Pore diffusion resistance consequences

Strong Pore-Diffusion Resistance Some Consequences... 

Here, we consider the consequences of being in the region of strong pore-diffusion resistance (77 - 11(f)" as apparent activation energy (f)" is given by equation 8.5-20b. 

The experiments with different inlet concentrations showed that the apparent gas-solids mass transfer coefficient value increases when the inlet concentration decreases. This indicated the existence of the pore diffusion. The pore diffusions resistance could not be eliminated completely, so the first assumption of the model is not reliable. Consequently, the mass transfer coefficients presented in the work are conservative estimates. 

Table 6.7 displays some physical properties of zeolites. A study issued from industry [8] demonstrates the significant role of mass-transfer resistances, even for small particles below 1 mm, reporting that pore diffusion may decrease the effectiveness from low to very low values (0.4 to 0.06). The external mass-transfer resistance is much less important. In consequence, in commercial operation only a small part of the catalyst is effectively used, typically less than 10%. Since the... 

Non-isothermal reaction with the diffusion resistance in the pore In an exothermal reaction, temperature gradients will arise within the pellet and, consequently, the temperature of pellet will be elevated compared with its surroundings. As a result, the reaction will be faster than the isothermal counterpart. AT in the film as well in the pellet can be theoretically predicted and, consequently, the maximum AT between the outer surface (Ts) and the inside of the catalyst (T n) calculated, when there c = 0 ... 

The molecular diffusivities were calculated from a weighted average of the binary diffusion resistances which in turn were estimated horn the formula of Fuller et al. The calculation of the Knudsen diffusivities requires information about the pore size. The catalyst had a bimodal pore distribution. Furthermore, via electron microscopy, it was found that the catalyst consisted of crystallites of about 5 p, separated by voids of about 1 ft. Consequently, the maximum length of the micro-... 

It is widely accepted that mass transfer effects are determinant for the FTS consequently, the selectivity toward certain olefins depends on the path of the species dixring the process, making difficult the establishment of unequivocal relationships between the diffusion resistances and the selectivity. One of the principal features of this complex process is that the diffusion of gaseous species H2 and CO through the pores of the catalysts filled with liquid waxes... 

As a consequence of this, i enever bulk dlffusional resistance domin ates Knudsen diffusional resistance, so that 1, it follows that fi 1 also, and hence viscous flow dominates Knudsen streaming. Thus when we physically approach the limit of bulk diffusion control, by increasing the pore sizes or the pressure, we must simultaneously approach the limit of viscous flow. This justifies a statement made in Chapter 5. 

As a rule, ACF not only presents a higher adsorption capacity than conventional GAC, but the pore network is also different due to the fibril structure, which ensures a much higher adsorption kinetics. The reason is that in GAC, the adsorbate must diffuse throughout the macro and mesopores before reaching the micropore or adsorption sites, whereas the micropores are directly accessible ftom the external surface in the ACF (Fig. 23). Consequently, there is no resistance to the diffusion of adsorbates through to the adsorption pores because there is no meso/macropore network. 

It thus appears that in real zeolite crystals diffusion over long distances reflects the influence of surface and internal barriers rather than the pore structure of the idealized framework. As a result the apparent intracrystalline diffusivities often show a strong dependence on the length scale of the measurement. Measurements by QENS and neutron spin-echo methods over distances corresponding to a few unit cells often approach the theoretical values derived by MD calculations for an ideal lattice. Similar values are often obtained by PEG NMR when the measurement is made over short distances. Measurements by most macroscopic methods are on the length scale of the crystals, and these tend to yield lower apparent diffusivities as a consequence of the intrusion of surface barriers and internal resistances due to structural defects. 

When concretes with the same moisture content are compared, resistivity is higher for the lower wIc ratio, for higher levels of hydration and for the use of additional components like fly ash, SF, etc. Resistivity increases with age and hardening of concrete because the pore system becomes less permeable there is an inverse correlation between concrete resistivity and the chloride diffusion rate. This fact is exploited for the measurement of concrete resistivity in view of assessing its susceptibility to chloride penetration and, consequently, to corrosion of reinforcement for both its initiation and rate of progress. The resistivity of external concrete cover is the most important and is expressed by the formula ... 

---