Dense Oxygen Permeable Membranes And Membrane-based Process

Some dense ceramics allow oxygen to permeate under a differential oxygen partial pressure at elevated temperatures. This thesis is to prepare and characterize ceramic materials permeable to oxygen and to explore their applications in production of syngas through partial oxidation of methane and combustion of fossil fuel.Chapter 1 gives an overview of membrane-based processes for production of syngas from methane and combustion of fossil fuel. In Chapter 2, the literature on the oxygen-permeable ceramic membrane materials is reviewed, and the principle of oxygen permeation through the dense ceramics is described as well.For the proposed reactor applications, the membrane is required to possess high oxygen permeability as well as good chemical and mechanical stability at elevated temperatures and under a large oxygen gradient. And it has been found extremely difficult for a single-phase material to meet all these requirements, improvement in one aspect of the material is often at the expense of the other aspect. One possible way round this problem is to use dual-phase composites as the membrane, in which oxygen ions and electrons transport through respective phases. In Chapter 3, a ceramic composite of oxygen ionic conductor Zr_0.8Y_0.2O_1.9 (60 vol %) and electronic conductor La_0.8Sr_0.2CrO_3-δ is investigated. An appreciable oxygen permeation flux was observed by exposing the outer shell of a membrane tube to air and sweeping its inner wall with pure helium or CO (balanced with helium). An oxygen flux of 9.2×10^-9 mol cm^-2 s^-1 was measured at 950 °C under air/helium gradient, and a larger flux of 3.2×10^-8 mol cm^-2 (membrane surface area) s^-1 at 930°C under air/CO gradient. The overall oxygen permeation process is shown to be controlled by the surface oxygen exchange step, and an increase of oxygen permeation flux is expected through the modification and activation of the membrane surface. The membrane tube was found to remain intact after the oxygen permeation measurements, and its phase composition and microstructure were largely retained. In Chapter 4, another dual-phase composite...