Metallic membrane support

Ceramic, Metal, and Liquid Membranes. The discussion so far implies that membrane materials are organic polymers and, in fact, the vast majority of membranes used commercially are polymer based. However, interest in membranes formed from less conventional materials has increased. Ceramic membranes, a special class of microporous membranes, are being used in ultrafHtration and microfiltration appHcations, for which solvent resistance and thermal stabHity are required. Dense metal membranes, particularly palladium membranes, are being considered for the separation of hydrogen from gas mixtures, and supported or emulsified Hquid films are being developed for coupled and facHitated transport processes. 

The teehniques of membrane extraetion permit an effieient and modern applieation of elassieal liquid-liquid extraetion (LLE) ehemistry to instmmental and automated operation. Various shorteomings of LLE are overeome by membrane extraetion teehniques as they use none or very little organie solvents, high enriehment faetors ean be obtained and there ai e no problems with emulsions. A three phase SLM system (aq/org/aq), where analytes are extraeted from the aqueous sample into an organie liquid, immobilized in a porous hydrophobie membrane support, and further to a seeond aqueous phase, is suitable for the extraetion of polar eompounds (aeidie or basie, ehai ged, metals, ete.) and it is eompatible with reversed phase HPLC. A two-phase system (aq/org) where analytes ai e extraeted into an organie solvent sepai ated from the aqueous sample by a hydrophobie porous membrane is more suitable for hydrophobie analytes and is eompatible with gas ehromatography. 

The solubilities of the various gases in [BMIM][PFg] suggests that this IL should be an excellent candidate for a wide variety of industrially important gas separations. There is also the possibility of performing higher-temperature gas separations, thanks to the high thermal stability of the ILs. For supported liquid membranes this would require the use of ceramic or metallic membranes rather than polymeric ones. Both water vapor and CO2 should be removed easily from natural gas since the ratios of Henry s law constants at 25 °C are -9950 and 32, respectively. It should be possible to scrub CO2 from stack gases composed of N2 and O2. Since we know of no measurements of H2S, SO, or NO solubility in [BMIM][PFg], we do not loiow if it would be possible to remove these contaminants as well. Nonetheless, there appears to be ample opportunity for use of ILs for gas separations on the basis of the widely varying gas solubilities measured thus far. 

The titanosilicate version of UTD-1 has been shown to be an effective catalyst for the oxidation of alkanes, alkenes, and alcohols (77-79) by using peroxides as the oxidant. The large pores of Ti-UTD-1 readily accommodate large molecules such as 2,6-di-ferf-butylphenol (2,6-DTBP). The bulky 2,6-DTBP substrate can be converted to the corresponding quinone with activity and selectivity comparable to the mesoporous catalysts Ti-MCM-41 and Ti-HMS (80), where HMS = hexagonal mesoporous silica. Both Ti-UTD-1 and UTD-1 have also been prepared as oriented thin films via a laser ablation technique (81-85). Continuous UTD-1 membranes with the channels oriented normal to the substrate surface have been employed in a catalytic oxidation-separation process (82). At room temperature, a cyclohexene-ferf-butylhydroperoxide was passed through the membrane and epoxidation products were trapped on the down stream side. The UTD-1 membranes supported on metal frits have also been evaluated for the separation of linear paraffins and aromatics (83). In a model separation of n-hexane and toluene, enhanced permeation of the linear alkane was observed. Oriented UTD-1 films have also been evenly coated on small 3D objects such as glass and metal beads (84, 85). 

Porous metals have long been commercially available for particulate filtration. They have been used in some cases as microfiltration membranes that can withstand harsh environments, or as porous supports for dynamic membranes. Stainless steel is by far the most widely used porous metal membrane. Other materials include silver, nickel. Monel, Hastelloy and Inconel. Their recommended maximum operating temperatures range from 200 to 650°C. Elepending on the pore diameter which varies from 0.2 to 5 microns, the water permeability of these symmetric membranes can exceed 3000 L/h-m -bar and is similar to that obtained with asymmetric ceramic microfiltration membranes. Due to the relatively high costs of these membranes, their use for microfiltration has not been widespread. 

In this chapter membrane preparation techniques are organized by membrane structure isotropic membranes, anisotropic membranes, ceramic and metal membranes, and liquid membranes. Isotropic membranes have a uniform composition and structure throughout such membranes can be porous or dense. Anisotropic (or asymmetric) membranes, on the other hand, consist of a number of layers each with different structures and permeabilities. A typical anisotropic membrane has a relatively dense, thin surface layer supported on an open, much thicker micro-porous substrate. The surface layer performs the separation and is the principal barrier to flow through the membrane. The open support layer provides mechanical strength. Ceramic and metal membranes can be either isotropic or anisotropic. 

Following the work of Bloch and Vofsi, other methods of producing immobilized liquid films were introduced. In one approach, the liquid carrier phase was held by capillarity within the pores of a microporous substrate, as shown in Figure 11.3(a). This approach was first used by Miyauchi [7] and by Largman and Sifniades and others [8,9], The principal objective of this early work was to recover copper, uranium and other metals from hydrometallurgical solutions. Despite considerable effort on the laboratory scale, the first pilot plant was not installed until 1983 [10], The main problem was instability of the liquid carrier phase held in the microporous membrane support. 

Belkhouche, N.E., Didi, M.A., Romero, R., Jonsson, J.A. and Villemin, D. (2006) Study of new organophosphorus derivates carriers on the selective recovery of M(II) and M(III) metals, using supported liquid membrane extraction. Journal of Membrane Science, 284, 398. Schlosser, S. (1997) Method and equipment for mass and heat transfer among several liquids (in Slovak), Slovak pat. No. 278547. 

Electrodialysis In this process, dialysis is carried out under the influence of electric field (figure 3) Potential is applied between the metal screens supporting the membranes. This speeds up the migration of ions to the opposite electrodes. So, dialysis is greatly accelerated. 

Experiments were conducted at the University of Magdeburg to examine the partial oxidation of ethane to ethylene by dosing oxygen into the fluidized bed of porous catalysts using immersed sintered metal and ceramic membranes. These studies were related to a DFG (German Research Association) research group (DFG-Nr. FOR 447/1-1) Membrane supported reaction engineering in the subproject Fluidized-bed membrane reactor . 

Tubular composite (X-AI2O3 -based supports for Pd-containing metal membrane have been developed. Their distinction consists in using metal nickel for the modification of the porous structure of ceramic supports. Nickel is analog of palladium in many respects it is also effective catalyst for molecular hydrogen... 

Thin film deposition for producing dense membranes has been presented in Sections 3.1.1 and 3.1.2. The processes can also be used to prepare porous membranes by adjusting the operating conditions. For example, transition metals and their alloys can be deposited on a porous ceramic, glass, or stainless steel support by the thin-film deposition process to produce porous metal membranes with small pore sizes [Teijin, 1984]. 

Modifications with metal membranes. The above modification techniques mostly deposit non-metal materials onto the membrane pores or surfaces. For enhanced separation or catalytic effects, metal layers are sometimes added to porous supports. 

Dense inorganic or metallic membranes for gas separation are usually ion-conducting materials, while membranes with carriers are polymers or supported liquid membranes (SLM). For transport through these materials, different flux equations should be applied. Figure 4.2 sums up and generalizes the various types of transport, which may take place in gas-separation membranes [21]. 

The porous structure of ceramic supports and membranes can be first described using the lUPAC classification on porous materials. Thus, macroporous ceramic membranes (pore diameter >50 nm) deposited on ceramic, carbon, or metallic porous supports are used for cross-flow microfiltration. These membranes are obtained by two successive ceramic processing techniques extrusion of ceramic pastes to produce cylindrical-shaped macroporous supports and slip-casting of ceramic powder slurries to obtain the supported microfiltration layer [2]. For ultrafiltration membranes, an additional mesoporous ceramic layer (2 nm<pore diameter <50 nm) is deposited, most often by the solgel process [11]. Ceramic nanofilters are produced in the same way by depositing a very thin microporous membrane (pore diameter <2 nm) on the ultrafiltration layer [4]. Two categories of micropores are distinguished the supermicropores >0.7 nm and the ultramicropores <0.7 nm. 

Tosti S. Supported and laminated Pd-based metallic membranes. Int. J. Hydrogen Energy 2003 28 1445-1454. 

Figure 8.42 shows the basic configuration of electrofiltration, where an electric field is applied across micro or ultrafiltration membranes in flat sheet, tubular, and SWMs. The electrode is installed on either side of the membrane with the cathode on the permeate side and the anode on the feed side. Usually, the membrane support is made of stainless steel or the membrane itself is made of conductive materials to form the cathode. Titanium coated with a thin layer of a noble metal such as platinum could, according to Bowen [93], be one of the best anode materials. Wakeman and Tarleton [94] analyzed the particle trajectory in a combined fluid flow and electric field and suggested that a tubular configuration should be more effective in use of electric power than flat and multitubular module. 

Due to the favorable thermodynamic conditions created at the F/M interface some components are selectively extracted from the F and transported into the membrane liquid. Simultaneously, at the M/R interface, conditions are such that the back extraction is favored. Various factors that could affect the transport of a metal ion through the LM are (a) the (transport) resistance encountered by the metal ion in the F and R phases, (b) the physicochemical properties of the carrier and diluent, and (c) the nature of membrane support such as its pore size, porosity, tortuosity, hydrophilicity, surface tension, and surface area to volume ratio encountered in the transport process. 

Water purification can be achieved by decomposition or destruction of the pollutant on contact with an electrically charged surface. In several cases permeable electrodes are used in a flow-through mode to ensure contact between the pollutant and the electrode. The type of electrode ranges from metal meshes and carbon cloths to ceramic membrane supports coated with a porous layer of electroconductive material. 

The high cost, limited lifetime, and low permeability are relevant limits of Pd and Pd-alloy membranes. To overcome these drawbacks, many studies have been carried out for the preparation of supported metallic membranes in which a thin metallic layer is supported on a thicker sublayer. However, the preparation technology of metalhc membranes is still today not sufficiently mature and more work is necessary to produce defect free and stable membranes at acceptable costs. 

Basically, an analytical pervaporator consists of the elements shown in Fig. 4.17A, namely an upper acceptor chamber (a) with inlets and outlets through which the acceptor stream is circulated and in which the gaseous analyte (or its reaction product if the analyte is not volatile) is collected a lower, donor chamber (d) that contains the solid sample or through which the feed stream of liquid or slurry sample is circulated a thin (ca. 1 mm) membrane support (b) made of polytetrafluoroethylene (PTFE) or metal and spacers (c) of variable thickness (2-10 mm) that can be placed below or above the membrane support in order to increase the volumes of the corresponding chambers. 

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