Distribution of reactant

Could poor mixing or inefficient distribution of reactants and heat sources result in undesirable side reactions, hot spots, runaway reactions, fouling, etc ... 

FIGURE 11.3 Distributions of reactant concentration near the electrode after different times of galvanostatic operation (a) t < (b) > t. ... 

Figure 3. Distribution of reactants (D, M) and products (M, P) observed in the (n-Bu)3SnH reduction of DCP. D = DCP, M = 2-chloropentane and P = pentane (see Figure 2).

In Fig. 9, the distribution of reactant C is shown in each environment. As cc is a linear combination of and Y2 (Eq. 78), we can distinguish features of both Fig. 7 and Fig. 8 in the plots in Fig. 9. In particular, because C is injected in the right-hand inlet stream, cC2 and 2 appear to be quite similar. Finally, as shown in Liu and Fox (2006), the CFD predictions for the outlet conversion X are in excellent agreement with the experimental data of Johnson and Prud homme (2003a). For this reactor, the local turbulent Reynolds number ReL is relatively small. The good agreement with experiment is thus only possible if the effects of the Reynolds and Schmidt numbers are accounted for using the correlation for R shown in Fig. 4. Further details on the simulations and analysis of the CFD results can be found in Liu and Fox (2006). 

Provided that equilibrium is maintained between the aqueous and micellar pseudophases (designated by subscripts W and M) the overall reaction rate will be the sum of rates in water and the micelles and will therefore depend upon the distribution of reactants between each pseudophase and the appropriate rate constants in the two pseudophases. Early studies of reactivity in aqueous micelles showed the importance of substrate hydropho-bicity in determining the extent of substrate binding to micelles for example, reactions of a very hydrophilic substrate could be essentially unaffected by added surfactant, whereas large effects were observed with chemically similar, but hydrophobic substrates (Menger and Portnoy, 1967 Cordes and Gitler, 1973 Fendler and Fendler, 1975). 

It is easy to understand the lower reactivity of non-ionic nucleophiles in micelles as compared with water. Micelles have a lower polarity than water and reactions of non-ionic nucleophiles are typically inhibited by solvents of low polarity. Thus, micelles behave as a submicroscopic solvent which has less ability than water, or a polar organic solvent, to interact with a polar transition state. Micellar medium effects on reaction rate, like kinetic solvent effects, depend on differences in free energy between initial and transition states, and a favorable distribution of reactants from water into a micellar pseudophase means that reactants have a lower free energy in micelles than in water. This factor, of itself, will inhibit reaction, but it may be offset by favorable interactions with the transition state and, for bimolecular reactions, by the concentration of reactants into the small volume of the micellar pseudophase. 

This section gives tabulated examples of recent work on micellar effects upon chemical and photochemical reactions. In general the examples given in this section do not duplicate material covered elsewhere in the chapter for example micellar effects on some photochemical reactions and reactivity in reversed micelles are listed here although they are neglected in the body of the text. For many ionic reactions in aqueous micelles only overall rate effects have been reported, in many cases because the evidence did not permit estimation of the parameters which describe distribution of reactants between aqueous and micellar pseudophases. These reactions are, nevertheless, of considerable chemical importance, and they are briefly described here. 

Fig. 2 When all of reactant B was fed into tank 1 with flow rate FB, = 0.003 (curves 1, 3). The productivity was slightly lower than in the case for equal distribution of reactant B to all tanks (2, 4). Higher reaction order with respect to B gave higher productivity (nB2 = 2 curves 1, 2). The maximum productivity possible was 0.001.

Part II - The distribution of reactants and products near a reacting surface. Combustion Science and Technology 8, 29-35. 

Surfactant solutions critical micelle concentration distribution of reactants among particles surfactant aggregation numbers interface properties and polarity dynamics of surfactant solutions partition coefficients phase transitions influence of additives... 

Intraparticle diffusion limits rates in triphase catalysis whenever the reaction is fast enough to prevent attaiment of an equilibrium distribution of reactant throughout the gel catalyst. Numerous experimental parameters affect intraparticle diffusion. If mass transfer is not rate-limiting, particle size effects on observed rates can be attributed entirely to intraparticle diffusion. Polymer % cross-linking (% CL), % ring substitution (% RS), swelling solvent, and the size of reactant molecule all can affect both intrinsic reactivity and intraparticle diffusion. Typical particle size effects on the... 

Figure 7.3 Concentration distribution of reactant within a sphericai cataiyst particie.

One of the drawbacks of this CAVERN device is the occurrence of a nonuniform distribution of reactant on catalysts because adsorption occurs on a deep bed of catalyst packed in a MAS rotor. To overcome this problem, we developed several shallow-bed CAVERN devices (95), and Fig. 10 shows a version of one such design. A thin layer of catalyst is supported on a glass trapdoor, and the device is evacuated. A furnace is clamped in place so that the catalyst can be activated if necessary. The catalyst is cooled with a cryogen bath, and a controlled amount of adsorbate is introduced from the vacuum line. The trapdoor is raised, the loaded catalyst falls into the MAS rotor, and the seal is driven into place. Finally the cold, sealed rotor is manually transferred into the cold MAS probe. The added advantages of the shallow-bed CAVERN is that all manipulations can be carried out without using a glovebox in any step. 

A reaction between solutes A and B in a solvent occurs at a rate k(t) [A] [B] when both reactants are distributed randomly throughout the solution. However, when A and B represent the result of bond fission (by photolysis or radiolysis), the distance to which geminate A and B pairs separate may be very small compared with the separation between pairs of A and B, unless very intense pulses of light or radiation were used. A very marked correlation in the distribution of A about B exists from the moment that recombination begins. This affects the subsequent rate of reaction and the probability that A and B will survive recombination. In Fig. 41, two initial distributions and their respective rate coefficients are shown. With the possible exception of some ESR techniques, such as 3-pulse electron spin echo, there are no methods for determining the initial distribution of reactant pairs. Indeed, as was mentioned in Chap. 6, Sect. 2 and Chap. 7, Sect. 2, the rate of reaction and survival probability of... 

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