(31a) Mixed-Conducting Metal Oxide Materials–from Membrane to Adsorption

Many metal oxides of cubic lattice structure, including perovskite, brownmillerite and fluorite type materials, contain a large amount of oxygen deficiencies. These materials conduct oxygen ions and electrons at high temperatures (mixed-conductivity), and have been extensively studied in the past decade as membranes. Membranes made of this group of materials are highly permselective for oxygen, with an extremely high oxygen permeation flux at temperatures above 800C. There are major industrial efforts in developing large scale membrane processes utilizing this group of metal oxides for applications in air separation and conversion of methane to syngas. The oxygen non-stochiometry of this group of metal oxides is a strong function of oxygen partial pressure and temperature. This leads to a new, but quite different applications of this group of metal oxides ? as sorbents in fixed-bed processes for air separation and production of oxygen enriched carbon dioxide streams. The perovskite type metal oxides preferentially adsorb oxygen at high temperatures, and can be regenerated through temperature or pressure swing,to produce pure nitrogen and oxygen. If carbon dioxide is used for regeneration, an oxygen enriched carbon dioxide stream with oxygen concentration larger than 30% can be produced. The membrane and adsorption processes using this group of materials offer complementing features making them attractive for many separation and reaction applications involving oxygen.