| CO2 is the most important anthropogenic emission of greenhouse gas which cause global warming.Facing CO2separation needs in flue gas treatment,natural gas processing,biogas purification and many chemical industries,continuous improvement of carbon capture technology is of great significance to alleviate the greenhouse effect and energy shortage.The membrane contact absorption technology is a new type of decarburization process that combines the traditional absorption method with the membrane separation technology to overcome the sizeable energy consumption and operational problems of the conventional absorption system.In order to solve the most important problem of PVDF membrane wetting during contactor operation and obtain higher absorption flux and long-term operation stability,in the first part of the study,the PVDF-PTFE hollow fiber composite membranes were fabricated through polymer blending method by dispersing more hydrophobic PTFE nanoparticles into PVDF solution and adopting Phosphate(PA)as nonsolvent additive.In the second part,the hydrophobically modified nano-SiO2was dispersed in the inner spinning solution,and the inner coagulation bath was changed to prepare the inner hydrophobic layer PVDF-SiO2 dual-layer composite membrane by co-extrusion approach.In order to verify the feasibility of the preparation method and obtain the optimal preparation conditions,composite membranes were characterized by the SEM morphological analysis,structural parameters,gas permeability and CO2 absorption performance.The specific research results are as follows:(1)Compared with the pure PVDF membrane,PVDF-PTFE composite membrane displayed larger proportion of the porous zone,more finger-like structure in the porous region and larger effective surface porosity.The mean pore size and contact angle of the membrane surface increased with the increase of PTFE loading,so composite membranes showed improvements in both gas permeability and wetting resistance,which resulted in improvement in CO2 absorption performance.The composite membrane with addition of 5 wt%PTFE presented moderate mean pore size and maximum surface effective porosity,and gained the highest CEPw value and CO2 absorption flux in the PTFE series membranes.(2)When the PTFE loading was 5 wt%,the porous zone in the PVDF-PTFE composite membranes and the finger-like structure in the porous zone decreased with the increase of PA content,and the membrane surface pore size and contact angle decreased,so that the wetting resistance was not significantly improved,but the gas permeability of the membrane declined with higher concentration of PA.The PVDF-PTFE composite membrane with 4 wt%PA addition in the spinning dope had high gas permeability and wetting resistance,it gained the optimum CO2absorption flux of 2.17×10-3 mol·m-2·s-1 at an inlet gas flow rate of 100mL/min by using aqueous diethanolamine solution(1M)as absorbent with the absorption operation mode of gas flowing through the lumen side.Moreover,it showed better long-term operation performance than pure PVDF membrane regardless of different types of absorbent.(3)The bore fluid containing 80%NMP induced a high-roughness PVDF spherulitic structure on the membrane inner surface,so the PVDF-PTFE composite membrane displayed higher CO2 flux when the liquid flows through the lumen side and the gas flows through the shell side.Therefore,in order to make full use of the structural advantages and reduce the gas mass transfer resistance,the second part of this paper prepared dual-layer composite membrane with the inner hydrophobic layer.(4)The cross-sectional morphology of the dual-layer membrane prepared by the co-extrusion approach exhibited a highly hydrophobic,relatively thin PVDF-Si02 composite inner layer and a highly open,fully finger-like structural outer layer.Compared with the single-layer composite membrane,the CEPw value of the dual-layer membranes was not significantly improved due to the enlargement of the outer surface pore size and the supporting outer layer of low tortuosity,but the gas permeability was higher than that of the single-layer membrane,so the dual-layer membranes had higher absorption fluxes.(5)When the content of NMP in the bore liquid is 80%,the increase of Si02 content in the inner spinning dope had little effect on the cross-sectional morphology and outer surface structure.The thickness of the inner hydrophobic functional layer and the size of the inner surface spherulitic particles were reduced,the contact angle was significantly increased,and the wetting resistance was slightly improved.The composite membrane with 4 wt%SiO2 addition presented the highest gas permeability and obtained the highest absorption flux of 3.12×10’3 mol m-2·s-1 with the absorption operation mode of liquid flowing through the lumen side.(6)When the SiO2 loading in the inner layer dope was 4wt%,the inner layer finger-like structure of the dual-layer membrane cross-section became shorter and disappeared with the increase of NMP concentration in the bore liquid,while the outer layer gradually developed from the small finger-like pore to fully finger-like open structure.The mean pore size on the outer surface and gas permeability of the membrane body were improved,the smooth inner skin layer was gradually replaced by a high-roughness spherulitic structure,the contact angle was significantly increased and the wetting resistance showed an increasing trend.When the bore fluid contained 65%NMP,the membrane inner surface presented a special network structure between the smooth skin and the spherulitic particles,and the wetting resistance was the strongest,so it exhibited the best long-term operation stability,indicating gas permeability and inner surface contact had a greater impact on absorption flux in short-term operation,while wetting resisitance was the key factor for long-term operation stability. |
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