Olefin/paraffin separation performance of asymmetric hollow fiber membrane of 6FDA/BPDA-DDBT copolyimide

Gas permeation properties of asymmetric hollow fiber membrane of copolyimide prepared from equimolar portion of 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) and 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) with 3,7-diamino-2,8(6)-dimethyldibenzothiophene sulfone (DDBT) were investigated for single-component light gases, olefins and paraffins and for mixed components of C3H6/C3H8 and C4H6/C4H10. The gas permeability of the copolyimide film was close to that of 6FDA-DDBT polyimide and much larger than that of BPDA-DDBT. Gas permeance of the asymmetric copolyimide hollow fiber membrane decreased significantly in the first several months and leveled off after about 10 months of aging. The skin layer thickness calculated from the gas permeability and permeance was in the range of 0.6 (for H-2) to 1.7 mum (for C3H6) and about 10 times larger than the thickness estimated from the SEM observation. These results indicated that the significant densification of the skin layer was caused by physical aging. The silicone rubber coating hardly changed the selectivity for light gas pairs such as H-2/CH4 and O-2/N-2, but enhanced that for C3H6/C3H8 and C4H6/C4H10 significantly especially at low temperatures. The evaluation of membrane quality based on the resistance model suggested that the extremely small surface porosity of defect pores significantly reduced the selectivity for the larger gas pairs. The asymmetric copolyimide hollow fiber membrane displayed better performance for C3H6/C3H8 and C4H6/C4H10: for example, permeances of C3H6 and C4H6 of 3.6 and 7.4GPU, respectively, and separation factors of 15 and 69 for C3H6/C3H8 and C4H6/C4H,(10) (50/50 mol% in feed) at 373 K and 1 atm. (C) 2002 Elsevier Science B.V. All rights reserved.