Diffusion compared

Table 10.4 gives critical data for the most common solvents used in high-pressure extraction. Table 10.5 illustrates the favorable mass transport properties that can be achieved in the supercritical area owing to a low viscosity and a high diffusivity, compared with the liquid phase. 

A corollary to the conclusion of the above discussion is that the number of lattice oxygen available for reaction at the active site depends not only on the atomic details of the site, but also on the rate of lattice diffusion compared with the rate of surface reaction. If the rate of lattice diffusion is slow relative to surface reaction, then the number of lattice oxygen atoms available for the reaction during the residence of an intermediate could be estimated from the atomic structure of the site, as in the examples above. On the other hand, if lattice diffusion is fast, a lattice oxygen atom will be replenished immediately after its removal. Then the behavior of the catalyst no longer depends as much on the stoichiometry of the active site, but on the degree of reduction of the active site at steady state, which would determine the heat of removal (or the reactivity) of the lattice oxygen. 

Determine the thermal-difffusive mass-flux profiles for all the species. Discuss the results in the context of the solution profiles. Which species have have important contributions to thermal-diffusive mass flux In what regions is the thermal diffusion important What is the direction of the thermal diffusion for the various species How does thermal diffusion compare in magnitude to ordinary diffusion for this problem ... 

It can be seen that, if the contributions of diffusing A and B atoms to the process of formation of the ApBq layer are equal, the time necessary for the layer to reach any given thickness x is two times less compared to the case where only one component is diffusing (compare equations (1.8), (1.15) and (1.16) on the one hand with equations (1.27)-(1.29) on the other). 

C. For the conditions of B, how long does it take on average for sucrose to move 10 mm and 1 m in the sieve tube (ignore concomitant diffusion) Compare your values with answers to A. 

In the Fiirth hole model for molten salts, the primary attraction is that it allows a rationalization of the empirical expression = 3.741 r p. In this model, fluctuations of the structure allow openings (holes) to occur and to exist for a short time. The mean hole size turns out to be about the size of ions in the molten salt. For the distribution function of the theory (the probability of having a hole of any size), calculate the probability of finding a hole two times the average (thereby allowing paired-vacancy diffusion), compared with that of finding the most probable hole size. 

The diffusivity of the second bed catalyst (catalyst C) was increased by the regeneration, but it was still lower than that of the fresh catalyst (catalyst A). This indicates that both coke and metal sulfides have a responsibility for a decrease of the diffusivity. Comparing the diffusivity between the catalyst B, on which only coke deposited, and the regenerated catalyst C, the initial coke deposit lowers the diffusivity more than the ultimate metal deposit. The fourth bed catalyst (catalyst F) showed the least diffusivity due to a large amount of coke deposit. 

The morphological changes discussed in Section 15.4.1.2 were obtained for monolayers at assembled states in 2D. In such an assembled state, secondary effects such as generation of 3D collapse are accompanied on the solid surface. This should be ascribed to the limited allowance of lateral diffusion compared to the rate of the photoisomerization process. For observation of intrinsic photoresponse of the monolayer, separation of the polymer chain is highly desired, ideally on a single-chain level. 

TABLE 2.2 Spreading of a Point Source through Diffusion Compared with Convective Transport ... 

Example 21.1. (a) For the diffusion of solute A through a layer of gas to an absorbing liquid, with = 0.20 and y, - = 0.10, calculate the transfer rate for one-way diffusion compared to that for equimolal diffusion. (6) What is the value oiy halfway through the layer for one-way diffusion ... 

---