Permeation MIEC membranes

Oxygen partial pressure between the two sides of the membrane is the driving force of oxygen permeation. Figure 3.1c shows a dual-phase MIEC membrane, which can be visualized as a dispersion of a continuous electronic conducting phase into an SE matrix. The electronic conducting phase is usually made from precious metal or metal oxides. 

Table 3.1 Oxygen permeation fluxes of some MIEC membrane materials...

Figure 18.1 Schematic of oxygen permeation across the MIEC membranes ( oxygen vacancy o-lattice oxygen h-electron hole).

For the oxygen permeation in MIEC membranes, (i) the overall charge balance is applied or = 0 and (ii) the local velocity of inert marker is... 

Combining eqns (18.9) and (18.10) gives the specific permeation rate through the MIEC membranes in terms of the oxygen partial pressures and membrane dimensions as ... 

Figure 4.3 Schematic of the counter-current oxygen transport mechanism of a symmetric (dense) MIEC membrane exploited for oxygen separation applications (a) and the change in oxygen permeation flux with the membrane thickness (b). fSource Reproduced from Ref [ 10], with permission from Elsevier)...

Engels, S., Markus, T., Modigell, M. and Singheiser, L. (2011) Oxygen permeation and stability investigations on MIEC membrane materials under operating conditions for power plant processes. Journal of Membrane Science, 370,58-769. 

Figure 3.2 Schematic illustration of oxygen permeation through mixed ionic-electronic conducting (MIEC) membranes when air is used as the feedstock stream (Zhu Yang, 2011). Reprinted with permission hy Elsevier.

Equations (14.14) and (14.18) can be used as starting point for generating equations describing O2 and H2 permeation within single-phase perovskite membranes. Key to these equations is the nature of the boundary conditions at the feed/membrane and permeate/membrane surfaces. To this aim, one needs to address appropriate defect point thermodynamics to establish equilibrium and surface exchange relations for all potential species that can play a role during permeation. As a general rule, the law of mass action can be used to predict the concentration of ionic vacancies, protons, electrons, and electron holes in the membrane. Below we describe a series of models that can be deduced for ID steady-state permeation within perovskite and extensively other MIEC membranes. 

The influence of carbon dioxide on the oxygen permeation of MIEC membranes has been extensively studied compared with the effect of other gases, such as SO2, which can also be present in the gas streams. Of course, CO2 will be the major component of a hypothetical gas mixture where the MIEC membrane should work (e.g., oxy-fuel process). Nevertheless, the influence of SO2 must be mandatorily evaluated and represents one of the biggest concerns in this area. For example, the SO2 concentration in the flux gas can be around 400 ppm or even higher than 1000 ppm, as reported before the scrubber system in some power plants in China [82]. Up to 2011, few reports can be found dealing with membrane operation in SO2 environments. However, on related topics, for example, catalysis, several studies revealed the formation of sulfetes and sulfldes... 

Last but not least, the fundamental understanding of the permeation mechanisms within perovsldte and more extensively MIEC membranes is still in its infancy. The most extended models are based on the Nernst-Planck equations (e.g., the Wagner equation) providing a macroscopic view of the permeation process itself. These models usually cannot afford the description of heterogeneous materials including impurities and occluded bubbles, as is the case for most real perovskite layers. To this aim, the development of meso- or microscale models with a proper description of diffusion effects and vacancy generation would be desirable. 

The most important consideration for a MIEC membrane is the delivery of a stable continuous transmembrane flux. This flux is central to membrane performance and therefore oxygen permeation studies in MIEC membrane characterization are fundamental. From an economic standpoint, an oxygen flux of 1 to 10 ml cm min (STP) has been cited as the requirement for future needs [25,26]. In order to provide a broad overview of this research area we will briefly outline synthesis and characterization methods applied to the MIEC perovskites before moving on to look at selected MIEC membranes currently used in laboratory-scale tests. 

Another approach has been to correlate the oxygen permeation flux to oxygen partial pressures pelectronic conductivity is high and constant within the MIEC membrane, the oxygen flux can be written as ... 

Figure 5.1 (Continued) (c) oxygen permeation through MIEC membrane (d) oxygen permeation through dual-phase membrane.

Figure 5.3 demonstrates schematically the oxygen permeation process through an MIEC membrane including the following steps in series (i) oxygen molecular diffusion from the gas stream to the membrane surface... 

Following the same process, the oxygen permeation flux through tubular MIEC membranes can be given by [19]... 

MIEC membrane reactors (MIEC-MRs). In an MIEC-MR, oxygen permeates through the membrane under the oxygen partial pressure gradient at high temperatures without the need for electrodes... 

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