Simultaneous application of vacuum and sweep gas in a polypropylene membrane contactor for the recovery of dissolved methane from water

Abstract

Vacuum and nitrogen as sweep gas were used simultaneously (combination mode) to generate the driving force in a microporous polypropylene membrane contactor for the recovery of dissolved methane from water. Experiments were carried out under different liquid flow-rates (5.0–28.0 L h−1), N2 flow-rates (0.5–1.5 L h−1) and vacuum pressures (0–480 mbar). The maximum removal efficiency of methane was >90% at moderate values of gas-to-liquid ratios (G/L) and vacuum of 0.2 and ≤ 200 mbar, respectively. Removal efficiencies obtained in combination mode were usually higher than when using vacuum or sweep gas, separately. To achieve a methane content >35% in the recovered gas, G/L values lower than 0.03 are necessary, with removal efficiency up to 55%. Even at these soft conditions, gas phase mass transfer resistance was demonstrated as being negligible, so the limiting resistance was in the liquid phase. G/L ratios increased until values of 0.3, with a vacuum of 200 mbar, maximised the energy output (>250 kJ m−3 of treated water), in the case of recovered methane mixed with biogas produced in an anaerobic reactor. The results show that combination mode could be used to improve the performance of methane degassing in membrane contactors.