Tortuosity support structure

Criteria for ILM support selection can be divided into two categories chemical properties and structural properties. Chemical properties consist of support surface properties and the reactivity of the polymer support toward fluids in contact with it. Structural characteristics include porosity, pore size distribution, tortuosity, support thickness and geometry. 

CNFs appear to be well-attached to the supporting structures and a good explanation for this observation is still lacking. The layers of CNFs are indeed highly macroporous and should have low tortuosity, and the eapability to allow fast mass and heat transfer has been demonstrated for hydrazine deeomposition. 

Higher intrapellet residence times increase the contribution of chain initiation by a-olefins to chain growth pathways. This intrapellet delay, caused by the slow diffusion of large hydrocarbons, leads to non-Flory carbon number distributions and to increasingly paraffinic long hydrocarbon chains during FT synthesis. But intrapellet residence time also depends on the effective diameter and on the physical structure (porosity and tortuosity) of the support pellets. The severity of transport restrictions and the probability that a-olefins initiate a surface chain as they diffuse out of a pellet also de-... 

Way, Noble and Bateman (49) review the historical development of immobilized liquid membranes and propose a number of structural and chemical guidelines for the selection of support materials. Structural factors to be considered include membrane geometry (to maximize surface area per unit volume), membrane thickness (<100 pm), porosity (>50 volume Z), mean pore size (<0.1)jm), pore size distribution (narrow) and tortuosity. The amount of liquid membrane phase available for transport In a membrane module Is proportional to membrane porosity, thickness and geometry. The length of the diffusion path, and therefore membrane productivity, is directly related to membrane thickness and tortuosity. The maximum operating pressure Is directly related to the minimum pore size and the ability of the liquid phase to wet the polymeric support material. Chemically the support must be Inert to all of the liquids which It encounters. Of course, final support selection also depends on the physical state of the mixture to be separated (liquid or gas), the chemical nature of the components to be separated (inert, ionic, polar, dispersive, etc.) as well as the operating conditions of the separation process (temperature and pressure). The discussions in this chapter by Way, Noble and Bateman should be applicable the development of immobilized or supported gas membranes (50). 

Criteria for immobilized liquid membrane (ILM) support selection can be divided into two categories structural properties and chemical properties. Structural properties include geometry, support thickness, porosity, pore size distribution and tortuosity. Chemical criteria consist of support surface properties and reactivity of the polymer support toward fluids in contact with it. The support thickness and tortuosity determine the diffusional path length, which should be minimized. Porosity determines the volume of the liquid membrane and therefore the quantity of carrier required. The mean pore size determines the maximum pressure difference the liquid membrane can support. The support must be chemically inert toward all components in the feed phase, membrane phase, and sweep or receiving phase. 

The other support parameter determining the diffusional path length is the tortuosity, which is a measure of the deviation of the structure from cylindrical pores normal to the support surface. 

The support is a microporous PVC-silica sheet having a porosity in the 70-80% range. The pore size as determined by mercury intrusion porosimetry is in the 0.2 u to 2.0 urn range. The support is extremely hydrophilic, has a negative charge, and a surface area of 80 m /g. The material is non-compressible under normal conditions, is steam sterilizable, and has a low dry density of 0.45 g/cm. The microporous support has received FDA approval for direct food contact. The tortuosity of the pore structure requires that the substrate make intimate contact with the active enzyme as it passes through the support material. The active sites are attributed to the silica contained within the porous matrix which allows the addition of organic functionality. 

Support for the idea of a percolated structure connected by solid bridges may be found in the Frisk and Laurent [ 1996] patent on the reduction of permeability through a polyester wall container reduction by a factor of 100 was reported after addition of w = 5 wt% clay (aspect ratio p = 1000 to 1500). This large effect cannot be explained by the standard mechanism of tortuosity [see Utracki, 2004, vol. 2], but it is logical if combined with the intrinsie reduction of matrix permeability caused by solidification of polymer on clay platelets that form a continuous barrier to CO2 fiux. 

Different supports are used (see Section 11.6.4) with different geometry (discs or tubes), thickness, porosity, tortuosity, composition (alnmina, stainless steel, silicon carbide, mullite, zirconia, titania, etc.), and synunetry or asymmetry in their structure. Tubular supports are preferable compared to flat supports because they are easier to scale up (implemented as multichannel modules). However, in laboratory-scale synthesis, it is usually found that making good-quality zeolite membranes on a tubular support is more difficult than on a porous plate. One obvious reason is the fact that the area is usually smaller in flat supports, which decreases the likelihood of defect appearance. In Figure 11.1, two commercial porous tubular supports, one made of a-alumina (left side) and the other of stainless steel (right side) used in zeolite-membrane synthesis, are shown. Both ends of the a-alumina... 

The A, B and C terms are related to flow anisotropy, molecular longitudinal diffusion and mass transfer processes, respectively. The theoretical support for the Knox equation was derived by Horvath [12]. The A term cannot be expressed simply. The theoretical treatment links A to structural parameters of the column packing, porosity, pore volume, pore diameter and tortuosity [12]. A is related to the flow pattern and the general band spreading due to "eddy" diffusion [13]. The B term (longitudinal molecular diffusion) was written as [13] ... 

The characteristics of membrane employed as support for SLM preparation influence both flux and stability in principle, higher porosity, less thickness and lower tortuosity of membrane matrix provide a higher flux. On the other hand, a membrane support with a thin and less tortuous structure is not favorable in terms of stability. SLM stability can also be affected by the organic solvent used in the LM phase and the method of preparation (Yang et al., 2007). 

If all this is known, then what is left to do The fact is that most of these demonstrations have been at the laboratory level and not much beyond that level. Preparation of these materials is still an art and not a science. The science begins once a useful membrane has been prepared in the lab prior to that point, despite much effort, the steps of preparation remain art. Can this be overcome Certainly it can be. Similarly, it is clear that being able to manufacture materials readily, reproducibly and at low cost, remains a barrier to adoption and apphcation Then there is the science associated with the synthesis of the pore stractures. Still too little is known about the details of the pore structure in many carbon membranes, and this has tended to limit the science that seeks to understand the mechanisms of separation—especially nanoscale kinetic separations. Eventually, synthesis of regular pore stractures must be the goal, so that we can have carbon membranes tailored for each application with pore stractures having optimal orientation for regular transport and low tortuosity, with pores that are sized for the separation to be done and on support media that lend themselves to ready incorporation into a module. 

Conventional FO membranes have a similar structure with UF, NF, and RO, consisting of a top thin barrier layer and a thick support layer. The drawback is due to the phenomenon of internal concentration polarization (ICP), caused by the tormous and dense support layer hindering the compensate diffusion passing through the support layer. ICP leads to a lower water flux, and it gets worse with solute concentration increase [94]. Most of the conventional FO membranes have water flux rate of less than 25 L/m h. Loeb and co-workers described that the appropriate support layer for FO should have low tortuosity, high porosity, and a thin structure [95]. 

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