Pore blockage

Equation 46 states that when complete pore blockage occurs, the intensity of the increase in the total resistance with increasing filtrate volume is proportional to the square of the flow resistance. 

Equation 61 states that the intensity of increase in total resistance with increasing filtrate amount is proportional to resistance to the 3/2 power. In this case, the total resistance increases less sensitively than in the case of total pore blockage. 

The filtration time corresponding to total pore blockage, when Ap->°o may be estimated from ... 

On ferrierite, ZSM-22 and EU-1 zeolite catalysts, 10MR monodimensional zeolite structures (ID), the main reaction is the isomerization of ethylbenzene (figure la). ZSM-5, 10MR three-dimensional structure (3D) zeolite is very selective in dealkylation (90%) (figure lb) and no deactivation was observed within 8 hours of reaction. This particular selectivity of the zeolite ZSM-5 can be partly explained by the presence of strong acid sites and its porous structure that on one hand promotes the containment of molecules in the pores (presence of 8-9A cages at the intersection of channels) and on the other hand prevents the formation of coke and therefore pore blockage. 

Pore blockage by carbon or heavy products may cause a loss in activity over time. Niemela and Krause39 reported a loss of turnover frequency for Co/Si02 FTS catalysts due to preferential blocking of the narrowest catalyst pores by carbon. Puskas74 found unusually high amounts of wax in the pores on a Co/Mg/ diatomaceous earth catalyst tested in the FTS at 190°C, 1-2 bar, H2/CO = 2.55 for 125 days. In a separate study it was concluded that pore plugging by the waxy products resulted in a fast deactivation of such catalysts.75... 

Iron Sponge Also called Dry box. An obsolete process for removing hydrogen sulfide from gas streams by reaction with iron oxide monohydrate. The ferric sulfide that is formed is periodically re-oxidized to regenerate ferric oxide and elemental sulfur. When this process becomes inefficient because of pore-blockage, the sulfur is either oxidized to sulfur dioxide for conversion to sulfuric acid, or is extracted with carbon disulfide. 

The mesoporous materials reported above are usually prepared from relatively expensive surfactants. Some of them have poor hydrothermal stability. Furthermore, the MCM-41 host structure has a one-dimensional pore system with consequent pore blockage and diffusion limitations. Shan et al. (52) reported the synthesis of a three-dimensional and randomly connected mesoporous titano-silicate (Ti-TUD-1, mesopore wall thickness = 2.5-4 nm, surface area — 700-1000 m2/g, tunable pore size —4.5-5.7 nm) from triethanolamine (TEA). Ti-TUD-1 showed higher activity (about 5.6 times) for cyclohexene epoxidation than the framework-substituted Ti-MCM-41. Its activity was similar to that of the Ti-grafted MCM-41 (52). 

It is seen that the intracrystalline MgO induces pore blockage in a fraction of the pore system and alters the porosity as well as D0, and/or r with the latter factors contributing most to the reduced diffusivity. In contrast, the coke modifier appears to affect mainly the surface-to-volume ratio and suggests that the effective surface area, number of available entrance ports, is reduced by two orders of magnitude. 

Li, Y., Chung, T.S., Cao, C., and Kulprathipanja, S. (2005) The effects of polymer chain rigidification, zeolite pore size and pore blockage on polyethersul-fone (PES)-zeolite A mixed matrix membranes./. Memhr. Sci., 260, 45-55. 

This parameter is widely used to describe the ionic transport by providing information on the effect of pore blockage. A tortuosity factor r = 1, therefore, describes an ideal porous laody with cylindrical and parallel pores, whereas values of r > 1 refer to more hindered systems. Higher tortuosity is good for dendrite resistance but can lead to higher separator resistance. 

Following the normal reactivity order, tertiary carbon atoms are more reactive than secondary ones, which in turn are far more reactive than primary ones [63-64,67]. Turn-over numbers in the oxidation of methyl cyclohexane with PhIO on FePcY decrease with increasing loadings of the phthallocyanine on the zeolite, as shown in Figure 4 [49-50,63-64,69]. This is due to pore blockage by the catalyst molecules themselves. 

This estimate could prove to be valuable input for more accurate representation of the pore blockage effect in the macroscopic two-phase fuel cell models. Figure 24 shows the variation of the effective oxygen diffusivity with liquid water saturation from the correlation in Eq. (29) along with the typical macrohomogeneous correlation with m = 1.5 and b = 1.5 otherwise used arbitrarily in the macroscopic fuel cell modeling literature. 

Figure 24. Pore blockage relation as function of liquid water saturation for the CL structure.

Figure 25 displays the anisotropic effective oxygen diffusivity variations with liquid water saturation in the GDL based on the evaluated pore blockage correlations. Furthermore, the impact of GDL compression on the pore blockage effect was also investigated.67... 

Figure 25. Pore blockage correlations for the uncompressed GDL structure.

This estimate could prove to be a valuable input for more accurate representation of the GDL pore blockage effect in the macroscopic two-phase fuel cell models. 

The pore blockage effect comes into play through the oxygen concentration, co2 distribution given by the following equation ... 

With the evaluated site coverage and pore blockage correlations for the effective ECA and oxygen diffusivity, respectively, and the intrinsic active area available from the reconstructed CL microstructure, the electrochemistry coupled species and charge transport equations can be solved with different liquid water saturation levels within the 1-D macrohomogeneous modeling framework,25,27 and the cathode overpotential, q can be estimated. 

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