Dense metal membrane materials

Dense Metal Membrane Materials, Configurations, Mechanisms of Transport, and Permeability... 

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 first membrane reactor studies made use of dense metallic membranes, but due to certain limitations of these dense materials (sec below) and due to the rapid progress in the development of (micro)porous... 

In dense, non-porous membranes, surface limitations to oxygen permeation are a common phenomenon as can be understood from the very low adsorption levels and large activation energies on the dense membrane materials (see Chapter 10). For hydrogen permeation in dense metal membranes estimates have been made by Govind [105]. 

Multi-phase catalytic reactions have attracted some attention but the area has not in our opinion been fully exploited. Previous studies have demonstrated that the yields obtained with the catalytic membrane reactors are often better than the yields obtained with more conventional reactors. Future research in this area must involve reactions with more immediate industrial applications. Examples of such reactions could be the hydrogenation reactions studied by Gryaznov and co-workers with dense metallic membranes which we discussed earlier. New materials like zeolite membrane could offer some advantages here with their enhanced regio- or chemioselectivity. 

However, dense metal membranes present a few challenges that have, to date, prevented their widespread industrial implementation. The best performing materials, Pd and Pd-alloys, are expensive. Pure Pd displays high H2 permeability and can operate at elevated temperatures, but suffers performance degradation because of poisoning and corrosion from exposure to conmum contaminants (H2S, NH3, CO, CO2, and Hg), hydride formation, and other factors, which will be described in following sections. Pd-alloys can display better chemical stability and mechanical strength than pine Pd, but often at the expense of permeability. 

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). 

Membrane surface contamination. Although not as hydrogen selective as Pd and its alloys, other metals such as niobium and vanadium in dense form also have moderate to high hydrogen permselectivity and potentially can be considered as membrane materials. Inevitably the membrane surface is contaminated with non-metal impurities prior to or during separation or membrane reactor applications. 

Thermal shock resistance. Temperature swing as part of the normal cycles of operation or regeneration of the membranes or membrane reactors can lead to deleterious thermal shock. The materials for the various components in a membrane reactor should be carefully selected to impart good thermal sh k resistance. This is particularly important for high temperature reactions. Also listed in Table 9.5 is a summary of various membrane materials along with qualitative description of their resistance to thermal shock. Again, the available data apply to dense materials. While various metal oxides have been made into commercial inorganic membranes, they tend to be affected by thermal shock much more than other ceramic materials. 

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 most simple form is a single, uniformly structured wall of a certain material, the so-called symmetric, stand-alone membranes. Examples are dense metal or oxide tubes and porous hollow fibres. To obtain sufficient mechanical strength, single-walled symmetric systems usually have a considerable thickness. 

Hydrogen selective inorganic membranes can be mesoporous (2 nm < pore diameter < 50 nm ceramic, glass or carbon) microporous (pore diameter < 2 nm ceramic, carbon or zeolite) or dense (ceramic or metal). These membranes can be used from ambient temperatures up to about 600°C for mesoporous materials, up to about 500°C for microporous inorganic membranes and up to about 800°C for dense inorganic membranes [14-16]. These temperatures are only a rough indication, because of the different materials which can be used and the test conditions at which the membranes have to operate. 

The complex phase diagrams and rich crystal chemistry of the transition metal-containing oxide systems, and great diversity in the defect chemistry and transport properties of mixed-conducting materials known in these systems, make it impossible to systematize all promising compositions in a brief survey. The primary attention here is therefore centered on the comparison of major families of the oxide mixed conductors used for dense ceramic membranes and porous electrodes of SOFCs and other high-temperature electrochemical devices. 

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