(Invited) Graded Oxygen Transport Membranes for Energy Efficient Separation Applications
Authors: Meulenberg Wilhelm A. 
Published: February 2020

Oxygen transport membranes (OTM) have attracted great interest due to their ability to separate oxygen from air in an energy-efficient way. One major driver in the last decade was the potential CO2 sequestration, enabled by the Oxyfuel technology, i.e. combustion of fossil fuel with pure oxygen instead of air. Process simulations show that oxygen generation by ceramic membranes leads to significantly lower efficiency losses compared to conventional separation technologies. Another application of interest is the development of membrane reactors, combining the separation tasks with a chemical reaction, which is normally catalytically promoted. Such process intensification could play a significant role in future sustainable industrial production processes. Recently, a growing attention again is paid in petrochemical applications such as partial oxidation of methane to synthesis gas or oxidative coupling of methane to higher hydrocarbons such as ethylene, propylene, or even aromatics, which are important intermediates for the chemical industry. However, the demand on the materials stability in harsh operation conditions is much higher compared to the sole oxygen separation from air.
OTMs are gastight Mixed Ionic Electronic Conductors (MIEC), which allow oxygen diffusion through oxygen vacancies and simultaneous conduction of electrons in the opposite direction. These processes require operation temperatures typically above 700°C. Obviously a key issue is the progress in the development of gastight oxygen separation membranes, which should have high oxygen permeation rates and at the same time sufficient thermal, chemical, and mechanical stability permitting a long term reliable operation. Since materials stability and permeability exhibit a trade-off a compromise has to be found. A promising approach is maximizing the oxygen permeation rate of stable materials by reducing the membrane thickness. However, a sole thickness reduction below a characteristic thickness Lc is not the solution as surface exchange kinetics becomes more and more rate limiting.
The focus of the presentation lies on the development of materials and graded membrane arrangements for different applications. However, highly permeable membrane materials show a chemical instability against CO2, SOx and other flue gas components. One major challenge faced is therefore to identify and develop membrane materials, components, and a Proof of Concept-module for the 4-end mode OTM integration. The selected desired membrane assemblies will consist of a thin membrane layer supported on substrate with engineered porosity and oxygen reduction catalysts with high and stable activity in flue gas.
The presentation is focussed on the material and microstructure aspect of stable perovskite and dual phase membranes. The manufacturing of thin supported membranes by sequential tape casing and the sintering procedure is shown and the characterisation of these membranes regarding performance (flux and conductivity), properties e.g. thermal and chemical expansion, stability and microstructure, etc. is shown.