Kinetic segregation

Yu.I. Boiko, V. Kaizer, G. Pettsov, A. Frish, Kinetic segregation on pores in the process of sintering a homogeneous solid solution, Powder Metall. Metal Ceram., 32, 401-403 (1993). 

An important parameter characterizing the laminar regime is the intersegment spacing over which the species must diffuse to be able to react. For kinetic control, as opposed to diffusion control of the reaction, this spacing must not exceed a certain scale of segregation given by [53J ... 

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)...

Surface composition. The principle of surface segregation in ideal systems is easy to understand and to derive thermodynamically the equilibrium relations (surface concentration Xg as a function of the bulk concentration Xb at various temperatures) is also very easy (4,8). Even easier is a kinetic description which can also comprise some of the effects of the non-ideality (9). We consider an equilibrium between the surface(s) and the bulk(b) in the exchange like ... 

Stamenkovic V, Schmidt TJ, Ross PN, Markovic NM. 2003. Surface segregation effects in electrocatalysis Kinetics of oxygen reduction reaction on polycrystalline PtsNi alloy surfaces. J Electroanal Chem 554 191 -199. 

The kinetics in terms of the intensities of segregation and the mean concentrations can be shown to be... 

In an emulsion polymerization, the reaction mixture is initially heterogeneous due to the poor solubility of the monomer in the continuous phase. In order for a reaction to take advantage of the desirable Smith-Ewart kinetics [96], the monomer and initiator must be segregated with the initiator preferentially dissolved in the continuous phase and not the monomer phase. Because of the kinetics of an emulsion polymerization, high molecular weight polymer can be produced at high rates. The polymer which results from an emulsion polymerization exists as spherical particles typically smaller than one pm in diameter. However, due to the high solubility of most vinyl monomers in C02, emulsion polymerization in C02 probably will not be a very useful process for commercially important monomers. 

To simplify the treatment for an LFR in this chapter, we consider only isothermal, steady-state operation for cylindrical geometry, and for a simple system (A - products) at constant density. After considering uses of an LFR, we develop the material-balance (or continuity) equation for any kinetics, and then apply it to particular cases of power-law kinetics. Finally, we examine the results in relation to the segregated-flow model (SFM) developed in Chapter 13. 

In this case, the results for /A2 are different, and this is typical for nonlinear processes. The lower result is for case (ii) in which earlier mixing occurs that is, when the CSTR precedes the PFR. Thus, although the kinetics, RTD, and degree of segregation (microfluid) are the same, a different mixing pattern in terms of location of mixing may result in a different performance. 

In addition to these two macromixing reactor models, in this chapter, we also consider two micromixing reactor models for evaluating the performance of a reactor the segregated flow model (SFM), introduced in Chapters 13 to 16, and the maximum-mixedness model (MMM). These latter two models also require knowledge of the kinetics and of the global or macromixing behavior, as reflected in the RTD. 

Comparison of the segregated-flow and maximum-mixedness models, with identical RTD functions, shows that the former gives better performance. This is consistent with the observations of Zwietering (1959), who showed that for power-law kinetics of order n > 1, the segregated-flow model produces the highest conversion. 

The segregation or demixing is a purely kinetic effect and the magnitude depends on the cation mobility and sample thickness, and is not directly related to the thermodynamics of the system. In some specific cases, a material like a spinel may even decompose when placed in a potential gradient, although both potentials are chosen to fall inside the stability field of the spinel phase. This was first observed for Co2Si04 [39]. Formal treatments can be found in references [37] and [38],... 

Dynamics of an Equalisation Basin 560 Dimensionless Kinetics in a Batch Reactor 235 Batch Reactor with Complex Reaction Sequence 240 Single Solute Batch Extraction 442 Mixing and Segregation 394... 

For an alloy droplet, the post-recalescence solidification involves segregated solidification and eutectic solidification. 619 Droplet cooling in the region (1),(2) and (6) can be calculated directly with the above-described heat transfer model. The nucleation temperature (the achievable undercooling) and the solid fraction evolution during recalescence and post-recalescence solidification need to be determined additionally on the basis of the rapid solidification kinetics. 154 156 ... 

H. Berry, Monte Carlo simulations of enzyme reactions in two dimensions Fractal kinetics and spatial segregation. Biophys. J. 83(4), 1891 1901 (2002). 

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