(407e) Directionally Dependent Transport in a Mixed Ionic-Electronic Conducting Membrane for Separating Oxygen from Air

Tubular mixed ionic-electronic conducting (MIEC) La_0.5Sr_0.5Fe_0.8Ga_0.2O_3-d membranes are used in the reforming of methane to synthesis gas in order to separate oxygen from air for supply to the partial oxidation reaction. Resistance to ionic oxygen transport through a MIEC membrane is dominated by two separate mechanisms: exchange of oxygen at the MIEC surfaces and bulk transport of the oxygen ions through the membrane. The surface exchange coefficient, k_io [cm/sec], and the bulk diffusion, D_a [cm²/sec], are related by the ?characteristic thickness? (L_Do [cm]) of the material, which is the ratio D_a/k_io. The characteristic thickness is the membrane thickness above which bulk diffusion limitations dominate the transport equation and below which surface exchange limitations dominate. Typical values for L_Do are in the range of 0.01 ? 1 cm. Operation of a membrane with a thickness that is within at least one order of magnitude of L_Do will result in mixed control whereby prediction of the ionic transport requires accounting for both limiting mechanisms. The currently accepted ionic transport model for a tubular membrane under mixed control predicts directional dependence for the magnitude of flow. The model projects that outward ionic flux from the tube to the shell side (F'') exceeds the inward flux from the shell to the tube side (F') for the same differential partial pressure of oxygen. Under typical conditions the outward flux (F'') is predicted to be about 1.05 times larger than the inward flux (F'). The directional dependence of the flux disappears when either one of the two resistances is negligible. Experiments were conducted to test the theoretical predictions. Ionic oxygen flow is through a dense membrane tube with an ID of 0.57 cm and an OD of 0.84 cm. The oxygen flux was measured at 1000°C with an oxygen partial pressure of 0.1 - 0.3 atm on one side and ~0.001 ? 0.045 atm on the second. The outward ionic oxygen flow was higher than the inward flow in each of nine experiments. During a typical experiment the inward flow was 0.381 mL/min while the outward flow was 0.456 mL/min, a factor of 1.2 increase. The average ratio between the two fluxes (F''/F') for all cases was 1.4 approximately. The experimentally measured enhancement was larger than that predicted by the model in every case. Detection of directional dependence confirmed that ionic flow was in a mixed control regime. The enhancement results suggest that commercial reactors should operate with the oxygen concentrate gas flowing through the tube side of the membrane.