Experimental investigations of oxygen-separating ion transport membranes for clean fuel synthesis

Abstract

Ion transport membranes (ITMs) are dense ceramic membranes which have the potential for 100% selective separation of oxygen from air. ITMs operate at extreme temperatures (>800°C), necessary for the mobility of lattice oxygen ions: this can result in significant experimental challenges. Specifically, the local gas compositions at both the high oxygen (air) and low oxygen (sweep) surfaces influence the oxygen flux: these experimental measurements have not been available until now. A novel ITM research reactor has thus been developed which can directly sample gases at the membrane surface at high temperature flux conditions. This ITM reactor has been scaled up to allow for gas-probing instruments to be used without overly disrupting the experimental flowfield. The ITM stoichiometry investigated in this study is La₀.₉9Ca₀.₁1FeO3-[delta] (LCF), and has been chosen for its chemical stability attributes and consequent applicability to industry. Two modes of operation have been investigated using the LCF ITM in the reactor: inert (using CO₂ sweep gas to carry away an oxygen-enriched stream) and reactive (using CO₂:CH₄ sweep gas resulting in fuel reactions with the permeating oxygen). There is a huge advantage to running ITMs reactive: the oxygen flux can be enhanced by an order of magnitude or more, whilst useful fuel synthesis reactions can be actively enhanced by the catalytic ITM surface. This study therefore utilizes the local measurement capabilities of the novel ITM reactor to develop a physical understanding through oxygen flux models for both modes of operation: inert and reactive. Both flux models enable the prediction of the oxygen lux with the operating conditions necessary as input parameters. They are therefore useful tools for future optimization of ITM reactor designs. Further insight using the flux models is also provided. The inert flux model is used to determine the surface oxygen vacancy concentration which drives the oxygen flux. The reactive flux model is used in preliminary numerical simulations of ITM reactors to produce flux performance maps based on the input operating conditions.