Packings diffusion through

Equimolar Counterdiffusion. Just as unidirectional diffusion through stagnant films represents the situation in an ideally simple gas absorption process, equimolar counterdiffusion prevails as another special case in ideal distillation columns. In this case, the total molar flows and are constant, and the mass balance is given by equation 35. As shown eadier, noj/g factors have to be included in the derivation and the height of the packing is... 

In a packed absorption column, the flow pattern is similar to that in a packed distillation column but the vapour stream is replaced by a mixture of carrier gas and solute gas. The solute diffuses through the gas phase to the liquid surface where it dissolves and is then transferred to the bulk of the liquid. In this case there is no mass transfer of the carrier fluid and the transfer rate of solute is supplemented by bulk flow. 

There remains one very curious feature of the polyimide films discussed above. Some of the side chains removed contain in excess of 16 carbon atoms and are therefore relatively massive. Neverthless, they appear to diffuse through a tightly packed polymer multilayer consisting of up to 200 monolayers. Undoubtedly this process takes place, but it is hard to understand the mechanism. 

Flow through packed beds (eddy or multipath diffusion). In chromatography, component zones are carried through a bed of randomly packed particles. The streamlines in such flow veer back and forth to find passage between the particles (see Figure 5.4) and fluctuate in velocity... 

Fig. 7.5. Schematic representation of some of the redox mediator processes at a whole cell biosensor. Lipohilic mediators may be reduced at redox active sites in the plasma membrane or at sites within the cytoplasm or both processes may occur—depending on the cell type and the mediator. Lipophobic mediators can only be reduced at sites on the outside edge of the plasma membrane. The oxidized form of the mediator. O, may be present in excess, but much of the reduced form. R, may need to diffuse between packed cells (dotted arrows) or through the cytoplasm (squiggly arrows). The subscripts aq, cyt, elec, and surf represent mediator in the aqueous phase, within the cytoplasm, at the electrode surface, and at the plasma membrane-aqueous interface, respectively.

Therefore, the effect of the monolayer is brought down to additional resistance of the equivalent by thickness aqueous layer h. It was shown that the permeability of the adsorption layer depends on surface tension (packing density) and size of the diffusing gas molecules [482], For many surfactants h is within the range of 7 to 12 nm. This means that the permeability of thick films is determined by the rate of molecular diffusion, while for black films (h 10 nm) Eq. (3.147) is valid and their permeability is determined by the properties of the surfactant monolayers. Electrolytes do not affect significantly the permeability of monolayers. It was considered that gas diffusion through the monolayer occurred as a result of creation of microscopic vacancies between the surfactant molecules. This model was called model of energy barrier. However, later this model proved unsatisfactory. [483]... 

The jump vector. A, wUl obviously depend on the mechanism and the structure. For example, an atom diffusing through the octahedral interstitial sublattice in an FCC metal, with lattice spacing a (Fig. 6.6), must jump the distance between interstitial sites, A = fl/V2. This is, of course, the same distance an atom diffusing by the vacancy mechanism must jump. It will be recalled that for every atom in a close-packed stmcture, there are two tetrahedral interstitial sites and one octahedral interstitial site. The reader might ask if the distances between the tetrahedral sites ate the same. 

The theory described above is applied to the binary hard-sphere system with the size ratio (Jijci = 0.2 and the concentration of small particles ci = 0.5. Figure 1 shows the diffusion constants D, of small (s = 1) and big (s = 2) particles versus the total packing fraction 9. The dotted lines show the power-law fit D = f o 9 — fj with 7=1.31 and 2.36 for small and big particles, respectively. The diffusion constant of the big particles is found to vanish at 95 =0.52, which is close to the liquid-glass transition point (9=0.516) of a one-component hard-sphere system, while Di becomes zero at 9 =0.53 (> 9b). This means that for 9b < 9 < 9x there exists a new phase (delocalized phase) with mobile small particles diffusing through the voids of a glassy structure formed by the immobile big particles. 

Reactants and products must diffuse through high-molecular-weight liquid hydrocarbons during FT synthesis. The liquid phase may be confined to the mesoporous structure within catalyst pellets or extend to the outer surface and the interstitial spaces between pellets, depending on the reactor design and hydrodynamic properties. In packed-bed reactors, the characteristic diffusion distance equals the radius of the pellets plus the thickness of any liquid boundary layer surrounding them. Intrapellet diffusion usually becomes... 

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