Membrane dense solid oxide

The consideration of thermal effects and non-isothermal conditions is particularly important for reactions for which mass transport through the membrane is activated and, therefore, depends strongly on temperature. This is, typically, the case for dense membranes like, for example, solid oxide membranes, where the molecular transport is due to ionic diffusion. A theoretical study of the partial oxidation of CH4 to synthesis gas in a membrane reactor utilizing a dense solid oxide membrane has been reported by Tsai et al. [5.22, 5.36]. These authors considered the catalytic membrane to consist of three layers a macroporous support layer and a dense perovskite film (Lai.xSrxCoi.yFeyOs.s) permeable only to oxygen on the top of which a porous catalytic layer is placed. To model such a reactor Tsai et al. [5.22, 5.36] developed a two-dimensional model considering the appropriate mass balance equations for the three membrane layers and the two reactor compartments. For the tubeside and shellside the equations were similar to equations (5.1) and... 

Figure 4 Dense solid oxide membranes (a) anion conductor with external circuit (b) mixed conductor with no external circuit...

For those dense solid electrolyte membranes using metal oxides, the degree of stabilization can make a difference in the resulting thermal shock resistance. For example, the fully stabilized zirconia has poor thermal shock resistance compared to the partially stabilized zirconia. 

The inorganic membranes had until the late nineties received fairly little attention for applications in gas separation. This has mainly been due to their porous stmcmre, and therefore lack of ability to separate gas molecules. Within the group of inorganic membranes there are however the dense metallic membranes and the solid oxide electrolytes these are discussed separately in Section 4.3.5. With reference to Section 4.2, the possible transport mechanisms taking place in a porous membrane may be summarized as in Table 4.4 below, as well as the ability to separate gases (+) or not (—). Recent findings [29] have however documented that activated Knudsen diffusion may take place also in smaller pores than indicated in the table. 

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. 

Ceramic electrochemical reactors are currently undergoing intense investigation, the aim being not only to generate electricity but also to produce chemicals. Typically, ceramic dense membranes are either pure ionic (solid electrolyte SE) conductors or mixed ionic-electronic conductors (MIECs). In this chapter we review the developments of cells that involve a dense solid electrolyte (oxide-ion or proton conductor), where the electrical transfer of matter requires an external circuitry. When a dense ceramic membrane exhibits a mixed ionic-electronic conduction, the driving force for mass transport is a differential partial pressure applied across the membrane (this point is not considered in this chapter, although relevant information is available in specific reviews). 

In the area of dense membrane applications for hydrogenation reactions a number of recent studies also report the use of proton conducting solid oxide membranes (Otsuka and Yagi [2.86], Panagos et al. [2.87], Mamellos and Stoukides [2.88]). As noted previously, this is an exciting class of new materials with significant potential applications. 

The materials of membrane construction can be classified as either dense or porous. Dense metal materials include palladium membranes that are semiperme-able to hydrogen, and silver membranes that are semipermeable to oxygen. The low permeation rates for silver membranes have led to the more recent use of solid oxide electrolyte dense membranes such as modified zirconias and perovs-kites, which have higher O2 permeation rates at high temperatures. ... 

Dense membranes have been used to feed hydrogen for hydrogenation reactions.Improved yields have been observed, due not to the above kinetics reasons, but attributed to the better availability of the active available on the membrane surface. Most dense membrane use has been to feed oxygen. Early studies considered silver membranes, but cost and low permeation rates did not favor these. More recent work has used solid oxide electrolytes as membranes. Initially, investigators used yittria- and calcia-stabilized zirconias (YSZ or CSZ), which had reasonable oxygen anion conductivity. Their low electron conductivity dictated the use of an external circuit, as shown in Figure 4(a). 

The principles behind this membrane technology originate from solid-state electrochemistry. Conventional electrochemical halfceU reactions can be written for chemical processes occurring on each respective membrane surface. Since the general chemistry under discussion here is thermodynamically downhill, one might view these devices as short-circuited solid oxide fuel cells (SOFCs), although the ceramics used for oxygen transport are often quite different. SOFCs most frequently use fluorite-based solid electrolytes - often yttria stabUized zirco-nia (YSZ) and sometimes ceria. In comparison, dense ceramics for membrane applications most often possess a perovskite-related lattice. The key fundamental... 

W.X. Kao, M.C. Lee, T.N. Lin, C.H. Wang, Y.C. Chang, and L.F Lin, A Novel Process for Fabrication of a Fully Dense Electrolyte Layer Embedded in a High Performance Membrane Electrode Assembly (MEA) (Unit Cell) of Solid Oxide Fuel Cell, EURO-Patent NO EP 2083465A1 (101-08-01). 

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