Oxygen transport dense membranes

In gas separation two completely different types of membranes can be used in this process (alchou in different regimes of application) a dense membrane where transport takes place via diffusion, and a porous membrane where Knudsen flow occurs. A commercial application of gas separation membranes occurs in hydrogen recovery, the separation of air (oxygen/nitrogen) and of methane and carbon dioxide provide other examples. Pervaporation and vapour permeation make use of a dense separating layer. 

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... 

Dense ceramic ion-conducting membranes (CICMs) are emerging as an important class of inorganic membranes based on fluorite- or perovskite-derived crystalline structures [18]. Most of the ion-conducting ceramics discovered to date exhibit a selective ionic oxygen transport at high temperatures >700°C. Ionic transport in these membranes is based on the following successive mechanisms [25] ... 

FIGURE 6.32 Schematic diagram of an integrated distributor/extractor membrane reactor based on the combination of dense ceramic oxygen and hydrogen transport membrane for syngas production. 

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... 

Membrane architecture concepts are as varied as the material concepts and are equally important in determining the success of an oxygen transport device. The architecture can be considered from a membrane standpoint (thick dense membrane, supported thin film, sandwich structure) and from a system standpoint (tubular, planar, monolith). 

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. 

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

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... 

Air separation membranes are typically dense ceramic (typically perovskite) membranes, which selectively permeate oxygen in ionic form. Over the past two decades. Air Products (ITMs) and Praxair (oxygen transport membranes [OTMs]) have worked towards the commercial scale-up of these membranes for applications in power generation, gasification, and gas to liquid conversion [94]. Air Products has focused on a planar configuration, whereas Praxair on tubular membranes. 

---