Membranes oxygen transport

A separate Oxygen Transport Membrane (OTM) Syngas Alliance was formed in 1997 and includes Amoco Production Company, British Petroleum, Praxair, Statoil, Foster Wheeler and Sasol Technology. This alliance is developing ceramic membrane technology that will economically convert natural gas to synthesis gas190. 

Chen J, Sirman J, lane J, Chen HC, van hassle B, Apte P, and Prasad R. Advances in oxygen transport membranes for gas separation. Proceedings of the Eight International Conference on Inorganic Membranes, Cincinnati, OH, July 18-22, 2004 38M-1. 

The process that is the subject of this article consists of the use of high temperature ceramic membranes that selectively transport oxygen. They are referred to with several acronyms of which ITM (Ion Transport Membranes), OTM (Oxygen Transport Membranes) and MIEC (Mixed Ionic Electronic Conducting) membranes prevail. We will use ITM throughout this article. 

The Evolution of Materials and Architecture for Oxygen Transport Membranes... 

Table 6.1 summarizes the materials projterties desired for operation of a pressure-driven ceramic oxygen transport membrane. 

Tab. 6.1 Desired materials properties for ceramic oxygen transport membranes used for oxygen separation...

The step change in pore structure can be fabricated in discrete steps. It also allows the use of dissimilar materials for the porous structure, as used in early Westinghouse SOFC cathode designs and discussed by Thorogood et al. for oxygen transport membranes [12]. 

A preferred embodiment of an oxygen transport membrane would thus have a thin porous support on the feed side to improve oxygen exchange, a thin dense separation membrane, a fine pore structure interfacial layer to facilitate oxygen transfer out of the membrane and a coarse porous support to maximize product flow and provide the structural support. An example is shown in Fig. 6.4. The coarse porous support material could be made out of inert material because it is not chemically active in the transport of oxygen. This allows the use of less expensive materials which may also have better strength characteristics. 

In the case of a solid oxide fuel cell, the anode or cathode support can be relatively thin because the component does not need to bear a very high load. However, in the case of oxygen transport membranes, the porous support needs to withstand a differential pressure of 20 atm or greater. Therefore, porous supports which are several millimeters thick are often considered. Alternatively, other concepts for strengthening the support structure are considered. These can include internal structures such as multichannel tubes, distinct solid porous inserts in tubes [21] and support braces in planar geometries. Examples of such structures are shown in Fig. 6.5 and 6.6. 

Figure 7.1 Principles behind dense oxygen transport membranes for promoting spontaneous value added partial oxidation chemistry.

For a given transition metal within the perovskite lattice, electron transport is promoted by overlap between the transition metal B-site and 0 orbitals. This is maximized when the B-O-B bonds are linear, corresponding to a cubic lattice. While the orthorhombic lattice has a lower B-O-B bond overlap, resulting in reduced electronic conductivity, many of the candidate oxygen transport membranes being developed do in fact possess this lattice. 

The Application of Oxygen Transport Membranes to Partial Oxidation Chemistries 1193 7.3... 

Figure 7.5 Probable reaction sequence for spontaneous natural gas conversion to syngas across an oxygen transport membrane.

Oxygen transport membranes provide an option for the reforming of liquid hydrocarbon fuels into a carbon monoxide/hydrogen feed [54] which is then compatible for subsequent electrochemical oxidation in the anode compartment of a SOFC. Such a strategy addresses fuel storage issues for fuel cells, particularly in transportation applications. 

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