Gas permeation properties of asymmetric carbon hollow fiber membranes prepared from asymmetric polyimide hollow fiber

Asymmetric carbon hollow fiber membranes were prepared by pyrolysis of an asymmetric polyimide hollow fiber membrane, and their mechanical and permeation properties were investigated. The carbon membrane had higher elastic modulus and lower breaking elongation than the polyimide membrane. Permeation experiments were performed for single gases such as H₂, CO₂, and CH₄, and for mixed gases such as H₂/CH₄ at high feed pressure ranging from 1 to 5 MPa with or without toluene vapor. The permeation properties of the carbon membranes and the polyimide membrane were compared. There was little change in the properties of the carbon membranes with a passage of time. The properties were hardly affected by the feed pressure, whether the feed was accompanied with the toluene vapor or not, because the carbon membranes were not affected by compaction and plasticization.     

Polybenzimidazole (PBI) nanofiltration hollow fiber membranes applied in forward osmosis process

For the first time, the potential of polybenzimidazole (PBI) nanofiltration membrane as a forward osmosis membrane has been investigated. PBI was chosen mainly because of its unique nanofiltration characteristics, robust mechanical strength and excellent chemical stability. The MgCl₂ solutions with different concentrations and other different salt solutions were employed as draw solutions to test the water permeation flux through the PBI membrane during forward osmosis. High water permeation flux and excellent salt selectivity were achieved by using the PBI nanofiltration membrane which has a narrow pore size distribution. Effects of membrane morphology, operation conditions and flowing patterns of two feed streams within the membrane module on water transport performance have been investigated. It may conclude that PBI nanofiltration membrane is a promising candidate as a forward osmosis (FO) membrane.     

Oxygen production at low temperature using dense perovskite hollow fiber membranes

A dense perovskite hollow fiber made of BaCo_xFe_yZr_zO_3−δ (BCFZ) was evaluated for the oxygen separation at low temperatures (400–500 °C). An oxygen permeation flux of 0.45 ml/min cm² was obtained at 500 °C, which is the first oxygen permeation data reported at such low temperature so far. A degradation of the oxygen permeation at 500 °C was observed, but the oxygen fluxes through the hollow fiber membrane can be regenerated by thermal treatment at 925 °C for 1 h in air. Energy-dispersive X-ray spectroscopy (EDXS) shows that a strong element segregation occurs in the membrane during operation at low temperature.     

New polyimide-polyaniline hollow fibers: Synthesis, characterization and behavior in gas separation

New polyimide-polyaniline hollow fibers were produced by dissolution of the polymers in NMP and dry/wet spinning of the resulting solution in a non-solvent (H₂O). The morphology and thermal properties of the fibers, were examined by means of SEM and TGA, and FTIR spectroscopy was used for the study of their chemical structure. Permeability and selectivity measurements in different gases (He, H₂, CH₄, CO₂, O₂ and N₂) were performed in order to evaluate the performance of the membrane in gas separation applications. The results indicate that the novel membrane is a well structured hollow fiber, thermally stable up to 500°. The introduction of polyaniline into the polyimide matrix, results in a great enhancement in fiber permeability (60-600 times) possibly due to increase of the total free volume due to the introduction of shorter polyaniline molecules in the matrix, allowing larger quantities of gases to pass through the composite membrane. Perm-selectivity ratios for the composite membranes H₂/CH₄, He/N₂, H₂/N₂ and H₂/CO₂ were found lower by a factor of 6.4, 8.9, 7.7 and 1.47, respectively, compared to membranes produced using only polyimide. The opposite effect was observed for CH₄/CO₂ and N₂/CO₂ perm-selectivity ratios that showed an increase by a factor of 3.52 and 5.2, respectively. The ratio CH₄/CO₂ is of particular interest for natural gas purification purposes.     

Thin Pd membrane on α-Al2O3 hollow fiber substrate without any interlayer by electroless plating combined with embedding Pd catalyst in polymer template

Palladium acetate and poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) were dissolved in chloroform to form a homogeneous solution. Using this solution, thin polymer template film with embedded Pd catalyst was coated on a porous α-Al₂O₃ hollow fiber substrate. The Pd in the template film was used as the catalyst for electroless plating of Pd membranes. After the template was removed by heat treatment, the thin Pd membranes without any intermediate layers and substrate penetration were synthesized successfully. The as-synthesized Pd composite membranes of thickness less than 5 μm not only have a very high hydrogen permeance/permeability but also have a good hydrogen selectivity. Moreover, the good membrane stability was verified by the long-term operation under the condition of hydrogen permeation and the gas exchange cycles between pure hydrogen and pure helium. The good membrane stability was interpreted by estimating the shear stress of the special membrane configuration with small gap between Pd membrane and porous substrate layer.     

Composite hollow fiber membranes for CO2 capture

Composite hollow fibers membranes were prepared by coating poly(phenylene oxide) (PPO) and polysulfone (PSf) hollow fibers with high molecular polyvinylamine (PVAm). Two procedures of coating hollow fibers outside and respective inside were investigated with respect to intrinsic PVAm solution properties and hollow fibers geometry and material.The influence of operating mode (sweep or vacuum) on the performances of membranes was investigated. Vacuum operating mode gave better results than using sweep because part of the sweep gas permeated into feed and induced an extra resistance to the most permeable gas the CO₂. The composite PVAm/PSf HF membranes having a 0.7–1.5 μm PVAm selective layer, showed CO₂/N₂ selectivity between 100 and 230. The selectivity was attributed to the CO₂ facilitated transport imposed by PVAm selective layer. The CO₂ permeance changed from 0.006 to 0.022 m³(STP)/(m² bar h) in direct correlation with CO₂ permeance and separation mechanism of the individual porous supports used for membrane fabrication. The multilayer PVAm/PPO membrane using as support PPO hollow fibers with a 40 nm PPO dense skin layer, surprisingly presented an increase in selectivity with the increase in CO₂ partial pressure. This trend was opposite to the facilitated transport characteristic behaviour of PVAm/porous PSf. This indicated that PVAm/PPO membrane represents a new membrane, with new properties and a hybrid mechanism, extremely stable at high pressure ratios. The CO₂/N₂ selectivity ranged between 20 and 500 and the CO₂ permeance from 0.11 to 2.3 m³(STP)/(m² bar h) depending on the operating conditions.For both PVAm/PSf and PVAm/PPO membranes, the CO₂ permeance was similar with the CO₂ permeance of uncoated hollow fiber supports, confirming that the CO₂ diffusion rate limiting step resides in the properties of the relatively thick support, not at the level of 1.2 μm thin and water swollen PVAm selective layer. A dynamic transfer of the CO₂ diffusion rate limiting step between PVAm top layer and PPO support was observed by changing the feed relative humidity (RH%). The CO₂ diffusion rate was controlled by the PPO support when using humid feed. At low feed humidity the 1.2 μm PVAm top layer becomes the CO₂ diffusion rate limiting step.     

Analytical theory for permeation of condensable gases in a nanopore

The permeation of a condensable gas mixture in a pressure gradient is examined within a dynamic density functional theory (DDFT). The non-equilibrium density and flux profiles of gas molecules trapped within a nanopore are calculated for each species as a function of feed gas density. Because of important fluid–fluid interaction close to condensation the flux and density gradients are not related by constant transport diffusivities with the thermodynamic correction of uniform density. For long narrow pores the relation of the selectivity to the equilibrium adsorption isotherms is validated. Improved separation is achieved by combining preferential wall interaction and advantageous attraction between gas molecules of different species and examples are discussed. Results from experiments and simulations of permeation in binary mixtures near condensation are still rare and the theory provides a simple basis to study qualitative trends using known parameters.     

Conductive diamond hollow fiber membranes

Boron-doped diamond hollow fiber membrane (BDD–HFM) was fabricated as a novel type of porous conductive diamond. BDD–HFM was obtained by deposition of BDD polycrystalline film onto a quartz filter substrate consisting of quartz fibers, followed by etching of the substrate in HF/HNO₃ aqueous solution. Cross-sectional scanning electron microscope (SEM) observation showed the inner diameter and wall thickness of the BDD hollow fibers were in the range of 0.4–2 and 0.2–2 μm, respectively. The BDD–HFM electrode exhibited a relatively large double-layer capacitance (ca. 13 F g⁻¹) in 0.1 M H₂SO₄. Electrochemical AC impedance properties were simulated using an equivalent circuit model containing a transmission line model, which indicated characteristics of a porous electrode material.     

Optimization of substrate conversion in hollow fiber reactor

The crude jackbean preparation was used as urease to determine the Km_(app), V_(app), and substrate conversion (%) at different flow rates in a hollow fiber device. It was found that, as the flow rate increases, the V_(app) increases, whereas the Km_(app) initially decreases and again increases. The maximum substrate conversion (96.25%) was obtained at the lowest Km_(app). No significant loss was observed in substrate conversion after 2 mo at room temperature.     

Evaluation of multiplexed hollow fiber flow field-flow fractionation for semi-preparative purposes

A multiplexed hollow fiber flow field-flow fractionation (MxHF5) is introduced to increase throughput of an HF5 channel system for semi-preparative purposes. HF5, a cylindrical version of the flow field-flow fractionation (FlFFF) operated with a porous, hollow fiber membrane by controlling the ratio of radial and axial flow rates, is capable of fractionating proteins, cells, and macromolecules by size. An advantage of HF5 is its inexpensive channel construction, allowing for disposability that can reduce run-to-run carryover problems. MxHF5 constructed in this study was made with six parallel HF5 modules connected to seven-port manifolds for the semi-preparative scale separation of proteins or biological particles. For the evaluation of MxHF5 separation efficiency, protein standards were utilized to test peak recoveries, band broadening, and throughput. The assembly showed the possibility of handling up to 50 μg of proteins without incurring overloading. The developed channel was applied to demonstrate size sorting of lipoproteins for the future study of size dependent lipidomic and proteomic analysis. The current trial offers a unique advantage of scaling up HF5 separation without using wide-bore, hollow fibers which sacrifice separation speed.     

Non-invasive monitoring of fouling in hollow fiber membrane via UTDR

Fouling is the most critical problem associated with membrane separations in liquid media. But it is difficult to control the inevitable membrane fouling because of its invisibility, especially on the inside surface of hollow fiber membranes. This study describes the extension of ultrasonic time-domain reflectometry (UTDR) for the real-time measurement of particle deposition in a single hollow fiber membrane. A transducer with a frequency of 10 MHz and polyethersulfone hollow fiber membranes with 0.8 mm inside diameter (ID) and 1.2 mm outside diameter (OD) were used in this study. The fouling experiments were carried out with 1.8 g/L kaolin suspension at flow rates 16.7 and 10.0 cm/s. The results show that UTDR technique is able to distinguish and recognize the acoustic response signals generated from the interfaces water/upper outside surface of the hollow fiber, lumen upside surface/water, water/lumen underside surface and lower outside surface/water in the single hollow fiber membrane module in pure water phase. The systemic changes of acoustic responses from the inside surfaces of the hollow fiber in the time- and amplitude-domain with operation time during the fouling experiments were detected by UTDR. It is associated with the deposition and formation of the kaolin layer on the inside surfaces. Further, the acoustic measurement indicates that the deposited fouling layer is denser on the lumen underside surface of the hollow fiber than that on the lumen upside surface as a result of weight. Moreover, it is found that the fouling layer grows faster on the inside surface of the hollow fiber at a flow rate of 10.0 cm/s than that at 16.7 cm/s due to the lower shear stress. The fouling layer formed is thicker at a flow rate of 10.0 cm/s than that at 16.7 cm/s. The flux decline data and SEM analysis corroborate the ultrasonic measurement. Overall, this study confirms that UTDR measurement will provide not only a new protocol for the observation of hollow fiber membrane fouling and cleaning, but also a quantitative approach to the optimization of the membrane bioreactor system.     

Experimental verification of pressure drop models in hollow fiber membrane

A new experimental method to obtain internal pressure profiles in a hollow fiber membrane was demonstrated. The experimentally obtained internal pressure profiles were compared with the theoretically calculated ones based on Hagen–Poiseuille equations. The experimental and theoretical results agreed very well in clean water conditions only when accurate membrane permeabilities and effective internal diameters were available. New experimental methods to obtain the two parameters were demonstrated. The same experimental technique was also applied for the submerged hollow fiber membranes filtering activated sludge to find out how internal pressure profiles were changing with time. Based on the pressure profiles, evidences that indicated the local flux near membrane exit was lower than those in adjacent area were found. This observation contradicted to the filtration models based on critical flux concept. It was considered that the cake layer collapse near the membrane exit was the cause. Though there was some degree of delay in pressures detection, the method demonstrated in this study provided a great accuracy when pressure profiles did not change rapidly.     

On the structure of the single-phase flow field in hollow fiber membrane modules during filtration

Whereas the structure of the flow field generated in the hollow fiber membrane modules operating as liquid–liquid or gas–liquid contactors has been studied extensively theoretically, this is not the case for the much more complex flow field generated during the filtration operation of the same type of modules. The present work is a first approach to analyze and understand the particular flow field. The mathematical problem is formulated and then geometrical and physical simplifications are introduced in order to decompose the full problem in a series of simpler sub-problems. In addition to the presentation of the general structure and the features of the problem, the focus of the present work is to derive all possible analytical solutions of the sub-problems. Several results based on analytical or simple numerical approaches are also presented.     

Synthesis,characterization, and application of hollow ceramic microsphere based Pd catalyst for hydrogenation of 2-ethylanthraquinone

Palladium metal has been used extensively in the hydrogenation reactions due to their great affinity towards hydrogen atoms. In the present study, the catalyst preparation attempted with Pd supported Hollow Ceramic Microspheres using wet impregnation method and its use as catalysts is explored in the hydrogenation of 2-ethylanthraquinone studing the effect of the reaction time, temperature, volume of working solution and the catalyst dosages on the conversion of 2-ethylanthraquinone and yield of hydrogen peroxide. The hydrogenation reaction of 2-ethylanthraquinone is the key step in the anthraquinone method for the industrial production of the hydrogen peroxide. The Pd supported catalyst was characterized by XRF, FTIR, and BET to confirm the composition of the prepared catalyst, Pd deposition, and the surface area. The highest catalyst activity was found to be 9.42 ​g/L with the maximum conversion of 96% at 70°C, 0.3 ​MPa. The kinetics of the heterogeneous hydrogenation reaction of 2-ethylanthraquinone with Pd supported on Hollow Ceramic Microspheres as catalyst was also investigated. This paper is in contribution of our earlier publication.     

The fabrication of hollow fiber membranes with double-layer mixed-matrix materials for gas separation

The concept of fabricating hollow fibers with double-layer mixed-matrix materials using the same polymeric matrix has been demonstrated for gas separation. Polyethersulfone (PES)–beta zeolite/PES–Al₂O₃ dual-layer mixed-matrix hollow fiber membranes with enhanced separation performance have been fabricated. This study presents an innovative approach of utilizing low cost PES and Al₂O₃ to replace expensive polyimides as the supporting medium for dual-layer mixed-matrix hollow fibers and eliminating interlayer de-lamination problems. The incorporations of 20 wt% beta zeolite in the outer selective layer and 60 wt% Al₂O₃ in the inner layer coupled with spinning at high elongational draw ratios yield membranes with an O₂/N₂ selectivity of 6.89. The presence of Al₂O₃ particles enables the membrane to retain its porous substructure morphology in the course of annealing above the glass transition temperature of PES. Moreover, spinning at high elongational draw ratios results in the re-distribution of Al₂O₃ particles towards both edges of the inner layer. Not only do the permeance and selectivity of the fibers increase, but also greater mechanical properties and lower degree of shrinkages are obtained. Therefore, the combination of PES–beta zeolite and PES–Al₂O₃ nanoparticles with a reasonable draw ratio may be another promising approach to produce hollow fibers with double-layer mixed-matrix materials.     

Effect of surface morphology on membrane fouling by humic acid with the use of cellulose acetate butyrate hollow fiber membranes

A serious problem faced during the application of membrane filtration in water treatment is membrane fouling by natural organic matter (NOM). The hydrophilicity, zeta potential and morphology of membrane surface mainly influence membrane fouling. The aim of the present study is to reveal the correlation between membrane surface morphology and membrane fouling by use of humic acid solution and to investigate the efficiency of backwashing by water, which is applied to restore membrane flux. Cellulose acetate butyrate (CAB) hollow fiber membranes were used in the present study. To obtain the membranes with various surface structures, membranes were prepared via both thermally induced phase separation (TIPS) and nonsolvent-induced phase separation (NIPS) by changing the preparation conditions such as polymer concentration, air gap distance and coagulation bath composition. Since the membrane material is the same, the effects of hydrophilicity and zeta potential on membrane fouling can be ignored. More significant flux decline was observed in the membrane with lower humic acid rejection. For the membranes with similar water permeability, the lower the porosity at the outer surface, the more serious the membrane fouling. Furthermore, the effect of the membrane morphology on backwashing performance was discussed.     

Bio-ceramic hollow fiber membranes for immunoisolation and gene delivery: I: Membrane development

Zirconia bio-ceramic hollow fiber membranes were developed using a sequence of mixing, extrusion, phase inversion and sintering steps. ZrO₂ partially stabilized by Y₂O₃ was chosen as the starting membrane material. The prepared membranes were characterized by SEM, EDX, XRD and gas permeation techniques. Effects of the starting ZrO₂ particle size and sintering temperature on the physical properties of the resulted hollow fiber membranes were extensively studied. Sintered at 1400 °C for 10 h, membranes made from 80 nm sized ZrO₂ particles display cubic fluorite as the major crystalline phase and give rise to interesting microstructure for cell response. Without any surface modification, this tailor-made membrane with high mechanical strength and pore size less than 1 μm was selected for further test of osteoblast attachment. In vitro bio-compatibility was evaluated by using mouse MC-3T3-E1 osteoblast cell culture. A series of cell interactions with fiber surface (i.e. cell adhesion, proliferation, formation of bone nodules, mineralization, etc.) verified the bio-compatibility of the prepared membranes.     

Synthesis and applications of diethylstilbestrol-based molecularly imprinted polymer-coated hollow fiber tube

A novel molecularly imprinted polymer-coated polypropylene hollow fiber tube (MIP-HFT) was photoinitiated for the copolymerization of diethylstilbestrol (DES) as a template molecule, α-methacrylic acid as a functional monomer, and ethylene glycol dimethacrylate as a crosslinking agent. The characteristics and applications of the MIP-HFT were investigated. In order to compare its characteristics with those of a non-imprinted polymer-immersed hollow fiber tube, the selectivity of the MIP-HFT was investigated using dienestrol and hexestrol as the structural analogues of a DES template, and phenol and methylbenzene were taken as reference compounds. The MIP-HFT was employed in the HPLC analysis of spiked milk samples. The detection limits of the method were found to be in the range 2.5-3.3 μg L⁻¹ for DES, dienestrol and hexestrol and the RSD% were in the range 6.4-8.9. The limits of quantitation were found to be in the range 8.7-9.4 μg L⁻¹ in milk for DES, dienestrol and hexestrol, and their average recoveries were 83.7-90.6% in the spiked milk samples. The experimental results revealed that the MIP-HFT provides a good carrier for the selective adsorption of DES and its chemical structure analogs, and can be used for the preconcentration of these compounds in complicated samples.     

Superior gas separation performance of dual-layer hollow fiber membranes with an ultrathin dense-selective layer

A concept demonstration has been made to simultaneously enhance both O₂ and CO₂ gas permeance and O₂/N₂ and CO₂/CH₄ selectivity via intelligently decoupling the effects of elongational and shear rates on dense-selective layer and optimizing spinning conditions in dual-layer hollow fiber fabrication. The dual-layer polyethersulfone hollow fiber membranes developed in this work exhibit an O₂/N₂ selectivity of 6.96 and an O₂ permeance of 4.79 GPU which corresponds to an ultrathin dense-selective layer of 918 Å at room temperature. These hollow fibers also show an impressive CO₂/CH₄ selectivity of 49.8 in the mixed gas system considering the intrinsic value of only 32 for polyethersulfone dense films. To our best knowledge, this is the first time to achieve such a high CO₂/CH₄ selectivity without incorporating any material modification. The above gas separation performance demonstrates that the optimization of dual-layer spinning conditions with balanced elongational and shear rates is an effective approach to produce superior hollow fiber membranes for oxygen enrichment and natural gas separation.     

ZIF-8 based polysulfone hollow fiber membranes for natural gas purification

Mixed matrix membranes (MMMs) have received worldwide attention for natural gas purification due to their superior performance in terms of permeability and selectivity. The zeolitic imidazole framework-8 (ZIF-8) blended polysulfone (PSf) membranes have been fabricated for natural gas purification. ZIF-8 was selected due to its low cost, remarkable thermal and chemical stabilities, and tunable microporous structure. The neat PSf hollow fiber membrane and mixed matrix hollow fiber membranes incorporated with the various ZIF-8 loadings up to 1.25% were fabricated. The prepared membranes were evaluated using field emission scanning electron microscopy (FESEM), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM), and gas separation performance. The low loading of ZIF-8 nanoparticles to the MMM improved thermal stability and glass transition temperature and yielded low surface roughness. MMMs were tested using pure gases with a significant improvement of 36% in CO₂ permeability and 28% in CO₂/CH₄ selectivity compared to the neat membrane. However, the high ZIF-8 loading reduced the separation performances. Moreover, CO₂/CH₄ selectivity decreased at elevated pressure (8 and 10 bar) due to CO₂-induced plasticization. Previously, the incorporation of ZIF-8 particles has primarily been subjected to the fabrication of flat sheet membranes, whereas this work focused on hollow fiber membranes which are rarely investigated. Hence, the promising results obtained at low feed pressure in this study demonstrated the potential of ZIF-8 based hollow fiber membrane for natural gas purification.