Micro-segregation

Many mixing models which utilize the simplified concepts of micro-mixing and segregation have been introduced. Most notable of these are the two-environment models of Chen and Fan (19), Kearns and Manning (20), and others (21, 22), and the dispersion models of Spielman and Levenspiel (23), and Kattan and Adler (24). 

Figure L Growth characteristics for seed polymer in CFSTR environments (a) growth characteristics for polymer chains in a micro-mixed environment (b) growth characteristics for polymer chains in a segregated environment...

Figure 7. Theoretical polymer distributions, based on kinetic description of Tan-lak (14) for micro-mixed and totally segregated CFSTRS with polymer feed (CFSTRS CMO = 0.5M FT = O.OIM 6 = 20.0 min XM = 0.70)...

Segregated CFSTR Micro-mixed CFSTR With Dead Polymer Fraction D D /D D D /D... 

Calculated Molecular Weight Distributions. The calculated weight fraction distributions for the micro-mixed, segregated, and micro-mixed reactor with dead-polymer models for Runs 2, 5,... 

The effects of micro-mixing on the molecular weight distribution are much more pronounced than those of segregation. According to Patterson (33) only a small increase in micro-mixing over that of total segregation will yield a pol)nner distribution very similar to that of micro-mixed reactor. 

In spite of visual indications of at least partial segregation, the concept of micro-mixing proved to be most useful in modeling the laboratory reactor. 

Suppose is a function of a alone and that neither dSt Ajda nor d Alda change sign over the range of concentrations encountered in the reactor. Then, for a system having a fixed residence time distribution. Equations (15.48) and (15.49) provide absolute bounds on the conversion of component A, the conversion in a real system necessarily falling within the bounds. If d S A/dc > 0, conversion is maximized by maximum mixedness and minimized by complete segregation. If d 0i A/da < 0, the converse is true. If cf- A/da = 0, micro-mixing has no effect on conversion. 

The irradiation (or ion bombardment) of solid solutions, where a scavenger can be present, should also be explored further. Here it will be important to ensure that the solids are indeed solutions before conclusions can be safely drawn. It is curious to note that the yields observed in frozen solutions are in several cases very similar to the yields in the pure crystalline solutes. This suggests the possibility that the frozen targets had segregated, and that the solute was in fact present as micro crystals. (If this is the case, it may well be that a new method can be developed on this basis for making phase studies at high dilution.)... 

In [53], segregated catalyst and polymer particles act as micro reactors where the polymerization process takes place. Each particle is an individual reactor with its own energy and material balance. During polymerization, the catalyst particles undergo a change in volume by a factor of 10 -10, thereby generating the corresponding polymer particles. The particle size distributions of catalyst and polymer are the same. 

Tschierske, C. (1998) Non-conventional liquid crystals - the importance of micro-segregation for self-organisation. Journal of Materials Chemistry, 8,1485-1508. 

Halpem M., Jia C. and Shapiro L. (1998). Segregated pathways in the vomeronasal system. Micros Res Tech 41, 519-529. 

DICTRA (Diffusion simulation software) (Borgenstam et al. 2000). DICTRA (coupled with Thermo-Calc) is a software for the simulation of diffusion in multi-component alloy systems (processes which can be simulated are for example the homogenization of alloys, micro-segregation during solidification, carburizing, and decarburizing, of steels, etc.). 

In this model, and also in the next one, the interaction is no longer homogeneously distributed. As a consequence, the segregation is not pure microscale segregation, but a combination of micro- and macroscale segregation. 

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