Fabrication Of Polyimide Hollow Fiber Membranes And Their Gas Separation Performance

Greenhouse effect and energy issues have attracted considerable attention.It is the challenges to control CO₂ emissions from flue gas and to separate CO₂ from natural gas and biogas.Hollow fiber membranes play an important role in energy utilization and environmental protection due to their advantages of energy saving,environmental friendly as well as high packing density.Until now,the membranes have become one of the most effective methods for gas separation.Membrane materials are the core of membrane technology.Polyimide?PI?materials have excellent mechanical properties,thermal properties and permeability.Therefore it is expected to be widely used in the field of gas separation.Here,the three aspects,including synthesis of polyimides with gas separation performance,formation of hollow fiber membranes and the surface coating,are discussed in order to improve the permeability of PI hollow fiber membranes.To synthesize the functional polyimides,the functional groups were introduced into the polymer chains in order to increase the free volume and promote CO₂ mass transfer and improve the permeability and selectivity of polyimides.Polyimides were synthesized by the reaction of 4,4'-?hexafluoroisopropylidene?diphthalic anhydride?6FDA?and 4,4'-oxybisbenzenamine?ODA?,4,4'-?4,4'-isopropylidenediphenyl-1,1'-diyldioxy??BAPP?,2,2-bis[4-?4-aminophenoxy?phenyl]-hex afluoropropanane?HFBAPP?,4,4'-[sulfonylbis?p-phenyleneoxy?]dianiline?BAPS?in the solvent of N-methyl-2-pyrrolidone?NMP?.The prepared polyimide exhibit good solubility in some organic solvents,and the films show good mechanical properties.The density of films ranges from 1.388 to 1.427 g/cm3,and the free volume fraction is between 0.144 and 0.162.Glass transition temperature of the polymers is between 257 oC and 304 oC,and the contact angle is ranging from 53° to 93°.Among them,6FDA-HFBAPP polyimide has the best solubility and the maximum free volume is 0.162.The maximum contact angle reaches 93° and the minimum glass transition temperature is 257 oC.Therefore,the solution is used to prepare hollow fiber membranes.The above 6FDA-HFBAPP polyamic acid?PAA?solution was used to prepare the hollow fiber membrane with dense and porous supports by dry-wet method and followed post-treatment.The effects of spinning solution properties,the compositions of the internal and the external coagulation baths,the velocity of the internal coagulation solution,and the length of air gap on the structure and performance of the separation membranes were investigated.As a result,the 6FDA-HFBAPP solution is suitable for two-step spinning.The optimum spinning conditions is followed: the external coagulation is pure water,the internal coagulation composition is VH2 OVNMP ? = 10 / 90,the ratio of the internal coagulation flow rate to the spinning liquid flow rate is between 0.4 and 0.6,and the air gap is 6 cm.The prepared hollow fiber membrane exhibits the good performance and the permeance of CO₂ and CH4 is 89 GPU and 75 GPU respectively.To further improve the separation performance,the polyimide hollow fiber membrane as the base membrane was coated with the PDMS to block the defects in the dense layer.The results show that when the coating mass concentration is 5%,the coating shows the good effect.The permeate flux of CO₂ and CH4 is 42 GPU and 7.8 GPU respectively,and the separation coefficient is 5.4.The separation mechanism of the composite membrane was analyzed by Henis complex resistance model.