Dense inorganic membranes

Inorganic membranes can be categorized as shown in Table 2.1. The dense inorganic membranes consist of solid layers of metals (Pd, Ag, alloys) or (oxidic) solid electrolytes which allow diffusion of hydrogen (or oxygen). In the case of solid electrolytes transport of ions takes place. Another category of dense membranes consist of a porous support in which a liquid is... 

Y.S. Lin, Microporous and Dense Inorganic Membranes Current Status and Prospective, Sep. Purif. Tech. 25, 39 (2001). 

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. 

In contrast to dense inorganic membranes, the rate of advances toward industrial-scale applications of porous inorganic membranes has been rapid in recent years. In the early periods of this century, microporous porcelain and sintered metals have been tested for microfiltration applications and, in the 1940s, microporous Vycor-type glass membranes became available. Then in the mid-1960s porous silver membranes were commercialized. These membranes, however, have not seen large scale applications in... 

Vapor deposition. Both physical and chemical vapor deposition methods can be used to prq)are dense inorganic membranes. In either process, vaporization of the membrane material to be deposited is effected by physical means (such as thermal evaporation and sputtering) or chemical reactions. 

Compared to liquid-phase applications, commercial gas- or vapor-phase applications of inorganic membranes have been limited. Due to their low permeabilities, dense inorganic membranes arc utilized only in special and low-volume cases. Hydrogen... 

While dense inorganic membranes such as palladium-based or zirconia membranes provide extremely high-purity gases, their permeabilities are usually low, thus making the process economics unfavorable. Therefore, most of the recent investigations focus on porous inorganic membranes. 

Some dense inorganic membranes made of metals and metal oxides are oxygen specific. Notable ones include silver, zirconia stabilized by yttria or calcia, lead oxide, perovskite-type oxides and some mixed oxides such as yttria stabilized titania-zirconia. Their usage as a membrane reactor is profiled in Table 8.4 for a number of reactions decomposition of carbon dioxide to form carbon monoxide and oxygen, oxidation of ammonia to nitrogen and nitrous oxide, oxidation of methane to syngas and oxidative coupling of methane to form C2 hydrocarbons, and oxidation of other hydrocarbons such as ethylene, methanol, ethanol, propylene and butene. 

Lin YS-, Porous and dense inorganic membranes for gas separations. Presented at the AlChE Annual Meeting, Miami, FL, paper 23c (1992). 

Lin YS. Microporous and dense inorganic membranes Current status and prospective. Sep. Purif. Technol. 2001 25 39-55. 

Another class of dense inorganic membranes that have been used in membrane reactor applications are solid oxide type membranes. These materials (solid oxide electrolytes) are also finding widespread application in the area of fuel cells and as electrochemical oxygen pumps and sensors. Due to their importance they have received significant attention and their catalytic and electrochemical applications have been widely reviewed [94-98]. Solid materials are known which conduct a variety of cationic/anionic species [14,98]. For the purposes of the application of such materials in catalytic membrane reactor applications, however, only and conducting materials are of direct relevance. 

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. 

Typical characteristics of both porous and dense inorganic membranes are given in Table 14.1. Only applications with porous ceramic membranes will be dealt with in this chapter. 

Sammells AF, Mundschau MV, editors. Dense inorganic membranes. New York WUey-VCH 2006. 

Mundschau M, Xie X. Dense inorganic membrane for production of hydrogen from methane and coal with carbon dioxide sequestration. Catal Today. 2006 118(1-2) 12-23. 

Dense inorganic membranes for hydrogen separation may consist either of a metal or an alloy, a ceramic (oxide), or a mixture of both, a so-called cermet. One may also envisage a mixture of two ceramics ( cercer ). In the following, we discuss literature on hydrogen permeability in some of these classes of dense membranes. 

Ion transport membranes are new dense inorganic membranes able to be permeated only by oxygen (or hydrogen). They show good performance in terms of permeability and selectivity at a very high temperature (>600 °C) however, the main problem is related to their durability. Once the time of operation is passed, the formation of micro-pinholes depletes the membrane properties, significantly reducing the selectivity [28]. 

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