PTFE Hollow Fiber Research Articles

Surface Modification of PVDF and PTFE Hollow Fiber Membranes for Enhanced Nitrogen Removal in a Membrane-Aerated Biofilm Reactor

Surface Modification of PVDF and PTFE Hollow Fiber Membranes for Enhanced Nitrogen Removal in a Membrane-Aerated Biofilm Reactor

A novel hydrophilic modification method for polytetrafluoroethylene (PTFE) hollow fiber membrane using sacrificial template

In order to enhance the hydrophilicity of PTFE hollow fiber membrane, high concentration of polymer was needed during the modifying process, but this method resulted in severe pore blockage of hollow fiber membrane with reduced water flux. Herein, we resolve the above contradiction by using cross-linkable poly (vinylpyrrolidone-co-γ-methacryloxypropyltrimethoxysilane) (P (VP-co-KH570)) as hydrophilic polymer and calcium chloride as sacrificial template. The enhanced hydrophilicity and chemical washing resistance of PTFE hollow fiber membrane were achieved by the highly hydrophilic PVP segments and highly crosslinking density contributed by hydrolysis and condensation of the KH570 segments, respectively. The template is embedded in the polymer network formed in the membrane pores, and after template removal the previously occupied pores are released for water transport, thereby minimizing the loss of water flux. The high water flux (1477.14 ± 16.11 L/(m2·h)), superhydrophilicity (the water contact angle = 0°) and hydrophilic stability proved that PTFE hollow fiber membranes are successfully modified by the sacrificial template method. This work provides a facile strategy for designing hydrophilic PTFE hollow fiber membranes for water treatment.

Fabrication of harsh environment‐tolerant and robust reinforced poly (tetrafluoroethylene) hollow fiber membranes with optimal heat setting technology

AbstractThe high‐temperature industrial waste oil treatment has put forward new requirements for membrane materials. Poly (tetrafluoroethylene) (PTFE) is regarded as the excellent membrane material for membrane process, due to its excellent properties. Herein, the reinforced PTFE hollow fiber membrane (PTFE‐HFM) was prepared by the combination of wrapping and heat setting technology. The objection of this study was focusing on the outstanding chemical and thermal resistance of PTEF‐HFM for overcoming harsh waste oil treatment. PTFE‐HFM exhibited better chemical resistance in the experiment of acid and alkali compared to PVDF‐HFM, which the surface of PVDF‐HFM corroded obviously and none of PTFE‐HFM. The pore size and porosity were precisely adjusted by heat setting technology. Firstly, the morphologies of PTFE‐HFM was investigated by scanning electron microscopy, which exhibited favorable interfacial bonding state between PTFE separation layer and supporting layer. Subsequently, the prepared membranes exhibited high‐temperature waste oil permeability of 195 L m−2 h−1 bar−1with only 3% attenuation in long‐term operation. Besides, the rejection of lubricants contained activated carbon also reached to 99%. Overall, the facile preparation and excellent performance of robust reinforced PTFE‐HFM highlights its great potential in treating high‐temperature waste oil.

Study on two‐stage stretching strategy for microstructure improvement of polytetrafluoroethylene hollow fiber membrane

AbstractPolytetrafluoroethylene (PTFE) has excellent chemical resistance, thermal stability as well as hydrophobicity, and is regarded as a good material for membrane fabrication. PTFE hollow fiber membrane is commonly prepared by the paste extrusion–stretching–sintering process, where the stretching temperature is usually fixed in existing reports. In this paper, a new two‐stage stretching method (first at 50°C and then at 200°C) was proposed to optimize the microstructure of PTFE hollow fiber membranes. Meanwhile, the stretching experiments at low‐temperature (50°C) and high‐temperature (200°C) were also conducted for comparison. The chemical composition, surface morphology, surface porosity, overall porosity, maximum pore size, water contact angle, and mechanical property were discussed. Moreover, the ethanol permeation flux and bovine serum albumin (BSA) rejection of PTFE hollow fiber membranes prepared from different stretching manners were tested. The results showed that with the increase of stretching ratio, the porosity, and pore size gradually increased for all three membranes. However, for a specific stretching ratio, the membrane fabricated by two‐stage stretching process exhibited better comprehensive properties in terms of porosity and pore size, that is, higher porosity than the 50°C method and smaller pore size than the 200°C method. As stretching ratio was 2.0, the ethanol permeation flux and BSA rejection was 2467 L/m2 h and 95%, respectively. Overall, the PTFE hollow fiber prepared from “two‐stage” stretching exhibited comparable BSA rejection (both around 95%) and much higher permeation flux (increased by 43.1%) compared to the sample prepared by low‐temperature stretching at 50°C, and meanwhile, comparable permeation flux (decreased by 10.8%) and higher BSA rejection (95% against 81%) compared to the sample prepared by high‐temperature stretching at 200°C. The proposed method provided a good reference for the microstructure improvement of PTFE hollow fiber membrane fabricated by paste extrusion–stretching–sintering technology.

Development of Antibacterial PTFE Hollow Fiber Membranes Containing Silver-carried Zirconium Phosphate as Air Filtration Units for the Removal of Ultrafine Particles

Development of Antibacterial PTFE Hollow Fiber Membranes Containing Silver-carried Zirconium Phosphate as Air Filtration Units for the Removal of Ultrafine Particles

Decontamination of uranium contained low-level radioactive wastewater from UO2 fuel element industry with vacuum membrane distillation

The rapid development of nuclear industry has generated large quantities of low-level radioactive wastewater (LLRW) to pose a potential threat globally. In this study, the feasibility of vacuum membrane distillation (VMD) process for the decontamination of uranium contained LLRW from UO2 fuel element production has been studied using commercial PTFE hollow fiber membrane. The effects of operation conditions, such as feed temperature/pH, vacuum pressure and feed flow velocity were investigated and optimized with artificial UO2(NO3)2 solution. The real process wastewater from the UO2 fuel element factory were then treated by VMD process under the optimized conditions for 1400 min with the concentration factor up to 9. No clear evidence of membrane fouling or wetting were noticed during the VMD operation nor the membrane autopsy studies. With an average permeate flux of 7.3 L m−2 h−1, it exhibited excellent retention towards all major contaminates such as uranium, COD, and NH4+-N to meet the Integrated Wastewater Discharge Standard of China (GB 8978-1996) for direct discharge.

Regeneration of CO2 absorbent with membrane contactor via pressure swing

A new method to regenerate high pressure CO2 absorbent is presented in this paper, where PTFE hollow fiber membrane contactor was used as a gas–liquid separator in pressure swing process. CO2 desorption happened in the membrane contactor at the same time. The two mass transfer mechanisms were analysed through calculation of viscous flux and mass transfer coefficients. Experimental results showed that CO2 loading in regenerated absorbent increased with absorbent flow rate. This was due to decrease in membrane area available for CO2 desorption, as most of the membrane area was utilized for gas-liquid separation. Co-current configuration exhibited higher CO2 desorption driving force compared to counter-current configuration, hence its CO2 loading in regenerated absorbent was lower. When integrated with membrane contactor for CO2 absorption, stable and consistent performance was observed. Unlike conventional flash drum, membrane contactor did not have requirement on flashed gas velocity, because the membrane itself functioned as a screen to prevent fine liquid droplets from escaping the system along with flashed gas. Results from this study show that vessel size reduction up to 2.5 times could be achieved when PTFE membrane contactor was used, with absorbent regeneration energy as low as 0.94 GJ/ton CO2.

Simulation and experiment on direct contact membrane distillation regenerator in the liquid dehumidification air-conditioning system

Liquid dehumidification air-conditioning (LDAC) systems are a promising dehumidification technology that reduce energy consumption. However, in hot and humid climates, liquid regeneration deteriorates the performance of regenerators and limits their applications. Therefore, developing methods to ensure regeneration performance in harsh climates as well as to reduce the droplet splatter is a challenging task that must be undertaken to advance the technology and make it viable. In this study, a novel regeneration method using the direct contact membrane distillation (DCMD) regenerator was applied to the LDAC system. The mathematical model of the DCMD solution regeneration process was developed and verified through PTFE hollow fiber membrane liquid regeneration experiments. The impacts of membrane length, temperature, concentration, and solution volume flow on the temperature and concentration of the resultant solution at the outlet, regeneration capacity mass transfer coefficient, membrane water flux, and heat flow were analyzed. The results indicate that the concentration of the solution at the outlet linearly increases from 34.9% to 35.9% when the solution is heated from 40 to 85 °C. However, when the temperature of the solution is less than 45 °C, the solution concentration at the outlet is lower than that at the inlet, which indicates that the regeneration capacity is negative. Thus, the solution temperature at the DCMD regenerator inlet should be maintained at >65 °C to provide a higher and stable regeneration efficiency. Finally, it was established that a flow rate that is either extremely high or low would impair the DCMD regeneration, whilst a volume flow of 1800 ml min−1 is still most appropriate for the regenerator to regenerate high-concentration solutions. Research on the cold side indicates that when the temperature of the water does not exceed 30 °C, the regenerator still has the ability to regenerate, and increasing the volume flow of water can enhance the regeneration effect.

Performance Evaluation of a Thermophilic Anaerobic Membrane Bioreactor for Palm Oil Wastewater Treatment.

Anaerobic treatment processes have achieved popularity in treating palm oil mill effluent due to its high treatability and biogas generation. The use of externally submerged membranes with anaerobic reactors promotes the retention of the biomass in the reactor. This study was conducted in thermophilic conditions with the Polytetrafluoroethylene hollow fiber (PTFE-HF) membrane which was operated at 55 °C. The reactor was operated at Organic Loading Rates (OLR) of 2, 3, 4, 6, 8, and 10 kg Chemical Oxygen Demand (COD)/m3·d to investigate the treatment performance and the membrane operation. The efficiency of the COD removal achieved by the system was between 93–98%. The highest methane yield achieved was 0.56 m3 CH4/kg CODr. The reactor mixed liquor volatile suspended solids (MLVSS) was maintained between 11.1 g/L to 20.9 g/L. A dead-end mode PTFE hollow fiber microfiltration was operated with the constant flux of 3 LMH (L/m2·h) in permeate recirculation mode to separate the clear final effluent and retain the biomass in the reactor. Membrane fouling was one of the limiting factors in the membrane bioreactor application. In this study, organic fouling was observed to be 93% of the total membrane fouling.

A systematic study of enhanced ozone mass transfer for ultrasonic-assisted PTFE hollow fiber membrane aeration process

The ozone mass transfer efficiency based on deionized water of both the PTFE hollow fiber membrane aeration and ultrasonic-assisted PTFE hollow fiber membrane aeration processes have been studied and described by mass transfer coefficient (KLa). Bubble size distributions were also investigated via an image-technology to analyze mass transfer processes. All the PTFE hollow fiber membranes were hydrophobic and had an asymmetrical structure. Results showed that the maximum value of KLa (0.438 min−1) was obtained at an intake flow rate of 300 L/h with 1# membrane (0.22 µm) in single membrane aeration system. This gas-liquid mass transfer was strongly dependent on pressure. After the introduction of ultrasonic power, the KLa value reached 0.632 min−1 with an ultrasonic power of 1000 W. This enhancement was related to the periodical replacement of local liquid at the interface between the bulk solution and the liquid with high concentration of ozone. The degradation of sulfonated phenolic resin (SMP) confirms that its mineralization was promoted by the enhanced gas-liquid mass transfer of ozone and the generation of OH in the bulk solution.

Novel Ultrafine Fibrous Poly(tetrafluoroethylene) Hollow Fiber Membrane Fabricated by Electrospinning.

Novel poly(tetrafluoroethylene) (PTFE) hollow fiber membranes were successfully fabricated by electrospinning, with ultrafine fibrous PTFE membranes as separation layers, while a porous glassfiber braided tube served as the supporting matrix. During this process, PTFE/poly(vinylalcohol) (PVA) ultrafine fibrous membranes were electrospun while covering the porous glassfiber braided tube; then, the nascent PTFE/PVA hollow fiber membrane was obtained. In the following sintering process, the spinning carrier PVA decomposed; meanwhile, the ultrafine fibrous PTFE membrane shrank inward so as to further integrate with the supporting matrix. Therefore, the ultrafine fibrous PTFE membranes had excellent interface bonding strength with the supporting matrix. Moreover, the obtained ultrafine fibrous PTFE hollow fiber membrane exhibited superior performances in terms of strong hydrophobicity (CA > 140°), high porosity (>70%), and sharp pore size distribution. The comprehensive properties indicated that the ultrafine fibrous PTFE hollow fiber membranes could have potentially useful applications in membrane contactors (MC), especially membrane distillation (MD) in harsh water environments.

Preparation and properties of PTFE hollow fiber membranes for the removal of ultrafine particles in PM2.5 with repetitive usage capability.

This study reveals the first attempt to apply PTFE hollow fiber membranes for removing ultrafine particles in PM2.5. The asymmetric polytetrafluoroethylene (PTFE) hydrophobic hollow fiber membranes were prepared through a cold pressing method including paste extrusion, stretching and heating. The reduction ratio, stretching ratio and heating temperature have influences on the morphology, structure, porosity, shrinkage ratio, tensile strength and permeability of the PTFE hollow fiber membranes. The morphological properties of the PTFE hollow fiber membrane were studied using field emission scanning electron microscopy (FESEM). The increase of stretching ratio can improve the pore size and porosity of the hollow membrane, but be negative for the mechanical properties. By changing the reduction ratio we can obtain different inner diameter PTFE hollow fiber membranes. Finally, the PTFE hollow fiber membranes were tested for their performances in the removal of ultrafine particles in PM2.5. The PTFE hollow fiber membranes had the microstructure of nodes interconnected by fibrils, designed to possess the synergistic advantages of porous filters and fibrous filters with a sieve-like outer surface and a fibrous-like porous substrate. Under dead-end filtration, the filtration efficiency is related to the wall thickness, pore size and porosity of the membranes. The air filtration achieved was higher than 99.99% for PM2.5 and 90% for PM0.3, indicating that all the prepared PTFE hollow fiber membranes exhibited satisfactory removal of ultrafine particles performances. Because of the hydrophobicity, PTFE hollow fiber membranes have self-cleaning ability and a large dust-holding capacity of >120 g m−2, slowing down membrane fouling. The fouled filter media after washing retained a high filtration efficiency without obvious deterioration. The hydrophobic PTFE hollow fiber membranes developed in this work exhibited potential applications in air filtration.

Optimizing stretching conditions in fabrication of PTFE hollow fiber membrane for performance improvement in membrane distillation

PTFE is one of the most approved hydrophobic materials for MD membranes. However, PTFE membrane was researched and applied on MD insufficiently due to its complex fabrication process. In this work, stretching conditions were comprehensively investigated to form a membrane with high flux and salt rejection. It was found that stretching led to increased cracks at an early stage, and turned to lead to the elongation of fibrils and physical dimension shrinkage of hollow fibers. A porosity prediction model was established to evaluate the improvement of porosity. It was found that a low stretching temperature was helpful for pore size control and hence a relative high stretching ratio can be carried out to achieve a higher porosity. Therefore high flux and rejection was achieved due to the higher porosity and well controlled pore size. Stretching rate of 30%/s gave rise to the most concentrated pore size distribution. NaCl rejection rate was high up to 99.99% for the membrane fabricated at a stretching ratio of 2.4 and a stretching temperature of 40°C.

Enhancing CO2 absorption efficiency using a novel PTFE hollow fiber membrane contactor at elevated pressure

The internal structure design of membrane module is very important for gas removal performance using membrane contactor via physical absorption. In this study, a novel membrane contactor developed by weaving polytetrafluoroethylene (PTFE) hollow fibers was applied to remove CO2 from 60% N2 + 40% CO2 mixture (with CO2 concentration similar to that of biogas) at elevated pressure (0.8 MPa) using water as absorbent. Compared with the conventional module with randomly packed straight fibers, the module with woven PTFE fibers exhibited much better CO2 absorption performance. The weaving configuration facilitated the meandering flow or Dean vortices and renewing speed of water around hollow fibers. Meanwhile, the undesired influences such as channeling and bypassing were also eliminated. Consequently, the mass transfer of liquid phase was greatly improved and the CO2 removal efficiency was significantly enhanced. The effects of operation pressure, module arrangement, feed gas, and water flow rate on CO2 removal were systematically investigated as well. The overall mass‐transfer coefficient (KOV) varied from 1.96 × 10−5 to 4.39 × 10−5 m/s (the volumetric mass‐transfer coefficient KLa = 0.034–0.075 s−1) under the experimental conditions. The CO2 removal performance of novel woven fiber membrane contactor matched well with the simulation results. © 2017 American Institute of Chemical Engineers AIChE J, 64: 2135–2145, 2018

Ultrasonic irradiation control of silica fouling during membrane distillation process

Ultrasonic irradiation was introduced into membrane distillation process and the influence of ultrasonic irradiation on silica fouling control was investigated. A gradual decline of permeate flux during the concentration process of silica solution was observed due to the formation and deposition of colloidal polysilicic acid on the membrane surface. The PTFE hollow fiber membrane could maintain its mechanical properties and initial pore size distribution in the presence of ultrasonic irradiation. Ultrasonic wave brought significant mechanical and thermal effects, and generated powerful shock wave and microstreaming with high speed. The microstreaming, shock wave and acoustic vortex streaming stimulated the liquid–membrane interface continuously, therefore the membrane surface could be effectively kept clean and the permeate flux was hardly affected by the increasing of concentration factor. Due to the charge neutralization and compression of double charged layer induced by calcium ions, the silica colloid stability was eliminated and the colloids aggregated. A large amount of silica scaling was deposited on the membrane surface even at the beginning of concentration process. With ultrasonic irradiation, the permeate flux maintained stable and was enhanced by about 43%; the ultrasonic irradiation could effectively control silica fouling during membrane distillation.

CO<sub>2</sub> Desorption by Hydrophilic PTFE Hollow Fiber Membranes via a Membrane Flash Process

In this study, hydrophilic PTFE hollow fiber membranes were prepared and applied for CO2 desorption via a membrane flash process, which is a new CO2 desorption process by utilizing waste thermal energy. The methyldiethanolamine was selected as the absorbent. Effects of the flashing temperature, flashing pressure, rich solution flow rate and MDEA concentration on CO2 release ratio and CO2 desorption flux were deeply investigated. The results show that flashing temperature is positive to the CO2 release ratio and CO2 desorption flux. However, the flashing pressure, rich solution flow rate and MDEA concentration are negative to the CO2 release ratio and CO2 desorption flux.

Effect of the highly asymmetric structure on the membrane characteristics and microfiltration performance of PTFE wrapped hollow fiber membrane

This study aims at enhancing the permeate flux and filtration precision of PTFE hollow fiber membrane (HFM) in microfiltration by preparing the PTFE wrapped hollow fiber membrane (wHFM), in which pore size and porosity could be artfully manipulated by an asymmetric microporous structure that HFM as the inner layer was coated with PTFE sheet membrane as the skin layer. Stability of such asymmetric microporous structure and its effect on the filtration performance were carefully studied in consideration of stretching temperature, stretching ratio and wrapping tension. The results show that the highly asymmetric microporous structure was successfully fabricated owing to the interlocking of fibrils at the interface between the skin and inner layers. High stretching temperature, high wrapping tension and low stretching ratio for preparing the inner layer are positive to the stability of asymmetric structure. In addition, an unusual phenomenon that smaller pore can be fabricated at higher stretching ratio is revealed in preparation of the PTFE sheet membrane. Finally, CaCO3 particles are retained by the skin layer of the wHFM, which leads to the high flux and rejection due to the high porosity and bubble point, respectively. We hope such an asymmetric membrane would be potentially useful in separation and purification fields.

Pushing the limits of intensified CO2 post-combustion capture by gas–liquid absorption through a membrane contactor

The possibility to maximize the intensification (i.e., volume reduction) of post-combustion carbon dioxide capture by chemical absorption in a monoethanolamine (MEA) solution using a membrane contactor module is reported. The influence of MEA concentration (20–90wt%) and temperature (293–333K) on the CO2 capture ratio achieved by a PTFE hollow fiber membrane contactor has been investigated with a CO2/N2 (15/85vol%) gas mixture and the experimental results modeled. It is shown, through a systematic parametric analysis, that the intensification factor of the absorption unit, defined as the ratio of the CO2 volumetric absorption capacity of the membrane contactor and of the packed column (taken at 1mol CO2m−3s−1), can be largely increased when a concentrated (70wt%) MEA solution at 333K is used. A volume reduction up to a factor 9, together with a lower energy penalty due to pressure drop effects (kWh per ton of captured CO2) is potentially obtained compared to packed column performances, when a large enough membrane mass transfer coefficient is achieved (i.e., larger than 10−4ms−1). The remaining challenges which are required in order to develop this level of performances at industrial scale based on the existing membrane contactor materials are discussed.

An ultrasonic assisted direct contact membrane distillation hybrid process for desalination

A novel ultrasonic assisted direct contact membrane distillation (USDCMD) hybrid process was designed and the effects of feed temperature, feed concentration, feed velocity, ultrasonic power and frequency on mass transfer were investigated. Under ultrasonic irradiation, changes and damages in membrane structure were found on PVDF membrane, while the pore size and the stretching strain of PP fibers were also enlarged and declined, respectively. The PTFE hollow fiber was selected to carry out USDCMD. The results showed that ultrasonic irradiation could effectively enhance mass transfer. Under the ultrasonic irradiation of 20kHz and 260W, the maximum permeate flux enhancement of 60% was obtained under conditions of feed temperature of 53°C, feed velocity of 0.25m/s and feed salt concentration of 140g/L. The increment was enlarged with the decrease of feed temperature, feed velocity and ultrasonic frequency as well as the increase of feed concentration and ultrasonic power. Ultrasonic irradiation had no significant influence on the mechanical strength, pore size and hydrophobicity of the PTFE membrane in a 240h continuous USDCMD experiment, and the novel membrane distillation process exhibited satisfying performance stability, which indicated that ultrasonic irradiation can be applied to membrane distillation for mass transfer enhancement.

Preparation for PTFE Hollow Fiber Membranes Used in Water Treatment and Optimization for its Extrusion Process

For their excellent physical and chemical properties, PTFE hollow fiber membranes can be used in harsh environment of water treatment. But the membranes cannot be prepared by conventional methods, such as the melting processing methods and the dry-wet spinning processing methods, because of a high melting point of 327°C and chemical stability. In this paper a method of extrusion-stretching-sintering is used, and using this method we get the membranes with high fracture strength (10MPa) and high pure water flux (1500L/(m2*h)), also the retention rate of bovine serum albumin reaching 50%, which meets the laboratory microfiltration membrane requirements.