Liquid membrane-based CO2 reduction in a breathing apparatus

Hydrophilic porous hollow fiber membranes were used to study appropriate immobilized liquid membranes (ILMs) for selective separation of carbon dioxide from anesthesia breathing circuit gas mixture containing nitrous oxide (N2O), carbon dioxide (CO2), oxygen (O2) and halogenated hydrocarbons (HHCs). The pores in these hollow fibers were impregnated with the following liquids: Na-glycinate–glycerol solution; K-glycinate–glycerol solution and Na-glycinate–PEG 400–glycerol solution. Halogenated hydrocarbon liquids (HHCs) used in the permeation study were halothane, enflurane, sevoflurane and desflurane. Separation of CO2–N2O–O2–HHC mixtures was studied using a variety of hollow fiber support membranes and modules and at temperatures between 25 and 37 °C. Relatively low separation factors between CO2/N2O were observed in the case of the immobilized Na-glycinate–glycerol solution or K-glycinate–glycerol solution due to similar physicochemical characteristics of carbon dioxide and nitrous oxide. Highest CO2 permeances achieved using 5%CO2 inlet feed concentration at 37 °C with hydrophilic microporous polyethersulfone (PES) membranes immobilized either with 3 M-Na/K-glycinate–glycerol or 2 M-Na-glycinate in 50/50 glycerol/PEG 400 solution were in the order of 100–490 GPU (1GPU = 10−6 scc/cm2-s-cmHg). Observed HHC permeances were in the range of 0.02–2.6 GPU depending on the type of liquid membrane, carrier, temperature and hollow fiber used during this study. Reasonably high selectivity for CO2/HHC was observed at room and elevated temperatures (37 °C). A module having larger membrane area achieved considerable CO2 removal from the feed breathing gas mixtures at higher sweep gas flow rates (100–450 cm3/min) while using 5%CO2 inlet feed gas concentration. The highest carbon dioxide permeance of 1600 GPU was achieved with the breathing gas mixture containing 2%CO2 at the elevated temperature of 37 °C, where 5%CO2/70%N2O/O2 balance yielded a CO2 permeance of 490 GPU at 37 °C. An ultrathin hydrophobic porous coating on the outside surface of the PES hollow fibers prevented flooding of the liquid membrane by moisture condensation or flowing water on the outside of the fiber.