Membrane ultrafiltration in multipass hollow-fiber modules

Rising fluid velocity in the fiber tubes of a hollow-fiber module has two conflicting effects. One, the decrease in resistance to permeation due to reduction in concentration polarization, is good for ultrafiltration, while the other, the decrease in average transmembrane pressure due to increase in frictional pressure loss, is bad for ultrafiltration. It appears, therefore, that proper adjustment of fluid velocity as well as proper arrangement of the number of fiber passes in a hollow-fiber module of fixed total hollow fibers, might effectively suppress the undesirable resistance to permeation while still preserving an effective transmembrane pressure and thereby lead to improved performance of membrane ultrafiltration. It was found that considerable improvement in ultrafiltration is obtainable by employing a multipass hollow-fiber membrane module rather than using the one with only single fiber pass and with the same total fibers, especially for a dilute solution ultrafiltered with low volumetric flow rate and under high transmembrane pressure.     

Diameter control of electrospun polyacrylonitrile/iron acetylacetonate ultrafine nanofibers

Electrospinning is the process of producing ultrafine fibers by overcoming the surface tension of a polymer solution using high voltage. In this work, the effects of both solution properties (viscosity, conductivity, and surface tension) and operational conditions (voltage, feed rate, and spinneret‐collector distance), on the structure of electrospun polyacrylonitrile nanofibers, were systematically investigated. Iron acetylacetonate was added to the electrospinning solution to control fiber diameter by selectively adjusting solution properties. It was found that, with increased salt concentration, the fiber diameter increases and then passes through a maximum due to changes in solution viscosity, conductivity, and surface tension. In addition, the fiber diameter increases with increase in voltage, feed rate, and spinneret‐collector distance. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1611–1618, 2008     

Study on morphology of electrospun poly(vinyl alcohol) mats

Submicron poly(vinyl alcohol) (PVA) fiber mats were prepared by electrospinning of aqueous PVA solutions in 6-8% concentration. Fiber morphology was observed under a scanning electron microscope and effects of instrument parameters including electric voltage, tip-target distance, flow rate and solution parameters such as concentration on the morphology of electrospun PVA fibers were evaluated. Results showed that, when PVA with higher degree of hydrolysis (DH) of 98% was used, tip-target distance exhibited no significant effect on the fiber morphology, however the morphological structure can be slightly changed by changing the solution flow rate. At high voltages above 10 kV, electrospun PVA fibers exhibited a broad diameter distribution. With increasing solution concentration, the morphology was changed from beaded fiber to uniform fiber and the average fiber diameter could be increased from 87 ± 14 nm to 246 ± 50 nm. It was also found that additions of sodium chloride and ethanol had significant effects on the fiber diameter and the morphology of electrospun PVA fibers because of the different solution conductivity, surface tension and viscosity. When the DH value of PVA was increased from 80% to 99%, the morphology electrospun PVA fibers was changed from ribbon-like fibers to uniform fibers and then to beaded fibers. The addition of aspirin and bovine serum albumin also resulted in the appearance of beads.     

Effect of shell-side flows on the performance of hollow-fiber gas separation modules

A theoretical analysis of shell-side flow effects on the performance of hollow-fiber gas separation modules is presented. The theory uses Darcy’s law to relate fiber packing, pressure fields, and velocity fields within the shell. The resulting shell conservation equations are coupled to the lumen conservation equations through the permeation relationship. This two-dimensional (2-D) analysis quantifies the performance penalty associated with gas distribution across the fiber bundle at the shell inlet and outlet. Theoretical predictions for the production of nitrogen from air in a commercial shell-fed module are closer to experimental data than predictions obtained assuming one-dimensional (1-D) plug flow. Fluid flows primarily across fibers near the inlet and outlet ports, and along fibers between ports. Nitrogen composition increases along fluid streamlines, which leads to axial and radial concentration variations within the fiber bundle. Diffusional contributions to shell mass transfer are small for the modules considered here.     

Viscous Drag of Periodic Row of Polydisperse Fibers with Allowance for Slip Effect

Viscous drag in the gas flowing through an equidistant periodic row of parallel polydisperse fibers with the random fiber radius distribution over period was studied using an approximation of small Reynolds numbers. The case of viscous advancing flow perpendicular to the row was considered. The effect of gas slip at the fiber surface was taken into account. Expression was found for the resistance force averaged over random ensemble of fibers, which well describes the results of numerical simulation with the use of lognormal fiber radius distribution, including the case of high fiber polydispersity. The resistance of a very dense polydisperse row was analyzed within the framework of Keller's model with allowance for slip effect.     

Facile fabrication of novel cobalt-based carbonaceous coatings on nickel-titanium alloy fiber substrate for selective solid-phase microextraction

Fabrication of selective adsorption coatings plays a crucial role in solid-phase microextraction(SPME).Herein, new strategies were developed for the in-situ fabrication of novel cobalt-based carbonaceous coatings on the nickel-titanium alloy(Ni Ti) fiber substrate using ZIF-67 as a precursor and template through the chemical reaction of ZIF-67 with glucose, dopamine(DA) and melamine, respectively. The adsorption performance of the resulting coatings was evaluated using representative aromatic co...     

High resistance to thermal decomposition in brown cotton is linked to tannins and sodium content

Brown cotton fibers (SA-1 and MC-BL) studied were inferior to a white cotton fiber (Sure-Grow 747) in fiber quality, i.e., a shorter length, fewer twists, and lower crystallinity, but showed superior thermal resistance in thermogravimetric, differential thermogravimetric, and microscale combustion calorimetric (MCC) analyses. Brown cotton fibers yielded 11–23 % smaller total heat release and 20–40 % greater char. Washing fibers in water and a 1 % NaOH solution showed that rich natural inorganic components and the condensed tannins present in brown cotton are responsible for the unusual thermal property. The loss of inorganics from white cotton during a water wash increased the thermal decomposition temperature of cellulose, resulting in no char yield. However, the stronger binding of metal ions for brown cotton as well as its dominant adsorption of sodium ions after a 1 % NaOH wash facilitated the low-temperature thermal-reaction route; the sodium content showed a significant negative correlation with the heat release capacity of the fiber. Condensed tannins greatly enhanced the adsorption of sodium ions to the fiber and exhibited inherent thermal stability. The limiting oxygen indices (LOI) calculated from the MCC parameters indicated the slower burning characteristic of brown cotton, and its LOI was further increased upon adsorption of sodium ions.     

On the indirect polyelectrolyte titration of cellulosic fibers. Conditions for charge stoichiometry and comparison with ESCA

The effect of electrolyte (NaHCO3) concentration on the adsorption of poly-DADMAC (poly-diallyldimethylammonium chloride) onto cellulosic fibers with different charge profiles was investigated. Surface carboxymethylated fibers were obtained by grafting carboxymethyl cellulose (CMC) onto the fiber surface and bulk carboxymethylated fibers were obtained by reacting the fibers with monochloroacetic acid. It was shown that nonionic interactions do not exist between cellulose and poly-DADMAC, rather electrostatic interactions govern the adsorption. Charge stoichiometry prevails under electrolyte-free conditions, whereas surface charge overcompensation occurs at higher electrolyte concentrations. It was shown that charge stoichiometry prevails if the thickness of the electric double layer kappa(-1) was larger than the mean distance between the charges on the fiber surface, as predicted by polyelectrolyte adsorption theories, taking lateral correlation effects into account. In a second set of experiments the ESCA technique served to independently calibrate the polyelectrolyte titrations for determining the surface charge of cellulosic fibers. Various molecular masses of poly-DADMAC were adsorbed to carboxymethylated fibers having different charge profiles. The adsorption of low M(w) poly-DADMAC (7.0 x 10(3)), analyzed by polyelectrolyte titration, was about 10 times higher than that of the high M(w) poly-DADMAC (9.2 x 10(5)). Despite the difference in accessibility of these two polyelectrolytes to the fiber cell wall, ESCA surface analysis showed, as expected, only slight differences between the two polyelectrolytes. This gives strong credibility to the idea that surface charge content of cellulosic fibers can be analyzed by means of adsorption of a high-molecular-mass cationic polymer, i.e., by polyelectrolyte titration.     

Extraction of surfactin from fermentation broth with n-hexane in microporous PVDF hollow fibers: Significance of membrane adsorption

In order to avoid foaming behavior and the formation of stable emulsions in traditional extraction, non-dispersive extraction of surfactin from the fermentation broth of Bacillus subtilis ATCC 21332 culture with n-hexane was studied in microporous polyvinylidene fluoride (PVDF, pore size 0.2 μm) hollow fiber module. In this work, the broth was pretreated by acid precipitation and the precipitate was then dissolved in NaOH solution, and the treated broth was passed through the lumen side of the module and n-hexane was flowed across the shell side. Experiments were performed at a fixed pH of 8.0 and a flow rate of both phases of 2.5 mL min⁻¹ but at different surfactin concentrations (300–3000 mg L⁻¹). Under the conditions studied, it was shown that surfactin was adsorbed onto the surface of the fibers, instead of being extracted by n-hexane and transported through the pores of the fibers into bulk n-hexane phase. The adsorption capacity was determined and the adsorption dynamics was analyzed. The purity of surfactin desorbed from the fibers with ethanol was found to be higher than that obtained after solvent extraction with n-hexane.     

Diffusive transport across unconfined hollow fiber membranes

The mass transfer characteristics of gas permeable, hollow fiber membranes in a liquid jet mixed reactor are studied. A membrane module, operated in the sealed-end mode, was pressurized with oxygen at the base of the fibers and centered within a submerged jet discharge. Unlike conventional membrane module designs, this configuration did not have the hollow fibers enclosed within a tubular shell. The membranes were unconfined and free to move within the generated flow field. This design is especially well suited for use in waters containing high solid concentrations. The membranes have a greater degree of freedom for movement and are therefore less likely to become fouled due to solids being lodged within the fiber bundle. Mass transfer rates were measured over a practical range of physical and process parameters. A mass transfer correlation for the unconfined configuration is presented and the transfer performance of this configuration is compared with conventional membrane contactor designs.     

Flux enhancement during dean vortex tubular membrane nanofiltration. 10. Design,construction, and system characterization

Controlled centrifugal instabilities (called Dean vortices) resulting from flow in helical tubes have been used to reduce concentration polarization and membrane fouling during nanofiltration. These vortices enhance back-migration of solute through convective flow away from the membrane–solution interface and allow for increased membrane permeation rates. Based on the theory of Dean vortex flow, a new prototype vortex generating tubular nanofiltration element was designed. Two sets of nanofiltration modules were constructed; a linear module and a new module containing hollow fibers wrapped around rods of small diameter in helical geometry. Optimization of the design is discussed with respect to the diameter and thickness of the hollow fibers. Axial pressure drop and energy consumption measurements for the helical module agreed very well with available correlations for various experimental conditions. Water permeabilities for the helical modules were similar to those of the conventional linear modules. No significant effect of pH was observed on the water permeability.     

Composite Polymer/Oxide Hollow Fiber Contactors: Versatile and Scalable Flow Reactors for Heterogeneous Catalytic Reactions in Organic Synthesis

Flexible composite polymer/oxide hollow fibers are used as flow reactors for heterogeneously catalyzed reactions in organic synthesis. The fiber synthesis allows for a variety of supported catalysts to be embedded in the walls of the fibers, thus leading to a diverse set of reactions that can be catalyzed in flow. Additionally, the fiber synthesis is scalable (e.g. several reactor beds containing many fibers in a module may be used) and thus they could potentially be used for the large‐scale production of organic compounds. Incorporating heterogeneous catalysts in the walls of the fibers presents an alternative to a traditional packed‐bed reactor and avoids large pressure drops, which is a crucial challenge when employing microreactors.     

Integrating Perovskite Solar Cells into a Flexible Fiber

Perovskite solar cells have triggered a rapid development of new photovoltaic devices because of high energy conversion efficiencies and their all‐solid‐state structures. To this end, they are particularly useful for various wearable and portable electronic devices. Perovskite solar cells with a flexible fiber structure were now prepared for the first time by continuously winding an aligned multiwalled carbon nanotube sheet electrode onto a fiber electrode; photoactive perovskite materials were incorporated in between them through a solution process. The fiber‐shaped perovskite solar cell exhibits an energy conversion efficiency of 3.3 %, which remained stable on bending. The perovskite solar cell fibers may be woven into electronic textiles for large‐scale application by well‐developed textile technologies.     

Fabrication of electrospun poly(methyl methacrylate) nanofibrous membranes by statistical approach for application in enzyme immobilization

Response surface methodology based on a five-level, five-variable central composite rotatable design was used to model the average diameter of electrospun poly(methyl methacrylate) nanofibers. The effects studied include polymer concentration, distance, temperature, flow rate, and voltage. Fiber diameter was correlated to these variables by using a second-order polynomial function at a 95% confidence level. The analysis confirmed that polymer concentration, temperature, flow rate, and voltage were the significant factors affecting the diameter with the first three being the most significant ones. Also, no interaction effect terms were found to be significant. The coefficient of determination of the model was found to be 0.9443. The predicted fiber diameters were in agreement with the experimental results. The adequacy of the model was examined using additional independent experiments that were not employed in the model generation to fabricate 100–500 nm fibers with the average absolute relative deviation being 3.55%. A minimum fiber diameter of 36 nm was established and could be validated by experiments. When used for immobilization of Candida rugosa lipase by adsorption, the nanofibrous membranes provided enzyme loading as high as 332 mg lipase/g fibers, which is 5.2 times the reported maximum value.     

微波辅助制备螯合纤维及其对汞的吸附性能

在微波辅助下,以聚丙烯腈纤维(PANF)为基体材料,二乙烯三胺(DETA)和硫化钠为改性试剂,通过两步接枝反应快速制备了含有大量硫原子的螯合纤维吸附剂。利用傅里叶变换红外光谱仪(FT-IR)和热重分析仪(TGA)对改性前后的纤维进行表征,同时考察了pH值、初始浓度、吸附时间和温度对螯合纤维吸附汞离子的影响。结果表明,微波辅助是一种高效、节能、经济和绿色的改性方法,改性过程在无毒的水环境中进行,试剂用量少,且改性时间大大缩短。改性纤维对汞离子的吸附是一个准二级动力学过程,较好的符合Langmuir吸附模型。在pH=7的条件下,螯合纤维对汞离子的最大吸附容量达到333.1mg/g,是一种有效的去除水中汞污染的吸附材料。     

Effect of fiber variation on the performance of cross-flow hollow fiber gas separation modules

A quantitative analysis of the effects of variable fiber properties on the performance of a cross-flow hollow fiber gas separation module is presented. The effects of variations in size, permeance, and selectivity are considered. Fiber variability is detrimental to performance. The recovery and flow rate of an enriched retentate stream decrease as variability increases. Some fibers may actually stop producing product as purity increases. Additionally, performance is poorer if the permeate from all fibers is not well mixed. The results of this work can be used to determine quality control guidelines for fiber manufacture and evaluate process enhancements.     

High‐performance poly(ethylene‐2,6‐naphthalate) fiber prepared by high‐tension annealing

A high‐tension annealing (HTA) method has been applied to zone‐annealed poly(ethylene‐2,6‐naphthalate) (PEN) fibers in order to further improve their mechanical properties. The HTA treatment was carried out under an applied tension of 428 MPa at a treating temperature of 175 °C. The applied tension was close to the tensile strength at 175 °C. The resulting HTA fiber had a birefringence of 0.492 and degree of crystallinity of 57%. Wide‐angle X‐ray diffraction (WAXD) photographs of the HTA fibers showed three reflections (010, 100, and 1 10) attributed to an α form crystal, but no (020) reflection attributed to a β form was observed in the equator. The tensile modulus and tensile strength increased with processing, and the HTA fiber had a maximum modulus of 33 GPa, a tensile strength of 1.1 GPa, and a storage modulus of 33 GPa at 25 °C. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 61–67, 2000     

Dead-end ultrafiltration in hollow fiber modules: Module design and process simulation

Ultrafiltration in a hollow-fiber module operating with outside-in and dead-end flow at a constant flow rate was simulated using a model that takes into account the longitudinal pressure drops inside the fibers and within the fiber bundle. The model considers both the filtration phase during which the membrane is fouled by the formation of a filter cake and the backwash phase in which it is cleaned, so as to predict the net rate of production of the module during an operating cycle.The results show that there is a combination of packing density and fiber diameter that gives a maximum net flow rate. Furthermore, this model allows the influence of operating conditions and feed properties on the module performance to be estimated. This can be used to determine how operating parameters must be modified when there is a change in the feed properties.     

Deposition of Aerosol Particles on the Row of Polydisperse Fibers Due to Interception with Allowance for Slip Effect

The efficiency of deposition of aerosol particles due to interception during the gas flow through the equidistant periodic row of parallel polydisperse fibers with random fiber radius distribution over the period was studied in the approximation of small Reynolds numbers. The case of advancing viscous flow perpendicular to the row was considered. An effect of gas slip at the fiber surface was taken into account. Quality of such model filter was studied. Expressions for deposition efficiency and filter quality averaged over random ensemble of fibers were derived; they well describe the results of numerical simulation based on lognormal fiber radius distribution including high degrees of fiber polydispersity.     

溶剂对不同纺速PET纤维聚集态结构的影响

用密度法,DSC,PLM 等方法研究了在29.5±0.2℃温度下,低、中、高速纺 PET 纤维在1,2-二氯乙烷中浸渍不同时间的聚集态结构变化。并测量了它们的无负荷自由收缩。结果表明,良溶剂对不同纺速 PET 纤维的诱导结晶过程与无定形 PET[1]不同。映证了前文[2]关于不同纺速 PET 纤维结构特征的结论,并指出中速纺纤维结构的径向分布较低速和高速纺纤维更不均匀。