A review on hollow fiber membrane module towards high separation efficiency: Process modeling in fouling perspective

Hollow fiber microfiltration (MF) and ultrafiltration (UF) membrane processes have been extensively used in water purification and biotechnology. However, complicated filtration hydrodynamics wield a negative influence on fouling mitigation and stability of hollow fiber MF/UF membrane processes. Thus, establishing a mathematical model to understand the membrane processes is essential to guide the optimization of module configurations and to alleviate membrane fouling. Here, we present a comprehensive overview of the hollow fiber MF/UF membrane filtration models developed from different theories. The existing models primarily focus on membrane fouling but rarely on the interactions between the membrane fouling and local filtration hydrodynamics. Therefore, more simplified conceptual models and integrated reduced models need to be built to represent the real filtration behaviors of hollow fiber membranes. Future analyses considering practical requirements including complicated local hydrodynamics and nonuniform membrane properties are suggested to meet the accurate prediction of membrane filtration performance in practical application. This review will inspire the development of high-efficiency hollow fiber membrane modules.     

Development of a hollow fiber membrane module for using implantable artificial lung

This study examined the effects of multiple mechanical forces on gas exchange and hemolysis in intravascular lung assist devices (IVLAD). Specific attention was paid to on the effect of membrane vibration. This study adhered to the recommended practice for the assessment of hemolysis described by the American Society of Testing and Materials (ASTM). The results showed a higher oxygen (O₂) transfer rate and carbon dioxide (CO₂) removal rate in each excited frequency bandwidth than those without vibration. The maximum oxygen transfer and carbon dioxide removal rate occurred at frequency band of 7 Hz. The gas exchange improved maximum 52%. The plasma-free hemoglobin was 11.2 ± 0.57 and 14.4 ± 0.74 mg/100 ml by exciting a piezo-vibrator with a sinusoidal wave magnitude of DC10 V and DC50 V, respectively. The NIHO value was determined to be 59 ± 2.76 and 95 ± 4.32 mg/100 ml by exciting a piezo-vibrator with a sinusoidal wave magnitude of DC10 V and DC50 V, respectively. In conclusion, piezoelectric lead zirconate titanate (PZT) materials are exciting systems for improving the oxygen transfer efficiency and blood suitability of hollow fiber membrane in the development of new IVLAD.     

Voltage-controlled separation of proteins by electromobility focusing in a dialysis hollow fiber

Electromobility focusing (EMF) is a relatively new protein separation technique that utilizes an electric field gradient and a hydrodynamic flow. Proteins are focused in order of electrophoretic mobility at points where their electrophoretic migration velocities balance the hydrodynamic flow velocity. Steady state bands are formed along the separation channel when equilibrium is reached. Further separation and detection can be easily achieved by changing the electric field profile. In this paper. we describe an EMF system with on-line UV absorption detection in which the electric field gradient was formed using a dialysis hollow fiber. Protein focusing and preconcentration were performed with this system. Voltage-controlled separation was demonstrated using bovine serum albumin and myoglobin as model proteins. The limitations of the current method are discussed, and possible solutions are proposed.     

Membrane-moderated stripping process for removing VOCs from water in a composite hollow fiber module

The “stripmeation” process for removing volatile organic compounds (VOCs) from water has been introduced and studied. An aqueous solution of the VOC is passed through the bores of hydrophobic microporous polypropylene hollow fibers having a plasma polymerized silicone coating on the fiber outside diameter; a vacuum is maintained on the shell side of the fiber. The VOC is stripped into the gas-filled pores of the hydrophobic substrate, permeates through the nonporous silicone skin and is recovered by condensation of the shell-side permeate stream. Removal of trichloroethylene (TCE) present in a concentration range 200–1040 ppm has been studied at 25°C. Process performance has been obtained over a range of flow rates. The observed TCE permeation and removal behavior has been modeled using a resistances-in-series approach; the two important resistances are the tube-side aqueous boundary layer resistance and the vapor permeation resistance of TCE through the silicone coating. Employing the known Graetz solution for the tube-side flow and the measured vapor permeation resistance of TCE, values of the overall TCE mass-transfer coefficient have been obtained. These values compare well with the experimental values. The conventional pervaporation process where the liquid feed solution is in contact with the nonporous silicone membrane has also been studied by passing the feed on the shell side. The tube-side feed-based operation performs much better than the shell-side based operation.     

Absorption of CO2 in a transverse flow hollow fiber membrane module having a few wraps of the fiber mat

An experimental study of gas-liquid contacting was conducted to determine the mass-transfer performance characteristics of hollow fiber devices that employ transverse liquid flow over microporous hydrophobic fibers present in a mat wrapped around a central feeder tube. Gas flow was through the fiber lumen; water was employed to absorb CO₂ from pure CO₂ and from a mixture of CO₂N₂. The results are characteristics of the local crossflow mass transfer due to the limited number of wraps of fiber layers. A correlation for mass-transfer coefficient was developed and compared with correlations from the literature.     

Fabrication of multi-layer composite hollow fiber membranes for gas separation

Using multilayer composite hollow fiber membranes consisting of a sealing layer (silicone rubber), a selective layer (poly(4-vinylpyridine)), and a support substrate (polysulfone), we have determined the key parameters for fabricating high-performance multilayer hollow fiber composite membranes for gas separation. Surface roughness and surface porosity of the support substrate play two crucial roles in successful membrane fabrication. Substrates with smooth surfaces tend to reduce defects in the selective layer to yield composite membranes of better separation performance. Substrates with a high surface porosity can enhance the permeance of composite membranes. However, SEM micrographs show that, when preparing an asymmetric microporous membrane substrate using a phase-inversion process, the higher the surface porosity, the greater the surface roughness. How to optimize and compromise the effect of both factors with respect to permselectivity is a critical issue for the selection of support substrates to fabricate high-performance multilayer composite membranes. For a highly permeable support substrate, pre-wetting shows no significant improvement in membrane performance. Composite hollow fiber membranes made from a composition of silicone rubber/0.1–0.5 wt% poly(4-vinylpyridine)/25 wt% polysulfone show impressive separation performance. Gas permeances of around 100 GPU for H₂, 40 GPU for CO₂, and 8 GPU for O₂ with selectivities of around 100 for H₂/N₂, 50 for CO₂/CH₄, and 7 for O₂/N₂ were obtained.     

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 fiber variation on the performance of cross-flow hollow fiber gas separation modules

A quantitative analysis of the effects of variable fiber properties on the performance of a cross-flow hollow fiber gas separation module is presented. The effects of variations in size, permeance, and selectivity are considered. Fiber variability is detrimental to performance. The recovery and flow rate of an enriched retentate stream decrease as variability increases. Some fibers may actually stop producing product as purity increases. Additionally, performance is poorer if the permeate from all fibers is not well mixed. The results of this work can be used to determine quality control guidelines for fiber manufacture and evaluate process enhancements.     

The effects of spinneret dimension and hollow fiber dimension on gas separation performance of ultra-thin defect-free Torlon hollow fiber membranes

A defect-free as-spun hollow fiber membrane with an ultra-thin dense-selective layer is the most desirable configuration in gas separation because it may potentially eliminate post-treatments such as silicone rubber costing, simplify membrane manufacture, and reduce production costs. However, the formation of defect-free as-spun hollow fiber membranes with an ultra-thin dense-selective layer is an extremely challenging task because of the complexity of phase inversion process during the hollow fiber fabrication and the trade-off between the formation of an ultra-thin dense-selective layer and the generation of defects. We have for the first time successfully produced defect-free as-spun Torlon^® hollow fiber membranes with an ultra-thin dense layer of around 540 Å from only a one polymer/one solvent binary system at reasonable take-up speeds of 10–50 m/min. The best O₂/N₂ permselectivity achieved is much higher than the intrinsic value of Torlon^® dense films. This is also a pioneering work systematically studying the effects of spinneret dimension and hollow fiber dimension on gas separation performance. Several interesting and important phenomena have been discovered and never been reported: (1) as the spinneret dimension increases, a higher elongation draw ratio is required to produce defect-free hollow fiber membranes; (2) the bigger the spinneret dimension, the higher the selectivity; (3) the bigger the spinneret dimension, the thinner the dense-selective layer. Mechanisms to explain the above observation have been elaborated. The keys to produce hollow fiber with enhanced permselectivity are to (1) remove die swell effects, (2) achieve finer monodisperse interstitial chain space at the dense-selective layer by an optimal draw ratio, and (3) control membrane formation by varying spinneret dimension.     

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.     

Simulation of binary gas separation in hollow fiber asymmetric membranes by orthogonal collocation

Modeling of hollow fiber asymmetric membrane modules can provide useful guidelines to achieve desirable separations of gas mixtures. In this work the performance of a countercurrent flow separator was analyzed through a parametric study of the most important system variables as functions of basic design and operational parameters. Results refer to CO₂–N₂ separation from power station flue gases as a typical, potential process. The appropriate model equations were solved by orthogonal collocation to approximate differential equations, and to solve the resulting system of non-linear algebraic equations by the Brown method. This technique compared to other applied computational procedures minimized the computational time and effort and improved solution stability. This is very important if the pressure and concentration profiles along the permeator, both in the residue and the permeate streams, need to be determined. These profiles influence strongly the permeator performance and, under certain conditions such as moderate and high feed pressure, they may result in lower than expected permeate purity. The simulation results also indicate that the role of the basic design parameters may be of equal if not higher importance to membrane selectivity. Thus industrial permeator performance, as it is expressed by stage cut and permeate purity, is not very sensitive to membrane permselectivity beyond a modest value of 40–50, especially at moderate and high (15–20 bar) feed pressures. A desirable gas separation may then be achievable with a reasonably permeable, albeit not very selective membrane, provided that design and operating variables are selected appropriately.     

A variation in fiber properties affects the performance of defect-free hollow fiber membrane modules for air separation

A flexible simulation method capable of modeling the performance of a gas separation module with variations in fiber properties is presented. Variations in fiber skin thickness, diameter and selectivity are considered. For separation of a high-pressure side product such as nitrogen from air, these variations can result in a considerable deviation from the ideal separation characteristic especially at high purities. It is shown that equal fiber quality is essential for the production of high purity nitrogen. The results presented in this publication are a part of a detailed and comprehensive sensitivity analysis on hollow fiber gas permeation modules.     

Fabrication of Matrimid/polyethersulfone dual-layer hollow fiber membranes for gas separation

We have developed almost defect-free Matrimid/polyethersulfone (PES) dual-layer hollow fibers with an ultra-thin outer layer of about 10 × 10⁻⁶ m (10 μm), studied the effects of spinneret and coagulant temperatures and dope flow rates on membrane morphology and separation performance, and highlighted the process similarities and differences between single-layer and dual-layer hollow fiber fabrications. The compositions of the outer and inner layer dopes were 26.2/58.8/15.0 (in wt.%) Matrimid/NMP/methanol and 36/51.2/12.8 (in wt.%) PES/NMP/ethanol, respectively. It is found that 25 °C for both spinneret and coagulant is a better condition, and the fibers thus spun exhibit an O₂/N₂ selectivity of 6.26 which is within the 87% of the intrinsic value and a calculated apparent dense-layer thickness of about 2886 × 10⁻¹⁰ m (2886 Å). These dual-layer membranes also have impressive CO₂/CH₄ selectivity of around 40 in mixed gas tests. The scanning electron microscopy (SEM) studies show that low coagulant temperatures produce dual-layer hollow fibers with an overall thicker thickness and tighter interfacial structure which may result in a higher substructure resistance and decrease the permeance and selectivity simultaneously. The elemental analysis of the interface skins confirms that a faster inter-layer diffusion occurs when the fibers are spun at higher spinneret temperatures. Experimental results also reveal that the separation performance of dual-layer hollow fiber membranes is extremely sensitive to the outer layer dope flow rate, and the inner layer dope flow rate also has some influence. SEM pictures indicate that the macrovoid formation in dual-layer asymmetric hollow fiber membranes is quite similar to that in single-layer ones. It appears that macrovoids observed in this study likely start from local stress imbalance and weak points.     

Simultaneous reactive extraction separation of amino acids from water with D2EHPA in hollow fiber contactors

Reactive extraction separation of binary amino acids from water using a microporous hollow fiber has been studied, in which the acidic extractant di(2-ethylhexyl)phosphoric acid (D2EHPA) was selected as an active carrier dissolved in kerosene. l-Phenylalanine (Phe) was extracted from an aqueous solution through the shell side of module to the organic phase through the lumen of fiber in the extraction module, in which l-Phe was then back-extracted to stripping phase in stripping module. Experiments were conducted as a function of the initial feed concentration of equimolar Phe and l-aspartic acid (l-Asp) (5 mol/m³), feed pH (3–5), the carrier concentration (0.1–0.5 mol/dm³), and stripping acidity (0.1–2 mol/dm³). The effect of process variables on the separation factor of Phe/Asp and the possible transport resistances including aqueous-layer diffusion, membrane diffusion, organic-layer, and interfacial chemical reaction were quantitatively studied and discussed. The high separation factor (β) of Phe/Asp was obtained to be 18.5 at feed pH 5 and 2 mol/dm³ of strip solution (HCl). The extraction and stripping processes appear to rely on pH dependence of the distribution coefficient of amino acids in reactive extraction system. The separation factor (β) was enhanced in hollow fiber membrane (HFM) process compared with conventional solvent process, which was a result of the counter transport of hydrogen ions.     

Composite hollow fiber membranes for gas separation: the resistance model approach

A model is developed to explain the behavior of composite gas separation membranes which consist of a porous asymmetric substrate of one polymer, and a coating of a second polymer. An analogy between gas permeation and electrical flow is made, and the various portions of the composite membrane are described in terms of their resistance to gas permeation. It is shown that substrate porosity can vary significantly without altering the separating properties of such a composite, and that substrate and coating properties can be matched to optimize the flux and separation factor. Several major problems previously associated with the development of useful hollow fibers for gas separation are discussed, and it is shown that the use of Resistance Model composites can help to resolve these problems.     

Impact of operating pressure on the permeance of hollow fiber gas separation membranes

Asymmetric and slightly pressure dependent permeation properties of commercial air separation hollow fibers have been observed – phenomena that have not been mentioned in the literature before. In bore-side feed the permeance of oxygen, nitrogen and helium slightly increases with pressure while in shell-side feed the permeance decreases with pressure. At a feed pressure of 12.8 bar_a the permeance in bore-side feed is about 10–15% higher than in shell-side feed mode. Interestingly, the effect was more or less similar for different gases and different fibers. The results suggest that mechanical stress induced by the pressure difference across the membrane affects the permeability of the skin layer.     

High-affinity sulfonated materials with transition metal counterions for enhanced protein separation in dual-layer hollow fiber membrane chromatography

High-affinity membrane materials have been successfully synthesized through a combination of the polymer sulfonation reaction with transition metal counterion exchange treatment. This type of promising materials were embodied for the first time with the aid of dual-layer hollow fiber technology for protein separation. Three types of immobilized metal affinity membranes (Cu(2+), Ni(2+) and Zn(2+) forms) were developed in this work and they all exhibited enhanced protein separation performance compared to the as-spun hollow fiber in H(+) form due to the strong affinity between transition metal counterions and target protein molecules. Ultimately, the high-purity target protein (>99%, w/w) could be achieved via the membrane in Cu(2+) form.     

Selective separation of flavonoids with a polyvinyl alcohol membrane

In order to develop a selective membrane separation process for flavonoids, i.e. baicalin, baicalein and flavone extracted from a crude drug, “Wogon”, we have measured the permeabilities of these flavonoids through a polyvinyl alcohol membrane, together with their aqueous solubilities. The aqueous solubilities of baicalin and baicalein increase with increasing aqueous solution pH due to the acid dissociation of the saccharic carboxyl group of baicalin or the phenolic hydroxyl group of baicalein. The mass transfer coefficients of flavonoids experimentally obtained in both systems of single and mixed constituents agreed well with the calculated values based on a solution-diffusion model together with the acid dissociation of the carboxyl or hydroxyl groups of baicalin and baicalein, respectively. The selective mutual separation of flavonoids can be achieved from mixed solution of constituents in the neutral pH region. Furthermore, a quantitative discussion of the permeation behavior of flavonoids through the PVA membrane is provided from a molecular modelling computational viewpoint.