Pore blocking effect

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. 

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. 

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. 

This relationship gives a variation of the pore length that is related to the variation of the pore area, found from the pore size distribution and can be used to study the pore blocking effect. From equation (14) different interesting data can be obtained. (1) The differential pore length distribution as a function of pore diameter. (2) The cumulative... 

KG 100 showed no pore blocking effects, as is seen by the constant pore length, although there is a decrease in surface area reported. This because the total surface... 

Figure 3.8 Illustration of the pore-blocking effect caused by bulky grafting species reacting preferentially at the pore openings and thus giving rise to a nonhomogeneous distribution of the organic components within the channel system.

Recent work has drawn attention to the complexity of capillary condensation in pore networks and has indicated that a pore size distribution curve derived from the desorption branch of the loop is likely to be unreliable if pore blocking effects occur. It is significant that a very steep desorption branch is usually found if the lower closure point of the loop is located at the limiting p/p° (sec Section 11.2.1.5.C). In particular,... 

This cost differential can be tolerated only in applications in which polymeric membranes completely fail in the separation [78]. Demanding separation applications, where zeolite membranes could be justified, due to the high temperatures involved or the added value of the components, and have been tested at laboratory scale, are the following separation of isomers (i.e., butane isomers, xylene isomers), organic vapor separations, carbon dioxide from methane, LNG (liquefied natural gas) removal, olefines/paraffins and H2 from mixtures. In most cases, the separation is based on selective diffusion, selective adsorption, pore-blocking effects, molecular sieving, or combinations thereof. The performance or efficiency of a membrane in a mixture is determined by two parameters the separation selectivity and the permeation flux through the membrane. 

Assessment of the total pore volume liberated shows that thermal treatment of the mesophase up to 120°C does not liberate any porosity. Heating up to 150°C liberates up to 25% of the total porosity whereas only around 50% of the total porosity is liberated when the mesophase was heated up to 300°C. The maximum pore volume is obtained after thermal treatment up to 760°C. When comparing these results with those obtained by SCTA, it would seem that pore blocking occurs. Indeed, after heating to 150°C, with around 25% of the porosity liberated, around 45% of the surfactant is removed. After heating to 300°C, with around 50% of the porosity liberated, up to 70% of the surfactant is removed. We have observed that it is only after treatment to 500°C, with the loss of almost 80% of the total surfactant, that an equivalent amount of pore volume becomes accessible. This pore blocking effect may also explain why the nitrogen desorption isotherms at 77K do not rejoin the adsorption branches at relative pressures below 0.2. 

Vishnyakov, A. and Neimark, A.V. (2003). Monte Carlo simulation test of pore blocking effects. Langmuir, 19, 3240—7. 

The permeability of the unloaded tube B obviously increases compared with that of platinum loaded. In figure 4, the intercept value of the line II is nearly twice as large as that of line IV (1.373 and 0.78, respectively), but the slope of line II is half that of line IV (0.00679 and 0.00344, respectively), which shows that with prolonged dipping time, the Pt loading on the membrane also increases. The pore blocking effect of Pt particles become also more serious, so that the pore size distribution of the membrane is improved. 

Exanqiles of this pore blocking effect on the desorpdon branch of the isottom Imve been well docummited in the porosimetry literature. Several years ago Kraemer [11] pointed out that in an ink botde-type pore, capillary condensation on the adsorption branch would be dominated by the radius of... 

To avoid the pore blocking effect of the catalysts with coarse fly ash particles and ammonium sulphate, the destruction of dioxins/furans is usually performed after the cleaning stages or at the boiler exhaust (350 00°C). The POPs reduction rate depends mainly on the volume capacity of the installed catalyst, the reaction temperature and the contact time in the selective catal5dic reduction (SCR) reactor, as well as the presence of poisoning elements (As, Pb, P, K, etc...). 

When adsorption and pore blocking are included for the biopolymer, incremental recovery drops to 17.5%. When the pore-blocking effect is absent the recovery is further decreased to 15%. Thus, in this case, the pore-blocking levels for the biopolymer as presented in Figure 8.35 do assist the recovery but they contribute only a small amount to the final quantity of incremental oil. 

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