Polyvinylidene Fluoride Hollow Fiber Membranes Research Articles

Modeling of CO2 capture from gas stream emissions of petrochemical industries by membrane contactor

Process optimization of CO2 removal from natural gas by a polyvinylidene fluoride hollow-fiber membrane contactor is a major goal of many computational fluid dynamics (CFD) simulations in this area. In this study, a 2D CFD model based on mass transfer equation inside the tube, the membrane, and the shell section of a HMFC at steady state and laminar conditions is developed and solved by COMSOL Multiphysics with finite element approach. Simulation results show an excellent agreement with experimental data. The model predicts that higher liquid velocity and membrane porosity results in higher CO2 removal, because of enhancement of effective diffusion coefficient. Also, taller fiber length results in higher contact area and higher mass transfer of CO2 from natural gas into distilled water. Although higher temperature will decrease the CO2 removal.

Extraction Kinetics of As(V) by Aliquat-336 Using Asymmetric PVDF Hollow-Fiber Membrane Contactors.

This work focuses on the study of the mass transfer of arsenic(V) through asymmetric polyvinylidene fluoride hollow-fiber membrane contactors using Aliquat-336 as an extractant. In the first part of this work, the fibers were prepared and characterized by SEM and by determining their thickness and porosity. From SEM pictures, an asymmetric structure was obtained that was characterized by an inner sponge-like structure and outer finger-like structure with a pore radius and porosity about 0.11 µm and 80%, respectively. In the second part, the prepared fibers were used as membrane contactors for the study of mass transfer of arsenic(V), investigating the effect of several parameters such as pH, temperature, and initial concentration of the feed. The overall mass transfer coefficient of As(V) was around 6 × 10–6 cm/s.

The Effect of Diluent Mixture with Upper Critical Solution Temperature on Membrane Formation Process, Microstructure, and Performance of PVDF Hollow Fiber Membrane by TIPS Process.

Thermally induced phase separation (TIPS) is a technique to prepare commercial membrane. However, the quick polymer crystallization during the quenching process will bring about a dense and thick skin layer and thus decrease permeability markedly. In this paper, a diluent mixture with upper critical solution temperature (UCST) was used to prepare polyvinylidene fluoride (PVDF) hollow fiber membrane. That is, the separation between diluent (propylene carbonate (PC)) and non-diluent (dioctyl terephthalate (DOTP)) occurred during the quenching process when the temperature of the dope was lower than 110 °C. The effects of separation between PC and DOTP and the resulting coalescence of DOTP on the PVDF crystallization process, microstructure, and the permeability of the membranes were analyzed. The results showed that the suitable PC/DOTP weight ratio reduced the thickness of the skin layer near the outer surface markedly and resulted in a porous outer surface, and the microstructure evolution process was proposed. The maximum pure water flux for the prepared membrane is up to 128.5 L·m−2·h−1 even in a dry mode without using a hydrophilizing agent. The rejection rate of the carbonic particle is nearly 100%. This study presents a novel and simple way to fabricate the microporous membrane with the interconnected pore structure.

Enhancement of the crystallinity and surface hydrophilicity of a PVDF hollow fiber membrane on simultaneous stretching and coating method

A polyvinylidene fluoride hollow fiber membrane (PVDF membrane) was prepared by thermally induced phase separation (TIPS) and had low orientation and weak tensile strength. A hydrophobic PVDF membrane composed of a semi-crystalline structure was physically modified to improve the crystallinity degree and tensile strength values, and this was achieved by increasing the orientation of the PVDF polymer chains from the α-phase to the β-phase. PVDF membranes coating were simultaneously applied with the crosslinked poly (vinyl alcohol) (PVA) solution to increase water flux and fouling resistance using sonication during the stretching process.

Effect of the molecular weight and its distribution of polyvinylidene fluoride on the relationship between the spinning process, microstructure and properties of hollow fiber membranes via thermally induced phase separation

Polyvinylidene fluoride (PVDF) is an important material in the preparation of ultrafiltration membranes via the thermally induced phase separation (TIPS) method. In this paper, four PVDF hollow fiber membranes with different molecular weights were prepared via the TIPS method by using dibutyl phthalate and dioctyl phthalate as a mixed diluent. The relationship between the molecular weight of PVDF and its distribution, phase separation, crystallization behavior and spinning process has been systematically studied. The effects of three factors on the microstructure and properties of the PVDF membrane have been analyzed. The flow behaviors of the PVDF/diluent and PVDF melt were tested by a capillary rheometer and a melt flow rate instrument, respectively. A phase diagram of the membrane solution was determined by thermal polarizing microscope and differential scanning calorimetry. The crystallization behavior and angle of orientation of the membrane were tested by using a differential scanning calorimeter and a sound velocity orientation measurement instrument. The microstructures, such as the pore structure and crystalline grain structure, were observed by field emission scanning electron microscopy. Meanwhile, the properties of the membrane were examined from the view of water flux, porosity and tensile testing. The results showed that changes in the polymer molecular weights affected not only the dynamics but also the thermodynamics of phase separation in membrane formation. As the PVDF molecular weight increased, the phase separation region increased, but the membrane structure became denser. A wide molecular weight distribution easily produced large pores. Then, the water flux decreased first and then increased.

Anti-fouling characteristic of carbon nanotubes hollow fiber membranes by filtering natural organic pollutants

Membrane fouling is a universal problem for conventional membrane filtration that usually causes a deterioration in membrane performance. We used electro-assisted carbon nanotubes hollow fiber membranes (CNTs-HFMs) to investigate the anti-fouling properties using natural organic pollutants. Benefiting from the electro-assistance, the permeation flux of humic acid solution using CNTs-HFMs was 190.20 L/(m2·h·bar), which was about 1.5- and 4.4-times higher than those of CNTs-HFMs without electro-assistance and traditional polyvinylidene fluoride hollow-fiber membranes (PVDF-HFMs). And the permeation fluxes of bovine serum albumin, sodium alginate and supernatant of anaerobic bioreactor also presented similar results. The average COD removal rate of CNTs-HFMs (66.8%) at −1.0 V was higher than that of CNTs-HFMs without electro-assistance and PVDF-HFMs, which can be attributed to the formation of electrostatic repulsive force. It could reduce the deposition of pollutants on membrane surface under electroassistance.

Novel tri-bore PVDF hollow fiber membranes for the control of dissolved oxygen in aquaculture water

Novel tri-bore hollow fiber membranes have been developed from polyvinylidene fluoride (PVDF) for the control the dissolved oxygen (DO) in aquaculture denitrification process. The fabricated hollow fibers are characterized in terms of morphology, porosity, hydrophobicity and mechanical strength. Two membrane modules, each including 200 pieces of hollow fibers, are connected in series or parallel in order to determine the optimum operation mode. The deoxygenation test is firstly conducted for DI water and then for aquaculture water. Various methods including water flushing, air blowing or chemical cleaning have been applied to assure the cleaning efficiency after membrane fouling. A mathematical model has been developed by using the resistance-in-series concept by taking into account boundary layer and membrane characteristics. Overall mass transfer coefficient, radial and axial concentration profiles, and molar flux of oxygen at different water flow rates are calculated. This work has demonstrated that the developed tri-bore hollow fiber membranes are applicable for the control of dissolved oxygen in aquaculture water. The observations have provided solid evidence for the development of membrane-based denitrification system for recirculating aquaculture system (RAS).

Novel tri-bore PVDF hollow fiber membranes for the control of dissolved oxygen in aquaculture water

Novel tri-bore hollow fiber membranes have been developed from polyvinylidene fluoride (PVDF) for the control the dissolved oxygen (DO) in aquaculture denitrification process. The fabricated hollow fibers are characterized in terms of morphology, porosity, hydrophobicity and mechanical strength. Two membrane modules, each including 200 pieces of hollow fibers, are connected in series or parallel in order to determine the optimum operation mode. The deoxygenation test is firstly conducted for DI water and then for aquaculture water. Various methods including water flushing, air blowing or chemical cleaning have been applied to assure the cleaning efficiency after membrane fouling. A mathematical model has been developed by using the resistance-in-series concept by taking into account boundary layer and membrane characteristics. Overall mass transfer coefficient, radial and axial concentration profiles, and molar flux of oxygen at different water flow rates are calculated. This work has demonstrated that the developed tri-bore hollow fiber membranes are applicable for the control of dissolved oxygen in aquaculture water. The observations have provided solid evidence for the development of membrane-based denitrification system for recirculating aquaculture system (RAS).

Preparation and characterization of SiO2/PDMS/PVDF composite membrane for phenols recovery from coal gasification wastewater in pervaporation

Silica (SiO2)/polydimethylsiloxane (PDMS)/polyvinylidene fluoride (PVDF) composite membranes were prepared through a dynamic negative pressure method for phenols recovery from coal gasification wastewater (CGW) in pervaporation process. Multiple techniques including scanning electron microscopy (SEM), water contact angle (WCA) and fourier transform adsorption spectrum (FTIR) were used to examine the surface morphology, hydrophobic properties and functional groups of the modified PVDF hollow fiber membranes, respectively. The effects of SiO2 concentration, coating time and coating pressure on pervaporation performance were systematically studied. The operating conditions obtained are as follows: 12wt.% SiO2, coating time 60min, coating pressure 50kPa. Under the optimum conditions, the phenols flux and separation factor were obtained as 6.55gm−2h−1 and 2.59, respectively. Results show that separation factor in pervaporation process based on 12wt.% SiO2 filled PDMS/PVDF composite membranes was about 2.5 times higher than that of the PDMS/PVDF membrane.

Autopsy study of irreversible foulants on polyvinylidene fluoride hollow-fiber membranes in an immersed microfiltration system operated for five years

Membrane autopsies were performed to understand causes of irreversible fouling on fouled membranes that were used for five years to treat surface water. The results of the autopsies of the membranes taken from the 1st and 2nd stages, i.e., a dual stage configuration to obtain 99% water recovery, showed that fouling layers occurred in both stages and severe fouling of the 2nd stage membrane was obvious. The severe flux decline in the 2nd stage illustrated the importance of the feed water quality. Compositional analyses by scanning electron microscopy with energy dispersive X-ray spectroscopy noted a great increase in the amount of Al on the 2nd stage membrane. Infrared spectra revealed that carboxylic acids, polysaccharide, and aromatic compounds originating from the feed water remained in the membrane. This study suggests that coagulation pretreatment should be implemented with great care because large amounts of Al remain on the membrane surface. In addition, cleaning should be performed in response to both organic and inorganic foulants. The fouling layer on the membrane captured the substantial amounts of inorganics such as Al in addition to organic foulants such as polysaccharide/protein-like compounds.

Geothermal Membrane Distillation in Industrial Greenhouse Applications: Membrane Fabrication and Characterization

Abstract Membrane distillation is reported for the first time in the literature being used to desalinate geothermal brackish groundwater for industrial greenhouse applications. Asymmetric hollow fiber membranes were fabricated and characterized for their direct contact membrane distillation (DCMD) application to provide irrigation water at Masson Greenhouse, New Mexico. Membranes exhibited an asymmetric configuration with an external sponge layer and an internal macrovoid layer. The intention was to prevent pore wetting from the hot saline fluid with the sponge layer, while using the macrovoid layer to increase the water flux and thermal efficiency of the membrane. A water flux of 55 liters/m2-h (LMH) with a salt rejection greater than 99.99% was observed for the fabricated polyvinylidene fluoride (PVDF) hollow fiber membranes at a water vapor pressure gradient of 3.2 MPa/cm (shell-side inlet temperature of 80°C). Further investigation, however, revealed that pore wetting existed in the macrovoid layer at...

The unidirectional regulatory role of coagulation bath temperature on cross-section radius of the PVDF hollow-fiber membrane

In this paper, the low-temperature thermally induced phase separation (L-TIPS) method was adopted to prepare a high-performance polyvinylidene fluoride (PVDF) hydrophobic membrane for membrane distillation (MD) with non-solvent polyethylene glycol-400 (PEG400) and solvent triethyl phosphate as a fully water-soluble mixed diluent. In the process of L-TIPS membrane formation, the unidirectional regulatory role of coagulation bath temperature on cross-section radius of the PVDF hollow-fiber membrane was analyzed. The results show that the tensile strength of the membrane increases with the increase of the external coagulation bath water temperature, which is the opposite of the general rule. PEG400 is employed to regulate the membrane structure, and more specifically to increase the cellular structure (a positive impact on tensile strength) and reduce the sphere-packed aggregation structure (a negative impact on tensile strength). When the coagulation bath temperature is 60°C, the tensile strength of the membrane reaches 6.21MPa (more than 2–4 times higher than the hydrophobic membranes reported). Meanwhile, the PVDF hollow-fiber hydrophobic membrane also has a high flux, the vacuum membrane distillation flux reaches 30.6kgm−2h−1, which is 14.6 times higher than that at the coagulation bath temperature of 15°C.

Optimization of hydrophobic modification parameters of microporous polyvinylidene fluoride hollow-fiber membrane for biogas recovery from anaerobic membrane bioreactor effluent

Tailoring the hydrophobic properties of membranes is an important requirement in gas-liquid membrane contactor processes because the membrane can be wetted by liquid. This paper describes the hydrophobic modification of the chemical surface of a polyvinylidene fluoride hollow-fiber membrane using a commercial perfluoropolyether, Fluorolink S10. The contact angle of the modified membrane was influenced by the modification parameters, and hence parameter optimization was required to obtain a highly hydrophobic membrane. The sodium hydroxide concentration (pH 8, 10 and 12), dehydrofluorination time (30, 60 and 90min), and chemical solution grafting time (30, 60 and 90min) were identified as the control parameters of this modification and were optimized using the Taguchi approach. The optimized membrane was then applied to recover methane from anaerobic membrane bioreactor effluent and benchmarked with a commercial polypropylene membrane manufactured for degasification of water. The contact angle of the polyvinylidene fluoride membrane was improved by 32% after the surface modification (under the optimal modification condition indicated by the Taguchi method). The membrane mass transfer coefficient was measured as 1.53 × 10−4m/s. Our membrane showed better methane recovery performance than the commercial polypropylene membrane and a very stable flux without pore wetting during 10 days of operation with synthetic effluent (made by saturating a 60:40 methane/carbon dioxide mixture into tap water). In addition, the membrane prepared in this study could be operated without significant fouling (7% flux drop during 8 days of operation) with real anaerobic membrane bioreactor effluent.

CO2 Absorption with NaOH Solution through the Porous PVDF Hollow Fibre Membrane Contactor

In this study, porous hydrophobic polyvinylidene fluoride (PVDF) hollow fiber membranes were fabricated via a wet phase inversion process. In order to improve the phase inversion rate and provide porous membranes, 4 wt.% lithium chloride (LiCl) was used in the spinning dope. The prepared membrane morphology was studied using field emission scanning electron microscopy (FESEM). Chemical CO2absorption by NaOH solution (1M) was conducted through the PVDF hollow fiber membrane contactor. The effect of the main operating condition such as absorbent temperature, CO2 pressure and absorbent flow rate on the performance of CO2 absorption was investigated. From FESEM examination, the membrane possesses an almost sponge–like structure with ultra thin skin layer. Results of CO2absorption test showed that by increasing the absorbent flow rate the CO2 flux increased which confirmed the existence of liquid side mass transfer resistance. It was found that by increasing the absorbent temperature the CO2 flux considerably improved. Meanwhile, the effect of CO2 pressure on the absorption rate was insignificant. Therefore, it can be concluded that by applying a porous hydrophobic membrane with improved structure and optimizing the operating conditions, high CO2 removal efficiency can be achieved through gas–liquid membrane contactors

Using green solvent, triethyl phosphate (TEP), to fabricate highly porous PVDF hollow fiber membranes for membrane distillation

While membrane technology continues to thrive in seawater desalination, it would defeat the very purpose of saving the environment if toxic solvents are involved in membrane manufacture. As a paradigm of green chemistry, a non-toxic solvent, triethyl phosphate (TEP), was used for the first time to fabricate polyvinylidene fluoride (PVDF) hollow fiber membranes for direct contact membrane distillation (DCMD). The phase inversion kinetics of the PVDF/TEP system was closely investigated and compared with the conventional N-Methyl-2-pyrrolidone (NMP)/PVDF system. The former showed a less abrupt phase inversion rate and produced a more porous sponge-like structure than the latter. The 12/88wt% PVDF/TEP binary solution produced fibers with promising performance. They not only possessed robust mechanical properties and a liquid entry pressure up to 2.0bar but also exhibited an average flux of 20kg/m2·h at 60°C and a NaCl rejection of 99.99%. In addition, hollow fiber membranes spun from this PVDF/TEP system had porosity of greater than 83% for all conditions studied. Since there were no additives or non-solvents in the dope solutions and no post-treatments involved, the use of TEP as a green solvent could significantly reduce the complexity of membrane fabrication, scale up and commercialization. Clearly, the much safer solvent TEP is able to replace toxic solvents commonly used in membrane manufacture and to produce membranes with highly competitive performance.

Effect of Polyethylene Oxide Concentration on Blood Compatibility of PVDF Hollow Fiber Plasma Separation Membranes

The anticoagulant properties of commonly PES/PVP plasma separation modified membrane materials are not ideal enough. Based on that, in this paper, a newly developed plasma separator made of polyvinylidene fluoride (PVDF) hollow fiber membranes was prepared by dry/wet phase inversion method with blended hydrophilic macromolecular polyethylene oxide (PEO). The effect of PEO concentration was evaluated for its blood compatibility. The results showed that the anticoagulant properties of PVDF/PEO modified membrane were more excellent than the PES/PVP modified membrane. And attenuated total reflection flourier transformed infrared spectroscopy (ATR-FTIR) analysis confirmed that the PEO macromolecular additives remained within the PVDF modified membrane, which could effectively improve hydrophilicity of the membrane.

Fabrication of a super-hydrophobic polyvinylidene fluoride hollow fiber membrane using a particle coating process

Membranes with outstanding anti-wetting property are essential for membrane distillation (MD) applications. In this work, a hydrophobization treatment for porous polyvinylidene fluoride (PVDF) hollow fiber membranes using an etching method combined with an ultrafiltration coating process was developed to achieve this desired property. PVDF particle was subjected to an etching process to coarsen its surface. Etched PVDF particles were subsequently dispersed in a dispersant with a specific PVDF-dissolving capacity and then coated onto the PVDF hollow fiber membrane surface via ultrafiltration to construct a super-hydrophobic surface with a micro/nanostructure. After this process, the super-hydrophobic PVDF hollow fiber membrane with the surface contact angle of 163.8°was obtained, and the coating layer on the membrane surface was exceedingly stable. Moreover, the membrane pores were not blocked by the coating layer. The optimal etching conditions for the PVDF particles were as follows: solubility parameter of the etchant, 25.6 (J/cm3)1/2; etching time, 60min at 25°C. A super-hydrophobic surface with a stable particles coated layer is achieved in an optimal coating condition (solubility parameter of the dispersant, 25.87 (J/cm3)1/2; PVDF particles coated, 18.0g/m2). The direct contact membrane distillation (DCMD) experiments showed that after the surface modification process, the hydrophobicity of the membrane was significantly enhanced. In addition, the anti-wetting property of the membrane improved, the time for membrane to be wetted through the channel increased from 40 to 180min, the pure water DCMD flux increased from 26.0 to 29.9kgm−2h−1, and the critical wetting depth increased from 19.5 to 35.7µm.

Improved ultrafiltration performance and chlorine resistance of PVDF hollow fiber membranes via doping with sulfonated graphene oxide

Sulfonated grapheme oxide (SGO) nanohybrids were synthesized and employed to modify polyvinylidene fluoride (PVDF) hollow fiber ultrafiltration membranes accompanied with perfluorosulfonic acid (PFSA). Due to the synergism effects of PFSA and SGO, the SGO-blended PVDF membranes exhibit high permeability, anti-fouling performances and improved chemical resistance. SGO addition leads to thinner skin and porous sublayer of the membranes. The pure water permeability increases from 87.8 to 174.2L·m−2·bar−1·h−1 by adding 0.5wt% SGO nanohybrids in dope, and then slightly decreases to 156.2L·m−2·bar−1·h−1 for M1.0 with further addition to 1.0wt%. After five fouling-cleaning cycles, SGO-modified PVDF membranes show less flux decline and more stable solute rejection than the unmodified one, which demonstrate that SGO nanosheets act as a chlorine barrier for the PVDF membranes, resulting in an outstanding anti-fouling ability and an effective suppression of the membrane degradation under frequent oxidative cleaning.

Effect of Swelling on Performance of Surface-Imprinted Composite Membranes

Molecularly imprinted composite membranes (MIMs) are developed by thermally initiated copolymerization on the surface of polyvinylidene fluoride (PVDF) hollow fiber membranes for the selective binding to levofloxacin. The prepared membranes, before and after surface polymerization, are characterized by SEM and FT-IR. The water flux, swelling changes and selectivity of the membranes are evaluated. The highest specific separation factor of about 1.27 is achieved for MIMs to separate levofloxacin and ofloxacin. The results show that imprinted polymerization has obvious influence on the performance of membranes and swelling results in the decrease of specific recognition.

The Effect of Membrane Material and Surface Pore Size on the Fouling Properties of Submerged Membranes

We aimed to investigate the relationship between membrane material and the development of membrane fouling in a membrane bioreactor (MBR) using membranes with different pore sizes and hydrophilicities. Batch filtration tests were performed using submerged single hollow fiber membrane ultrafiltration (UF) modules with different polymeric membrane materials including cellulose acetate (CA), polyethersulfone (PES), and polyvinylidene fluoride (PVDF) with activated sludge taken from a municipal wastewater treatment plant. The three UF hollow fiber membranes were prepared by a non-solvent-induced phase separation method and had similar water permeabilities and pore sizes. The results revealed that transmembrane pressure (TMP) increased more sharply for the hydrophobic PVDF membrane than for the hydrophilic CA membrane in batch filtration tests, even when membranes with similar permeabilities and pore sizes were used. PVDF hollow fiber membranes with smaller pores had greater fouling propensity than those with larger pores. In contrast, CA hollow fiber membranes showed good mitigation of membrane fouling regardless of pore size. The results obtained in this study suggest that the surface hydrophilicity and pore size of UF membranes clearly affect the fouling properties in MBR operation when using activated sludge.