【作者】 李隽；

【导师】 邵嘉慧；

【作者基本信息】 上海交通大学 ， 环境科学与工程， 2020， 博士

【摘要】 近年来,膜蒸馏（Membrane Distillation,MD）作为一种高效的分离技术（理论截留达100%）,在诸如海水淡化、反渗透浓水和页岩气废水处理等领域受到越来越广泛的关注。膜蒸馏采用疏水微孔膜,在膜两侧温差引起的蒸气压差（传质驱动力）的作用下,料液中易挥发组分透过微孔膜孔进入渗透侧,并在渗透侧冷凝回收,而料液中的非挥发性物质则被截留。与传统压差分离的膜（如纳滤、反渗透）过程相比,膜蒸馏具有运行压力低、占地面积小的优势;与传统蒸馏（如多级闪蒸、低温多效蒸馏）相比,膜蒸馏具有运行温度低的特点,因而可充分利用太阳能、地热能、工厂余热等低品热源。膜蒸馏有望成为一种廉价的制取清洁水的方法。膜性能是决定膜蒸馏分离效果的关键要素,静电纺丝膜的高孔隙率和孔径可调,可作为理想膜蒸馏膜。聚偏氟乙烯（PVDF）疏水性好和易溶于极性溶剂,成为最常用的膜蒸馏用静电纺丝膜材料。然而传统静电纺丝膜蒸馏膜在膜蒸馏中易发生润湿、结垢和污染等问题,限制了该静电纺丝膜在膜蒸馏领域的应用。针对膜孔润湿、膜结垢及膜污染问题,本课题分别通过共混、水热生长和表面涂覆等手段,构筑具有疏水、超疏水、超疏液、疏水-亲水以及亲水-超疏液-亲水等不同结构的静电纺丝膜。系统研究不同表面性质的膜在膜蒸馏运行中抗润湿、抗结垢和抗污染方面的表现,分析不同表面性质的膜在膜蒸馏过程中的传质传热,实现膜表面特性的调控和优化。具体研究成果包括:采用响应面分析法,探明静电纺丝制膜参数及膜蒸馏运行参数对膜性能的影响。以接触角（CA）、膜厚和液体渗透压力（LEP）为指标,探究聚合物浓度、溶剂配比、推助速率、接收距离、滚筒转速等静电纺丝参数的影响规律并考察相互作用关系。结果表明:接触角主要受PTFE浓度和推助速率的影响;厚度主要受PVDF浓度和针头到接收器距离的影响;LEP大小则主要受PVDF浓度及其与溶剂配比的影响。进一步确定了最佳制膜参数:PVDF质量分数为10%,PTFE质量分数为0.9%,DMAc/丙酮为4,TCD为16 cm,电压为8 k V,推助速率为0.07 mm min^-1,滚筒转速为140 rpm。将最优化膜应用于直接接触式膜蒸馏（DCMD）中,采用响应面分析法探究膜蒸馏过程参数对通量、截留率的影响。获得最佳膜蒸馏运行参数:料液浓度为3.5%,原料液温度为80°C,原料液流速为1 L min^-1,渗透液流速为0.5 L min^-1。结果表明在最优条件下,膜通量可达67.5 kg m^-2 h^-1。采用一步静电纺丝法,以PVDF为膜基体高分子材料、ZnO为添加剂,以N,N-二甲基乙酰胺（DMAc）和丙酮为混合溶剂、1H,1H,2H,2H-全氟癸基三乙氧基硅烷（PDTS）为氟化改性剂,成功制备超疏水FZP纳米纤维膜。考察了FZP膜对质量分数为3.5%氯化钠溶液、十二烷基硫酸钠（SDS）和硫酸钙料液的膜蒸馏性能。结果表明:与原始膜和ZnO共混PVDF纳米纤维膜（ZP）相比,FZP膜发生润湿时间最晚、抗润湿性最强。这是由于FZP膜的水突破压力最高（32.6 k Pa）,料液润湿膜孔所需克服的阻力最大;同时FZP膜的跨膜温差最小（23.8°C）,有利于延缓膜蒸馏过程中表面润湿向完全润湿的转变,进一步提高膜的抗润湿性能。采用水热法在PVDF纳米纤维膜上原位生长ZnO纳米棒,并通过PDTS氟化改性,成功制备超疏液纳米纤维膜（FZnO-PVDF）。FZnO-PVDF膜表面均匀分布着ZnO纳米棒。与原始PVDF膜相比,FZnO-PVDF膜疏水（纯水接触角164.9°）和疏有机性（50%乙醇水溶液接触角121.1°）显著提升。对混合盐（CaCl₂+Na₂SO₄）料液膜蒸馏研究发现,当膜蒸馏运行350 min后,PVDF膜表面附着大量硫酸钙针状结垢,而FZnO-PVDF膜表面几乎无硫酸钙附着。Gibbs自由能计算表明,与原始PVDF膜相比,硫酸钙晶体在FZnO-PVDF膜表面异相成核所需自由能较高(34.2m J mol^-1)。此外,FZnO-PVDF膜表面滑移性能,显著降低膜表面边界层料液停留时间,进一步降低硫酸钙异相成核的可能性。采用两步静电纺丝法,以PVDF-PTFE为疏水层,以壳聚糖-聚环氧乙烷（CSPEO）为亲水层,成功制备疏水-亲水复合纳米纤维膜。SEM、FTIR、接触角分析表明该复合膜具有非对称润湿结构,复合膜上表面疏水（144°,55μm）而下表面亲水（20°,75μm）。纯水膜蒸馏实验表明,疏水-亲水复合膜通量(19 kg m^-2 h^-1)显著大于单一疏水膜通量(15 kg m^-2 h^-1)。这是由于复合膜中亲水层的存在能及时吸收渗透侧冷凝水,从而提供较大传质驱动力（14.26 k Pa）,比单一疏水纳米纤维膜提高10%。采用浸渍涂覆技术,在FZnO-PVDF膜上下表面涂覆聚多巴胺,成功制备外亲水内超疏液PFZnO-PVDF纳米纤维膜。SEM、XPS和FTIR表征均表明膜表面呈亲水性,与原始PVDF膜接触角（138.5°）相比,该PFZnO-PVDF膜接触角仅为41.2°,水下油接触角可达124°。氯化钠料液膜蒸馏实验表明,PFZnO-PVDF膜可运行多达6个循环,而原始PVDF膜在2个循环后便出现润湿问题,且PFZnOPVDF膜平均通量(18.6 kg m^-2 h^-1)大于原始PVDF膜的通量(10.0 kg m^-2 h^-1)。这是由于PFZnO-PVDF膜具有更高的传质系数、温差极化系数等。油水乳液膜蒸馏实验表明,FZnO-PVDF膜可运行3个循环之久,而原始PVDF膜在20 min内丧失其脱盐性能。这是因为十六烷沉积在PFZnO-PVDF膜上需克服的能垒（0.12k T）,大于其沉积在原始PVDF膜上所需克服的能垒（0.03 k T）。上述系列研究表明,通过对静电纺丝制膜参数及膜蒸馏工艺参数的调控,探明各个参数与膜性能的相互影响关系;通过制备超疏水、超疏液纳米纤维膜,可显著提升膜的抗润湿性;通过制备疏水-亲水、外亲水内超疏液静电纺丝膜,可在增加膜通量的同时提高抗污染性能。经计算水突破压力、跨膜温差、Gibbs自由能、十六烷与膜表面相互作用能,从理论角度充分验证所制备膜材料的抗润湿、抗污染性能。本研究可为膜蒸馏用膜的发展提供一定指导,加深对膜孔润湿、膜污染机理的理解。更多还原

【Abstract】 Among various membrane technologies,membrane distillation（MD）with theoretical separation rate of 100%,has been applied in freshwater production,desalination and shale gas produced wastewater treatment.MD is a remarkable desalination process using vapor pressure gradient across membrane to deliver water vapor from a hot salty feed side to a cold fresh permeate side.Non-volatile dissolved solutes cannot pass through the membrane.Compared with traditional pressure-driven membrane process like reverse osmosis（RO）,MD technique has its unique advantages such as low operation pressure and small footprint.Compared with traditional distillation like multi-stage flash（MSF）,the operation temperature of MD is low.Therefore,some low-grade heat such as solar energy,geothermal energy and factorial waste heat can be fully utilized in MD process.MD is expected to be an inexpensive technology for producing clean water.In MD process,membrane is the key factor.Among the various membrane fabrication techniques,as v versatile technique for generating durable and efficient membranes for MD application,the electrospinning technique for fabricating nanofibrous membrane has gained lots of attention.Electrospun membrane possesses several advantages involving high porosity and interconnected open pore structures.Polyvinylidene fluoride（PVDF）is widely used in electrospinning process due to its remarkable hydrophobicity and high solubility in polar solvent.However,large-scale commercialization of MD has not been fully achieved yet because of some technical challenges,such as wetting,fouling and scaling of membrane.In this study,a series of membranes including hydrophobic,superhydrophobic,omniphobic and hydrophobic-hydrophilic membranes were fabricated to mitigate the afore mentioned wetting,fouling and scaling problems.The main achievements of this study are listed as follows:Effectively modeling and optimization of electrospinning parameters and membrane distillation process by response surface methodology.In this chapter,seven variables:concentration of PVDF,concentration of PTFE,mass ratio of DMAc to acetone,tip to collector distance（TCD）,voltage,flow rate and rotation rate were optimized by RSM in consideration of contact angle,liquid entry pressure and thickness.Results showed LEP value was mainly determined by PVDF concentration and its interaction effects with mass ratio of DMAc to acetone and voltage.CA was mostly dependent on the interaction effect of PTFE concentration and flow rate.Thickness was mainly determined by PVDF concentration and TCD.The optimizes electrospinning parameters were:10%PVDF,0.9%PTFE,4 of DMAc/acetone,16 cm TCD,8 k V voltage,0.07 mm min-1 flow rate,140rpm rotation rate.Under optimized condition,the corresponding CA,thickness and LEP value were 146.2°,70μm and 53.5 k Pa,respectively.Furthermore,operation parameters of DCMD process were also optimized by RSM.The optimized parameters were:3.5%Na Cl feed,80°C of feed temperature,1 L min^-1 feed flow rate and 0.5 L min^-1 permeate flow rate.A maximal distillation flux of 67.5 kg m^-2 h^-1 was obtained under the optimum conditions.Successfully fabrication of superhydrophobic membrane（FZP）by fluorinating of zinc oxide（ZnO）blended electrospun PVDF membrane.The FZP membrane showed a stable superhydrophobicity with contact angle of 162.3°and sliding angle of 9.8°When desalinating 35 g L^-1 sodium chloride（Na Cl）feed,the water flux of FZP membrane was found to be stable for 68 h with an average of 14.8 kg m^-2 h^-1 due to its superhydrophobicity,which was more than 2 times longer than neat membrane.For desalination of sodium dodecyl sulfate（SDS）contained feed,neat membrane was easily wetted after SDS concentration reached 0.1 m M,whereas,FZP was not wetted until the concentration of SDS reached 0.2 m M.Mechnisms analysis showed FZP membrane exhibited the highest breakthrough pressure among three membranes.Further,FZP membrane could induce a low transmembrane temperature graduate（23.88°C）that may further postpone the occurrence of fully wetting.An omniphobic FZnO-PVDF membrane was successfully fabricated through fluorinating a ZnO hydrothermally grown membrane.Various membrane characterizations such as SEM,TEM,3D profiles,FTIR,XPS and XRD,all confirm the successful growth of ZnO.Contact angle of 50%ethanol droplet on FZnO-PVDF membrane surface was 121.1°.Na₂SO₄ and Ca Cl₂ were used as feed in DCMD process to evaluate the anti-scaling property of resultant membrane.Results showed flux of PVDF membrane decreased after 350 min operation.Whereas,the flux of FZnO-PVDF membrane kept almost stable during the whole desalination process.Theoretically study showed Gibbs free energy for gypsum depositing on FZnO-PVDF membrane was 34.94m J mol^-1,which was higher than that of PVDF membrane with 29.26 m J mol^-1.Compared with prisinte membrane,it was more difficult for gypsum to deposite on FZnO-PVDF membrane.After fabricating the optimized PVDF-PTFE electrospun membrane in chapter 2,ahydrophobic-hydrophilic composite membrane was further prepared by two-step electrospinning process.The composite membrane was composed of a PVDF-PTFE hydrophobic layer,a PET support layer and a chitosan-polyethylene oxide hydrophilic layer（labeled as TL-M）.Membrane characterizations including SEM,FTIR and contact angle,all confirm the asymmetric structure of TL-M membrane.The contact angle and thickness of hydrophobic layer were 144°and 55μm,respectively.The contact angle and thickness of hydrophilic layer were 20°and 75μm,respectively.TL-M membrane and DL-M membrane without hydrophilic layer were tested in DCMD process using 3.5%Na Cl feed.Results showed TL-M membrane with 19 kg m^-2 h^-1 average flux was higher than that of DL-M membrane(average flux:15 kg m^-2 h^-1).Mean driving force of TL-M was 14.26 k Pa,which was higher than that of DL-M membrane.This indicated the incorporation of hydrophilic layer could increase the driving force,and largely enhance the membrane flux.A novel inner super-hydrophobic and external hydrophilic nanofibrous membrane（labeled as PFZnO-PVDF）was fabricated via fluorination and polydopamine coating to alleviate the wetting and fouling problems.Results of SEM,XPS and contact angle showed super-hydrophobic modification and hydrophilic coating of PFZnO-PVDF were successful.When treating Na Cl feed solution,PFZnO-PVDF membrane had higher average flux(18.6 kg m^-2 h^-1)and longer operation time（for 6 cycles）than PVDF membrane with 10 kg m^-2 h^-1 average flux and 2 cycles.Compared to PVDF membrane,the higher flux of PFZnO-PVDF membrane was ascribed to its higher mass transfer coefficient.Furthermore,when dealing with oil-in-water emulsion,PFZnO-PVDF membrane exhibited robust anti-fouling properties,while the performance of PVDF membrane can be easily deteriorated.The remarkable anti fouling property of PFZnO-PVDF membrane was due to high energy barrier of 0.12 k T for oil deposition.In summary,MD performance of electrospun membrane can be largely enhanced by optimization of electrospinning parameters and preparing membranes with different structure.The main objective of this study was to illustrate the relationship between membrane structure and performance.更多还原