Membranes separation layer

Porous ceramic membrane layers are formed on top of macroporous supports, for enhanced mechanical resistance. The flow through the support may consist of contributions due to both Knudsen-diffusion and convective nonseparative flow. Supports with large pores are preferred due to their low resistance to the flow. Supports with high resistance to the flow decrease the effective pressure drop over the membrane separation layer, thus diminishing the separation efficiency of the membrane (van Vuren et al. 1987). For this reason in a membrane reactor it is more effective to place the reaction (catalytic) zone at the top layer side of the membrane while purging at the support side of the membrane. 

Finally it should be noted that the characterisation of membranes is more demanding than most other porous materials. Firstly, the membranes separation layer is generally thin and supported, which requires a sensitive technique capable of analysing a sample in such a form. The characterisation of a powder "equivalent" to the membrane carmot in all cases be considered as representative of the membrane texture. Secondly, the structure is frequently anisotropic and moreover often microporous. Assessment of the microporosity is much less advanced compared to meso- and macro-porosity, despite emphasis given to this in the recent lUPAC symposia [6-8]. The current and widespread interest in the characterisation of microporous matericds is well illustrated by the numerous and varied publications found in these symposia proceedings. These highlight recent developments in characterisation techniques, their applications and limitations. The particular features of importance in membrane studies will be considered in the light of the characterisation techniques to be described. 

The membrane thickness, the application temperature, and other species present further complicate the selection of a membrane polymer for a specific gas-separation. Equations show that the gas fluxes through the membrane are inversely proportional to the membrane thickness, 1. Most of the published gas-permeabihty data arises from pure gas measurements on flat, thick, dense films supported on a porous backing medium to accommodate the applied pressure differential. Thus while a specific polymer may be chosen for its favorable selectivity, it may very well be impractical if it cannot be fabricated into a very thin membrane-separating layer that is stable in the intended application. 

The above equation assumes that the swelling of the membrane separating layer is negligible. It is similar to the well-known equation presented by Wij-... 

Most polymeric OSN membranes have an asymmetric structure and are porous with a dense top layer. This asymmetry can be divided into two major types the integral type, where the whole membrane is composed of the same material, and the thin-film composite (TFC), where the membrane separating layer is made of a different material. 

In filtration processes for the extracorporeal treatment of renal failure, partially rejeeted proteins accumulate at the membrane separation layer [i.e., concentration polarization (CP) phenomena occur] to an extent that depends on protein eoncentralion in the bulk blood, and the fluid dynamics of the blood compartment. Higher protein concentrations at the membrane surface cause the membrane sieving coefficient to be higher than that expeeted, based on the intrinsic membrane separation properties. However, once the latter are known, the actual sieving coefficient can be estimated from the operating conditions and module geometry as follows (Klein et al., 1978) ... 

FIGURE 4.17 FESEM images on the permeate side of a bare porous alumina substrate (a) before and (b) after adsorption of 10 PAH/PSS bilayers on the membrane separation layer. (Reprinted with permission from Harris, J. J. et al., Chem. Mater. 12, 1941-1946, 2000. Copyright 2000 American Chemical Society.)... 

Cellulose acetate Loeb-Sourirajan reverse osmosis membranes were introduced commercially in the 1960s. Since then, many other polymers have been made into asymmetric membranes in attempts to improve membrane properties. In the reverse osmosis area, these attempts have had limited success, the only significant example being Du Font s polyamide membrane. For gas separation and ultrafUtration, a number of membranes with useful properties have been made. However, the early work on asymmetric membranes has spawned numerous other techniques in which a microporous membrane is used as a support to carry another thin, dense separating layer. 

The phenomenon of concentration polarization, which is observed frequently in membrane separation processes, can be described in mathematical terms, as shown in Figure 30 (71). The usual model, which is weU founded in fluid hydrodynamics, assumes the bulk solution to be turbulent, but adjacent to the membrane surface there exists a stagnant laminar boundary layer of thickness (5) typically 50—200 p.m, in which there is no turbulent mixing. The concentration of the macromolecules in the bulk solution concentration is c,. and the concentration of macromolecules at the membrane surface is c. 

Most commercially available RO membranes fall into one of two categories asymmetric membranes containing one polymer, or thin-fHm composite membranes consisting of two or more polymer layers. Asymmetric RO membranes have a thin ( 100 nm) permselective skin layer supported on a more porous sublayer of the same polymer. The dense skin layer determines the fluxes and selectivities of these membranes whereas the porous sublayer serves only as a mechanical support for the skin layer and has Httle effect on the membrane separation properties. Asymmetric membranes are most commonly formed by a phase inversion (polymer precipitation) process (16). In this process, a polymer solution is precipitated into a polymer-rich soHd phase that forms the membrane and a polymer-poor Hquid phase that forms the membrane pores or void spaces. 

A fundamental difference exists between the assumptions of the homogeneous and porous membrane models. For the homogeneous models, it is assumed that the membrane is nonporous, that is, transport takes place between the interstitial spaces of the polymer chains or polymer nodules, usually by diffusion. For the porous models, it is assumed that transport takes place through pores that mn the length of the membrane barrier layer. As a result, transport can occur by both diffusion and convection through the pores. Whereas both conceptual models have had some success in predicting RO separations, the question of whether an RO membrane is truly homogeneous, ie, has no pores, or is porous, is still a point of debate. No available technique can definitively answer this question. Two models, one nonporous and diffusion-based, the other pore-based, are discussed herein. 

An important consideration is that the goodness of the separation is almost independent of the membrane thickness, z, and the rate of the process is inversely proportional to z, giving rise to a major emphasis on maldng the separating layer of a membrane very thin. It is rare that z is known for a commercial membrane, and Jj is stated without regard to z. 

From what we know today about PET in biological and synthetic membrane or layered systems, we may expect that the non-biological apparatus providing photogeneration of spatially separated one-electron reductant and oxidant is likely to be developed in a rather universal way and may be expected to accomplish in the future not only water cleavage, but also various other redox reactions, such e.g., as photochemical synthesis of ammonia via the hv... 

Complete equilibration of two solutions separated by a membrane is a very slow process. Often quasiequilibrium systems are used, where there is no equilibrium between the outer solutions (their composition is that arbitrarily given at the outset), although each of these solutions is in equilibrium with an adjacent thin membrane surface layer there is no equilibrium within the membrane between these surface layers. 

Pericytes lie periendothelially on the abluminal side of the microvessels (Figure 15.3). A layer of basement membrane separates the pericytes from the endothelial cells and the astrocyte foot processes. Pericytes send out cell processes which penetrate the basement membrane and cover around 20-30% of the micro-vascular circumference [18]. Pericyte cytoplasmic projections encircling the endothelial cells provide both a vasodynamic capacity and structural support to the microvasculature. They bear receptors for vasoactive mediators such as catecholamines, endothelin-1, VIP, vasopressin and angiotensin II. Pericytes become mark-... 

In a previous section, the effect of plasma on PVA surface for pervaporation processes was also mentioned. In fact, plasma treatment is a surface-modification method to control the hydrophilicity-hydrophobicity balance of polymer materials in order to optimize their properties in various domains, such as adhesion, biocompatibility and membrane-separation techniques. Non-porous PVA membranes were prepared by the cast-evaporating method and covered with an allyl alcohol or acrylic acid plasma-polymerized layer the effect of plasma treatment on the increase of PVA membrane surface hydrophobicity was checked [37].The allyl alcohol plasma layer was weakly crosslinked, in contrast to the acrylic acid layer. The best results for the dehydration of ethanol were obtained using allyl alcohol treatment. The selectivity of treated membrane (H20 wt% in the pervaporate in the range 83-92 and a water selectivity, aH2o, of 250 at 25 °C) is higher than that of the non-treated one (aH2o = 19) as well as that of the acrylic acid treated membrane (aH2o = 22). 

The qualitative problems involved in this development also were formidable. UFg is chemically very aggressive, which limits the choice of possible materials. Some metals and ceramics are among the candidate materials. Typically, tubular membranes were developed, which comprised a macropo-rous support, one or several intermediate layers of decreasing thickness and pore diameter, and the separating layer. The separating layer covered the internal surface of the tube (Charpin and Rigny 1990). 

For Eurodif and for Pierrelatte, the supports were made by private industrial companies, the final separating layer by SPEC and the CEA developed the process and had the overall technical responsibility. A handful of companies were competing to manufacture the membrane support structure. Finally, two companies proposing ceramic oxide based supports, Ceraver (the new name of CGEC) and Euroceral (a 50/50 joint venture between Norton and Desmarquest) each won 50% of the market. This happened in 1975. Within a matter of 6 years, each company had to deliver more than 2,000,000 m of supports which SPEC would convert into more than 4,000,000 m of membranes (Charpin and Rigny 1990). Special plants were built at a very rapid pace. These were close to Tarbes for Ceraver, close to Montpellier for Euroceral and close to the Eurodif site for SPEC. 

The use of a multichannel support made of a sintered oxide carrying a separation layer deposited on the surface of the channels was not a new concept. This was described in the patent literature as far back as the 1960s (Manjikian 1966). The multichannel geometry is particularly attractive in terms of its sturdiness, lower production cost compared to the single tube or tube-bundle geometry and lower energy requirement in the cross-flow recirculation loop. However, Ceraver was the first company to industrially produce multichannel membranes. Since 1984 these membranes, which have 19 channels per element with a 4 mm channel diameter are sold under the trademark Membralox. ... 

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