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Polytetrafluoroethylene (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. [ABSTRACT FROM AUTHOR]

Nafion has been widely used in electrochemistry, but there are only a few reports on its application in other fields, such as, gas separation, even though it exhibits good performance. The primary reason for that is the high cost of Nafion and making a composite membrane with a thin Nafion layer is a potential solution to solve this problem. In this study, a novel Nafion‐PTFE composite hollow fiber membrane, which had a thin (~5 μm) and detect‐free Nafion layer on PTFE surface, without Nafion filling substrate pores was developed, differing from the reported ones in which Nafion resin was required to impregnate into porous PTFE membrane as thorough as possible to ensure the ion conductivity and operation stability. The surface morphology, crystallite, solubility in ethanol/water mixture, and water uptake of membranes were systemically investigated. The gas permeance tests were also conducted. The permeances of different gasses of prepared composite membranes were significantly enhanced compared with the commercial membranes due to the decrease of Nafion thicknesses, while the selectivity remained the same, verifying the detect‐free structure of Nafion layer on PTFE substrate. This study provided a good reference for the preparation and application of low‐cost Nafion composite membranes. [ABSTRACT FROM AUTHOR]

The process intensification in carbonylation of formaldehyde by active and passive enhancement was investigated in stirred tank reactor and membrane contactor, respectively. The increase of rotational speed and decrease of reaction solution volume was advantageous to the yield of glycolic acid in active strengthening experiments, indicating that the reaction was greatly affected by the mass transfer of CO. In passive reinforcement mode, the membrane contactor composed of PTFE hollow fibers with inner diameter of 0.85 mm or 0.42 mm was used as a novel reactor to improve the process mass transfer. The effects of reaction temperature, operation pressure, reaction time and reactant concentration were systematically studied. The results showed that membrane contactor was an effective reactor for the carbonylation of formaldehyde compared to the conventional tank reactor. This study provided a reference for the process intensification of similar gas-liquid reactions. [ABSTRACT FROM AUTHOR]

The batch and continuous synthesis of glycolic acid via carbonylation of formaldehyde using high-temperature and high-pressure poly(tetrafluoroethylene) (PTFE) membrane contactor was demonstrated in this paper. The experiments in conventional stirred tank reactor were also conducted for comparison. The results showed that the yield of glycolic acid was significantly improved in membrane contactor system compared to the stirred tank reactor. At reaction temperature of 120 °C and reaction pressure of 5.0 MPa, the yield of glycolic acid was higher than 90% when the reaction time was 1 h, while it was only about 45% in the conventional stirred tank reactor. The enhancement of yield was due to the improvement of mass transfer or dispersion of CO in membrane contactor system, which could provide larger gas-liquid contact area and shorter mass transfer distance. The stability of PTFE hollow fibre membranes was also investigated by Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), liquid entry pressure (LEP), bubble point (BP) and contact angle (CA) measurements. The characterizations indicated that PTFE membranes exhibited good chemical and thermal stability in this reaction system. This study provided new idea and reference for the carbonylation of formaldehyde and similar gas-liquid reactions. • Batch and continuous synthesis of glycolic acid via carbonylation of formaldehyde in membrane contactor was demonstrated. • Mass transfer of CO is greatly enhanced in membrane contractor system compared to the conventional tank reactor. • Membrane contactor can provide larger contact area and shorter diffusion path for gas-liquid reaction. • PTFE hollow fibers exhibited good stability under high temperature and high pressure conditions. [ABSTRACT FROM AUTHOR]

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 [ABSTRACT FROM AUTHOR]

In this study, the effect of sintering conditions including manner, temperature and duration on properties of polytetrafluoroethylene (PTFE) hollow fibre membrane fabricated by extrusion method was intensively investigated. Different from un-sintered and relaxed sintered, the fixed sintered PTFE hollow fibre membrane was observed to generate a uniform ‘fibril–node’ porous structure and a main crystal transformation to folded chain crystal with smaller size. Consequently, it was found that for fixed setting sintering, both temperature increase from 340°C to 400°C and duration prolongation obviously improved pore size, ethanol permeation performance and mechanical strength. Additionally, the test results revealed that the membrane sintered below virgin melting point (350°C) had a noticeable higher porosity but poorer ethanol permeation performance that could be primarily attributed to increased ratio of closed pore. The sintering condition exhibited evident influence on PTFE hollow fibre membrane thermal stability, though it showed no alteration to the thermal decomposition of PTFE. The obtained PTFE hollow fibre membrane was tested to evaluate their vacuum membrane distillation (VMD) performances. It was found that PTFE membrane from lower sintering temperature delivered a better salt rejection; on the other hand, the permeate flux was improved by increased vacuum pressure during VMD operation. [ABSTRACT FROM AUTHOR]

Polytetrafluoroethylene (PTFE) porous hollow-fiber membranes were prepared by a method involving billet preform, paste extrusion, stretching, and sintering. The influence of stretching parameters including stretching ratio, temperature, and speed was systematically investigated. The structure and properties of these membranes were characterized by field emission scanning electron microscopy, filtration performance, and mechanical strength test. The results indicate that the stretching ratio is the most significant influencing factor for the formation of various regions in the stress-strain curve. With increasing stretching ratio, the porosity, maximum pore size, and ethanol flux increase remarkably while bovine serum albumin rejection is reduced. These findings have the potential to facilitate PTFE membrane fabrication with finely tuned microporous structure for various application processes. [ABSTRACT FROM AUTHOR]

Nafion-PTFE hollow fiber composite membranes were fabricated and applied in vacuum membrane distillation (VMD) process to improve the anti-fouling and anti-wetting performance in the presence of oil and surfactant. The composite membrane exhibited an underwater super-oilphobic property according to the contact angle test. The VMD performance when treating saline feed containing 3.5 wt% NaCl and 1000 ppm mineral oil for the composite membrane behaved a stable flux for 40 h. In construct, an immediate and sharply-declined flux was observed for the pristine PTFE, indicating the significant improvement in the anti-fouling property. Meanwhile, the composite membrane also alleviated the wetting phenomena caused by the surfactant comparing with the pristine PTFE membrane, and the Nafion-PTFE composite membrane behaves a stable flux and high salt rejection higher than 99.99% for 60 h treating saline feed containing O/W emulsions. The pressure depression effect of the Nafion layer on the mass transfer was analyzed. It was found that the depression could be reduced with the increasing of the feed temperature and the enhancement of the corresponding effective pore diameter of the Nafion layer, that the pressure depression factor decreased from 0.23 to 0.04 with the feed temperature increasing from 60 °C to 75 °C. Image 1 • An asymmetric composite membrane with a thin Nafion layer on the outside of a PTFE membrane was fabricated for VMD. • The application of Nafion-PTFE membrane to VMD was first studied and reported. • The Nafion-PTFE composite membrane showed good anti-fouling and anti-wetting property. [ABSTRACT FROM AUTHOR]

Water condensation is a possible cause of membrane wetting in the operation of membrane contactors, especially under high-temperature conditions. In this study, water condensation in pores of polytetrafluoroethylene (PTFE) hollow fiber membranes was investigated during high-pressure CO2 absorption around 70 °C. It was found that the liquid accumulation rate in the treated gas knock-out drum was constant during continuous operation for 24 h when all experimental conditions were fixed, indicating a stable degree of membrane wetting. However, as the operating parameters were changed, the equilibrium vapor pressure of water within membrane pores could change, which may result in a condensation-conducive environment. Water condensation in membrane pores was detected and proven indirectly through the increase in liquid accumulation rate in the treated gas knock-out drum. The Hagen–Poiseuille equation was used to correlate the liquid accumulation rate with the degree of membrane wetting. The degree of membrane wetting increased significantly from 1.8 × 10−15 m3 to 3.9 × 10−15 m3 when the feed gas flow rate was reduced from 1.45 kg/h to 0.40 kg/h in this study due to water condensation in membrane pores. The results of this study provide insights into potential operational limitations of membrane contactor for CO2 absorption under high-temperature conditions. [ABSTRACT FROM AUTHOR]

• Membrane contactor was used as a gas-liquid separator in pressure swing process. • The two mass transfer mechanisms were analysed through calculation of viscous flux and mass transfer coefficients. • When integrated with membrane contactor for CO 2 absorption, stable and consistent performance was observed. A new method to regenerate high pressure CO 2 absorbent is presented in this paper, where PTFE hollow fiber membrane contactor was used as a gas–liquid separator in pressure swing process. CO 2 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 CO 2 loading in regenerated absorbent increased with absorbent flow rate. This was due to decrease in membrane area available for CO 2 desorption, as most of the membrane area was utilized for gas-liquid separation. Co-current configuration exhibited higher CO 2 desorption driving force compared to counter-current configuration, hence its CO 2 loading in regenerated absorbent was lower. When integrated with membrane contactor for CO 2 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 CO 2. [ABSTRACT FROM AUTHOR]

[Display omitted] • Hydrophobic FAS-PDMS/PEEK composite membranes were prepared by dip-coating method. • Honeycomb-like structures were observed dispersing on composite membrane surface. • The wetting resistance of membrane was improved after coating with FAS-PDMS layer. • Effects of gas and liquid flow velocities on CO 2 absorption performance were studied. • FAS-PDMS/PEEK composite membrane contactor exhibited good application potential. In this study, as an efficient technology membrane absorption was applied with newly developed polyetheretherketone (PEEK) hollow fiber membrane (HFM) to remove CO 2 from flue gas. To alleviate membrane wetting issue, 1H,1H,2H,2H-perfluorodecyltrimethoxysilane (FAS) was integrated into polydimethylsiloxane (PDMS) and then crosslinked FAS-PDMS was coated on porous PEEK membranes to prepare composite membranes. Surface characterization verified that PDMS layer containing fluoroalkylsilane had been successfully formed on membrane surface. As mass ratio of FAS to PDMS in coating solution raised, an interesting honeycomb-like structure was observed dispersing on composite membrane surface. Meanwhile, water contact angle was improved from 90.0° to 133.5° indicating enhanced membrane hydrophobicity. The effects of gas and liquid flow velocities on CO 2 absorption performance of FAS-PDMS/PEEK HFM were systematically studied. The highest CO 2 removal efficiency of 99.6% was achieved when gas and liquid velocity were 0.027 and 0.019 m s−1. When gas velocity raised to 0.110 m s−1, the maximum absorption flux of 1.22 × 10-4 mol m−2 s−1 and overall mass-transfer coefficient of 2.69 × 10−5 m s−1 could be obtained. Compared with the original membrane, the CO 2 absorption flux of FAS-PDMS-0.6 HFM was increased by 50.9%, and the performance had no degradation in 36 h test. This study provides a novel strategy to alleviate the membrane wetting problem during CO 2 membrane absorption process. [ABSTRACT FROM AUTHOR]

• A novel aqueous-organic membrane contactor process was applied. • The novel membrane contactor process was more favorable to treat low-ammonia-content feeds. • Accompanying water transport was avoided. Nafion-PTFE hollow fiber composite membranes were applied in the ammonia removal and recovery with a novel aqueous-organic membrane contactor (AOMC) process, in which an aqueous solution and an organic solution were separated by the composite membrane. A kerosene-diluted di-(2-ethylhexyl) phosphoric acid (D2EHPA) solution was used to receive the ammonia, which could be recycled and reused after regeneration by the stripping agent (a dilute solution of H 2 SO 4 , HNO 3 , or H 3 PO 4). The new-type membrane contactor can avoid the accompanying water transport in conventional membrane contactors. The effects of experimental factors including the pH, the velocity and the initial ammonia concentration of the feed on the overall mass transfer of the ammonia were investigated. Meanwhile, how the regeneration experiment parameters influenced the stripping ratio was also studied. The results showed that the AOMC process was more favorable to treat ammonia feed with low ammonia concentrations. The ammonia removal could be over 99% under certain conditions. Additionally, the ammonia was concentrated 26.4 times after four cycles. It was found that the accompanying water transport was non-negligible in the traditional aqueous-aqueous membrane contactor (AAMC) process when treating ammonia feed of low concentrations, with the maximum water flux two orders of magnitude higher than ammonia. [ABSTRACT FROM AUTHOR]

Polytetrafluoroethylene (PTFE) is considered as a promising material for membrane contactors due to its intrinsic hydrophobicity and excellent chemical stability. In most published works concerning CO 2 absorption, PTFE hollow fiber membranes with diameter larger than 1.2 mm even 2.0 mm were applied, which lost the advantage of membrane contactor in high specific surface area and also increased the mass transfer resistance. In this study, biogas purification by membrane contactor was systematically investigated based on the smaller-diameter PTFE hollow fibers (0.9 mm) developed recently in our laboratory. CH 4 –CO 2 (40%) mixture was used as simulated biogas, and deionized water was used as absorbent for all tests. The influence of module configuration, liquid flow pattern, membrane microstructure, and gas-liquid flow velocity on CO 2 removal performance were studied in detail. Moreover, the mass transfer resistance and CH 4 recovery rate were also discussed. CO 2 content in purified biogas could be below 3% by adjusting the operating parameters. CO 2 flux and overall mass transfer coefficient (K OV) under experimental conditions ranged from 0.61 × 10−3 to 1.35 × 10−3 mol/(m2. s) and from 1.67 × 10−5 to 6.34 × 10−5 m/s, respectively. Moreover, CO 2 removal performance was stable in an operation of 36 h. PTFE hollow fibers with smaller diameter exhibited preeminent CO 2 removal efficiency, which proved great application potential in biogas upgrading by membrane contactor. [Display omitted] • PTFE hollow fiber (diameter of 0.9 mm) for biogas upgrading by membrane contactor. • Weaving configuration design of membranes significantly improved mass transfer. • Effects of fluid flow pattern and membrane microstructure were also investigated. • Smaller-diameter PTFE membrane contactor exhibited good application potential. [ABSTRACT FROM AUTHOR]

• TR-polymer hollow fiber membranes with high flexibility were prepared. • Thermal densification was employed to induce selective skin layer. • Solvent exchange drying caused severer shrinkage than direct drying. • Coating can effectively obtain ultrathin skin and high selectivity. Flexible poly(benzoxazole-co-amide) (PBOA) gas separation hollow fiber membranes were successfully prepared deriving from ether-containing polyamide precursors by thermal densification with or without a supplementary coating treatment. Compared with polyimide precursors, thermally rearrangement in this work required lower treatment temperature but generated better mechanical properties. Both spinning process and thermal treatment conditions were systemically manipulated to intensify the membrane densification aiming at achieving greatly improved selectivity of the obtained TR-PBOA hollow fiber membranes. It was found that draft ratio had little contribution to selectivity enhancement. Separation performance was substantially improved when precursor fibers were dried using ethanol-hexane exchange method and then thermally treated at 350–390 °C TR hollow fibers obtained by combined thermal treatment with coating method exhibited the highest selectivity values in this study. [ABSTRACT FROM AUTHOR]