Membranes macroporous

Gas-permeable membranes Macroporous PIPE foil Dissolved gases (O2 CI2)... 

Air is commonly run with tube-side feed. The permeate is run countercurrent with the separating sldn in contact with the permeate. (The feed gas is in contact with the macroporous back side of the membrane.) This configuration has proven to be superior, since the permeate-side mass-transfer problem is reduced to a minimum, and the feed-side mass-transfer problem is not limiting. 

The pores of tire separating membrane are to be most uniformly distributed and of minimum size to avoid deposition of metallic particles and thus electronic bridging. One distinguishes between macroporous and microporous separators, the latter having to show pore diameters below I micron (/urn ), i.e., below one-thousandth of a millimeter. Thus the risk of metal particle deposition and subsequent shorting is quite low, since active materials in storage batteries usually have particle diameters of several microns. 

A packed-bed nonpermselective membrane reactor (PBNMR) is presented by Diakov et al. [31], who increased the operational stability in the partial oxidation of methanol by feeding oxygen directly and methanol through a macroporous stainless steel membrane to the PB. Al-Juaied et al. [32] used an inert membrane to distribute either oxygen or ethylene in the selective ethylene oxidation. By accounting for the proper kinetics of the reaction, the selectivity and yield of ethylene oxide could be enhanced over the fixed-bed reactor operation. 

Tennikov, M. B., Gazdina, N., Tennikova, T. B., and Svec, F., Effect of porous structure of macroporous polymer supports on resolution in high-performance membrane chromatography of proteins, J. Chromatogr. A, 798, 55, 1998. 

Besanger, T.R., Easwaramoorthy, B. and Brennan, J.D. (2004) Entrapment of highly active membrane-bound receptors in macroporous sol-gel derived silica. Analytical Chemistry, 76, 6470-6475. 

Recently, the LbL technique has been extended from conventional nonporous substrates to macroporous substrates, such as 3DOM materials [58,59], macroporous membranes [60-63], and porous calcium carbonate microparticles [64,65], to prepare porous PE-based materials. LbL-assembly of polyelectrolytes can also be performed on the surface of MS particles preloaded with enzymes [66,67] or small molecule drugs [68], and, under appropriate solution conditions, within the pores of MS particles to generate polymer-based nanoporous spheres following removal of the silica template [69]. 

Macroporous materials with various shapes such as particles, tubes, rods, fibers, membranes, and 3DOM have been designed to cater for different applications. Three... 

Fig. 7.n SEM images of the 3D macroporous functionalized macroporous zeolitic membrane zeolitic membrane used as a support for enzyme bioreactor was prepared via the LbL electrostatic immobilization by the LbL procedure. Images assembly of PEs and enzyme (catalase) on the 3D (A-D) are cross-sections of the membrane at macroporous membrane. (Reprinted from [59] different magnifications. A biomacromolecule- with permission of Wiley-VCH). 

Figure 6.6 ULtrafiLtration separates molecules based on size and shape, (a) Diagrammatic representation of a typical laboratory-scale ultrafiltration system. The sample (e.g. crude protein solution) is placed in the ultrafiltration chamber, where it sits directly above the ultrafilter membrane. The membrane, in turn, sits on a macroporous support to provide it with mechanical strength. Pressure is then applied (usually in the form of an inert gas), as shown. Molecules larger than the pore diameter (e.g. large proteins) are retained on the upstream side of the ultrafilter membrane. However, smaller molecules (particularly water molecules) are easily forced through the pores, thus effectively concentrating the protein solution (see also (b)). Membranes that display different pore sizes, i.e. have different molecular mass cut-off points, can be manufactured, (c) Photographic representation of an industrial-scale ultrafiltration system (photograph courtesy of Elga Ltd, UK)...

The microstructure of a catalyst layer is mainly determined by its composition and the fabrication method. Many attempts have been made to optimize pore size, pore distribution, and pore structure for better mass transport. Liu and Wang [141] found that a CL structure with a higher porosity near the GDL was beneficial for O2 transport and water removal. A CL with a stepwise porosity distribution, a higher porosity near the GDL, and a lower porosity near the membrane could perform better than one with a uniform porosity distribution. This pore structure led to better O2 distribution in the GL and extended the reaction zone toward the GDL side. The position of macropores also played an important role in proton conduction and oxygen transport within the CL, due to favorable proton and oxygen concentration conduction profiles. 

Terms such as symmetric and asymmetric, as well as microporous, meso-porous and macroporous materials will be introduced. Symmetric membranes are systems with a homogeneous structure throughout the membrane. Examples can be found in capillary glass membranes or anodized alumina membranes. Asymmetric membranes have a gradual change in structure throughout the membrane. In most cases these are composite membranes... 

In gas separation applications, polymeric hollow fibers (diameter X 100 fim) are used (e.g. PAN) with a dense skin. In the skin the micropores develop during pyrolyzation. This is also the case in the macroporous material but is not of great importance from gas permeability considerations. Depending on the pyrolysis temperature, the carbon membrane top layer (skin) may or may not be permeable for small molecules. Such a membrane system is activated by oxidation at temperatures of 400-450 C. The process parameters in this step determine the suitability of the asymmetric carbon membrane in a given application (Table 2.8). 

Inorganic membranes employed in reaction/transport studies were either in tubular form (a single membrane tube incorporating an inner tube side and an outer shell side in double pipe configuration or as multichannel monolith) or plate-shaped disks as shown in Figure 7.1 (Shinji et al. 1982, Zaspalis et al. 1990, Cussler 1988). For increased mechanical resistance the thin porous (usually mesoporous) membrane layers are usually supported on top of macroporous supports (pores 1-lS /im), very often via an intermediate porous layer, with pore size 100-1500 nm, (Keizer and Burggraaf 1988). 

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. 

The porous membrane templates described above do exhibit three-dimensionality, but with limited interconnectedness between the discrete tubelike structures. Porous structures with more integrated pore—solid architectures can be designed using templates assembled from discrete solid objects or su-pramolecular structures. One class of such structures are three-dimensionally ordered macroporous (or 3-DOM) solids, which are a class of inverse opal structures. The design of 3-DOM structures is based on the initial formation of a colloidal crystal composed of monodisperse polymer or silica spheres assembled in a close-packed arrangement. The interconnected void spaces of the template, 26 vol % for a face-centered-cubic array, are subsequently infiltrated with the desired material. 

The electrotransfer of proteins onto (non-specific) binding membranous sheets is named Western hlot in contrast to the transfer of DNA (Southern hlot) and of RNA (Northern hlot). The main advantage of blotting procedures lies in the immobilization and presentation of macromolecules on the surface of a solid planar material. This presentation leads to an easy access of reactants in the opposite to the diffusion-controlled motion of reaction partners within gels or macroporous spheres. 

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