Pore plugging

In selecting cloths made from synthetic materials, one must account for the fact that staple cloths provide a good retentivity of solid particles due to the short hairs on their surface. However, cake removal is often difficult from these cloths - more than from cloths of polyfilament and, especially, monofilament fibers. The type of fiber weave and pore size determine the degree of retentivity and permeability. The objective of the process, and the properties of particles, suspension and cake should be accounted for. The cloth selected in this maimer should be confirmed or corrected by laboratory tests. Such tests can be performed on a single filter. These tests, however, provide no information on progressive pore plugging and cloth wear. However, they do provide indications of expected filtrate pureness, capacity and final cake wetness. 

The ways in which inhibitive anions affect the corrosion of zinc are mainly similar to those described above for iron. In inhibition by chromate, localised uptake of chromium has been shown to occur at low chromate concentrations and in the presence of chloride ions Thus under conditions unfavourable for inhibition, pore plugging occurs on zinc. Inhibitive anions also promote the passivation of zinc, e.g. passivation is much easier in solutions of the inhibitive anion, borate , than in solutions of the non-inhibitive anions, carbonate and bicarbonate , A critical... 

Although the FTS is considered a carbon in-sensitive reaction,30 deactivation of the cobalt active phase by carbon deposition during FTS has been widely postulated.31-38 This mechanism, however, is hard to prove during realistic synthesis conditions due to the presence of heavy hydrocarbon wax product and the potential spillover and buildup of inert carbon on the catalyst support. Also, studies on supported cobalt catalysts have been conducted that suggest deactivation by pore plugging of narrow catalyst pores by the heavy (> 40) wax product.39,40 Very often, regeneration treatments that remove these carbonaceous phases from the catalyst result in reactivation of the catalyst.32 Many of the companies with experience in cobalt-based FTS research report that these catalysts are negatively influenced by carbon (Table 4.1). 

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... 

Enhanced biorestoration is thus the process of providing all the materials necessary for optimum degradation of the contaminant of concern. This process can include direct injection of air or nutrients, circulation of groundwater to distribute materials, addition of cometabolites (such as sugar) where additional sources of carbon and attached oxygen are needed, or injection of special inhibitors to limit precipitation of pore-plugging iron oxides. 

Zinc oxide beds are limited to operation at temperatures below 430°C probably because of pore plugging during sulfur removal and sintering. Thermodynamics also favors lower temperatures. At the higher temperatures, the H2S cannot be reduced to levels low enough for shift catalyst or to reach fuel cell limits. 

Homogenous (multi) layers in the pores Plugs in the pores (constrictions) Plugs/layers on top of the pores... 

The concentration of metals in the feedstock can also have a major impact on catalyst life. Figure 53 compares the relative catalyst lifetimes for a typical HDS catalyst processing the high-metals Maya residuum and an Arabian Heavy residuum. As is evident, a higher concentration of metals in the feedstock increases the rate of deactivation of both the intermediate period and the final pore-plugging phase. New catalyst systems are required to handle heavy feeds that have metal concentrations of this magnitude. 

This review will only focus on the modeling efforts in pore diffusion and reaction in single-catalyst pellets which have incorporated pore plugging as a deactivation mechanism. A broad literature exists on the deactivation of catalysts by active site poisoning, and it has been reviewed by Froment and Bischoff (1979). The behavior of catalytic beds undergoing deactivation is... 

Newson (1975) was among the first to develop a pore plugging model of demetallation to predict catalyst life. By using the pore structure model of Wheeler (1951), the pellet was assumed to have N pores of identical length but with a specified distribution of pore radii. Metal deposition was assumed to be a first-order reaction over an outer fraction of the pore length and to have a uniform thickness. This model showed that the broadness of the size distribution had little effect on the catalyst life for the same average radii, but that increasing the radii from 45 to 65 A more than doubled the catalyst life. The restricted form of the diffusivity (see Section IV,B,5) was not employed in this model. 

Haynes apd Leung (1983) formulated a similar configurational diffusion model combining the effects of active site poisoning as well as pore plugging on the HDM reaction. In this case the reaction form in the conservation equation is multiplied by a deactivation function which accounts for the loss of intrinsic activity, (1 - ) is frequently chosen, where x is the fractional coverage of the sites. Other forms of the site deactivation function have been discussed by Froment and Bischoff (1979). The deactivation was found to depend on a dimensionless parameter given by... 

The effects of metal deposition on catalyst pore plugging were not included in this computation of Wei and Wei (1982). Rather, this model... 

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