Preparation of PET threads reinforced PVDF hollow fiber membrane

PET threads were incorporated in the support layer of hollow fiber membrane in axial direction as a special reinforcement material for the purpose of improving the mechanical properties of PVDF hollow fiber membranes. It was found that the reinforcement threads had a limited effect on the separation-related properties of the membrane, such as porosity and pore size, but the tensile strength of the reinforced membrane was improved several folds. Also, the criterion of choosing reinforced fiber materials was suggested.     

Preparation and properties of PVDF/PVA hollow fiber membranes

On principle of polymer blend phase separation, PVDF/PVA hollow fiber membranes were prepared using phase inversion method. The membrane morphology and performance varied with the blending ratio. The PVDF/PVA blends showed incompatibility by the results of dynamic mechanical analysis (DMA) and infrared attenuated total reflection (FTIR-ATR) sampling technique. Based on bursting pressure and tensile strengths results, we suggest that the mechanical properties of PVDF/PVA blend membranes are worse than that of PVDF membrane. PVA can improve the hydrophilicity of PVDF/PVA hollow fiber membranes, which could be illuminated by the decrease in contact angle, the increase in equilibrium water content (EWC) and the variety in dynamic moisture regain. The pure water flux increases while the rejection ratio decreases with PVA content increasing. Moreover, PVA can improve the anti-fouling property of PVDF/PVA hollow fiber membranes, which could be illuminated by the result of increase coefficient of resistance.     

Hydrophobic PVDF hollow fiber membranes with narrow pore size distribution and ultra-thin skin for the fresh water production through membrane distillation

Polyvinylidene fluoride (PVDF) hydrophobic asymmetric hollow fiber membrane was fabricated through the dry-jet wet phase inversion process. It is found that the PVDF hollow fiber has an ultra-thin skin layer and a porous support layer from the morphology study. The fully porous membrane structure has the advantage of decreasing the vapor transport resistance and enhancing the permeation flux. The fabricated PVDF membrane has a mean pore size of in diameter and a narrow pore size distribution. The rough external surface produces an advancing contact angle of 112±3^° with water. During direct contact membrane distillation (MD) of 3.5 wt% salt solution, PVDF hollow fibers produced a water permeation flux of (based on the external diameter of hollow fiber) and a NaCl rejection of 99.99% with a hot salt solution at 79.3 °C and cold distillate water at 17.5 °C. This performance is comparable to or superior to most of commercially available PVDF hollow fiber membranes, indicating that the newly developed PVDF may be suitable for MD applications.     

硼氢化钠在氢氧化钠溶液中的密度和黏度

在291、298、303 K下,将不同质量的硼氢化钠固体分别加入到质量分数为13.35%的氢氧化钠溶液中,用密度瓶法和Ubbelohde黏度管分别测量溶液密度和黏度。根据溶液的密度计算硼氢化钠溶液的表观摩尔体积,讨论浓度的变化对混合溶液密度的影响,可知随着硼氢化钠浓度的加大,密度逐渐减小。根据溶液黏度数据和BH4- 半径计算了不同温度下硼氢化钠和BH4- 的黏度系数B,结果表明,BH4- 是结构促进型离子,且随着温度升高,黏度系数B逐渐减小。     

Poly(vinyl chloride) (PVC) hollow fibre membranes for gas separation

Poly(vinyl chloride) (PVC) gas separation hollow fibre membranes were produced from multicomponent dopes using dry/wet forced convection spinning. Membranes spun from a low polymer content solution exhibited disappointing gas separation properties. Their low selectivities were indicative of thick skins and high surface porosities. In contrast, high polymer content spun fibres showed good gas separation properties. Selectivities were high, active layers relatively thin and surface porosities moderate. Coating with poly(dimethylsiloxane) nullified the surface pores. The favourable performance of the high polymer content spun fibres was also related to shear rate and forced convection residence time during spinning. To the knowledge of the authors, this work represents the first reported success in producing PVC hollow fibre membranes with morphologies suitable for gas separation. The development of PVC hollow fibres relates to the ultimate quest to produce membranes capable of reliably separating oxygen and ozone gas mixtures.     

氢氧化钠对生漆膜腐蚀行为的研究

本文用氢氧化钠水溶液对天然生漆漆膜进行腐蚀,并用红外分析、元素分析、热重分析和扫描电镜分析等手段来研究漆膜的腐蚀机理。结果表明,漆膜在氢氧化钠水溶液腐蚀下,漆膜的酚羟基氧化生成醌、侧链不饱和键生成醛或酮,醚键上发生断裂,漆膜中的部分多糖和糖蛋白发生降解而流失,漆膜出现多而大的孔洞,吸水率升高,光泽度和耐热性降低。     

An analytical index for evaluating manufacturing cost and performance of low-pressure hollow fibre membrane systems

Water and waste-water treatment through the use of the membrane filtration technology is one of the processes utilized currently to meet the growing demand for water. While new technologies can harness water from various non-traditional sources such as oceans, there remains the possibility of making drinking water more expensive through the use of costly water treatment equipment. To prevent this and ensuing catastrophes in the world, the water industry needs a strategy that keeps the price of water and price of products aiding the treatment of water controlled in to the future.     

Analysis of hollow fibre membrane systems for multicomponent gas separation

This paper analysed the performance of a membrane system over key design/operation parameters. A computation methodology is developed to solve the model of hollow fibre membrane systems for multicomponent gas feeds. The model represented by a nonlinear differential algebraic equation system is solved via a combination of backward differentiation and Gauss–Seidel methods. Natural gas sweetening problem is investigated as a case study. Model parametric analyses of variables, namely feed gas quality, pressure, area, selectivity and permeance, resulted in better understanding of operating and design optima. Particularly, high selectivities and/or permeabilities are shown not to be necessary targets for optimal operation. Rather, a medium selectivity (<60 in the given example) combined with medium permeance (∼300–500 × 10⁻¹⁰ mol/s m² Pa in the given case study) is more advantageous. This model-based membrane systems engineering approach is proposed for the synthesis of efficient and cost-effective multi-stage membrane networks.     

膜生物反应器中PVDF中空纤维膜亲水性变化特征

为研究膜生物反应器(MBR)中PVDF中空纤维膜的亲水性变化特征,提出以接触角变化速率的绝对值(λ)表征亲水性的方法 ,按轴向长度将膜纤维划分为4个位置,考察过滤和清洗对膜亲水性的影响。结果表明:在MBR运行过程中,膜污染的加剧与膜的亲水性变化有关,膜纤维通量越高的位置,膜的亲水性越差。采用物理反洗和短时间的化学浸泡清洗,膜的亲水性能得到一定恢复,但效果有限。延长化学浸泡清洗时间发现,浸泡3 h膜纤维的亲水性最好。     

Absorption of SO<Subscript>2</Subscript> using PVDF hollow fiber membranes with PEG as an additive

In this study, removal of SO₂ from gas stream was carried out by using microporous polyvinylidene fluoride (PVDF) asymmetric hollow fiber membrane modules as gas-liquid contactor. The asymmetric hollow fiber membranes used in this study were prepared polyvinylidene fluoride by a wet phase inversion method. Water was used as an internal coagulant and external coagulation bath for all spinning runs. An aqueous solution containing 0.02 M NaOH was used as the absorbent. This study attempts to assess the influence of PEG additive, absorbent flow rate, SO₂ concentration, gas flow rate and gas flow direction on the SO₂ removal efficiency and overall mass transfer coefficient. The effect of liquid flow rate on SO₂ removal efficiency shows that at very low liquid flow rate, the NaOH available at the membrane surface for reacting with SO₂ is limited due to the liquid phase resistance. As liquid flow rate is above the minimum flow rate which overcomes the liquid phase resistance, the SO₂ absorption rate is controlled by resistance in the gas phase and the membrane. The SO₂ absorption rate with inlet SO₂ concentration was sharply increased by using hollow fiber membranes compared to a conventional wetted wall column because the former has higher gas liquid contacting area than the latter. The mass transfer coefficient is independent of pressure. When the gas mixture was fed in the shell side, the removal efficiency of SO₂ declined because of channeling problems on the shell side. Also, the addition of PEG in polymer dopes increased SO₂ removal efficiency. This work was presented at the 6 ^th Korea-China Workshop on Clean Energy Technology held at Busan, Korea, July 4–7, 2006.     

Removal of As(V) by PVDF hollow fibers membrane contactors using Aliquat-336 as extractant

In this work, the removal of As(V) from an aqueous stream by non-dispersive solvent extraction in a hollow fibre membrane contactor was investigated. To this purpose, microporous hydrophobic poly(vinylidene fluoride) hollow fibres were prepared by the dry/wet spinning technique. Fibres were spun, changing the pore forming additives, to obtain micro-porous sponge-like structure membranes, with pore diameters of about 0.2 μm and porosity of 80%. The produced fibres were used to perform As(V) extraction experiments in a membrane contactor device, using Aliquat-336 as extractant. Different tests were carried out in order to study the effect of temperature, initial concentration, pH of the feed solution and membrane properties. The results showed that the extraction of As(V) by non-dispersive solvent extraction was influenced by the pH of the feed, with an optimum reached for neutral values, increased with the arsenic content in the feed and was not affected by temperature. The extraction was favoured by working with thinner membranes. The highest removal of arsenic achieved was around 70% after 6 h of operation. The produced fibres kept their performance for all the experimental activity, giving reproducible results with time.     

Transport of thorium from nitric acid solution by non-dispersive solvent extraction using a hollow fibre contactor

Membrane based non-dispersive solvent extraction (NDSX) of Th(IV) from aqueous nitric acid medium was carried out using di-n-hexyl octanamide (DHOA) in normal paraffin hydrocarbon (NPH) using a commercial hollow fiber module containing microporous hydrophobic polypropylene capillaries. The NDSX operation was carried out with pumping various concentrations of nitric acid (1–6 M) containing Th(IV) through the tube side and organic extractant (usually 1.1 M DHOA in NPH) through the shell side of the hollow fibre capillaries at aqueous and organic phase flow rates of 3.5 mL/s and 4.5 mL/s, respectively. Extraction studies were performed under different hydrodynamics conditions and the overall mass transfer was evaluated under cuonter-current flow condition. The percentage NDSX of Th(IV) increased with the increase in the extractant concentration (from 0.1 M DHOA to 1.1 M DHOA) as well as with nitric acid concentration (from 1 M to 6 M). Stripping studies were carried out using both distilled water as well as oxalic acid as the strippant. The possibility of the separation of U from Th was also evaluated.     

Effects of pre-oxidant (KMnO4) on structure and performance of PVDF and PES membranes

The structure and performance of membrane materials are very important to the efficient and stable operation in membrane drinking water purification technology. Potassium permanganate (KMnO₄), which can change the characteristics of organic matters and control membrane surface fouling, has been widely used as pre-oxidant in the front of membrane drinking water process. This study investigates the evolution of membrane surface structure and performance when polyvinylidene fluoride (PVDF) and polyethersulfone (PES) were exposed to 10, 100 and 1000 mg·L^-1 KMnO₄ solution for 6 and 12 d, respectively. The aged membrane physicochemical characteristics such as membrane surface morphology, chemical composition, hydrophilicity, porosity and zeta potential were evaluated by modern analytical and testing instruments. The anti-fouling property of membrane surface was also investigated by the filtration-backwash experiment. The results indicated that the different concentrations and exposure time of KMnO₄ led to a different variation on PVDF and PES membrane surface structure and performance, which could further affect the membrane separation performance and the membrane fouling behaviors. The membrane surface pore size and porosity increased due to the dislodgment and degradation of membrane additive (PVP), which improved membrane permeability and enhanced the adsorption and deposition of pollutants in the membrane pores. With the increase of exposure time, the membrane surface pore size and porosity reduced for the reactions of chain scission and crosslinking on membrane materials, and the backwashing efficiency declined, leading to a more serious irreversible fouling. Compared with PVDF membranes, the formation of sulfonic group for PES membranes increased the negative charge on membrane surface due to the oxidation of KMnO₄. The present study provides some new insights for the regulation of the pre-oxidant dose and the selection of the membrane materials in KMnO₄ pre-oxidation combined with membrane filtration system.     

饱和铬酸钠共存苛性碱中铬铁矿的氧化分解行为

为考察铬铁矿苛性碱氧化浸出反应体系循环苛性碱溶液对铬铁矿氧化分解行为的影响,以90℃下饱和铬酸钠共存苛性碱(代替单一苛性碱溶液)作为反应介质,研究了苛性碱浓度、氧气分压、铬铁矿粉粒度、反应温度、反应时间等参数对铬铁矿氧化分解过程的影响规律,分析了铬酸钠与苛性碱的分离效果以及铬酸钠与铝、硅杂质元素的分离效果。研究结果表明,铬铁矿氧化分解的较佳工艺条件为苛性碱浓度50wt%、氧气分压3.2 MPa、铬铁矿粒度小于48 μm、反应温度250℃、反应时间240 min。在较优工艺条件下,铬铁矿中铬元素的浸出率达到95.24%;铬铁矿中浸出的铬有96.36%以铬酸钠晶体的形式在后续浸出渣溶解分离过程中进入溶晶液,而铬铁矿中浸出的铝、硅分别有84.92%和95.04%进入到浸出液中;同时,未参与反应的苛性碱有89.41%保留在浸出液中。与苛性碱单一反应介质氧化分解铬铁矿的过程相比,本工作采用的反应方式将从铬铁矿中浸出的主元素铬、杂质元素铝和硅,以及未参与反应的苛性碱分别调整到溶晶液和浸出液中,获得了较好的分离效果。     

Diverse morphologies of PVDF hollow fiber membranes and their performance analysis as gas/liquid contactors

A systematic investigation on the morphology development of polyvinylidene fluoride hollow fiber membrane made using various N‐methyl‐2‐pyrrolidone (NMP) aqueous solutions as an inner coagulant was carried out. The cross‐sectional and inner surface morphology were analyzed with scanning electronic microscopy (SEM). It is found that with increase on NMP concentration, the morphology of the resultant membranes gradually shifted from a double‐skin to a single‐skin structure. When 40.0 ～ 55.0 wt.% NMP solution was used, some unexpected macrovoids near the inner region were observed. This special morphology feature was attributed to the reduced solidification rate of the inner surface as a result of increase on NMP concentration, which sharply weakened the inner skin strength. While the existence of centralized stress formed in the phase inversion process, such as shrinkage stress from syneresis, resulted in fractured points in the nascent skin surface that finally made it difficult to maintain a uniform structure. Investigations on effects of the dope flow rate and the bore fluid velocity on the morphology of PVDF fiber membranes experimentally confirmed the suggestion. Three model membranes with double skins, single skin and single skin with macrovoids structures, respectively, were used to test their permeation performance in a CO₂ membrane contactor system. The experimental results show the membranes without an inner skin present higher permeability and lower mass transfer resistance than the membrane with a double skin structure. © 2010 Wiley Periodicals, Inc. Journal of Applied Polymer Science, 2010     

酚酞型聚醚砜对聚偏氟乙烯中空纤维膜结构及性能的影响

以二苯甲酮和N,N-二甲基乙酰胺为稀释剂,酚酞型聚醚砜(PES-C)为添加剂,通过热致相分离法制备了聚偏氟乙烯(PVDF)中空纤维膜。采用扫描电镜观察了膜的结构,测试了膜的纯水通量。在膜生物反应器(MBR)中测试了膜的污水通量和出水的化学需氧量及氨氮指标。该法制得的膜具有较为致密的皮层结构和疏松的支撑层结构,添加质量分数为2%的PES-C制备的PVDF膜与PVDF膜相比纯水通量增加60%,污水通量增加37.8%,出水COD去除率增加了3.32%,NH4+—N去除率增加了2.2%,且MBR出水达到排放标准。     

多效膜蒸馏技术用于氢氧化钠稀溶液的浓缩

研究了多效膜蒸馏(MEMD)过程对氢氧化钠稀溶液的浓缩回收。结果表明,该过程可将Na OH稀溶液浓缩至250 g/L以上,且截留率维持在99.9%以上;虽然Na OH溶液的黏度和沸点随着浓度上升而急剧上升使得膜通量和造水比都显著下降,但是当Na OH质量浓度达到200 g/L时膜通量和造水比仍可达3.05 L/(m2·h)、5.04,达到相当于7效蒸发器的节能效果。膜组件在连续运行的60 d内保持了良好的操作稳定性。