Mixing segregation intensity

Chemical reactions will take place only when the reactant molecules are in intimate contact. In some cases, especially with very fast reactions or viscous liquids, segregation of the reactants can exist, which make the reaction rates and selectivities dependent on the mixing intensity. In chemical reactor engineering, the assumption is usually made that only mean concentrations need be considered. In reality, concentration values fluctuate about a mean, and in some cases these fluctuations must be considered in detail. This field is very complex and is still the subject of much research. This example serves only to introduce these concepts and to show how simulations can be made for certain simple situations. 

If the scalar mixing time is constant, then the classical exponential-decay result is recovered for the intensity of segregation defined by (3.23), p. 66 ... 

The mixing efficiency was characterized by a dimensionless parameter, called the intensity of segregation, for the six scenarios [68], This parameter is based on the distribution of the mass fractions of the species. The results given in Table 1.4 show the importance of the z-component of the flow velocity to induce penetration of species and thus to promote mixing. 

Despite the utility of the physical methods described above, characterization of entities on the nanometer scale is still a problem. Well-mixed nanoparticles are not necessarily completely homogeneous, and one metal may preferentially segregate to the nanoparticle surface. Subtle differences in surface stoichiometries are presently extremely difficult to quantitatively evaluate with spectroscopy, even when the metal of interest has an intense surface plasmon. 

Fig. 2. Central slice of a rotating tube 75% full, with an aspect ratio of 3, comprising mustard seeds (MR active) and polystyrene beads (MR inactive). The mustard seeds are identified as the light grey pixels. The polystyrene beads give no signal intensity. In images (a)-(c) the mustard seeds and polystyrene beads are seen to segregate rapidly but retain a non-mixing core region. Reproduced with permission from Metcalfe et at. (1999).

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