2.5. Gas permeability measurement
The gas permeability of the polymer membrane and MMMs were measured using a constant-volume varying-pressure system [52]. The membrane was held in the permeation cell which was divided into two parts by the membrane and sealed with O-rings. The entire permeation apparatus was placed in an oven to keep the temperature constant. In order to measure the gas permeability of the membranes precisely, the system was evacuated for 24 h to eliminate residual air or other gases. The feed gas was introduced on the upstream of the membrane while the downstream was held at high vacuum. The pressures on both sides were monitored online using pressure transducers. Permeate from the membrane was collected in a constant volume downstream and was measured directly by evaluating its pressure increase using a Sensys pressure transducer. The trans-membrane pressure was kept constant by maintaining the upstream pressure unchanged and ensuring that the downstream pressure remains under 100 mbar during the gas permeability measurement. This 100 mbar (∼0.1 bar) is negligible in comparison with the applied upstream pressures (>2 bar) in all the experiments; thus a constant trans-membrane pressure was obtained. The gas permeability measurements for pure CH4 and CO2 gases were carried out at 35 °C and with feed pressures of 2, 4 and 6 bar. Circular membrane discs with 5 cm diameter and the effective permeation area of about 11.4 cm2 and the thickness of about 150 μm were used for flat sheet gas permeation tests. Each gas was run through a membrane three times and the average of the results was reported.