Pore blocking/closure

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,... 

Filtration of colloids at pH extremes served as a baseline of colloids, which are neither aggregated nor stabilised by an adsorbed organic layer. As expected from the literature review, the particle size closest to the membrane pore size (250 nm) caused largest flux decline. Rejection occurred in this case due to a combination of hydrophobic, specific, and van der Waals forces, and could not be explained by charge interaction only. Blocking law analysis showed evidence of pore blocking, cake formation, adsorption and pore closure at some stage of the filtration process. 

Figure 6.2 depicts 3 ways in which microporous membranes can foul (a) pores can suffer closure or restriction, (b) pores or porosity can be blocked or plugged and (c) a surface cake or layer can cover the membrane. All three mechanisms could apply, probably in sequence (a) then (b) followed by (c). Nonporous membranes are fouled by cake or surface layers (c). 

At lower temperatures hydrated lime, Ca(OH)2, can be injected into the flue gas stream near the economiser zone (300-650 C). In this temperature range, CaCOj can be formed, which is undesirable because it not only consumes sorbent but pore closure also blocks the access of SO2 to the active sorbent surface. Carbonation significantly increases with reaction temperature, and therefore, the flue gas duct process where the temperature is about 150 C, may be more effective. This process yields S02-removal efficiencies of approx, 80% in actual commercial installations if small particles with an open pore structure are applied. 

On the basis of the unit-cell size and pore diameter, one can estimate that the distance between adjacent spherical mesopores of the LP-FDU-12 sample calcined at 450°C was 6 nm. This distance is likely to correspond to the pore entrance length. Given that the pore entrance diameter (in the narrowest point) was 2 nm, the pore entrance length-to-diameter ratio was 3 1, and thus one can readily envision that the reduction of the pore entrance diameter through the thermal treatment would eventually lead to the closure of the entrance. The obtained ordered closed-pore silicas were white powders. It is clear that the block copolymer template was removed without the formation of the carbon residue, which was observed in an ordered closed-pore silica templated by PEO-PS copolymer. Pluronic (PEO-PPO-PEO) copolymers usually decompose at relatively low temperatures, so they may be inherently more convenient than PEO-PS copolymers, whose PS block has an appreciable tendency to carbonize. 

The suggested mechanism of the thermally induced pore closure process is illustrated in Scheme 11.1. The starting point of the synthesis is the formation of an ordered silica-copolymer (or organosilica-copolymer) composite, in which the copolymer micelles are embedded in the silica (or organosilica) framework. More specifically, the hydrophilic blocks (i.e., poly(ethylene oxide), PEO, blocks) of the copolymer are occluded in the silica (or organosilica) framework, while the hydro-phobic blocks (i.e., poly(propylene oxide), PPO, blocks) form separate domains. Such a structure of silica/copolymer composites emerged from NMR studies and provided the basis of the explanation of the pore connectivity in the structure of SBA-... 

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