Strong diffusion

Although hydrogenation of 4-CBA over Pd/C is very fast, there is strong diffusion resistance. Furthermore, apparent kinetic equations on different catalyst particle sizes have been obtained from experimental data. 

Specific catalytic activity of the composites obtained was at least several times higher than the same value for the random copolymer Nafion (even in an esterification reaction considered to be a diffusion-uncontrolled reaction). For the oligomerization reaction of decene-1 with strong diffusion control, the specific catalytic activity of the composites was 35 times higher than that for the random copolymer. Esterification of acrylic acid and alkylation of mesitilene by a substituted phenol were also performed using the composite catalyst. 

Therefore, concentrations of the polyvinyl monomer, the branching and network formation proceeds as shown in Figure 12. Mlcrogel-llke species are formed first which are highly internally crossllnked. The pendant double bonds in the interior are very Immobile so that their reactivity is strongly diffusion controlled and they almost cannot react at all, even with the monomers. Only the more mobile pendant double bonds in the periphery of these species can enter into reactions with macroradicals and participate in Interbinding of the species together. 

Let us next see whether the reactor is operating in the diffusion-free or strong diffusion regime. 

Consider the behavior of upstream diffusion as illustrated by the solutions in Fig. 16.8. Despite the fact that the unbumed reagents are only methane and air, it may be observed in the lower left-hand panel that significant levels of H2 are present at the inlet boundary. Had the burner-face boundary condition been specified as a fixed composition of methane and air (instead of the mass-flux fractions as in Eq. 16.100), the solution in the vicinity of the burner would have been different, since the H2 fraction would have vanished at the boundary. The influence of upstream diffusion, and hence the need for the mass-flux-fraction boundary condition, is increasingly important at low flow rates or low pressure. In either case relatively strong diffusive mass transport can cause reaction-product species to diffuse to the inlet boundary. 

The supression of the microscopic defect segregation by strong diffusion is well-observed for d = 3 - see Fig. 7.6. The main effect is seen for the correlation function of similar (rather than dissimilar) defects. At last, a comparison of Figs 7.7 and 7.4 demonstrates an effect of the diffusion upon... 

We studied samples of La lecithin with 16-29% water at room temperature. The samples were prepared between two microscope glass slides that were first treated with a sulfochromic solution, then carefully rinsed with distilled water and dried. They were observed by transmission polarizing microscopy. Just after deposition between the slides, the samples present a highly disordered texture which strongly diffuses light. After compression (squeezing) and/or shearing parallel to the plates, one obtains an overall homeotropic orientation that is perturbed by defects. The thickness of the samples is in the order of 20/. ... 

As the table and Fig. 30 indicate, reaction of Ni-T3MPP in the presence of basic additives (Cs, Na) is characterized by small Thiele moduli and uniform profiles (0m — 1), indicative of kinetic-controlled reactions. Acidic components (S, I, Cl) result in larger Thiele moduli (smaller 6m) and more sharp, U-shaped profiles, signaling strong diffusion-limited reactions. 

Under the ribs, the species diffusion in the direction orthogonal to the cell plane, i.e. cross-plane diffusion, is obviously impossible. Hence, the reactant concentrations on the electrode/electrolyte site under the ribs are driven only by in-plane diffusion. Due to the strong diffusion property of H2, in-plane diffusion allows H2 to penetrate under the ribs, while in the case of O2, a relevant concentration reduction is noticed. The effect of the ribs is significant at all operating conditions, but it becomes predominant at high fuel utilization (not shown in the figures). 

The high process - temperature (1100 - 2300 K) causes a strong diffusion of the coating into the substrate, resulting in a good attachment. The CVD technique does not suffer from the Tine-of-sight effect (cfr. PVD) the substrate is coated entirely with a relatively uniform layer. 

It plays the same role as the effectiveness factor in heterogeneous catalysis and is a measure of the film thickness uniformity. It represents the ratio of the total reaction rate on each pair of wafers to that we would obtain if the concentration in the cell formed by the two wafers were equal to the bulk concentration everywhere. Thus, if the surface reaction is the rate controlling step, n = 1, whereas if the diffusion between the wafers controls, n < 1. In the limit of strong diffusion resistance the deposition is confined to a narrow outer band of the wafers and a strongly nonuniform film results. 

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