Metal membrane

Historically there are two major types of dense inorganic membranes that have been studied and developed extensively. They are metal membranes and solid electrolyte membranes. 

Carbide in the 1960s (McBride and McKinley, 1965) to separate hydrogen from refinery off-gas. It utilized 25 pm thick Pd-Ag membrane foils (McBride et al., 1967). 

Gryaznov and his co-workers (e.g. IGryaznov, 1986]) have extensively explored the permselective properties of palladium and its alloys as dense membranes and membrane reactors. While their studies will be discussed in later chapters, it suffices to say that the palladium-based membranes have reached the verge of a commercialization potential for the process industry. 

Besides Pd and its alloys, other dense metal materials are also possible candidates for sq aration of fluid components. Notable examples are tantalum, vanadium and niobium which have high selectivities for hydrogen. Dense silver is known to be permselective to oxygen gas. 

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Metal-matrix composites Metal membranes Metal-metal bonds Metal naphthenates Metal oleates... 

Three forms of caustic soda are produced to meet customer needs purified diaphragm caustic (50% Rayon grade), 73% caustic, and anhydrous caustic. Regular 50% caustic from the diaphragm cell process is suitable for most appHcations and accounts for about 85% of the NaOH consumed in the United States. However, it caimot be used in operations such as the manufacture of rayon, the synthesis of alkyl aryl sulfonates, or the production of anhydrous caustic because of the presence of salt, sodium chlorate, and heavy metals. Membrane and mercury cell caustic, on the other hand, is of superior quaUty and... 

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. 

Because membranes appHcable to diverse separation problems are often made by the same general techniques, classification by end use appHcation or preparation method is difficult. The first part of this section is, therefore, organized by membrane stmcture preparation methods are described for symmetrical membranes, asymmetric membranes, ceramic and metal membranes, and Hquid membranes. The production of hollow-fine fiber membranes and membrane modules is then covered. Symmetrical membranes have a uniform stmcture throughout such membranes can be either dense films or microporous. 

FIGt 22-48 Transport mechanisms for separation membranes a) Viscous flow, used in UF and MF. No separation achieved in RO, NF, ED, GAS, or PY (h) Knudsen flow used in some gas membranes. Pore diameter < mean free path, (c) Ultramicroporoiis membrane—precise pore diameter used in gas separation, (d) Solution-diffusion used in gas, RO, PY Molecule dissolves in the membrane and diffuses through. Not shown Electro-dialysis membranes and metallic membranes for hydrogen. 

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. 

Gibbs TK, McCallum C, Pletcher D. 1977. The oxidation of carbon monoxide at platinum and gold metallized membrane electrodes. Electrochim Acta 22 525-530. 

Palladium-based dense metallic membranes have been known to be completely selective for hydrogen permeation and are used in commercially available small-scale hydrogen purification units (e.g., Johnson Matthey, 2007 REB Research, 2007 Power + Energy, 2007 ATI Wah Chang, 2007). These hydrogen purification units typically use palladium-alloy... 

ATI Wah Chang, Palladium-Based Metal Membrane, available at http //www.wahchang.com/ pages/products/data/pdf/Hydrogen%20Metal%20Membrane.pdf (accessed July 2007). 

Buxbaum, R.E., High Flux Metallic Membranes for Hydrogen Recovery and Membrane Reactors, Proceedings of2007 U.S. DOE Hydrogen Annual Merit Review Meeting, Arlington, VA, May 2007. 

Edlund, D.J. and W. Pledger, Thermolysis of hydrogen sulfide in a metal membrane reactor, /. Membr. Sci., 77, 255-264, 1993. 

Dense metal membranes, 15 800 Dense nonaqueous phase liquids... 

Micropore diffusion, 1 596, 597-599 Microporous catalysts, in bisphenol A manufacture, 14 420 Microporous metal membranes, 15 813t Microporous particles, apparent effective diffusivity and, 15 729-730 Microporous range, pore diameters within, 16 812... 

The type of apparatus described here has been used by many workers since the early 1900s, but was not used much until the work of Devanathan and Stachurski emphasized its potential in the early 1960s. The principle is quite simple and is illustrated in Fig. 25. Two electrochemical cells are separated by a metallic membrane, which acts as the working electrode in each of the cells. 

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. 

Dense metallic membranes have the advantage of very high selectivities since only certain species can be dissolved in their structural lattice. However, the permeabilities are lower by a factor of 100 than those of porous membranes (Ilias and Govind 1989, van Vuren et al. 1987, Itoh 1987, Suzuki, Onozato and Kurokawa 1987). For example, the permeability of... 

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