Bore Fluid Composition Research Articles

Preparation of PVDF hollow fiber ultrafiltration membrane via phase inversion/chemical treatment method

Poly(vinylidene fluoride) (PVDF) hollow fiber membranes were prepared via a phase inversion/chemical treatment method, using N,N-dimethylacetamide (DMAc) as solvent, Calcium carbonate as inorganic additive, and water and HCl as coagulation media. Effects of external coagulation bath and bore fluid composition on membrane performance were investigated by using scanning electron microscopy, mechanical properties, porosity, average bubble point pressure, and permeation performance. The results show that: (1) The cross-section morphology of membranes have no significant change, exhibiting a two-layer finger-like structure extended to the middle of the cross-section. (2) The roughness of the external surface increased considerably and the number of the surface pores increased as well when we introduced HCl as the second coagulation. The permeation experiments reveal that acid treatment had no impact on fibers which had precipitated completely. (3) The number and pore size underlying the inner skin layer increased when bore fluid changed from DI water to 10% acetic acid, leading to the decrease of porosity and tensile strength. Moreover, the pure water flux increased from 360 to 580 L/(m2 h) without the significant change of BSA rejection. The PVDF/CaCO3 membranes prepared from the dope with 10% acetic acid as the bore fluid and 1 mol/L HCl as second coagulation bath had the best ultrafiltration performance.

Effect of bore fluid composition on structure and performance of asymmetric polysulfone hollow fiber membrane contactor for CO2 absorption

Porous asymmetric polysolfone (PSf) hollow fiber membranes were fabricated using wet spinning process. Glycerol (4wt.%) was used as additive. Aqueous solutions of 1-methyl-2-pyrrolidone (NMP) with different composition (0, 50, 70 and 90wt.% NMP) were used as bore fluid. Distilled water was used to preparing aqueous NMP solutions. Structure and performance of resulting membranes in measuring of surface porosity, pore size, critical water entry pressure (CEPw) and CO2 absorption in hollow fiber membrane contactor were investigated. The cross-section, the inner skin layer and the inner surface of the membranes were examined via scanning electronic microscopy (SEM). Using distilled water as bore fluid provided the membrane structure with a finger-like macrovoids which extend from the inner and outer surfaces of the membranes to the middle of the hollow fiber wall. Other hollow fiber membranes formed with a finger-like structure near the other surface and a sponge-like structure with holes beneath, which resulted in a high CEPw and CO2 absorption rate. In addition, since a mixture of distilled water and NMP was used as the neutral bore fluid, the membranes showed a skinless inner surface. Results of gas permeation tests showed with increasing NMP concentration in the bore fluid the N2 permeance decrease significantly and the wetting resistance decrease slightly. CO2 absorption by distilled water was conducted through the gas–liquid membrane contactors. By employing 90% NMP in the bore fluid, the PSf membrane showed a CO2 flux of approximately 110% higher than the PSf membrane fabricated with distilled water as the internal coagulant at the absorbent velocity of 7.9×10−4m/s.

Preparation of poly (vinyl butyral) hollow fiber ultrafiltration membrane via wet-spinning method using PVP as additive

Polyvinyl butyral (PVB) hollow fiber membranes were prepared via wet-spinning method using polyvinylpyrrolidone (PVP) as additive. The effects of PVP and PVB concentration, bore fluid composition on the membrane formation were investigated by viscosity, light transmission, scanning electron microscopy (SEM) and ultrafiltration respectively. Results revealed that PVB and PVP have a good miscibility. Addition of PVP mainly changed the kinetic property of the dope, such as the increased viscosity, the delayed demixing, etc. The cross-sections of the resulting membranes showed a similar asymmetric structure. Ultrafiltration showed that the rejection of membrane increased, but the flux firstly increased then decreased with an increase in PVP concentration. The low PVB concentration in the dope caused a loose cross-sectional morphology, which improved the water flux of membrane. Moreover notable difference occurred at the internal surface of PVB hollow fiber membrane when N, N-dimethulacetamide (DMAc) was added to the bore fluid. As a result, membrane exhibited a high flux with no significant change in retention performance.

Morphology control of the perovskite hollow fibre membranes for oxygen separation using different bore fluids

La 0.6Sr 0.4Co 0.2Fe 0.8O 3−α (LSCF6428) perovskite hollow fibre membranes were fabricated through a phase inversion/sintering technique. The effects of the internal coagulant (bore fluid) compositions on the morphology of the resultant hollow fibres were systematically investigated. The bore fluids were specially designed to vary the solvent–nonsolvent exchange rates in the phase inversion stage by mixing different polymer solvent (1-methyl-2-pyrrolidinone–NMP) content inside the water or ethanol internal coagulant. The prepared hollow fibre membranes were characterized with scanning electron microscopy (SEM), porosity and bending strength measurements, gas-tightness examination and oxygen permeation tests at high temperatures. The results indicate that the bore fluid composition has dramatic effects on the microstructure of the resultant hollow fibre membranes. With the gradual increase of NMP concentration in the bore fluids, the membrane morphology evolution from multi-dense layers to one single dense layer has been clearly demonstrated. LSCF6482 hollow fibre membranes with one dense layer near the outside surface integrated with the highly porous structure showed the highest oxygen fluxes with a maximum value of 34.4 mmol m −2 s −1 under air/He gradient at 1000 °C. The relationships between bore fluid composition and membrane morphology change and their significant effects on oxygen permeation have been discussed extensively. This work shows the feasibility of achieving ceramic hollow fibre membranes with different architectures for various purposes in one single step simply by adjusting the bore fluid composition.

Sublayer structure and reflection coefficient and their effects on concentration polarization and membrane performance in FO processes

We have demonstrated through experimental and theoretical study that membrane sublayer structure has significant influence on the performance in forward osmosis (FO) processes. Cellulose acetate (CA) hollow fiber membranes with different sublayer structures have been fabricated by varying the bore fluid composition during dry-jet wet spinning and then characterized in terms of pore structure of the selective layer, porosity of the sublayer, pure water permeability (PWP) coefficient, salt permeability coefficient and salt rejection. The water flux and reverse salt flux of these membranes are evaluated in the pressure-retarded osmosis (PRO) mode and FO mode. It is observed that varying the sublayer structure does not have apparent influence on the FO performance under the PRO mode but has significant influence on the performance under the FO mode. The different performance under different operation modes is resulted from more severe concentration polarization (CP) in the FO mode. The characteristics of the membrane sublayer structure are the origin of CP phenomenon in the FO mode. Through NF tests and FO tests with low draw solution concentrations on one of the membranes, the reflection coefficients ( σ 0) are determined as 0.77 and 0.99 for NaCl and MgCl 2 draw solutes, respectively. A low reflection coefficient indicates a low effective driving force in FO processes resulted from serious leakage of draw solutes from the draw solution to the feed. Thus, the reflection coefficient cannot be assumed to be equal to 1 and its significance must be considered when modeling FO performance using NaCl as the draw solute. The theoretical study reveals that the sublayer porosity, tortuosity and thickness have great impact on membrane performance when the draw solution flows against the sublayer. The desired FO membrane should have no sublayer. Since the phase inversed membrane may always have a sublayer, the preferred sublayer has high porosity, low tortuosity and small thickness. However, FO membranes for water reuse may have less structure requirements than those for seawater desalination.

Molecular design of the morphology and pore size of PVDF hollow fiber membranes for ethanol–water separation employing the modified pore-flow concept

In this study, we have established the fundamental science and engineering of fabricating poly(vinylidene fluoride) (PVDF) asymmetric hollow fiber membranes for ethanol–water separation and elucidated the complicated relationship among membrane morphology, pore size, pore size distribution and separation performance using the concept of the modified pore-flow model proposed in our previous work. The variation of bore-fluid composition, air-gap distance and take-up speed results in membranes with various morphologies ranging from large-finger-like macrovoid to nearly perfect macrovoid-free structures. Interestingly, an increase in air-gap distance or take-up speed not only effectively suppress the formation of macrovoids but also results in the reduction of membrane pore size and the narrowing of pore size distribution, hence leading to the enhancement of membrane performance. The permeation flux is found to be mainly controlled by the overall porosity and the contribution of large pore sizes of the membrane, while the selectivity or separation factor is greatly determined by membrane pore size and pore size distribution, which is consistent with the modified pore-flow model proposed in our previous works. The newly developed PVDF asymmetric hollow fiber membranes demonstrates remarkable high fluxes of 3500–8800 g m −2 h −1 and reasonable ethanol–water separation factors of 5–8 compared to existing polymeric-based pervaporation membranes.

Novel rectangular membranes with multiple hollow holes for ultrafiltration

We have demonstrated, for the first time, rectangular membranes with multiple hollow holes, a hybrid of flat sheet membranes and hollow fibers, have been successfully produced for ultrafiltration applications. The spinning process was carried out with a specially designed spinneret containing a rectangular slit for the polymer solution and seven injectors for the bore fluid. The experimental results show that the membrane microstructure, outer surface contour and shapes of lumen holes strongly depend on the spinning conditions such as dope formulation, bore fluid composition, air-gap distance and external coagulants. The evolution of membrane morphology as a function of spinning conditions has been elucidated by light transmittance. By carefully tuning the spinning parameters, polyacrylonitrile (PAN) rectangular membranes with a pure water permeation flux of 238 L/m 2 h bar for ultrafiltration have been manufactured. This performance is comparable to some literature works when only simple binary dope formulation is considered and no modifications are applied to the membrane. Compared to hollow fiber and flat sheet membranes, rectangular membranes do show a combination of the characteristics in terms of mechanical strengths.

Elimination of die swell and instability in hollow fiber spinning process of hyperbranched polyethersulfone (HPES) via novel spinneret designs and precise spinning conditions

This study has successfully demonstrated that a proper combination of novel spinneret designs and spinning parameters can effectively counteract the die swell as well as flow instability phenomena, i.e. extrudate distortion, in the hyperbranched polyethersulfone (HPES) hollow fiber spinning. Attempts are also made to unravel the die swell and flow behavior differences between HPES and linear polyethersulfone (LPES) membranes spun using various spinneret designs and spinning conditions. In terms of flow stability, it is revealed that short conical spinnerets with a flow angle of 60° as well as short round flow channel spinneret with a flow angle of 30°, can reduce or eliminate extrudate distortions. Apart from spinneret designs, this study also accentuates the importance of a proper choice of spinning conditions for each specific spinneret to achieve flow stability and reduce die swell, namely: (1) bore fluid composition; (2) dope flow rate; (3) spinning temperature; and (4) take-up speed. Experimental results concluded that a proper combination of spinneret design and these four spinning parameters is the key to stabilize the spinning process. It is found that a high take-up speed spinning and a high non-solvent concentration in the bore fluid can fully eliminate die swell and enhance flow stability in the HPES hollow fiber spinning using short and conical or round spinnerets.

Factors affect defect-free Matrimid® hollow fiber gas separation performance in natural gas purification

Asymmetric hollow fibers have increasingly become attractive for high volume gas separation applications including natural gas purification. The formation of asymmetric structure with ultrathin defect-free selective layer and mechanically robust porous supporting layer without additional coatings is one of the major challenges. In this study, a commercially available polyimide material Matrimid® was used as the benchmark polymer material in fabrication of hollow fibers for natural gas purification. Major fabrication process variables including polymer solution formulation, bore fluid composition, dope and bore flow rates, and air-gap length were systematically investigated and optimized for fabrication of hollow fibers to achieve high CO2/CH4 separation performance. SEM images of cross-section indicated asymmetric structure of hollow fiber with a skin layer and sponge-like supporting structure. Hansen's solubility parameters were calculated for different solvent systems and coagulation medium, and the predicted trends of their effect to the formation of skin layer and gas separation performance were consistent with the experimental observation. High CO2/CH4 separation factors (ranging up to 67) are achieved which are among the highest reported for purely polymeric hollow fibers without post-treatments and exceeded the commonly reported bulk values of Matrimid®.

Preparation of poly(lactic acid) hollow fiber membranes via phase separation methods

Poly(lactic acid) (PLA) is attracting a lot of attention as a bio-based polymer derived from plant resources. A hollow fiber membrane made from PLA is an alternative to the conventional hollow fiber membrane of petrochemical polymers. In this study, we succeeded in the preparation of a PLA hollow fiber membrane using phase separation method. PLA hollow fiber membranes with various structures were prepared by varying the preparation conditions (bore fluid composition and additive). The water permeability of the hollow fiber membrane increased as the DMSO content in the bore fluid and the PEG content in the PLA solution increased. The investigation of protein (BSA) rejection revealed that PLA hollow fiber membranes exhibited ultrafiltration performance. We found that controlling the experimental conditions allowed two different types of phase separation (liquid–liquid phase separation and solid–liquid phase separation) in the preparation of a PLA hollow fiber membrane, resulting in the variety of the microporous structures of the hollow fiber membrane.

Fabrication of poly(vinylidene fluoride- co-hexafluropropylene) (PVDF-HFP) asymmetric microporous hollow fiber membranes

In the present work, poly(vinylidene fluoride- co-hexafluropropylene) (PVDF-HFP) asymmetric microporous hollow fiber membranes were successfully fabricated using nonsolvent-induced phase inversion method which has not been reported previously. Analyses of thermodynamic characteristics of the PVDF-HFP/NMP/water system without/with PVP as an additive and the rheological property of the spinning dopes were carried out as a first step. Subsequently, the effects of spinning conditions (wet spinning, air gap, bore fluid composition), dope concentration and PVP addition on the resultant hollow fiber membranes were studied in terms of membrane morphology, structure, pure water flux and molecular weight cut-off (MWCO) as well as their hydrophobicity and mechanical properties. It was found that the wet spinning process produced hollow fibers with irregular inner contour under present experimental conditions. The strong viscoelastic property of PVDF-HFP material was thought to be associated with this phenomenon. A partial release of accumulated back stress in the nascent fibers might have caused the macromolecules to move inward in the transverse direction. Increasing the air gap could eliminate this inner contour deformation, as the outer surface of the nascent fiber was not vitrified prior to entering the water bath, thus allowing the orientated macromolecules to be relaxed. In addition, the nascent fiber is generally expected to experience an elongational stretch over the distance of the air gap, which might help to smooth the inner surface. Membranes made from the PVDF-HFP/NMP dopes were found to have small pure water fluxes no matter what the polymer concentration, the bore fluid composition and the air gap were used in the spinning processes. The pure water flux can be increased by adding PVP into the dope solution, up to 5 wt.% PVP in the dope can promote the formation of macrovoids in the membrane. When the PVP content was increased to 10 wt.%, the macrovoids were suppressed, but the membrane still presented a higher porosity, which was reflected from the significant increase in the pure water flux from 60 to 107 L/(h m 2 atm). In addition, PDVF-HFP hollow fiber membranes were more hydrophobic than those membranes made from pure PVDF, the former also exhibited pronounced ductile behavior when compared with the latter.

Investigation of corrugation phenomenon in the inner contour of hollow fibers during the non-solvent induced phase-separation process

In this paper by proposing a novel mechanism, we reveal one of the most controversial issues in the hollow fiber fabrication process regarding the instability leading to the deformed cross-section of fibers fabricated through non-solvent induced phase separation. We have analyzed possible instability mechanisms based on our experimental observations and then postulated that the principal instability occurs in the external coagulation bath where the pressure induced in the nascent fiber outer layer as a result of diffusion/convection, precipitation, densification and shrinkage buckles the rigid precipitated polymer shell in the dope and bore fluid interface. In addition, the effect of some spinning conditions such as air-gap distance, bore fluid composition, take-up speed, external coagulant and dope concentration on the final shape of the fiber cross-section have been investigated. The proposed mechanism is in good qualitative agreement with all our observations.

PEEKWC ultrafiltration hollow‐fiber membranes: Preparation, morphology, and transport properties

AbstractA series of hollow‐fiber membranes was produced by the dry–wet spinning method from PEEKWC, a modified poly(ether ether ketone) with good mechanical, thermal, and chemical resistance. The fibers were prepared under different spinning conditions, varying the following spinning parameters: polymer concentration in the spinning solution, height of the air gap, and bore fluid composition. The effect of these parameters on the water permeability, the rejection of macromolecules (using dextrane with an average molecular weight of 68,800 g/mol), and the morphology of the membranes was studied. The results were also correlated to the viscosity of the spinning solution and to the ternary polymer/solvent/nonsolvent phase diagram. The morphology of the cross section and internal and external surfaces of the hollow fibers were analyzed using scanning electron microscopy (SEM). All membranes were shown to have a fingerlike void structure and a skin layer, depending on the spinning conditions, varying from (apparently) dense to porous. Pore size measurements by the bubble‐point method showed maximum pore sizes ranging from 0.3 to 2 μm. Permeability varied from 300 to 1000 L/(h−1 m−2 bar) and rejection to the dextrane from 10 to 78%. The viscosity of polymer solutions was in the range of 0.2 to 3.5 Pa s. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 841–853, 2004

Effect of spinning conditions on the structure and the gas permeation properties of high flux polyethersulfone—polyimide blend hollow fibers

In this work, the effects of major spinning parameters, such as: polymer concentration, air gap distance, bore fluid composition, and take-up velocity on the structure and the permeation properties of polyethersulfone-polyimide gas separation hollow fibers are discussed in detail. It is shown that a spinning dope starts to exhibit significant chain entanglement at a critical polymer concentration. Fibers spun from this critical concentration exhibit theoretically the thinnest skin layer and minimum surface porosity. The longer the nascent hollow fiber membrane is exposed to a humid air-gap, the higher the water content in the top layer before demixing occurs. This results in higher surface porosity and gas permeance. Better mixing between the polymer solution and the bore liquid is achieved by adjusting the composition of the bore fluid (NMP/H 20). Finally, by increasing the velocity of the take-up drum, the permeance of both CO 2 and N 2 decrease while their permselectivity remains constant. Suitable selection of the spinning conditions results in gas separation hollow fibers with thin skin layers (0.1 μm), macrovoid-free substructure, high permeation rates (CO 2: 40–60 GPU) and selectivity coefficients (a CO 2/N 2: 40). These results compete directly with the performance of commercial gas separation membranes.

Fabrication of fluoropolyimide/polyethersulfone (PES) dual-layer asymmetric hollow fiber membranes for gas separation

By using co-extrusion and dry-jet wet-spinning phase inversion techniques, we have developed delamination-free dual-layer asymmetric composite hollow fiber membranes for gas separation. Delamination-free is essential for dual-layer membranes to withstand high testing pressures. For concept demonstration, a 6FDA–durene–1,3-phenylenediamine (mPDA) (50:50) copolyimide was used to form the outer asymmetric separating layer, while polyethersulfone (PES) was employed to yield the inner interpenetrated porous supporting layer. A special assembly for outer-layer dope passage was firstly introduced into the dual-layer spinneret design in order to enhance the uniformity of dope distribution in nozzle orifice. A much thinner (≈10 μm) and uniform outer layer was achieved. The effects of spinning conditions, such as spinneret temperature, air gap, bore fluid chemistry, inner-layer dope concentration and formulation, and solvent exchange on the interface delamination between the dual layers were examined. Inner-layer dope concentration and bore fluid composition as well as the sequent solvent exchange were found to play important roles to produce delamination-free dual-layer membranes. Pure gas test results show that the resultant 6FDA–durene–mPDA/PES dual-layer membranes have an O 2/N 2 selectivity approaching to the intrinsic ideal selectivity value of 4.7 with a permeance of oxygen around 28 GPU (gas permeance unit) at room temperature, indicating the dual-layer hollow fiber membranes are apparently defect-free.