Molten salt facilitated transport membranes. Part 1. Separation of oxygen from air at high temperatures

Abstract

A new approach to the fabrication of facilitated transport (FT) liquid membranes for the separation of gases is described. The membranes are prepared by imbibing a thin, porous flat sheet support with a molten salt that reacts reversibly with the desired permeant gas, and which thus acts as a facilitated transport carrier of the gas. As an illustration of this concept, membranes consisting of molten lithium nitrate and sodium nitrate immobilized in thin porous metal supports were constructed and tested for the separation of oxygen from dried zero grade air at 350–524°C. These immobilized molten salt (IMS) membranes were shown to selectively permeate oxygen via a FT mechanism, which is based on the reversible reaction of oxygen with nitrite ions in the melts: M +NO − 2 + 1 2 O 2 ⇄ M + NO − 3, M  Li, Na For a NaNO 3-IMS membrane oxygen permeabilities ( PO O2) ranging from 58 to 1110 barrer with corresponding oxygen to nitrogen selectivities ranging from 4 to 80 were observed, at 452°C to 524°C and specified experimental conditions. There is a strong dependence of Po O2 on operating temperature, which is indicative of a nitrate ion dissociation rate determining step for the FT of oxygen through the membrane. The advantages and limitations of immobilized molten salt versus conventional immobilized liquid FT membranes are discussed.