Fractionation near walls

It is expected that as the strain rate increases, the overall coupling between the surface and the gas-phase increases, since the flame is pushed toward the surface. Figure 26.6a shows the wall heat flux that can be extracted from the system, and the fuel mole fraction near the surface vs. the inverse of the strain rate for 28% inlet H2 in air, at two surface temperatures. The end points of the curves in Fig. 26.6, at high-strain rates, are the extinction points. The conductive heat flux exhibits a maximum as the strain rate increases from low values, which is at first counterintuitive. In addition, with increasing strain rate the fuel mole fraction increases monotonically, while the mole fractions of NOj, decrease, as seen in Fig. 26.66. The species mole fractions show sharper changes with strain rate near extinction, as the mole fractions of radicals decrease sharply near extinction. 

In Chap. 2, the concept of the diffusion layer was established. It is a thickness, within which a large fraction of diffusional changes take place, and at a distance of several times this thickness, practically no more diffusional changes are observed. This layer will here be given the symbol 8b (D for diffusion). In fluid dynamics, there is a similar layer, within which most of the velocity changes occur. This is the hydrodynamic layer 8. It turns out that for diffusive mass transfer, 8b is usually much smaller than <5/,. This is fortunate, because it justifies to some extent the linearised velocity profiles often assumed near walls, making analysis easier. These relations are very lucidly discussed in a classic paper by Vielstich [560]. 

During cell fractionation, nearly all the cyclopenase activity was found in a fraction containing the cell wall together with the cytoplasmic membrane (83,84). From this fraction, the enzyme could be partially solubilized by detergents (e.g., Triton X-100) to yield a protein-phospholipid complex. By treatment with M-butanol, the solubilized enzyme preparation was split into the lipid fraction and the enzyme protein which retained a considerable part of the total enzyme activity. Compared with that of the membrane-bound enzyme, the substrate affinity of the solubilized protein-lipid complex was decreased, whereas... 

Measured radial gas fraction profiles are plotted in Figure 5. It should be noticed that the reproducibility is quite good. Profiles of CO, CO2 and CH4 show a parabolic behaviour, while fractions near the wall can be up to 3 times higher than fractions in the centre. The profiles are not completely symmetrical this can be ascribed partly to the method of measurement and die gas-solid flow One could think of several hypothetical causes for the radial profiles ... 

Example 4.1 Oscillatory Variation of Void Fraction Near the Walls of Packed Beds... 

Figure 4.4 shows the result of the analysis. Notice the wide fluctuations of void fraction near the wall, decaying gradually as we move toward the center of the bed. According to Govindarao and Froment (1986), when D/dp > 10, the radial void fraction approaches a constant asymptotic value (Eb) at a distance from the wall of approximately 5 particle diameters, consistent with the results shown in Figure 4.4. 

Figure 4.4 Oscillatory variation of void fraction near the walls of a packed bed.

C. Void fraction near the walls of annular packed beds... 

Williams, P.S. Lee, S. Giddings, J.C. Characterization of hydrodynamic hft forces by field-flow fractionation. Inertial and near-wall hft forces. Chem. Eng. Commun. 1994,130, 143. 

Figure 10.12 Comparison of the axial profiles of (a) propane mass fraction, (b) wall and bulk gas temperature and (c) Nusselt number obtained from CFD simulations (symbols), pseudo-2D model with Nu/Sh fits (solid lines) and pseudo-2D model with constant Nu/Sh (dashed lines) near extinction, i.e. with ks = 20W/m K ...

The heterogeneous reactivity of propane was assessed by varying the reactor pressure and surface temperature, and monitoring the near-wall bending of the propane transverse profiles. In Case 2 (Fig. 4.1, p = 3 bar) the catalytic fuel conversion is already appreciable at surface temperatures < 850 K, while mass-transport-limited conditions are approached for = 1064 K (manifested by the low propane mole fractions near both walls in Fig. 4.1 (2d)). 

All the above results are true and are caused by the increasing void fraction of the bed near the wall, as the previous authors recognized. Pressure drop is a very sensitive function of the void fraction. Because in a... 

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