Production, characterization and modeling of hollow fiber membranes for biogas purification

One type of technology for biogas purification which has experienced substantial growth during past decades is membrane-based technology which presents the advantage of high energy efficiency, simplicity in design and environmental compatibility. The market for CO₂ is dominated by polymeric membranes due to their relatively low manufacturing cost. In this work, the performance of different P84 hollow fibers membranes for CO₂/CH₄ separation is investigated. Different spinning parameters are studied on their effect on gas separation performance of the produced fibers. The attention is focused on air gap length and spinning temperature, since they both affect the formation of the selective layer during the spinning process. Scanning Electron Microscopy (SEM) is used to investigate the morphological characteristics of the developed fibers. Permeation rates of CO₂ and N₂ are measured using the pressure increase method. The achieved ideal selectivity was 30/40 for CO₂/N₂ separation, while permeances are quite low. The permeance of CO₂ through asymmetric hollow fibers increases with pressure because of plasticization. Both gases permeance increases by increasing the air gap and higher spinning temperatures lead to lower permeances. Finally, NE-LF model is used to calculate CO₂ and CH₄ solubility and CO₂/CH₄ solubility selectivity in P84. Large deviation from ideal conditions is predicted underlining a marked competition which affects more CH₄ than CO₂ and results in a real solubility selectivity higher than the ideal one. By considering that real mixed gas perm-selectivity generally presents negative deviation, it follows that diffusivity of CH₄ in P84 is enhanced in presence of CO₂ lowering the diffusivity-selectivity.