Self-supported thin membranes

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]. 

Self-supporting inorganic membranes can be formed with or without a substrate. In either case the precursor sol, consisting of either a colloidal suspension or a polymeric solution, must be formed. To produce a membrane supported on a substrate (i.e. a supported membrane), a preformed porous support is dipped in the precursor sol and a gel forms at the surface of the support typically by the slipcasting method [48]. Another approach is spin coating, in which an excess amount of liquid is deposited onto a substrate and then thinned uniformly by centrifugal force [12]. To produce a non-supported membrane, the liquid is simply poured into a mold of appropriate shape and allowed to dry. All these processes need to be done before the gel point which is accompanied by a large increase in viscosity. 

Even if the detailed mechanism for specific response to analyte binding is not always clear yet, the data for ultrathin and thin MIP films suggest that the permeability of the entire MIP film is involved. Hence, MIP membranes are most versatile in triggering—and potentially amplifying—signals. Beside the supported MIP films described above, self-supported MIP membranes could, in principle, be used in sensors but also for larger-scale separations (see SectionV.A). 

The set-ups developed for studying membranes cannot be directly applied to thermotropic liquid crystals, as most of them - especially the nematics - are usually not able to form self-supporting thin films they require solid containers. The technique developed by the authors copes with this... 

This chapter also discusses how the stability of self-supported Pd membranes needs an appropriate design of the membrane module. The finger-like design of membrane modnles in both single- and multitube configuration avoids any mechanical stress of the thin-walled permeators, thus ensuring the durability of the membranes. 

There are two types of membranes available for ultra-pure H2 production composite and self-supported thin ones. In both cases, the major aim of the study concerning Pd-based membranes is the reduction of their thickness. The H2 flux increases as the membrane gets thinner, therefore the overall costs decrease because of both the reduced operating and material costs. In accordance with the applications where ultra-pure H2 production is required, commercial self-supporting dense Pd-based membranes with wall thicknesses larger than 100 j.m keep a sufficient mechanical strength and assure a defect-free surface and complete H2 selectivity. However, these membranes are too thick to obtain a satisfactory Hj flux. In addition to the low H2 permeance (as customarily used for composite membranes and defined as the ratio of Pch 15), thick Pd-based membranes are too expensive. Indeed,... 

As the name suggests, flat-sheet membranes are flat, like a sheet of paper, and can be made as thin as less than 1 pm. However, they need special holders to hold them in place. Hollow-fiber membranes are shaped like tubes (200 to 500 pm ID), allowing fluids to flow inside as well as on the outside. Hollow fibers are self-supported and offer the advantage of larger surface area per unit volume and high packing density. A large number of parallel fibers can be packed into a small volume. 

Alternative to fibers and 2D and 3D woven or nonwoven networks thereof formed as either self-supporting structures or as a hydrogel, it is possible to self-assemble peptides into thin self-assembled monolayers (SAMS) or multilayer structures. Such structures have been reported to act as membranes for controlled diffusion of ions and controlled movement of body fluids and contaminants (Ellis-Behnke et al., 2006, 2007 Holmes et al., 1999). Alternatively, various techniques have been put in place to provide coatings on various substrates ranging from tissue to metals and inorganics, for example, mica (Boden et al., 2002 Haynie, 2005,2007 Haynie and Zhi, 2007 Yoo et al., 2008). 

Pt(IV) than Pd(n) [49], An interesting system, with asymmetric inorganic membranes, was used for selective metal ion separation. The membrane phase was a self-assembled monolayer of alkyl thiols as a hydrophobic phase for a trialkyl phosphate and phosphine oxide-based metal ion carrier. This organic mixture was attached on alumina porous supports with thin layers of gold. The thin membrane layer gave high fluxes and high selectivity, while metal ions transport was carrier limited [50]. 

These numbers show that for Ci-utilization, quite thick membranes ca. 1 mm) can be used, which can be self-supporting. For dehydrogenation reactions very thin films are needed, which will be a difficult challenge for materials scientists to obtain phase and mechanical stability under process conditions. Further evidence of the need for highly selective materials for dehydrogenation is presented by Harold et al. and Sheintuch. ... 

Reformate obtained from low to moderate sulfur liquid fuels (< 15-30 ppm S by weight) would contain about 1-2 ppm sulfin species by volume. This sulfur concentration is low enough to be tolerated by the 60wt.% Pd-40wt.% Cu system according to the literature. Kulprathipanja et al. give an excellent discussion of the impact of sulfur [23] and other components of reformate [24], CO, CO2 and H2O on self supporting Pd-Cu foil and supported thin film Pd-Cu membranes. 

In parallel to development activities in the materials themselves, significant progress has been made in the architecture of devices. This includes a transition from dense, self-supported membranes to the use of thin films supported on porous substrates, as well as developments in tubular geometries and modified planar geometries. This chapter will provide an outiine of these development activities and provide possible insight into potential future developments. 

An essential element of the Hysep technology is the use of thin film palladium composite membranes to enable low cost and reliable hydrogen separation. The supported palladium layer in the Hysep module has a thickness as low as 3-9 pm, a substantial improvement over current commercial available palladium membranes, which are based on self supporting metal foils with a thickness of 20-100 pm. 

Finally, SINTEF has developed a technique for the manufacture of Pd-based hydrogen separation membranes based on a two-step process, allowing a reduction in membrane thickness. First, a defect-free Pd-alloy thin film is prepared by magnetron sputtering onto a silicon wafer. In the second step the film is removed from the wafer. These films may subsequently be either used self-supported or integrated with various supports of different pore size, geometry and size. This allows, for example, the preparation of... 

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