Xenon recovery from medical gas mixtures by polymer membranes: Effect of temperature on Xe/O₂ selectivity

Xenon is a high-cost precious inert gas having the great potential to use in clinical anesthesia. A promising way to reduce the cost of the medical procedure is recovery of Xe from the used anesthetic mixtures. Membrane gas separation can be prospective and effective technology for this purpose, however, the lack of data on xenon transport in polymeric membranes hinders exploitation and deployment in this area. This paper describes the study of temperature effect in the range 0-50 degrees C on Xe/O-2 selectivity of various membrane polymers and membranes. Theoretical estimation of xenon permeability at different temperatures was performed for PVTMS, PC, TMPC, TMHFPC, PTMSP, PDMS and polyarylate copolymer with PDMS (A-PDMS). Experimental data were obtained for industrial membrane MDK-1 (siloxane-based copolymer selective layer) and industrial PPO hollow fiber membrane (HF PPO). Theoretical and experimental results show that membrane polymers can be divided into two groups: (1) oxygen-selective PVTMS, TMHFPC, PC, TMPC, PPO, and (2) xenon-selective PDMS, A-PDMS, PTMSP, and MDK-1 membrane. Almost all considered polymers and membranes increase selectivity with temperature decrease. Minor selectivity changes among both groups demonstrate TMHFPC, while PVTMS, PC, TMPC, PPO, PDMS, A-PDMS, PTMSP and MDK-1 exhibit an increasing selectivity as the temperature decreases. HF PPO membrane provides the highest O-2/Xe selectivity and MDK-1 has as high Xe/O-2 selectivity as PDMS. The O-2/Xe selectivity of HF PPO rises from 8.5 to 15 and Xe/O-2 selectivity of MDK-1 improves from 3.0 to 4.7 while decreasing temperature from 50 to 0 degrees C, however, it leads to permeance decline of both O-2 and Xe in this case. Nevertheless, still high permeance of these membranes at 0 degrees C (216 GPU for Xe in MDK-1 and 47.4 GPU for O-2 in HF PPO) in combination with elevated selectivity represents attractive properties for the membrane separation system development. Modeling of a medical anesthetic O-2/Xe mixture (30/70 vol%) separation by one-, two-, and three stage membrane systems using HF PPO and MDK-1 membranes was performed. Single-stage separation with MDK-1 membrane is allowing for a maximum xenon purity of around 91 vol %. Single-stage separation with HF PPO demonstrates increasing the xenon recovery from 0.45 to 0.59 at a xenon purity of 99 vol% while lowering the temperature from 50 to 0 degrees C. For a two-stage HF PPO based membrane system the recovery of xenon increases by approximately 0.26 compared to single-stage separation at all temperatures. Introducing the supplement xenon-selective stage with MDK-1 membrane leads to additional increase xenon recovery by 0.11 reaching a value of 0.95 and 99 vol% of xenon purity at 0 degrees C. These separation characteristics exhibit corresponding values of sorption systems for xenon capture and recovery. Obtained results demonstrate potential of application gas separation systems combining membranes with xenon- and oxygen selective properties.