Transporteigenschaften gemischtleitender Hochtemperaturmembranen zur Sauerstoffabtrennung
- Mass transfer properties of mixed ion electron conducting membranes for the oxygen separation from air
Engels, Stephan; Modigell, Michael (Thesis advisor)
Aachen : Publikationsserver der RWTH Aachen University (2012)
Dissertation / PhD Thesis
Aachen, Techn. Hochsch., Diss., 2012
The work at hand deals with the mass transfer through mixed conducting high temperature membranes with the material composition Ba0.5Sr0.5Co0.8Fe0.2O3-d (BSCF), which are used for the oxygen separation from air. Thereby the analysis considers all relevant mass transport resistances and shows their influence on the oxygen mass transfer across the membrane. For this purpose, experimental studies on the O2-permeation rate and CFD-simulations of the test facilities are made and the mass transfer models are validated by an inverse parameter fitting. Thereby the CFD simulation gives detailed information about the conditions near the membrane surface, which are crucial for the oxygen transport across the membrane. By experimental measurement methods these information are normally not accessible. This approach makes it possible to determine the intrinsic material constants CWagner and KWagner of the Wagner-equation and the characteristic membrane thickness LC(T). Further possible concentration polarization effects in the gas phases and the mass transport resistances (bulk transport in the membrane material, surface exchange and the gas phase transport) can be determined. The work at hand shows, that the oxygen mass transfer through the BSCF membranes at temperatures above 800°C is limited only slightly by the surface exchange processes. Due to that an increase of the O2-permeation rate will be possible by further decreasing the membrane thickness. Furthermore there exist significant concentration polarization effects, which have an effect on the mass transport across the membrane. Here the feed and permeate gas phase resistances together can account for up to 40% of the total mass transfer resistance across the membrane. Further the influence of the flue gas components CO2, SO2 and evaporated chromium from steel on the oxygen mass transfer has been studied. Here the perovskite materials BSCF and Sr0.5Ca0.5Mn0.8Fe0.2O3-d(SCMF) and the perovskite related material La2NiO4+d have been studied experimentally and evaluated concerning their applicability in the OXYCOAL-AC power plant process. The results show that BSCF membranes have the highest oxygen permeation rate, but in contact with CO2-rich gas they show degradation effects. These lead to a decreasing of the O2-permeation rate. SCMF and La2NiO4+d show a better stability against CO2, but they are also not applicable, if they are exposed to 360 ppm SO2. Further it has been determined in long term tests that both SCMF and BSCF react with evaporated CrO3 from steel, which leads to a slow decrease of the O2-permeation rate.