Solid oxide membrane

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. 

The earlier solid oxide electrolytes were solid solutions of divalent or triva-lent metal oxides (Y2O3, Yb203 on CaO) in oxides of tetravalent metals having a fluorite type structure A4O8 like Zr02, Th02 or Ce02 [97]. The introduction of 

Hydrogen and CH4 (and most recently CH3OH) SOFCs have been the subject of intensive investigations since the early sixties and a complete review of the area certainly goes beyond the scope of this chapter. Hydrogen SOFCs have received the lion s share of attention. CH4 (and more recently CH3OH) SOFCs have shown promise but concerns still remain with carbon deposition and low catalytic activity. 

A significant recent effort in this area is a collaborative study by Amoco and the Argonne National Laboratory utilizing solid oxide type membranes [112-113]. The newly developed membranes show improved mechanical and thermal characteristics and are reported to remain stable for over 21 days at 900°C under CH4 partial oxidation conditions. The membrane used was tubular in shape. A CH4/Ar mixture was allowed to flow in the tubeside which was packed with a Rh based catalyst. Air was the source of oxygen on the outside 

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D. Eng, and M. Stoukides, Catalytic and Electrocatalytic Methane Oxidation with Solid Oxide Membranes, Catalysis Reviews - Science and Engineering 33, 375-412 (1991). 

Lin, Y. S., Wang, W., and Han, J. (1994). Oxygen permeation through thin mixed-conducting solid oxide membranes, AIChE J. 40(5), 786. 

The use of Pd-based membrane reactors can increase the hydrogenation rates of several olefins by more than 10 times higher than those in conventional premixed fixed>bed reactors. Furthermore, it has been pointed out that the type and state of the oxygen used to carry out partial oxidation of methane can significantly affect the conversion and selectivity of the reaction. The use of a solid oxide membrane (e.g., a yttria-stabilized zirconia membrane) not only can achieve an industrially acceptable C2 hydrocarbon yield but also may eliminate undesirable gas-phase reactions of oxygen with methane or its intermediates because oxygen first reaches the catalyst through the solid oxide wall [Eng and Stoukides, 1991]. 

An extensive review has been made on catalytic and elccu ocatalytic methane oxidation with solid oxide membranes including the fuel cell mode [Eng and Stoukidcs, 1991]. 

Lin Y.S., de Vries K.J., Brinkman H.W. and Buiggraaf A.J., Oxygen semipermeable solid oxide membrane composites prepared by electrochemical vapor deposition, 7. Membr. ScL 66 211 (1992). 

Eng D. and Stoukides M., Catalytic and electrocatalytic methane oxidation with solid oxide membranes, Catal. Rev.ScL Eng. 33 315 (1991). 

Y.S. Lin, K.J. de Vries, H.W. Brinkman and A.J. Burggraaf, Oxygen semipermeable solid oxid membrane composites prepared by electrochemical vapor deposition. J. Membr. Sci, 66 (1992) 211-226. 

Huintung Deng, Minyan Zhou and B. Abeles, Diffusion reaction in mixed-electronic solid oxide membranes with porous electrodes. Solid State Ionics, 74 (1994) 75-84. 

Data Oxygen Permeability of Solid Oxide Membranes... 

J.E. ten Elshof, B.A. Van Hassel and H.J.M. Bouwmeester, Activation of methane using solid oxide membranes. Catal. Today, 25 (1995) 397-402. 

Fig. 11,9. Methane conversion, CO and H2 selectivities and O2 permeation in a solid oxide membrane reactor. Reproduced from Balachandran et al. [113] with permission.

The model of Tsai et al. is of direct relevance to the experimental study of Balachandran and coworkers [112,113] that we have previously discussed. The same group [120] have also recently presented a modelling study of the application of solid oxide membrane reactors in the area of environmentally benign processes. With the emergence of solid oxide membranes and their use in membrane reactor applications, a number of models have appeared recently to... 

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. 

Figure 2.12. A schematic of the solid oxide membrane reactor for synthesis gas production. From Dyer et al. [2.232], with permission from Elsevier Science.

As was described in Section 2.3, Sammels et al [2.310] have recently reported the use of solid oxide membranes (brownmillerite) for gasifying coal. In the same study they have reported the use of such membranes in a catalytic MR for the autothermal reforming of logistic fuels (JP-8 and DF-2), in order to deliver a gaseous feedstock compatible for subsequent use in a SOFC. This is an important application, which makes it feasible for directly using such fuels in a SOFC. 

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...

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