z/w is constant, but the circulation and macromixing times are longer This relationship wiU be considered further in connection with micro-mixing and chemical reactions (see [462] and Section 1.4.6.4). 

The uncertainty in Eq. (6.5) arises because the induced flow is about twice the direct discharge from the turbine, and it is difficult to measure or predict the exact value. Since the blending time is about 36/ (Eq. 6.1), the macromixing time is roughly one-fourth the blending time, or about 2-10 seconds for a large tank. 

Exp. Batch Size (mL) Agitator Speed (rpm) Reynolds Flow (W/kg) (W/kg) Macromixing Time (s) Micromixing Time (s)... 

Macromixing time is a parameter characterizing the time required for the distribution of a compound (added instantly) throughout the entire volume of the tank. It is calculated as the time required to reduce the maximum difference of local concentrations of the added substance to approximately 1% of its final average value (under batch mixing conditions). 

Note that experiments 5 and 6 in Table 27.2 can be deemed to be internal scale-up experiments experiment 5 was designed such that the maCTomixing time was comparable to the macromixing time of experiment 1 experiment 6 was designed to reproduce the average power/mass in experiment 2. The corresponding resnlts are depicted in Table 27.3. 

From the data analysis one can see that neither macromixing time nor power/mass are good candidates as scale-up factors in this study. 

The first term describes the reduction of concentration fluctuations which are mostly reduced after approximately 10 up to 20 stirrer revolutions in the case of a fully turbulent flow with > 10. The second term depends on the limiting step in the microscale eddies and the degree of segregation. Many experimental results have shown that the minimrtm macromixing time (first term) can be... 

According to Schafer (Schafer 2001) the macromixing time can be described by the last equation with C acro for a special marine-type impeller, Cmacro = 6.5 for au inclined blade impeller, and = 8.5 for a six-blade... 

The macromixing time is mainly dependent on the mean specific power input and the vessel diameter D and increases with D for = const. 

Asa rule the micromixing time is short in comparison to the macromixing time however, in fluid boundary layers close to solid surfaces (walls, heat exchanger, tube bundles, etc.), the local specific power input e can be very low with the consequence of long micromixing times. 

Notice the sensitivity to the heat transfer cooling time tjj. Solid lines perfect mixing. Dashed lines imperfect mixing (macromixing time tj4 = 0.1, micromixing time t = 0.01). J = 200 K ... 

The thermal runaway can be caused by the formation of local hot spots. The causes for local hot spots are, for example, feed impurity, excess initiator in feed, poor feed distribution, inadequate mixing, mechanical friction, poor reactor temperature control system, feed temperature disturbance, and so forth [9]. Imperfect mixing in the polyethylene reactor has been considered as a primary cause for the runaway reaction phenomena. Short reactor residence time and comparable macromixing times may lead to imperfect... 

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