Thin selection

Ultra filtration. This process removes macromolecules, microorganisms, particulate matter, and pyrogens using a thin, selectively permeable membrane. Ultrafiltration caimot remove ions from water and is generally employed as a polishing process. 

Carotenoids are also present in animals, including humans, where they are selectively absorbed from diet (Furr and Clark 1997). Because of their hydrophobic nature, carotenoids are located either in the lipid bilayer portion of membranes or form complexes with specific proteins, usually associated with membranes. In animals and humans, dietary carotenoids are transported in blood plasma as complexes with lipoproteins (Krinsky et al. 1958, Tso 1981) and accumulate in various organs and tissues (Parker 1989, Kaplan et al. 1990, Tanumihardjo et al. 1990, Schmitz et al. 1991, Khachik et al. 1998, Hata et al. 2000). The highest concentration of carotenoids can be found in the eye retina of primates. In the retina of the human eye, where two dipolar carotenoids, lutein and zeaxan-thin, selectively accumulate from blood plasma, this concentration can reach as high as 0.1-1.0mM (Snodderly et al. 1984, Landrum et al. 1999). It has been shown that in the retina, carotenoids are associated with lipid bilayer membranes (Sommerburg et al. 1999, Rapp et al. 2000) although, some macular carotenoids may be connected to specific membrane-bound proteins (Bernstein et al. 1997, Bhosale et al. 2004). 

Zeolite/polymer mixed-matrix membranes can be fabricated into dense film, asymmetric flat sheet, or asymmetric hollow fiber. Similar to commercial polymer membranes, mixed-matrix membranes need to have an asymmetric membrane geometry with a thin selective skin layer on a porous support layer to be commercially viable. The skin layer should be made from a zeohte/polymer mixed-matrix material to provide the membrane high selectivity, but the non-selective porous support layer can be made from the zeohte/polymer mixed-matrix material, a pure polymer membrane material, or an inorganic membrane material. 

Although less discussed in the technical and scientific literature, permeate-side concentration polarisation may also become a problem when using thin selective films that require macroporous supports for mechanical stability [13]. 

Various thin selective layers, as discussed in Chapter 1, can be used to provide chemical selectivity. As with piezoelectric crystals, the acoustic properties of these films affect the performance of the SAW sensor in different ways by the change of... 

Anisotropic membranes are layered structures in which the porosity, pore size, or even membrane composition change from the top to the bottom surface of the membrane. Usually anisotropic membranes have a thin, selective layer supported on a much thicker, highly permeable microporous substrate. Because the selective layer is very thin, membrane fluxes are high. The microporous substrate... 

Currently, most solution-coated composite membranes are prepared by the method first developed by Riley and others [45,56,57], In this technique, a polymer solution is cast directly onto the microporous support. The support must be clean, defect-free and very finely microporous, to prevent penetration of the coating solution into the pores. If these conditions are met, the support can be coated with a liquid layer 50-100 xm thick, which after evaporation leaves a thin selective film 0.5-2 xm thick. A schematic drawing of the meniscuscoating technique is shown in Figure 3.25 [58], Obtaining defect-free films by this technique requires considerable attention to the preparation procedure and the coating solution. 

Figure 3.36 Apparatus to make composite hollow fiber membranes by coating a hollow fiber support membrane with a thin selective coating [105]...

Recently, attempts have been made to reduce the cost of palladium metal membranes by preparing composite membranes. In these membranes a thin selective palladium layer is deposited onto a microporous ceramic, polymer or base metal layer [19-21], The palladium layer is applied by electrolysis coating, vacuum sputtering or chemical vapor deposition. This work is still at the bench scale. 

During the last few years, ceramic- and zeolite-based membranes have begun to be used for a few commercial separations. These membranes are all multilayer composite structures formed by coating a thin selective ceramic or zeolite layer onto a microporous ceramic support. Ceramic membranes are prepared by the sol-gel technique described in Chapter 3 zeolite membranes are prepared by direct crystallization, in which the thin zeolite layer is crystallized at high pressure and temperature directly onto the microporous support [24,25],... 

Cross-section structure. An anisotropic membrane (also called asymmetric ) has a thin porous or nonporous selective barrier, supported mechanically by a much thicker porous substructure. This type of morphology reduces the effective thickness of the selective barrier, and the permeate flux can be enhanced without changes in selectivity. Isotropic ( symmetric ) membrane cross-sections can be found for self-supported nonporous membranes (mainly ion-exchange) and macroporous microfiltration (MF) membranes (also often used in membrane contactors [1]). The only example for an established isotropic porous membrane for molecular separations is the case of track-etched polymer films with pore diameters down to about 10 run. All the above-mentioned membranes can in principle be made from one material. In contrast to such an integrally anisotropic membrane (homogeneous with respect to composition), a thin-film composite (TFC) membrane consists of different materials for the thin selective barrier layer and the support structure. In composite membranes in general, a combination of two (or more) materials with different characteristics is used with the aim to achieve synergetic properties. Other examples besides thin-film are pore-filled or pore surface-coated composite membranes or mixed-matrix membranes [3]. 

An interesting pore-filled composite membrane, made by photograft copolymerization onto a solvent-stable PAN UF membrane, has been established [47]. High flux and selectivity for PV separation of organic-organic mixtures were achieved by a very thin selective barrier and prevention of swelling of the selective polymer in the pores of the barrier. 

Each membrane/module type has advantages and disadvantages [2,7]. Hollow fine fibers are generally the cheapest on a per-square-meter basis, but it is harder to make very thin selective membrane layers in hollow-fiber form than in flat-sheet form. This means the permeances of hollow fibers are usually lower than flat-sheet membranes made from the same material. Also, hollow fine fiber modules require more pretreatment of the feed to remove particulates, oil mist and other fouling components than is usually required by capillary or spiral-wound modules. These factors offset some of the cost advantage of the hollow fine fiber design. 

Membranes used for separation are thin selective barriers. They may be selective on the basis of size and shape, chemical properties, or electrical charge of the materials to be separated. As discussed in previous sections, membranes that are microporous control separation predominantly by size discrimination, charge interaction, or a combination of both, while nonporous membranes rely on preferential sorption and molecular diffusion of individual species. This permeation selectivity may, in turn, originate from chemical similarity, specific complexation, and/or ionic interaction between the permeants and the membrane material, or specific recognition mechanisms such as bioaffinity. 

This method, developed by Cadotte and coworkers of Film Tech in the 1970s, is currently most widely used to prepare high-performance reverse osmosis and nanofiltration membranes.A thin selective layer is deposited on top of a porous substrate membrane by interfacial in situ polycondensation. There are a number of modifications of this method primarily based on the choice of the monomers.However, for the sake of simplicity, the polycondensation procedure is described by a pair of diamine and diacid chloride monomers. 

As shown above, aromatic rings are connected by an amide linkage, -CONH-. While the aromatic ring attached to -NH- is m a-substituted, the ring attached to -CO- is the mixture of meta- and para-substitutions, which gives more flexibility to the polymeric material. Aromatic polyamide remains one of the most important materials for RO membranes because the thin selective layer of composite membranes is aromatic polyamide synthesized by interfacial in situ polymerization. 

According to the method, a relatively thick silicone rubber layer is coated on a thin selective layer of an asymmetric PS membrane. The thickness of silicone rubber is about 1 pm while the effective thickness of the selective PS layer is 1/10 of 1pm. While being coated, silicone rubber penetrates into the pores to plug them (Fig. 5). Thus, the feed gas is not allowed to leak through the defective pores. The selectivity of the membrane approaches that of the defect-free PS layer. Moreover, because the permeabilities of silicone rubber for gases are orders of magnitude higher than those of PS, the permeation rate is not affected very much even when a relatively thick silicone rubber layer is coated. 

Comparison of RO and CCRO. In virtually all RO membranes, a thin, selective skin layer is supported by a much thicker microporous sublayer. During RO operation, the composition of the permeate is determined by the selectivity of the skin layer, the feed solution composition, and the operating pressure. The concentration of the permeate is established as the feed solution flows through the skin layer, and it remains constant inside the sublayer. This concentration profile is shown in Figure 2a. 

Recently developed techniques for the fabrication of thin-film metal phosphates and phosphonates provide another method for the preparation of layered solids at surfaces. Layers of precisely controlled thickness can be built up by alternate immersion of a suitably pretreated surface in aqueous solutions of a soluble phosphate or phosphonate followed by an appropriate metal salt. This leads to the sequential build-up of thin metal containing films at the surface [72, 214] (see figure 6.18). The method is quite flexible and can be used to build up mixed microporous films on the surface which show molecular sieving properties [215, 216]. This building up approach looks very attractive for the systematic development of thin, selective films. 

Approach One approach to the development of a very thin membrane with gt mechanical strength is to integrate a structure with distmet selective and stqrpotting elements. An example of this would be a thin selective layer supported by a thick porous layer. Since the functionality of the two elements are now sqrarated, the flux and mechanical strength can be manipulated independently to meet the application requirements. 

Membrane performance also depends on the thickness of the selective layers of a silver-polymer complex, that is, the thinner the selective layer, the better the transport performance of the membrane. A thickness of several micrometers for the selective layer was the limitation in conventional methods used to prepare composite membranes. Based on exciting developments in the field of nanostructure science and technology, there have been several attempts to reduce the thickness of the selective layer to a nanometer length scale [41-43]. Schematic methods for preparing nano-thin selective layer membranes are generalized in Fig. 9-13, based on the use of nanometer-sized dendrimers, star... 

Figure 9-13. Schematic methods for the preparation of nano-thin selective layer...

Great improvements in the TFC membranes were also experienced by Chen et al. [56] by incorporating water-soluble amine reactants—sulfonated cardo poly(arylene ether sulfone) (SPES-NH2)—into an aqueous solution containing MPD. Under optimum preparation conditions, the TFC membranes prepared from SPES-NH2 showed remarkable increase in water permeability (51.2 L/m h) with a slight decrease in salt rejection (97.5% at 2000 ppm NaCl, 2 MPa) compared to membranes prepared without SPES-NH2 (37.4 L/m h and 99%). The improved results are attributed to the incorporation of hydrophilic SPES-NH2 to PAs and/or a higher degree of cross-linking formed in the thin selective layer. In view of the importance of hydrophilicity on TFC membrane performance, a novel amine monomer—3,5-diamino-A-(4-aminophenyl) benzamide (DABA)—with three amino... 

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