Natural gas purification and olefin/paraffin separation using cross-linkable dual-layer hollow fiber membranes comprising Β-Cyclodextrin

In this study, thermally cross-linkable co-polyimide dual-layer hollow fiber membranes grafted with Β-Cyclodextrin for separation of CO ₂/CH ₄ and propylene/propane have been fabricated. In order to find the best spinning condition, the performance of hollow fiber membranes at various take-up velocities and outer-layer dope flow rates was investigated. The fiber membranes were thermally cross-linked at different temperatures and the performance of the fibers before and after silicon rubber coating was studied using CH ₄, CO ₂, propane and propylene. It was observed that permeances of all gases decreased with an increase in take-up velocity and outer-layer dope flow rate. Selectivities of the membrane with respect to the take-up velocity initially increased and after a take-up velocity value of 7.4m/min started to decrease. This up and down trend was attributed to the influence of elongational draw ratio and change in surface porosity of the membrane. Optimum take-up velocity and outer-layer dope flow rate for as-spun fibers were 7.4m/min and 0.5ml/min, respectively. These conditions resulted in CO ₂/CH ₄ selectivities of 6.22 and 14.3 before and after silicon rubber coating, respectively. The results demonstrated that thermal treatment improves membrane selectivities and decreases membrane permeances. The enhancement of selectivities can be a result of cross-linking and reduction in permeances due to densification of the hollow fiber membranes. Selectivities of thermally treated fiber membranes at 350°C were slightly higher than those of the precursor fibers and this improvement was more significant for membranes treated at 400°C. This enhancement demonstrates that cross-linking is more severe at 400°C. The best separation performance of the annealed and silicone rubber coated hollow fibers in this study has a CO ₂ permeance of around 82GPU with a CO ₂/CH ₄ ideal selectivity of around 20 and a high C ₃H ₆ permeance of around 29GPU with a C ₃H ₆/C ₃H ₈ ideal selectivity of 15.3. It can also resist CO ₂ induced plasticization until 25atm. It is believed that, with these gas separation and anti-plasticization properties, the newly developed membranes may have high prospective for natural gas purification and olefin/paraffin separation.