Hydrogen separation dense metal membranes

Hydrogen selective dense metal membranes is another approach to separate hydrogen from CO2, at elevated temperatures. Again it can be nsed... 

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. 

There are essentially four different types of membranes, or semipermeable barriers, which have either been commercialized for hydrogen separations or are being proposed for development and commercialization. They are polymeric membranes, porous (ceramic, carbon, metal) membranes, dense metal membranes, and ion-conductive membranes (see Table 8.1). Of these, only the polymeric membranes have seen significant commercialization, although dense metal membranes have been used for commercial applications in selected niche markets. Commercial polymeric membranes may be further classified as either asymmetric (a single polymer composition in which the thin, dense permselective layer covers a porous, but thick, layer) or composite (a thick, porous layer covered by a thin, dense permselective layer composed of a different polymer composition).2... 

Thicker, self-supporting, dense metal membranes are known. These are tubular and are usually commercially successful palladium-silver hydrogen separation membranes were of this type.21 Currently, Power and Energy, Inc. also fabricates this type of membrane, although planar membranes are more common due to easier fabrication and a greater variety of fabrication methods. 

Tosti S, Borelli R, Borgognoni F, Favuzza P, Rizzello C, Tarquini P (2008) Study of a dense metal membrane reactor for hydrogen separation from hydroiodic acid decomposition, hit J Hydrogen Energy 33 5106-5114... 

Hydrogen can permeate selectively dense metal membranes, behaviour that permits the separation of hydrogen from gas mixtures. The mass transfer mechanism consists of several steps dissociation of hydrogen molecules into atoms, interaction of hydrogen atoms with the metal surface and their adsorption, diffusion of hydrogen into the metal lattice, and desorption of hydrogen atoms from the other metal surface and their recombination into molecules.The overall transport process through the metal wall is called permeation and is ruled by the expression ... 

Dense metal membranes have obtained huge successes in implementing reactors in this sense, the case of palladium membrane (for hydrogen separation) is very representative (Adhikari Fernando, 2006). 

Hydrogen can permeate selectively dense metal layers, and, accordingly, this phenomenon is exploited when metal membranes are applied for separating ultrapure hydrogen from gas mixtures [24-27]. The hydrogen mass transfer through dense metal membranes includes several transport mechanisms, which are discussed in the following sections. 

Based on the different compositions, structures and configurations, the dense metallic membranes can be prepared using various methods. As described earlier, the dense metallic hydrogen separation membrane can be... 

As explained in Chapter 5, the transport mechanism in dense crystalline materials is generally made up of incessant displacements of mobile atoms because of the so-called vacancy or interstitial mechanisms. In this sense, the solution-diffusion mechanism is the most commonly used physical model to describe gas transport through dense membranes. The solution-diffusion separation mechanism is based on both solubility and mobility of one species in an effective solid barrier [23-25], This mechanism can be described as follows first, a gas molecule is adsorbed, and in some cases dissociated, on the surface of one side of the membrane, it then dissolves in the membrane material, and thereafter diffuses through the membrane. Finally, in some cases it is associated and desorbs, and in other cases, it only desorbs on the other side of the membrane. For example, for hydrogen transport through a dense metal such as Pd, the H2 molecule has to split up after adsorption, and, thereafter, recombine after diffusing through the membrane on the other side (see Section 5.6.1). 

Dense membranes are made from solid layers of metals (e.g. Pd alloys) for hydrogen separation, or of mixed (electronic, ionic) conducting oxides for oxygen separation. A special form are the LIMs (liquid immobilised membranes) which consist of a porous support filled with a liquid or molten salt which is semipermeable. 

Inorganic membrane development is still in progress [57] (see also Section 14.2.2). Microporous silica membranes have been developed at several universities and research institutes. Membrane selectivities of 15 and 20 for the separation of H2 from CO2 have been reported. Even higher selectivities for H2 arid CO, CH4 and N2 have been measured [20,57]. Most measurements reported in the literature have been performed on a laboratory scale. However, it has been shown that it is possible to upscale these microporous ceramic membranes to, at least, bench scale [31,57]. With other membranes such as noble (Pd) metal membranes and dense ceramic membranes very high and almost infinite selectivities for hydrogen are possible [58]. The permeation of these membranes is generally smaller than the permeation of microporous membranes. 

The application of polymer membranes is generally limited to temperatures below 475 K and to the separation of mixtures that are chemically inert. Otherwise, membranes made of inorganic materials can be used. These include mainly microp-orous ceramics, metals, and carbon, and dense metals, such as palladium, that allow the selective diffusion of very small molecules such as hydrogen and helium. 

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