等离子体聚合法制备膜蒸馏用疏水微孔复合膜

本文以八氟环丁烷为单体,采用等离子体聚合法将亲水性硝酸纤维素微孔膜改性,制得疏水硝酸纤维素微孔复合膜。所得疏水微孔复合膜可用于膜蒸馏,并具有优良的膜蒸馏性能,其通量达到反渗透水平。利用扫描电镜、X-射线显微分析和XPS等分析手段研究了聚合条件对所得复合膜结构性能的影响.     

基于相转化全过程的聚合物微孔膜功能化研究进展

传统的相转化过程主要通过相分离的热力学及动力学因素对聚合物膜的微孔结构进行调控,该过程是一个典型的物理过程,不涉及膜的表/界面功能调控。微孔膜的表/界面功能如抗污染性能、抗菌性能和抗凝血性能等对于分离性能具有重要影响。本文提出了基于相转化全过程赋予聚合物微孔膜表/界面特定化学功能的方法,即"化学相转化",其本质是围绕相转化全过程控制功能分子在膜及微孔表面的迁移路径及固定方式,实现了膜的表/界面功能化。总结了通过"化学相转化"进行聚合物微孔膜的表/界面功能化策略,根据功能分子迁移路径可分为:基于铸膜液的由内而外(Inside-out)迁移及原位交联功能化策略、基于凝固浴的由外而内(Outside-in)迁移及离域交联功能化策略、基于微孔膜的自上而下(Top-down)迁移及界面交联功能化策略,从而实现了聚合物微孔膜的表/界面功能化改性及其在水处理、油水分离和血液净化等方面的应用。"化学相转化"理论为高性能、多功能聚合物微孔膜的制备及其分离应用提供了新的研究思路。功能分子的引入也会对相转化过程产生影响,从而影响膜的微孔结构,本课题组将从微孔结构调控和表/界面功能化这两个方面完善"化学相转化"理论。     

聚全氟乙丙烯中空纤维膜研究

以聚全氟乙丙烯(FEP)为成膜聚合物,复合无机粒子为成孔剂,邻苯二甲酸二辛酯(DOP)为稀释剂,采用熔融纺丝工艺制备得到FEP中空纤维膜.分析和讨论了不同成膜体系对FEP中空纤维膜热性能、动态力学性能和力学性能的影响,并对膜的纯水通量和孔径分布进行表征.用扫描电子显微镜(SEM)观察了膜的横断面和表面形貌.结果表明,所得FEP中空纤维膜为由溶出微孔和界面微孔组成的海绵状孔结构.随着成孔剂含量的增加,成孔剂在成膜体系中分散程度变差,容易发生团聚,最终导致膜孔径变大,孔径分布变宽.成孔剂和稀释剂对FEP中空纤维膜的热性能和动态力学性能影响较小.当FEP含量增加到70 wt％时,膜表面容易形成一层致密层,降低了膜的通透性.     

同质编织管增强型聚丙烯腈中空纤维膜研究

利用二维编织技术将聚丙烯腈(PAN)纤维编织成中空编织管,以聚丙烯腈为成膜聚合物,以聚乙二醇为成孔剂,配制铸膜液,采用同心圆纺丝法制备同质编织管增强型聚丙烯腈中空纤维膜.研究结果表明,所得同质编织管增强型聚丙烯腈中空纤维膜的表面分离层具有类似于非对称膜的结构,铸膜液可渗入编织管纤维束中;随着编织管编织节距的增大,同质编织管增强型聚丙烯腈中空纤维膜表面分离层厚度减小,同时膜的平均孔径增大,膜的纯水通量随之增大;铸膜液渗入编织管纤维束的现象未影响膜的通透性能;编织管的断裂强度最大可达100 MPa以上.通过水浴振荡、超声波水浴振荡及等力拉伸3种方法测试了同质编织管增强型中空纤维膜和异质编织管增强型中空纤维膜中编织管与表面分离层之间的界面结合性能,结果表明前者的界面结合性能优于后者.     

磺化聚醚酰亚胺/聚醚酰亚胺共混膜的结构与性能

将亲水性的磺化聚醚酰亚胺(SPEI)和疏水性的聚醚酰亚胺(PEI)共混,以N,N-二甲基乙酰胺(DMAc)为溶剂,制备了SPEI(Na型)／SPEI中空纤维超滤膜．研究了纺丝过程中内凝固浴组成比例和空气间隙距离变化对膜结构以及膜分离性能的影响。实验结果表明,随着内凝固浴中DMAc含量的提高,纤维内指状孔减少,水通量下降,而截留率则不受影响;随着空气间隙距离的增大,从膜内壁侧出发的指状孔结构前端逐渐向外壁发展,而膜的外壁侧则逐渐变得致密,同时,膜的外表面可能出现较大的微孔结构,导致膜的水通量随着空气间隙距离的增加而迅速增加后随之下降,而截留率则一直保持下降趋势。     

纤维编织管增强型中空纤维膜研究

采用二维编织技术将聚丙烯腈(PAN)长丝编织成中空纤维编织管作为增强体,分别以聚丙烯腈(PAN)和聚偏氟乙烯(PVDF)为成膜聚合物,N,N-二甲基乙酰胺(DMAc)为溶剂,聚乙烯吡咯烷酮(PVP)为添加剂,调制铸膜液,采用同心圆挤出-涂覆法制备了PAN纤维编织管同质增强型PAN中空纤维膜和异质增强型PVDF中空纤维膜.研究表明,所得PAN纤维编织管增强型中空纤维膜断裂强度最大可超过75 MPa,在伸长率10%范围内,表面分离层与增强体之间界面结合良好;表面分离层具有类似于非对称膜的结构,铸膜液可浸入纤维编织管纤维空隙中,铸膜液浸入部分固化后未影响膜的通透性能;随成膜聚合物浓度增加,膜纯水通量减小,牛血清白蛋白(BSA)截留率增大;随添加剂PVP浓度增加,膜的纯水通量先增大后减小,在8 wt%左右达最大值,BSA截留率随PVP浓度增加而单调增加;同质增强型中空纤维膜界面结合程度优于异质增强型.     

内凝固浴流速对聚醚酰亚胺中空纤维膜结构与性能的影响

以N,N-二甲基乙酰胺为溶剂,水为内、外凝固浴,制备聚醚酰亚胺中空纤维膜,研究了内凝固浴流速对膜的形态结构、分离性能以及力学性能的影响。实验结果表明：随着内凝固浴流速的提高,纤维外径略有下降、内径有较大程度的提高、壁厚减小,其它结构无明显变化。与此相对应,膜的力学性能有一定程度的下降,而水通量有很大程度的提高,截留率变化不大。当内凝固浴流速／纺丝液流速大于0．4时,纤维内壁表面产生较大的轴向应力,把处于相分离早期的分子链或分相微区拉开,产生了微孔。     

氯化锂为添加剂制得的疏水聚偏氟乙烯不对称微孔膜的形态结构及其膜蒸馏性能

结合膜的形态结构研究了以 LiCl为添加剂制得的疏水 PVDF膜的膜蒸馏性能。与来用水溶液高分子添加剂制得的PVDF微孔膜相比,膜蒸馏性能有了较大提高,尤其具有更高的截留率。制得的微孔膜的蒸馏通量已接近商品膜的膜蒸馏通量,表明以LiCl为添加剂制得的PVDF疏水微孔膜是一种适用于膜蒸馏的较理想的疏水微孔膜。     

内凝固浴组成对聚醚酰亚胺中空纤维膜结构和性能的影响

以N,N-二甲基乙酰胺(DMAc)为溶剂,水为外凝固浴,乙醇为内凝固浴,DMAc为内凝固浴添加剂,采用相转化法制备了聚醚酰亚胺(PEI)中空纤维膜,研究了内凝固浴组成和纺丝液浓度对膜结构和性能的影响。实验结果表明,随着内凝固浴中DMAc含量在一定范围内增加,纤维断面指状孔有所减少,内表面由无孔到有微孔出现,但膜的水通量下降,截留率不受影响;随着纺丝液浓度提高,膜的水通量下降,截留率提高。     

填充液压力对聚醚砜中空纤维膜微结构和性能的影响

研究了聚醚砜／二甲基亚砜(PEs／DMS0)体系中填充液压力的变化对PES中空纤维结构及性能的影响。结果表明,随着填充液压力的增大,中空纤维膜的水通量增大,轴向取向度下降。为纺制具有合适性能的中空纤维膜提供参考。     

Effect of solution extrusion rate on morphology and performance of polyvinylidene fluoride hollow fiber membranes using polyvinyl pyrrolidone as an additive

<正>Polyvinylidene fluoride(PVDF) hollow fiber membranes prepared from spinning solutions with different polyvinyl pyrrolidone(PVP) contents(1%and 5%) at different extrusion rates were obtained by wet/dry phase process keeping all other spinning parameters constant.In spinning these PVDF hollow fibers,dimethylacetamide(DMAc) and PVP were used as a solvent and an additive,respectively.Water was used as the inner coagulant.Dimethylformamide(DMF) and water(30/70) were used as the external coagulant.The performances of membranes were characterized in terms of water flux,solute rejection for the wet membranes.The structure and morphology of PVDF hollow fiber were examined by BET adsorption,dry/wet weight method and scanning electron microscopy(SEM).It is found that the increase in PVP content and extrusion rate of spinning solution can result in the increase of water flux and decrease of solute rejection.The improvements of interconnected porous structure and pore size are induced by shear-thinning behavior of spinning solution at high extrusion rates,which could result in the increase of water flux of hollow fiber membranes.The increase of extrusion rate also leads to the increase of membrane thickness due to the recovery effect of elastic property of polymer chains.     

Non-invasive monitoring of fouling in hollow fiber membrane via UTDR

Fouling is the most critical problem associated with membrane separations in liquid media. But it is difficult to control the inevitable membrane fouling because of its invisibility, especially on the inside surface of hollow fiber membranes. This study describes the extension of ultrasonic time-domain reflectometry (UTDR) for the real-time measurement of particle deposition in a single hollow fiber membrane. A transducer with a frequency of 10 MHz and polyethersulfone hollow fiber membranes with 0.8 mm inside diameter (ID) and 1.2 mm outside diameter (OD) were used in this study. The fouling experiments were carried out with 1.8 g/L kaolin suspension at flow rates 16.7 and 10.0 cm/s. The results show that UTDR technique is able to distinguish and recognize the acoustic response signals generated from the interfaces water/upper outside surface of the hollow fiber, lumen upside surface/water, water/lumen underside surface and lower outside surface/water in the single hollow fiber membrane module in pure water phase. The systemic changes of acoustic responses from the inside surfaces of the hollow fiber in the time- and amplitude-domain with operation time during the fouling experiments were detected by UTDR. It is associated with the deposition and formation of the kaolin layer on the inside surfaces. Further, the acoustic measurement indicates that the deposited fouling layer is denser on the lumen underside surface of the hollow fiber than that on the lumen upside surface as a result of weight. Moreover, it is found that the fouling layer grows faster on the inside surface of the hollow fiber at a flow rate of 10.0 cm/s than that at 16.7 cm/s due to the lower shear stress. The fouling layer formed is thicker at a flow rate of 10.0 cm/s than that at 16.7 cm/s. The flux decline data and SEM analysis corroborate the ultrasonic measurement. Overall, this study confirms that UTDR measurement will provide not only a new protocol for the observation of hollow fiber membrane fouling and cleaning, but also a quantitative approach to the optimization of the membrane bioreactor system.     

Effect of surface morphology on membrane fouling by humic acid with the use of cellulose acetate butyrate hollow fiber membranes

A serious problem faced during the application of membrane filtration in water treatment is membrane fouling by natural organic matter (NOM). The hydrophilicity, zeta potential and morphology of membrane surface mainly influence membrane fouling. The aim of the present study is to reveal the correlation between membrane surface morphology and membrane fouling by use of humic acid solution and to investigate the efficiency of backwashing by water, which is applied to restore membrane flux. Cellulose acetate butyrate (CAB) hollow fiber membranes were used in the present study. To obtain the membranes with various surface structures, membranes were prepared via both thermally induced phase separation (TIPS) and nonsolvent-induced phase separation (NIPS) by changing the preparation conditions such as polymer concentration, air gap distance and coagulation bath composition. Since the membrane material is the same, the effects of hydrophilicity and zeta potential on membrane fouling can be ignored. More significant flux decline was observed in the membrane with lower humic acid rejection. For the membranes with similar water permeability, the lower the porosity at the outer surface, the more serious the membrane fouling. Furthermore, the effect of the membrane morphology on backwashing performance was discussed.     

Experimental and theoretical study on propylene absorption by using PVDF hollow fiber membrane contactors with various membrane structures

Five kinds of asymmetric poly(vinylidene fluoride) (PVDF) hollow fiber membranes with considerable different porosities at the inner and outer surfaces of the membrane were prepared via thermally induced phase separation (TIPS) method and applied for propylene absorption as gas–liquid membrane contactors. A commercial microporous poly(tetrafluoroethylene) (PTFE) hollow fiber membrane was also used as a highly hydrophobic membrane. Experiments on the absorption of pure propylene into silver nitrate solutions were performed and the effects of membrane structure, inner diameter, silver nitrate concentration and absorbent liquid flow rate were investigated at 298 K. PVDF membranes prepared by using nitrogen as bore fluid had lower inner surface porosity than the membranes prepared with solvent as bore fluid. Except the membrane with a skin layer at the outer surface, propylene absorption flux was inversely proportional to the inner diameter of the hollow fiber membrane, and propylene absorption rate per fiber was almost the same. Propylene flux increased with increasing the silver nitrate concentration and also with increasing the absorbent flow rate.A mathematical model for pure propylene absorption in a membrane contactor, which assumes that the membrane resistance is negligibly small and the total membrane area is effective for gas absorption, was proposed to simulate propylene absorption rates. Experimental results were satisfactorily simulated by the model except for the membrane having a skin layer. The model also suggested that propylene is absorbed in silver nitrate solutions accompanied by the instantaneous reversible reaction. This paper may be the first experimental and theoretical study on propylene absorption in membrane contactors.     

Fabrication of blend hydrophilic polyamide imide (Torlon®)-sulfonated poly (ether ether ketone) hollow fiber membranes for oily wastewater treatment

Blend hydrophilic polyamide imide (PAI)-sulfonated poly (ether ether keton) (SPEEK) hollow fiber membranes were fabricated for oil-water emulsion separation. The structure and performance of the membranes were examined by FESEM analysis, N₂ permeation, overall porosity, collapsing pressure, water contact angle, pure water flux, molecular weight cutoff (MWCO), and oil rejection tests. By studying ternary phase diagrams of polymer/solvent-additive/water system, the higher phase-inversion rate was confirmed for the solutions prepared at higher PAI/SPEEK ratio. A more open structure with larger finger-likes was observed by increasing PAI/SPEEK ratio. Mean pore size of 81 nm, overall porosity of 79% and water contact angle of 58° were obtained for the improved membrane prepared by PAI/SPEEK ratio of 85/15. Increasing SPEEK ratio resulted in lower mechanical stability in terms of collapsing pressure. Pure water flux of about 2.5 times of the plain PAI membrane was found for the improved membrane. MWCO of 460 kDa was found for the improved blend membrane. From oil rejection test, all the membranes demonstrated an oil rejection of over 95%. The improved membrane showed a lower rate of permeate flux reduction compared to the plain membrane which was related to the smaller fouling possibility. Less fouling resistance of the improved membrane was related to the higher flux recovery ratio (about 92%). For all the membranes, the dominant fouling mechanism was found to be the cake filtration. The improved PAI-SPEEK hollow fiber membranes was found to be practical for ultrafiltration of oily wastewaters.     

Preparation and characterization of polyvinylidene fluoride (PVDF) hollow fiber membranes

Polyvinylidene fluoride (PVDF) hollow fiber membranes were prepared by dry/wet and wet phase inversion methods. In spinning these PVDF hollow fibers, dimethylacetamide (DMAc) and polyvinyl pyrrolidone (PVP) were used as a solvent and an additive, respectively. Water was used as the external coagulant. Water or ethanol was used as the internal coagulants. The membranes were characterized in terms of water flux, molecular weight cut-off for the wet membranes. Gas permeation fluxes and effective surface porosity were determined by a gas permeation method for the dried membranes. The cross-sectional structures were examined by scanning electron microscopy. The effects of polymer concentration, air-gap, PVP molecular weight, PVP content in the polymer dope, and the internal coagulant on the permeation properties and membrane structures were examined. Highly permeable PVDF hollow fiber membranes could be prepared from a polymer dope containing low molecular weight PVP and using ethanol as the internal coagulant.     

Analysis of constant permeate flow filtration using dead-end hollow fiber membranes

Dead-end filtration of colloids using hollow fibers has been analysed theoretically and experimentally. A mathematical model for constant flux filtration using dead-end hollow fiber membranes has been developed by combining the Hagen–Poiseuille equation, the (standard) filtration equation, and cake filtration theory of Petsev et al. [D.N. Petsev, V.M. Starov, I.B. Ivanov, Concentrated dispersions of charged colloidal particles: sedimentation, ultrafiltration and diffusion, Colloid Surf. A: Physicochem. Eng. Aspects, 81 (1993) 65–81.] to describe the time dependence of the filtration behavior of hollow fiber membranes experiencing particle deposition on their surface. Instead of using traditional constitutive equations, the resistance of the cake layer formed by the deposited colloids has been directly correlated to the cake structure. This structure is determined by application of a force balance on a particle in the cake layer combined with the assumption that an electrostatically stable cake layer of mono-sized particles would be ordered in a regular packing geometry of minimum energy. The developed model has been used to identify the relationship between the filtration behavior of the hollow fiber membrane and the particle properties, fiber size, and imposed average flux. Filtration experiments using polystyrene latex particles of relatively narrow size distribution with a single dead-end hollow fiber membrane demonstrate good consistency between experimental results and model prediction. The developed model has been used to simulate the distribution of the cake resistance, transmembrane pressure, and flux along the hollow fiber membrane and used to assess the effect of fiber size, particle size, zeta potential, and the average imposed flux on the suction pressure-time profiles, flux, and cake resistance distributions. These results provide new insights into the filtration behavior of the hollow fiber membrane under constant flux conditions.     

Polyamide-imide/polyetherimide dual-layer hollow fiber membranes for pervaporation dehydration of C1–C4 alcohols

To circumvent the common swelling and deteriorated performance of integral asymmetric hollow fiber membranes for pervaporation dehydration, we have developed novel polyamide-imide (PAI)/polyetherimide (PEI) hollow fiber membranes with synergized performance with the aid of dual-layer spinning technology. Dehydration of C1–C4 alcohols has been conducted and the orders of their fluxes and permeances have been analyzed. The hollow fibers spun at 2 cm air gap and annealed at 75 °C exhibit the highest pervaporation performance: separation factors for t-butanol/water and iso-butanol/water binary systems are greater than 50,000 with flux more than 700 g/m² h. A comparison with literature data shows that the newly developed membranes outperform most other polymeric membranes for the dehydration of IPA and butanols. The dual-layer hollow fiber membranes also exhibit good long-term stability up to 200 h. The superior performance can be attributed to (1) the balanced properties of PAI as the selective layer for dehydration pervaporation; (2) the low water uptake and less swelling characteristic of the PEI supporting layer; and (3) the desirable membrane morphology consisting of a fully porous inner layer, a porous interface, and an ultrathin dense-selective outer skin.     

Experimental verification of pressure drop models in hollow fiber membrane

A new experimental method to obtain internal pressure profiles in a hollow fiber membrane was demonstrated. The experimentally obtained internal pressure profiles were compared with the theoretically calculated ones based on Hagen–Poiseuille equations. The experimental and theoretical results agreed very well in clean water conditions only when accurate membrane permeabilities and effective internal diameters were available. New experimental methods to obtain the two parameters were demonstrated. The same experimental technique was also applied for the submerged hollow fiber membranes filtering activated sludge to find out how internal pressure profiles were changing with time. Based on the pressure profiles, evidences that indicated the local flux near membrane exit was lower than those in adjacent area were found. This observation contradicted to the filtration models based on critical flux concept. It was considered that the cake layer collapse near the membrane exit was the cause. Though there was some degree of delay in pressures detection, the method demonstrated in this study provided a great accuracy when pressure profiles did not change rapidly.