Pores blocking

Everett concludes that in systems where pore blocking can occur, pore size distribution curves derived from the desorption branch of the isotherm are likely to give a misleading picture of the pore structure in particular the size distribution will appear to be much narrower than it actually is. Thus the adsorption branch is to be preferred unless network effects are known to be absent. 

In a pore system composed of isolated pores of ink-bottle shape, the intrusion curve leads to the size distribution of the necks and the extrusion curve to the size distribution of the bodies of the pores. In the majority of solids, however, the pores are present as a network, and the interpretation of the mercury porosimetry results is complicated by pore blocking effects. 

Finally, for the case of intermediate filtration, the intensity of increase in total resistance with increasing filtrate volume is less than that occurring in the case of gradual pore blocking, but greater than that occurring with cake filtration. It may be assumed that the intensity of increase in total resistance is directly proportional to this resistance ... 

It is important to note that pore blocking occurs when suspensions have the following characteristics ... 

The transition from pore-blocked filtration to more favorable cake filtration can therefore be achieved with a suspension of low settling particles by initially feeding it to the filter medium at a low rate for a time period sufficient to allow surface accumulation. This is essentially the practice that is performed with filter aids. 

Generally, the effectiveness of the separation is determined not by the membrane itself, but rather by the formation of a secondary or dynamic membrane caused by interactions of the solutes and particles with the membrane. The buildup of a gel layer on the surface of an ultrafiltration membrane owing to rejection of macromolecules can provide the primary separation characteristics of the membrane. Similarly, with colloidal suspensions, pore blocking and bridging of... 

Uojtanowicz, A.K., Krilov, Z., Langlinais, J.P. "Study on the Effect of Pore Blocking Mechanisms on Formation Damage," SPE paper 16233, SPE Production Operations Symposium, Oklahoma City, OK, March 8-10, 1987. 

Nitrogen adsorption/desorption isotherms on Zeolite and V-Mo-zeolite are very similar and close to a type I characteristic of microporous materials, although the V-Mo-catalysts show small hysterisis loop at higher partial pressures, which reveals some intergranular mesoporosity. Table 1 shows that BET surface area, microporous and porous volumes, decrease after the introduction of Molybdenum and vanadium in zeolite indicating a textural alteration probably because of pore blocking by vanadium or molybdenum species either dispersed in the channels or deposited at the outer surface of the zeolite. The effect is far less important for the catalysts issued from ZSM-5. 

In this way, the conjunct polymers serve as a reservoir of hydride ions. Under some conditions, the polymers are a source of hydride ions, but they accept these ions under other conditions. Substantial amounts of the saturated products are supposedly formed via this route with sulfuric acid. In zeolites, species similar to conjunct polymers also form. The heavy hydrocarbon molecules, which deactivate the catalyst by pore blocking or by site blocking, are generally termed soft coke or low-temperature coke , because of the absence of aromatic species. 

The bad performance of zeolites at low reaction temperatures is most likely a consequence of the hindered diffusion of bulky molecules under such conditions. The catalyst will be prematurely deactivated by pore blocking. These diffusion... 

For polymer solutions, equation 4.29 applies only to flow through unconsolidated media since, otherwise, the pore dimensions may be of the same order of magnitude as those of the polymer molecules and additional complications, such as pore blocking and adsorption, may arise. 

Measurement of heat of adsorption by means of microcalorimetry has been used extensively in heterogeneous catalysis to gain more insight into the strength of gas-surface interactions and the catalytic properties of solid surfaces [61-65]. Microcalorimetry coupled with volumetry is undoubtedly the most reliable method, for two main reasons (i) the expected physical quantities (the heat evolved and the amount of adsorbed substance) are directly measured (ii) no hypotheses on the actual equilibrium of the system are needed. Moreover, besides the provided heat effects, adsorption microcalorimetry can contribute in the study of all phenomena, which can be involved in one catalyzed process (activation/deactivation of the catalyst, coke production, pore blocking, sintering, and adsorption of poisons in the feed gases) [66]. 

X-ray absorption spectroscopy has proved the presence of rhenium dioxide within this nanostructure [12]. Extraction of the surfactant with various solvents remained inefficient since either the surfactant persists within the composite or the nanostructure is lost. Calcination at mild temperatures as low as 300-350°C in nitrogen atmosphere leads to a mass loss under these pyrolytic conditions of about 50% with only little loss of the nanostructure. Similar results are obtained when the composite is oxidatively treated in an oxygen plasma for not more than ten minutes. Physisorption measurements on the calcined or plasma treated samples show only very small surface areas, which cannot be assigned to a mesoporous structure. Right now we believe that residual carbon may introduce some pore blocking effects within the nanostructure preventing good access of the inner pore surfaces. 

In the second mechanism the topology of the pore network plays a role [394], During the desorption process, vaporization can occur only from pores that have access to the vapor phase, and not from pores that are surrounded by other liquid-filled pores. There is a pore blocking effect in which a metastable liquid phase is preserved below the condensation pressure until vaporization occurs in a neighboring pore. Therefore, the relative pressure at which vaporization occurs depends on the size of the pore, the connectivity of the network, and the state of neighboring pores. For a single ink bottle pore this is illustrated in Fig. 9.15. The adsorption process is dominated by the radius of the large inner cavity while the desorption process is limited by the smaller neck. 

The 78 amino acid m-agatoxin IIIA blocks P/Q-type channels with nanomolar affinity, but also potently inhibits L-type and N-type channels (Mintz 1994 Mintz et al. 1991). This toxin is much larger than the co-conotoxins (which are typically 25-30 amino acids in length), but nonetheless acts by a pore blocking mechanism and is thus distinct in action from aga IVA (Mintz 1994). Yet, pore block by this peptide is incomplete suggesting that toxin-bound channels can still conduct a small amount of calcium. Due to its nonselective nature, and its failure to elicit complete channel block, this toxin is not very useful experimentally. 

Huang, L. and Morissey, M. (1998) Fouling of membranes during microfiltration of surimi wash water Roles of pore blocking and surface cake formation. Journal of Membrane Science, 144, 113-123. 

A gradual increase of the reaction temperature accentuates the pore-blocking effect. Depending on the reaction temperature polymeric amino-boranes (H2N- BH2)n and borazines (B3N3H6) were observed in the zeolites. 

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