Metallic membranes technology developments

In order to maintain a definite contact area, soHd supports for the solvent membrane can be introduced (85). Those typically consist of hydrophobic polymeric films having pore sizes between 0.02 and 1 p.m. Figure 9c illustrates a hoUow fiber membrane where the feed solution flows around the fiber, the solvent—extractant phase is supported on the fiber wall, and the strip solution flows within the fiber. Supported membranes can also be used in conventional extraction where the supported phase is continuously fed and removed. This technique is known as dispersion-free solvent extraction (86,87). The level of research interest in membrane extraction is reflected by the fact that the 1990 International Solvent Extraction Conference (20) featured over 50 papers on this area, mainly as appHed to metals extraction. Pilot-scale studies of treatment of metal waste streams by Hquid membrane extraction have been reported (88). The developments in membrane technology have been reviewed (89). Despite the research interest and potential, membranes have yet to be appHed at an industrial production scale (90). 

The understanding of bio- and chemo-catalytic functionalities, their integration in recognizing materials (doped materials, membranes, tubes, conductive materials, biomarker detection, etc.) and the development of smart composite materials (e.g., bio-polymer-metal) are all necessary elements to reach above objectives. It is thus necessary to create the conditions to realize a cross-fertilization between scientific areas such as catalysis, membrane technology, biotech materials, porous solids, nanocomposites, etc., which so far have had limited interaction. Synergic interactions are the key factor to realizing the advanced nanoengineered devices cited above. 

Figure 21.4 Porous membranes developed for emulsification processes, (a) Shirasu porous glassy membrane (from SPG Technology Co., LTD, Japan), (b) metallic membrane (From Micropore Technologies, United Kingdom).

In these sensors the technology developed for ISFET construction is used in conventional electrodes. Links between the membrane and internal reference are metallic (ohmic contact), by deposition of the metal on the membrane (solid state membranes), or by deposition of an ion-selective membrane on a metal. This latter is an integrated sensor. 

This is the most recent development in membrane technology. It is currently being explored for a variety of applications, includii recovery of metals from low grade ores by hydrometallury and recovery of metal ions from a variety of waste waters for pollution control. 

Membrane technology may become essential if zero-discharge mills become a requirement or legislation on water use becomes very restrictive. The type of membrane fractionation required varies according to the use that is to be made of the treated water. This issue is addressed in Chapter 35, which describes the apphcation of membrane processes in the pulp and paper industry for treatment of the effluent generated. Chapter 36 focuses on the apphcation of membrane bioreactors in wastewater treatment. Chapter 37 describes the apphcations of hollow fiber contactors in membrane-assisted solvent extraction for the recovery of metallic pollutants. The apphcations of membrane contactors in the treatment of gaseous waste streams are presented in Chapter 38. Chapter 39 deals with an important development in the strip dispersion technique for actinide recovery/metal separation. Chapter 40 focuses on electrically enhanced membrane separation and catalysis. Chapter 41 contains important case studies on the treatment of effluent in the leather industry. The case studies cover the work carried out at pilot plant level with membrane bioreactors and reverse osmosis. Development in nanofiltration and a case study on the recovery of impurity-free sodium thiocyanate in the acrylic industry are described in Chapter 42. 

Development and application of composites containing nanostructured forms of metals open new horizons in creation of novel composite materials with controlled-quality properties and their usage in various kinds of human activity medicine, nanophotonics, catalytic chemical and refining processes, membrane technology, ecology, climate-monitoring systems, pure energetic, etc. 

Due to low prices for transition metals, it becomes increasingly true that with the development of effective and efficient ligands the costs of these ligands exceed those of the transition metals. Appropriately modified, asymmetric, re-immobilized ligands which yield a lower enantioselectivity in an aqueous medium [29] may therefore have a good chance in the future in connection with membrane technology. 

Initial hydrogen diffusion experiments with metal membranes began as early as the middle of the nineteenth century with palladium [3-5], Since these experiments, Pd has been the most widely investigated membrane material with the most acknowledged studies being conducted in the mid-twentieth century [6-11], Additionally, several comprehensive reviews have been published that summarize the developments of palladium membrane technologies over the past 150 years [12, 13],... 

Commercial eind laboratory applications of liquid membrane technology are discussed including gas transport, sensor development, metal ion recovery, waste treatment, biotechnology and biomedical engineering. Immobilized liquid membranes, emulsion or liquid surfactant membranes, and membrane reactors are discussed. Economic data from the literature for liquid membrane processes are presented and compared with existing processes such as solvent extraction and cryogenic distillation of air. 

Metallic membranes, (Pd-Ag) alloys, are typically used for separation of H2, either as an unsupported foil or a supported thin film. In these membranes, the hydrogen transport is by adsorption and atomic dissociation on one side of the membrane, dissolution in the membrane, followed by diffusion, and finally desorption (on the other side). Due to the H2 dissociation step, H2 separation is driven by a transmembrane difference of the square roots of the hydrogen partial pressures. The preparation technologies of both unsupported and supported Pd-Ag membranes are well developed and such membranes are commercially available. Since the membrane reformer performance is limited by separation capability, optimization of membrane permeability is one of the important issues. 

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