Thin selective membrane layers

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. 

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. 

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. 

Most gas separation processes require that the selective membrane layer be extremely thin to achieve economical fluxes. Typical membrane thicknesses are less than 0.5 xm and often less than 0.1 xm. Early gas separation membranes [22] were adapted from the cellulose acetate membranes produced for reverse osmosis by the Loeb-Sourirajan phase separation process. These membranes are produced by precipitation in water the water must be removed before the membranes can be used to separate gases. However, the capillary forces generated as the liquid evaporates cause collapse of the finely microporous substrate of the cellulose acetate membrane, destroying its usefulness. This problem has been overcome by a solvent exchange process in which the water is first exchanged for an alcohol, then for hexane. The surface tension forces generated as liquid hexane is evaporated are much reduced, and a dry membrane is produced. Membranes produced by this method have been widely used by Grace (now GMS, a division of Kvaemer) and Separex (now a division of UOP) to separate carbon dioxide from methane in natural gas. 

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]. 

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. 

Thin film composite membranes consist of a thin, selective polymer layer atop a porous support. In this membrane type, the separation and mechanical functions... 

Sorption of the solvent molecules in polymer at the liquid-membrane interface. The membrane swells and behaves like a thin selective solvent layer. This step is generally fast. The sorption selectivity is mainly responsible of the efficiency of the membrane. 

Contact modalities and concentration profiles in catalytic membrane reactors for three-phase systems.The concentration of reactants is represented on the y-axis and the spatial coordinate along the membrane cross-section is represented on the x-axis. Below the scheme of each case the sequence of the mass transfer (MT) resistances and of the reaction event (R) are reported. (a)Traditional slurry reactor (b) supported thin porous catalytic layer with the liquid impregnating the porosity and the gas phase in contact with the catalytic layer (c) supported thin porous catalytic layer with the liquid impregnating the porosity and the liquid phase in contact with the catalytic layer (d) supported dense membrane which is perm-selective to the gas-phase reactant (e) dense catalytic membrane perm-selective to both reactants in the gas and liquid phases (f) forced flow of the liquid phase enriched with the gas-phase reactant through the thin catalytic membrane layer. 

In pervaporation it is more advantageous to use the inside-out type to avoid increase in permeate pressure within the fibers, but the outsidc-in concept can be used as well with short fibers. Another advantage of the inside-out concept is that the very thin selective top layer is better protected, whereas a higher membrane area can be achieved with the outside-in concept. 

The selective layer of a membrane should be as thin as possible since the flux is inversely proportional to the membrane thickness. Thus, membranes for industrial applications are composite structures with a thin selective dense layer (0.5-5 micron) on a porous and mechanically stable support (Figure 20.2a) [20, 21]. This issue is particularly important for a soft and elastic material such as PDMS. The nature of the support can be organic (e.g., cellulose acetate, polysul-fone, polytetrafluoroethylene, polyvinylidine fluoride, etc.) or inorganic (e.g., alumina, titania). The support is often backed by a highly porous non-woven layer. In principle, the role of the support on the mass transport should be negligible, but it could be predominant in some cases [22]. 

The use of a polymer for supporting the preparation of composite membranes with thin selective palladium layers was first established by Gryaznov etal (1969). 

The explicit mathematical treatment for such stationary-state situations at certain ion-selective membranes was performed by Iljuschenko and Mirkin 106). As the publication is in Russian and in a not widely distributed journal, their work will be cited in the appendix. The authors obtain an equation (s. (34) on page 28) similar to the one developed by Eisenman et al. 6) for glass membranes using the three-segment potential approach. However, the mobilities used in the stationary-state treatment are those which describe the ion migration in an electric field through a diffusion layer at the phase boundary. A diffusion process through the entire membrane with constant ion mobilities does not have to be assumed. The non-Nernstian behavior of extremely thin layers (i.e., ISFET) can therefore also be described, as well as the role of an electron transfer at solid-state membranes. 

Immobilized enzymes used in conjunction with ion-selective electrodes provide very convenient methods of analysis. The immobilized enzyme may be held in a gel or membrane around the electrode and the substance to be measured diffuses into the enzyme gel. Its conversion to the product alters the ionic equilibrium across the ion-selective membrane (Figure 8.23). It is important that the enzyme layer is thin, to minimize any problems caused by slow diffusion rates through the layer. 

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. 

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],... 

Yet another approach to stabilizing facilitated transport membranes is to form multilayer structures in which the supported liquid-selective membrane is encapsulated between thin layers of very permeable but nonselective dense polymer layers. The coating layers must be very permeable to avoid reducing the gas flux through the membrane materials such as silicone rubber or poly(trimethylsilox-ane) are usually used [26],... 

These observations have several practical consequences for membrane processes where the selective layers are as thin as or even thinner than the low end of the range studied here. First, it is clear that use of thick film data to design or select membrane materials only gives a rough approximation of the performance that might be realized in practice. Second, because the absolute permeability of a thin film may be severalfold different than the bulk permeability, use of the latter type of data to estimate skin thickness from flux observations on asymmetric or composite membranes structures is also a very approximate method. Finally, these data indicate that one could expect... 

Good quality RO membranes can reject >95-99% of the NaCl from aqueous feed streams (Baker, Cussler, Eykamp et al., 1991 Scott, 1981). The morphologies of these membranes are typically asymmetric with a thin highly selective polymer layer on top of an open support structure. Two rather different approaches have been used to describe the transport processes in such membranes the solution-diffusion (Merten, 1966) and surface force capillary flow model (Matsuura and Sourirajan, 1981). In the solution-diffusion model, the solute moves within the essentially homogeneously solvent swollen polymer matrix. The solute has a mobility that is dependent upon the free volume of the solvent, solute, and polymer. In the capillary pore diffusion model, it is assumed that separation occurs due to surface and fluid transport phenomena within an actual nanopore. The pore surface is seen as promoting preferential sorption of the solvent and repulsion of the solutes. The model envisions a more or less pure solvent layer on the pore walls that is forced through the membrane capillary pores under pressure. 

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