(Invited) Dense Gas Separation Membranes - Current Status and Future Challenges

The application areas of oxygen transport membranes (OTM) and the potential benefits they can provide will be described. The importance of close thermal integration with the process to which the oxygen is supplied will be elucidated by balance-of-plant studies and techno-economical evaluations in two different applications; biomass gasifiers and cement production. The status of the technology will be presented with emphasis on results from several national and EU-funded membrane development projects.  There are still several challenges to overcome before use of the technology will be wide spread. One is the required upscale of the technology another one is ensuring the required reliability and durability of the membranes. Recently developed methods for characterizing the mechanical properties of OTM materials and components at operating conditions and a theoretical analysis of stresses encountered during operation will be presented and based on these strategies to ensure fail-safe design/operation. An OTM material should ideally have high ionic and electronic conductivity as well as good chemical stability under both oxidizing and reducing conditions. Further, the mechanical properties (strength, toughness, creep rate) must match the requirements of the application. Candidate materials involve a broad class of Fe-, Co- and Ti-based perovskites. Recent results on selected materials from this broad class of materials will be presented. Challenges with reaching both the required transport properties and the required mechanical reliability with such single phase perovskites has spurred renewed interest in dual phase membranes where a good ionic conductor (typically based on zirconia or ceria) is combined with a second phase providing electronic (or mixed) conductivity. Our most recent results on such dual phase membranes will be presented involving both zirconia and ceria based systems. On both tubular and planar LSF/CGO membranes high oxygen fluxes >10 Nml min-1 cm-2 (850°C) have been demonstrated between air on one side of the membrane and a strongly reducing “syngas” mixture (CO/H2/H2O/CO2) on the other side. The tubular membranes were prepared by dip coating on an extruded low cost MgO support and the planar ones by phase inversion tape casting providing a highly oriented pore structure in the support layer. The processes limiting the flux in these systems were investigated in some detail by conductivity relaxation.  Whereas the addition of CGO to LSF has the expected effect of increasing the ionic transport in the bulk it also had a more surprising effect of enhancing the surface exchange rate. Possible explanations for this will be presented.  Finally, selected results on zirconia based composite membranes optimized for high pO2 applications will be presented including ZnO-ZrO2 and (MnCO)3O4 -ZrO2 membranes. For the latter system (MnCo2O4/(Y2O3)0.01(Sc2O3)0.10(ZrO2)0.89) fluxes around 1 Nml min-1 cm-2 at 850°C have recently been demonstrated between air and a nitrogen purge (pO2~10-3 atm.) using a ~10 μm thick supported membrane.