Application of Cardo-type polyimide (PI) and polyphenylene oxide (PPO) hollow fiber membranes in two-stage membrane systems for CO2/CH4 separation

In this study, a two-stage membrane system for the separation of CH4/CO2 was designed based on a series of laboratory-scale experiments obtained using two types of commercial hollow fiber membranes including polyphenylene oxide (PPO) and Cardo-type polyimide (PI). The former showed high permeability and low to moderate selectivity, while the latter exhibited high selectivity and moderate permeability. In this design, the permeate from the first stage – rich in CO2 but containing considerable amount of methane – is fed into the second stage to recover its methane content. The performances of double- and single-stage system designs were compared in terms of methane loss (the ratio of methane content in permeate to that of feed) and recompression needed for the second stage at a given retentate CO2 concentration. It was concluded that the methane loss was minimum when PI was used in both stages, while the required membrane area was minimum when PPO was used in both stages. Since in latter case the obtained methane loss was the highest, the proposed design to attain both low methane loss and membrane area was using PI in the first and PPO in the second stages.