Study Of Preparation Of Superhydrophobic Hollow Fiber Membrane For Membrane Contact Absorption And Its The Mass Transfer Performance For CO₂

Environmental problems and climate change caused by the greenhouse gases released from fossil fuels have attracted much attention.Carbon dioxide(CO₂)is the most predominant greenhouse gas,which is mainly emitted from combustion engines and fossil fuel-fired power plants.Therefore,the capture of CO₂ from large emission sources for geological storage is considered as an important strategy to decrease the global greenhouse gas emissions.The common capture methods of CO₂ included chemical absorption methods,adsorption methods,membrane separation methods,and low-temperature distillation methods.Among them,the chemical absorption method is the most widely used process in industry.However,the traditional tower-type chemical absorption methods have the large equipment,expensive prices,the uncontrollable of the gas and liquid phase flow rates,and it is easy to cause the problems such as mist entrainment,flooding,and leakage during the absorption process.Compared with the traditional liquid absorption process,membrane contactor technology for CO₂absorption has many advantages,including high interfacial area between the feed gas and liquid absorbent,flexible operation,low cost,and linear amplification,as a result,CO₂ capture by membrane contact absorption is considered as the most promising alternative to conventional CO₂ absorption technology,which has been widely studied by scholars at home and abroad.In the research and application of membrane contact absorption technology,hollow fiber membranes are widely used due to its advantages such as large surface area,compact structure and light weight.However,in the process of membrane contacting absorption,the hollow fiber membrane is easily wetted by the absorption liquid,which leads to a sharp drop in the CO₂ mass transfer flux.This shortcoming limits its application in industrial practice.Therefore,improving the wetting resistance of hollow fiber membranes is the focus and difficulty of the research in membrane contact absorption technology.Studies have shown that the wettability of the membrane is determined by the microstructure of the membrane and the surface chemical composition of the membrane.The anti-wetting performance of the membrane can be improved by regulating the membrane structure and the free energy of the membrane surface.Based on the above background,this research focuses on the preparation technology of highly hydrophobic hollow fiber membranes.The wet phase inversion method is used to prepare highly hydrophobic hollow fiber membranes based on two aspects of membrane structure regulation and surface free energy.Firstly,in order to improve the anti-wetting performance of the membranes,the non-solvent additives are used to induce and control the hollow fiber membrane structure.However,the prepared hollow fiber membranes have the poor hydrophobicity,and then further research is carried out,a hydrophobic modification method of defluorinated grafted silane is proposed to prepare a superhydrophobic hollow fiber membrane.However,the defluorination reaction will cause the strength of the prepared superhydrophobic hollow fiber membrane to decrease.Finally,by introducing inorganic nanoparticles into the casting solution,the hydrophobic modification method of defluorinated grafted silane is optimized,and the strength of the superhydrophobic hollow fiber membranes are improved.Through the scanning electron microscope(SEM),the fourier transform infrared spectrometer(ATR-FTIR),the nitrogen permeation experiment,membrane tensile experiment,contact angle experiment and water penetration pressure(CEP_W)experiments,the structure,surface chemical properties,and the performance characteristics of hollow fiber membranes are studied and analyzed.And the relationship between the microstructure,surface chemical properties and the anti-wetting performance of the hollow fiber membrane is discussed.At last,through the CO₂ absorption performance experiment,the relationship between the anti-wetting performance of the hollow fiber membrane and the CO₂ mass transfer flux was revealed.The main conclusions were as follows:(1)In order to study the influence the structure of PVDF hollow fiber membrane on the anti-wetting performance.Lithium chloride(Li Cl),phosphoric acid(PA)and their mixture over a range of concentrations(0–8 wt.%)were used as non-solvent additives,the morphological structure of hollow fiber membrane was adjusted by non-solvent additives concentrations which to prepare a strong hydrophobicity hollow fiber membrane.The regulatory effect of non-solvent additives concentration on the structure and performance characteristics of the membranes was revealed;the structure and performance characteristics of membranes with changes in CO₂ absorption performance was discussed by the CO₂ absorption experiments of the membrane contactor and the long-term stability experiments.The result shows that:as the concentration of non-solvent additives increases,the sponge-like structure in the membrane structure gradually increases,and the finger-like structure gradually decreases.The PVDF-PA-8 hollow fiber membrane with small average pores on the surface and a full sponge structure has the largest CEP_W.Using a CO₂/N₂ mixture(19vol%CO₂)as the feed gas and distilled water as the liquid absorbent,a maximum CO₂flux of 8.5×10^-4mol/m²s is achieved at a gas flow rate of 20 m L/min,a liquid flowrate of 50 m L/min,and an inlet gas relative pressure of 20 k Pa for the PVDF hollow fiber membrane with the addition of 8 wt%PA in the polymer dope solution.However,due to the increase of the sponge-like structure in the hollow fiber membrane structure,the mass transfer resistance of the hollow fiber membrane will increase,and the air permeability of the membrane will decrease.Therefore,the structure of PVDF hollow fiber membrane can be improved by introducing a high concentration of non-solvent additives into the polymer casting solution,which will improve the wetting resistance of the hollow fiber membrane prepared,but it cannot enable the membrane to obtain an ideal CO₂ mass transfer flux.(2)In order to enhance the surface hydrophobicity of the membrane,a preparation method of hydrophobically modified hollow fiber membranes with defluorinated and grafted was proposed.Using defluorination and grafting technology,PVDF is hydroxylated and then grafted with the hydrophobic hexadecyltrimethoxysilane(HDTMS)in the process of membrane preparation to prepare superhydrophobic PVDF-HDTMS composite hollow fiber membranes.The feasibility of this method was studied,and the influence of chemical grafting on the structure and properties of the membrane was analyzed;the CO₂ absorption experiments of the membrane contactor and long-term operation experiment are used to investigate the anti-wetting property and long-term operation stability of the membrane.The result shows that:the membranes exhibited super-hydrophobicity which was attributed to the formation of rough nanoscale microstructure and low surface free energy of the outer surface.The largest contact angle of water and diethanolamine(DEA)on the membrane outer surface attained 150°.Scanning electron microscopy images revealed that the membrane outer surfaces consisted of rough microscale hierarchical spherulitic particles with a nanoscale stereoscopic coralliform microstructure.The fabricated membranes possessed structures suitable for CO₂absorption in membrane contactors.The layer near the inner surface displayed a small finger-like structure,which is advantageous for gas permeation.The layer near the outer surface was made up of hierarchical spherulitic particles with rough nanoscale microstructure,which markedly enhanced the membrane hydrophobicity.The membranes showed excellent CO₂ mass transfer flux.The membrane contactor in which the membrane produced by adding 1.5 wt.%of HDTMS in the polymer dope achieved the maximum CO₂ mass transfer flux of 2.23×10^-3mol/m²s with the inlet gas(CO₂/N₂=19/81,v/v)at a flow rate of 20 m L/min and the absorbent liquid(1 mol/L DEA)at a flow rate of 50 m L/min.under the conditions used in this work.After 17 days of membrane contact absorption with 1 mol/L DEA as the absorbent,the CO₂ mass transfer flux of this membrane contactor decreased by only 17%and then remained stable.The hydrophobic modification of PVDF membranes proposed in this study is very simple and the fabricated membrane exhibited significantly improved surface hydrophobicity due to improved physical surface texture and chemical composition.As a result,the modified membrane displayed excellent CO₂ mass transfer performance in a long-term CO₂ membrane contact absorption experiment.However,due to the defluorination reaction of alkaline,the PVDF molecular structure is damaged,which causes the strength of the PVDF-HDTMS membrane to decrease.Although the membrane strength can meet industrial requirements,it needs to be further improved.(3)In order to improve the strength of PVDF-HDTMS membrane,based on the preparation method of PVDF-HDTMS,the hydrophilic Si O₂ and the casting liquid were blended to prepare PVDF-Si O₂-HTDMS composite hollow fiber membrane.The effect of hydrophilic Si O₂ on membrane structure and strength were studied;through the CO₂absorption experiments of the membrane contactor and long-term operation experiment,the relationship between the membrane structure,membrane surface physical and chemical properties of the PVDF-PA-8,PVDF-HDTMS-1.5 and PVDF-Si O₂-HDTMS membranes and the absorption performance characteristics are comprehensively discussed.The results show that the addition of hydrophilic Si O₂ greatly improves the mechanical strength of the membrane,and the hydrophilic Si O₂ nanoparticles have no agglomeration in the PVDF-Si O₂-HDTMS membranes.Compared with PVDF-PA-8and PVDF-HDTMS-1.5,this membrane has the strongest resistance to wetting,high mechanical strength and chemical stability,and shows excellent CO₂ mass transfer flux.The membrane contactor in which the membrane produced by adding 1.5%of Si O₂ in the polymer dope achieved the maximum CO₂ mass transfer flux of 2.39×10^-3mol/m²s with the inlet gas(CO₂/N₂=19/81,v/v)at a flow rate of 20 m L/min and the absorbent liquid(1 mol/L DEA)at a flow rate of 50 m L/min under the conditions used in this work.After 20 days of membrane contact absorption with 1 mol/L DEA as the absorbent,the CO₂ mass transfer flux of this membrane contactor decreased by only 3%.