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 beta-Cyclodextrin for separation of CO2/CH4 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 CH4, CO2, 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 7.4 m/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 7.4 m/min and 0.5 ml/min, respectively. These conditions resulted in CO2/CH4 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 degrees C were slightly higher than those of the precursor fibers and this improvement was more significant for membranes treated at 400 degrees C. This enhancement demonstrates that crosslinking is more severe at 400 degrees C. The best separation performance of the annealed and silicone rubber coated hollow fibers in this study has a CO2 permeance of around 82 GPU with a CO2/CH4 ideal selectivity of around 20 and a high C3H6 permeance of around 29 GPU with a C3H6/C3H8 ideal selectivity of 15.3. It can also resist CO2 induced plasticization until 25 atm. 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. (c) 2012 Elsevier B.V. All rights reserved.