Influence of gas flow rate on oxygen flux measurements for dense oxygen conducting ceramic membranes

Oxygen transport through a dense oxygen conducting ceramic membrane is driv en by an oxygen activity gradient imposed by the gas streams on either side of the membrane. The oxygen activity on each side of the membrane is alter ed as oxygen is transported across the membrane. Thus, the oxygen activity gradient will not be the same in each region of a membrane and will generat e a different oxygen Aux. This interaction between the oxygen gradient and the oxygen flux can produce a lower average oxygen Bur across the membrane, can mask differences between materials, and can explain the dependence of oxygen flux on oxygen activity. An oxygen flux model has been developed tha t takes into account the rate of oxygen transport across the membrane, the reducing gas flow rate and the reducing gas composition when determining th e oxygen activity gradient imposed on the membrane. The model was validated by collecting oxygen flux data for a dense perovskite ceramic membrane at 1000 degrees C under a wide range of concentrations and flow rates of CO an d CO2 on one side and air on the other side of the membrane. The results sh ow that an order of magnitude variation in the oxygen flux can be produced merely by changing the reducing gas flow rate. These results can help expla in differences in oxygen flux data reported over a range of conditions and shed light on the rate limiting step in oxygen transport through dense oxyg en conducting ceramic membranes. (C) 2000 Elsevier Science B.V. All rights reserved.