Macromixing time, measurement

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. 

A further important conclusion is that for a given C-curve or residence time distribution obtained from tracer studies, a unique value of the conversion in a chemical reaction is not necessarily obtainable unless the reaction is first order. Tracer measurements can certainly tell us about departures from good macromixing. However, tracer measurements cannot give any further information about the extent of micromixing because the tracer stimulus-response is a first-order (linear) process as is a first-order reaction. 

The experimental SCISR is the same as that used for the measurements of macromixing and residence time distribution, as shown in Fig. 10.2, while its major dimensions are shown in Fig. 10.6 and the equipment system scheme is illustrated in Fig. 10.7. 

Mixing times in tanks are usually measured either by tracer techniques or by visual observation. These techniques measure macromixing, i.e. they can be used to measure the time taken to achieve a given degree of homogeneity throughout the whole vessel. They are of no value if local concentrations close to a feed point are required. 

Figure 3.14. Microbial growth in dependence on micromixing and macromixing dimensionless cell mass concentration x as a function of internal recycle ratio r in a recycle reactor at different values of total Bo number (Bo ot as a measure of macromixing) and varying mean residence time t. (Adapted from Dohan and Weinstein, 1973. With permission from Ind. Eng. Chem. Fundam., 12, 64. Copyright American Chemical Society).

The residence time distribution measures features of ideal or nonideal flows associated with the bulk flow patterns or macromixing in a reactor or other process vessel. The term micromixing, as used in this chapter, applies to spatial mixing at the molecular scale that is bounded but not determined uniquely by the residence time distribution. The bounds are extreme conditions known as complete segregation and maximum mixedness. They represent, respectively, the least and most molecular-level mixing that is possible for a given residence time distribution. 

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