(Invited) Modeling Charged-Defect Transport within Calcium Doped Lanthanum Ferrite Oxide-Ion Transport Membranes

Abstract

Partial oxidation processes play important roles in the production of synthetic fuels and chemicals from methane feedstocks. Such processes include catalytic partial oxidation (CPOX) to produce synthesis gas and oxidative coupling of methane (OCM) to produce ethylene. In both cases, catalytic reactors with permselective oxygen-transport membranes can be designed to improve process performance through process intensification. Membrane reactors provide two distinct potential advantages compared to conventional catalytic reactors. First, oxygen can be separated from air within the fuel processor, thus eliminating a dedicated upstream air-separation reactor. Secondly, and perhaps more importantly, the spatially distributed introduction of oxygen can assist in thermal management of highly exothermic processes. Calcium-doped lanthanum ferrite (La 0.9Ca0.1FeO3- d, LCF), an A-site-doped perovskite, is particularly relevant for partial-oxidation membrane reactors applications. LCF membranes have good oxide-ion conductivity from 600 ˚C to 900 ˚C, and unlike other related ceramic oxide-ion conductors (e.g., La1-xSrxCo1-yFeyO3- d, LSCF) that are only stable on oxidizing environments, LCF is stable in both oxidizing and reducing environments. Membranes for partial-oxidation reactors must have chemical and structural stability on both the fuel (reducing) and air (oxidizing) sides of the membrane. As is the case with most doped perovskites, the LCF material is a mixed ionic-electronic conductor (MIEC). The three mobile charged defects are oxygen vacancies V•• as well as both reduced Fe’ and oxidized Fe• iron, which behave as small polarons associated with the iron B site. Two relevant reactions describe the oxygen incorporation and the iron disproportionation. Based on the thermodynamic properties reproted by Geary and Adler (Solid State Ionics, 253:88-93, 2013), Fig. 1 shows equilibrium defect concentrations as functions of oxygen partial pressure. With the relatively high concentration of polarons as shown in Fig. 1, the LCF material sustains reasonably high electronic conductivity, which makes it a candidate as a ceramic electrode material for applications such as solid-oxide fuel cells or electrolyzers. However, because of the high electronic conductivity, polarization does not enhance migration fluxes in LCF membranes. This presentation focuses on model-based analysis of the defect transport through LCF membranes, primarily within the context of partial-oxidation membrane reactors. The mixed-conduction problem is formulated in the Nernst-Planck-Poisson (NPP) setting in the dilute limit (i.e., neglecting direct defect-defect interactions). Fluxes are represented as a combination of diffusive contributions (i.e., proportional to concentration gradients) and migration contributions (i.e., proportional to electrostatic-potential gradients). Mass conservation is represented in terms of the divergence of the charge-defect fluxes. The electrostatic-potential profiles are determined via the Poisson equation, essentially preserving local charge neutrality. Additionally, the model must be formulated to preserve local lattice-scale site balances and charge neutrality. Boundary conditions are specified in terms of incorporation kinetics at the gas-ceramic interfaces. The surface kinetics may be kinetically limited or may remain essentaily equilibrated. The mathematical problem is represented as a system of partial differential equations, which can be solved computationally. The presentation explores membrane performance under a range of temperatures and gas compositions relevant to oxidation membrane reactors. Although overall reactor performance depends on interactions between membrane fluxes and fuel-side catalytic chemistry, this presentation focuses entirely on the membrane performance. Figure 1