Segregation effects

To evaluate the real behavior of fuels in relation to the segregation effect, the octane numbers of the fuel components can be determined as a function of their distillation intervals In this manner, new characteristics have been defined, the most well-known being the delta R 100 (A7 100) and the Distribution Octane Number (DON). Either term is sometimes called the Front-End Octane Number . 

A. Muehlbauer, A. Muiznieks, J. Virbulis. Analysis of the dopant segregation effects at the floating zone growth of large silicon crystals. J Cryst Growth 750 372, 1997. 

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. 

At any level in the transition region, there will be a balance between the mixing effects attributable to (a) axial dispersion and to (b) the segregating effect which will depend on the difference between the interstitial velocity of the liquid and that interstitial velocity which would be required to produce a bed of the same voidage for particles of that size on their own. On this basis a model may be set up to give the vertical concentration profile of each component in terms of the axial mixing coefficients for the large and the small particles. 

Segregation effects become possible and molecular weight distributions are then broadened [62]. 

In systems where different pigments are combined (e.g., white and colored pigments), segregation effects can occur which change the optical appearance (flooding... 

We have studied above a model for the surface reaction A + 5B2 -> 0 on a disordered surface. For the case when the density of active sites S is smaller than the kinetically defined percolation threshold So, a system has no reactive state, the production rate is zero and all sites are covered by A or B particles. This is quite understandable because the active sites form finite clusters which can be completely covered by one-kind species. Due to the natural boundaries of the clusters of active sites and the irreversible character of the studied system (no desorption) the system cannot escape from this case. If one allows desorption of the A particles a reactive state arises, it exists also for the case S > Sq. Here an infinite cluster of active sites exists from which a reactive state of the system can be obtained. If S approaches So from above we observe a smooth change of the values of the phase-transition points which approach each other. At S = So the phase transition points coincide (y 1 = t/2) and no reactive state occurs. This condition defines kinetically the percolation threshold for the present reaction (which is found to be 0.63). The difference with the percolation threshold of Sc = 0.59275 is attributed to the reduced adsorption probability of the B2 particles on percolation clusters compared to the square lattice arising from the two site requirement for adsorption, to balance this effect more compact clusters are needed which means So exceeds Sc. The correlation functions reveal the strong correlations in the reactive state as well as segregation effects. 

The kinetic equations are useful as a fitting procedure although their basis - the homogeneous system - in general does not exist. Thus they cannot deal with segregation and island formation which is frequently observed [27]. Computer simulations incorporate fluctuation and correlation effects and thus are able to deal with segregation effects but so far the reaction systems under study are oversimplified and contain only few aspects of a real system. The use of computer simulations for the study of surface reactions is also limited because of the large amount of computer time which is needed. Especially MC simulations need so much computer time that complicated aspects (e.g., the dependence of the results on the distribution of surface defects) in practice cannot be studied. For this reason CA models have been developed which run very fast on parallel computers and enable to study more complex aspects of real reaction systems. Some examples of CA models which were studied in the past years are the NH3 formation [4] and the problem of the universality class [18]. However, CA models are limited to systems which are suited for the description by a purely parallel ansatz. 

For a homogeneous chemical reaction of a chemical substance A (without mass transfer limitation and without a segregation effect) carried out in a CSTR the mass balance results in do = a + rkda when n is the chemical reaction order. (In this case the chemical reaction order is the same as the over-all conversion order.) It now follows that... 

Hildebrand (1949) designated nonideal solutions as regular if there was sufLcient thermal agitation to overcome the segregating effect of unequal cohesive densities between the solute and... 

An example of the importance of mixing effects in chemical reactors continuous free radical polymerization. One might now ask the question are segregation effects really important in practice or is micromixing "a solution in search of a problem"... 

Besides these laboratory experiments, the analysis of industrial reactors may also reveal segregation effects, as for instance in reactors for free radical polymerization of ethylene where the initiator feedstream is likely to be mixed by an erosive process (175). Polymerization and polycondensation reactors offers an especially interesting field for future applications of micromixing. 

Figure 24 Gibbsean segregation effects illustrated by ratio of profile decay length with an oxygen leak to that without an oxygen leak plotted versus oxide heat of formation. The substrate was silicon, and the primary ion was Ar+. (From Ref. 81.)...

In this context, Berry [277] studied the enzyme reaction using Monte Carlo simulations in 2-dimensional lattices with varying obstacle densities as models of biological membranes. That author found that the fractal characteristics of the kinetics are increasingly pronounced as obstacle density and initial concentration increase. In addition, the rate constant controlling the rate of the complex formation was found to be, in essence, a time-dependent coefficient since segregation effects arise due to the fractal structure of the reaction medium. In a similar vein, Fuite et al. [278] proposed that the fractal structure of the liver with attendant kinetic properties of drug elimination can explain the unusual... 

The resulting decrease in temperature in regions close to the surface of Cu components may also affect segregation effects. In that case the effect of... 

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