Numerical study of hydrogen permeation flux in SrCe0.95Yb0.05O3-α and SrCe0.95Tm0.05O3-α (II)

One of the main technical challenges presently faced in the production of hydrogen is the cost of separation from gas mixtures such as syngas. One of the major hydrogen separation processes, gas separation membranes, seems to be the most promising technology for hydrogen purification when compared to other alternative technologies. Attractive candidates for hydrogen separation membranes are dense ceramic membranes, commonly based on perovskite-type oxides such as 5%Ytterbium (Yb)-doped strontium cerate (SrCe0.95Yb0.05O3-α), and 5%Thulium (Tm)-doped strontium cerate (SrCe0.95Tm0.05O3-α). Our previous study estimated the hydrogen permeation flux of SrCe0.95Yb0.05O3-α and SrCe0.95Tm0.05O3-α and compared the model results with experimental data (Matsuka et al., 2007). It was found that the effect of the hydrogen partial pressure gradient on the hydrogen permeation in the model result was not as significant as indicated by the experimental results. It was suspected that the disagreements may possibly have arisen from one of the model assumptions where the influence of the O2 partial pressure gradient is assumed to have no influence on the hydrogen permeation flux. Therefore, the aims of this study are to 1) analyse the model with the additional terms to include the O2 partial pressure gradients; 2) analyse sensitivity properties of the modified model; and 3) tune model parameters in the modified model to predict hydrogen permeation flux in SrCe0.95Tm0.05O3-α. It is noted that Song et al. (2003) listed three cases (Case 1, 2, and 3) for hydrogen permeation flux calculations. However due to the nature of the model structure where PH2O is fixed for the stepwise calculations (i.e. PH2O is constant throughout the membrane.), Case 2 in Song et al. (2003) is not analysed in this study. Therefore, this study investigated Case 1 and Case 3 in Song et al. (2003) and they are referred to as Case A and Case B in this study, respectively. The results showed that the hydrogen permeation flux, calculated with the additional term for the O2 partial pressure gradients (Case B), agrees reasonably well with the experimental data. However the effect of hydrogen partial pressure gradients on the hydrogen permeation in the model result was still not as significant as indicated by the experimental results. This indicated that the discrepancy in the effect of hydrogen partial pressure gradients is not due to the influence of O2 potential gradients. Parametric sensitivity analysis showed that the model is generally more sensitive to the charge carrier mobilities, a result which disagrees with the previous study. This may be due to the modified method for defect concentration calculations which allows incorporation of the concentration constraints in the parametric sensitivity analysis. The effect of the thermodynamic equilibrium constants may be suppressed by the concentration constraints incorporated in the analysis, since the thermodynamic equilibrium constants greatly influence the defect concentrations, and the defect concentrations influence the conductivity of charge carriers (Eq. (8)-(12) and (15)). It was expected that the model may become more sensitive to hole mobility (μh) and the thermodynamic equilibrium constant for oxygen ion incorporation (KOX) in Case B, due to the incorporation of O2 partial pressure gradients. O2 partial pressures influence the hole concentration, and consequently the overall electrical conductivity of the membrane. Concentration of oxygen vacancy also depends on the value of KOX.. However, they remained the least sensitive parameters. Further there is no significant difference in the sensitivity of the model by comparing Case A and B. Thus, these suggest that the influence of O2 partial pressures on these types of membrane may not be significant. As a result of parameter tuning, the hydrogen permeation flux in SrCe0.95Tm0.05O3-α is fairly well predicted with the tuned parameters. The tuned parameters showed a similar trend to the previous study. As expected, the tuned values for oxygen vacancy mobility (μvo) and thermodynamic equilibrium constant for oxygen ion incorporation (KOX) in SrCe0.95Tm0.05O3-α resulted in their lower boundary values. This may further indicate a lower affinity of SrCe0.95Tm0.05O3-α to oxygen even under the influence of oxygen partial pressures.