LIQUID SALT HYDRATE ACID GAS ABSORBENTS - AN UNUSUAL DESORPTION OF CARBON-DIOXIDE AND HYDROGEN-SULFIDE UPON SOLIDIFICATION

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 t etrahydrate which as a melt absorbs 0.30 mol CO2/mol salt at 50 degree s 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 hyd rate melts containing absorbed CO2 were cooled to temperatures which r esulted in solidification, CO2 was spontaneously desorbed. For example , a sample of tetraethylammonium acetate tetrahydrate (TEAA) containin g 0.15 mol CO2/mol salt at 50 degrees C and 102 kPa desorbed 90% of it s bound CO2 upon cooling to 26 degrees C. Gas absorption and desorptio n are completely reversible, and the absorbent can be cycled by simply raising or lowering the temperature through the point of solidificati on. Similarly, a sample of TEAA containing 0.30 mol H2S/mol salt at 50 degrees C desorbed H2S upon cooling to 10 degrees C. A rationale for this un usual temperature-dependent desorption of acid gases from salt hydrates is presented. The modest decrease in temperature required fo r an abrupt release of gas from TEAA may supply a kind of ''on/off'' s witch for gas absorption which may be of considerable value for the se paration of acid gases, particularly H2S, from process streams.