Experimental extrusion of tubular multilayer materials for Oxygen Transport Membranes

Inorganic oxygen transport membranes (OTMs) are of interest for high purity oxygen production and for integration into membrane reactors where oxygen is required at high temperatures. Doped ceria is an interesting material for an OTM due to its high phase stability under both oxidizing and reducing atmospheres and its high ionic conductivity. Designing and developing a high performance oxygen transport membrane involves scientific challenges associated with material development, ceramic processing and integration of materials in a multi-layer structure. In this work an asymmetric based oxygen transport membrane based on gadolinia doped ceria oxide, (Ce0.9Gd0.1O1.95−δ), (CGO) was developed on a tubular, porous support structure based on cost-efficient magnesium oxide (MgO). The porous support structure was prepared by thermoplastic extrusion using MgO powder, thermoplastic binders and graphite pore former. An optimization of the thermoplastic feedstock has been carried out with the aim of improving gas permeability and mechanical properties of the resulting MgO supports. The influence of three types of pore former (graphite with different shapes and sizes, and polymethyl methacrylate (PMMA)) on the mechanical strength and gas permeation of the extruded MgO porous supports was investigated for sintering temperatures between 1250 and 1400 °C. The gas permeability through the MgO supports during membrane operation was highly dependent on the total open porosity and on the size of pore necks. As expected, for all samples with different pore former type the permeability decreased with sintering temperature and decreasing total porosity. Only for the porous support prepared with flaky graphite did the total porosity and gas permeability increase with increasing sintering temperatures above 1300 °C. Scanning electron microscopy showed that for samples sintered above 1300 °C there was a growth of macro-pores and opening of bottle-neck pores, resulting in improved pore connectivity and thus improved gas permeability. Mercury intrusion porosimetry experiments confirmed an increase in the average pore size for samples sintered above 1300 °C, despite a significant decrease in total porosity. The highest open porosity of 42.5 % and gas permeability of 4.7 ×10-16 m2 was obtained for an MgO support with spherical graphite as a pore former. Implementing a bimodal pore size distribution (by using a mixture of two pore formers with an average size of 5.5 and 10.5 μm) it seems feasible to increase the gas permeation value to 4 ×10-15 m2 and this route is also recommended for further studies. The characteristic strength of the MgO supports was characterized by Weibull measurements with a novel high temperature 4-point bending test method. The results revealed sufficiently high bending strength values of 60 MPa for the MgO support at an operation temperature of 850 °C, whereas the strength at room temperature was 77 MPa. The oxygen permeation flux on an asymmetric tubular CGO membrane, consisting of an MgO support (porous), catalytic layer on permeate side (NiO-CGO) (porous), CGO (dense), catalytic layer on feed side (porous, infiltrated nano LSC (La0.6Sr0.4CoO3−δ) particles on a porous CGO backbone layer), was tested at temperatures between 650 °C and 920 °C on a 30 mm long tube (inner/outer diameter of 9.8/11.4 mm) using atmospheric air and N2, H2 for feed and sweep side respectively. The oxygen permeation was 3.5 Nml min-1 cm-2 at 856 °C using a H2 flow of 200 Nml min-1 on the permeate side. After subsequent oxidation and reduction (redox-cycle) of the Ni-CGO catalytic layer in the membrane, the permeation flux of the membrane improved significantly especially at low temperatures, reaching 4 Nml min-1 cm2 at 850 °C. The improved performance is attributed to an improvement of the catalytic activity of the Ni-CGO structure after a redox-cycle. Finally, oxygen permeation tests on the asymmetric CGO membrane in methane/humidified hydrogen mixtures were performed which led to breaking of the membrane. Postmortem analysis of the membrane microstructure by scanning electron microscopy (SEM) after the oxygen permeation test in methane indicated detachment of the catalytic layer on the permeate side, most likely due to carbon formation.