Separation of CO2 from CH4 by using gas–liquid membrane contacting process

The separation of carbon dioxide (CO2) from methane (CH4) by using a gas–liquid membrane contactor was studied in order to confirm the potential of the process. The experiments were performed in a membrane contactor constructed with 0.2 μm pore size microporous PVDF hollow fibers. Pure water, aqueous sodium hydroxide (NaOH) solution and aqueous monoethanolamine (MEA) solution were employed as the absorbents. Sodium chloride (NaCl) was also used as an additive in the NaOH aqueous solution. The effects of operating parameters such as the gas and liquid velocity, concentration of NaOH solution, absorbent temperature, and NaCl concentration on the CO2 flux were investigated along with the mass transfer analysis of the process. In addition, the impacts of the flow pattern and the membrane module packing density as well as the long-term performance were also investigated, aiming to obtain a full picture and a deep insight on the system.It was found that the CO2 flux was enhanced by the increase of NaOH solution concentration, NaOH solution temperature and the CO2 volume fraction in the feed stream, but the increase of water temperature resulted in decreasing the CO2 flux. The retentate selectivity obtained in this work was not satisfactory due to the laboratory scale of the membrane module used. However, the percentage of CH4 recovery was very high, suggesting that no significant CH4 loss took place during the operation. Counter-current flow mode took the advantage of higher mass transfer over co-current one, and the CO2 flux increased with increasing membrane module packing density. Long-term performance tests showed that the NaOH solution was more suitable than other absorbents to be applied to the PVDF microporous hollow fiber contactors, as the NaOH aqueous solution can provide a higher separation performance and the CO2 flux was kept almost unchanged over a long period of operation. Furthermore, the PVDF membrane gave the membrane resistance around 22% of the total resistance for pure CO2 absorption in water.