Liquid Salt Hydrate Acid Gas Absorbents: An Unusual Desorption Of Carbon Dioxide And Hydrogen Sulfide Upon Solidification

Abstract

It was recently reported that certain salt hydrate melts can function as pressure swing absorbents for acid gases. The utility of these salt hydrates derives from their large and reversible acid gas absorption capacities. Typical is the salt hydrate tetramethylammonium fluoride tetrahydrate which as a melt absorbs 0.30 mol CO₂/mol salt at 50[ddot]C and 100 kPa CO₂. It has now been discovered that the reactivity of some of these salt hydrate melts with CO₂ and H₂S exhibits an unusual and unexpected temperature dependence. When certain specific salt hydrate melts containing absorbed CO₂ were cooled to temperatures which resulted in solidification, CO₂ was spontaneously desorbed. For example, a sample of tetraethylammonium acetate tetrahydrate (TEAA) containing 0.15 mol CO₂/mol salt at 50[ddot]C and 102 kPa desorbed 90% of its bound CO₂ upon cooling to 26[ddot]C. Gas absorption and desorption are completely reversible, and the absorbent can be cycled by simply raising or lowering the temperature through the point of solidification. Similarly, a sample of TEAA containing 0.30 mol H₂S/mol salt at 50[ddot]C desorbed H₂S upon cooling to 10[ddot]C. A rationale for this unusual temperature-dependent desorption of acid gases from salt hydrates is presented. The modest decrease in temperature required for an abrupt release of gas from TEAA may supply a kind of “on/off” switch for gas absorption which may be of considerable value for the separation of acid gases, particularly H₂S, from process streams.