Dense ceramic membranes principles

The dense ceramic membranes applied in MRs can be either MIECs or pure ionic conductors (electrolytes). In the MIEC-MRs, the membrane itself serves as the internal circuit for electron transport, whereas an external circuit for electron transport has to be provided in the pure ionic conductor MRs (EMRs). The principles of the dense ceramic MRs are illustrated schematically in Eigure 5.5. 

Figure 5.5 Principles of dense ceramic membrane reactors (a) electrochemical pump membrane reactor (EP-MR) (b) solid oxide fuel cell membrane reactor (SOEC-MR) (c) mixed ionic-electronic conducting membrane reactor (MIEC-MR).

Abstract Dense ceramic membrane reactors are made from composite oxides, usually having perovskite or fluorite structure with appreciable mixed ionic (oxygen ion and/or proton) and electronic conductivity. They combine the oxygen or hydrogen separation process with the catalytic reactions into a single step at elevated temperatures (>700°C), leading to significantly improved yields, simplified production processes and reduced capital costs. This chapter mainly describes the principles of various types of dense ceramic membrane reactors, and the fabrication of the membranes and membrane reactors. 

The most demanding support requirements are those for ultra thin micro-porous gas separation membranes, which are currently being developed in several research organisations worldwide including ECN (Petten, the Netherlands). In principle, a mesoporous Knudsen or UP membrane can serve as support for these membranes if the defect density in the substrate surface, i.e. the mesoporous layer, is low enough. Indeed, the quality of the Knudsen or UF membrane as support for a microporous gas separation membrane should be higher than is usually needed for the UF or Knudsen function [4]. This means that not every mesoporous ceramic membrane is a suitable support for micro-porous or dense amorphous gas separation membranes. 

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... 

This chapter will describe extensively the dense ceramic oxygen permeable membrane reactors from their principles, fabrication, and design to their applications. The prospects and critical issues of the dense... 

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