Asymmetric Hollow Fiber Membranes Research Articles

Dual thermally crosslinked hollow fiber membranes for plasticization-resistant helium recovery from natural gas

Membrane-based helium recovery from natural gas faces a major challenge due to membrane plasticization, which increases permeances but reduces selectivities. Thermal crosslinking has emerged as a promising approach to enhance the plasticization resistance of membranes by restricting segmental mobility and creating well-defined microporosity for gas separation. Herein, we develop novel dual thermally crosslinked asymmetric hollow fiber membranes (HFMs) using a 4,4′-diamino-2,2′-biphenyldicarboxylic acid-containing copolyimide. Decarboxylation-based dual crosslinking is achieved through heat treatment at various temperatures, resulting in the generation of C–C covalent bonds. The interchain distance increases from 5.33 to 5.76 Å, and the hierarchical pore size distributions exhibit ultra-micropore size between 5.6 and 6.8 Å as well as micropore size between 7.0 and 9.5 Å. Notably, the formation of stable C–C covalent bonds through dual crosslinking and the existence of bulky −CF3 groups in the 2,2′-bis(trifluoromethyl)-4,4′-biphenyldiamine moiety prevent substructure collapse by enhancing the chain rigidity and rotational barrier. Gas transport properties of the crosslinked HFMs are effectively tuned by adjusting the heat treatment temperatures. Particularly, the PI-TFMB-HF@400 membrane exhibits a He permeance of 25 GPU and He/CH4 selectivity of 269. Additionally, the crosslinked HFMs exhibit enhanced plasticization resistance. The PI-TFMB-HF@400 membrane, for instance, shows only a 24 % decrease in mixed-gas CO2/CH4 selectivity and an 80 % increase in mixed-gas [He/(CO2 + CH4)] selectivity when exposed to a high-pressure (40 bar) ternary mixed-gas feed of He/CO2/CH4 (0.3/49.7/50, v/v/v), and the mixed-gas [He/(CO2 + CH4)] selectivity increases with temperature. The dual thermally crosslinked HFMs in this study demonstrate the potential for helium recovery from aggressive natural gas.

Performance and stability of cellulose triacetate membranes in humid high H2S natural gas feed streams

The current study aims to provide a thorough understanding of the separation performance of the cellulose triacetate (CTA) membrane material in humid high H2S natural gas feed streams. A systematic study of the gas separation performances of CTA polymers has been carried out in self-standing symmetric films and asymmetric hollow fiber membranes. The separation performance in H2S-containing mixtures (CH4/CO2/H2S, 80/5/15 mol%) were evaluated at 30 and 50 °C at 20, 35 and 50 bar, including higher hydrocarbons (butane and toluene) in the presence of various levels of humidity, up to 4500 ppm or 80 % RH, depending on conditions of temperature and pressure. For the first time, the effect of these contaminants and humidity together under sour gas conditions has demonstrated the promising stability of CTA membrane separation performance, both in terms of H2S permeability and H2S/CH4 selectivity. The improvement in H2S permeability is due to a combination of H2S plasticization and competitive sorption making this phenomenon even more attractive and worth studying further.

Evaluation of the Permeation of Syngas and Its Components through a Synthesized Matrimid Asymmetric Hollow-Fiber Membrane

Evaluation of the Permeation of Syngas and Its Components through a Synthesized Matrimid Asymmetric Hollow-Fiber Membrane

Manufacturing of Asymmetric Hollow Fiber Membranes for Gas Separation Made of Poly(2,6-Dimethyl-1,4-Phenylenoxide)

Manufacturing of Asymmetric Hollow Fiber Membranes for Gas Separation Made of Poly(2,6-Dimethyl-1,4-Phenylenoxide)

Practical Manufacturing of Asymmetric Hollow Fiber Membranes for Gas Separation Made of Poly(2,6-dimethylphenylenoxide-1,4)

The regularities of manufacturing of hollow fiber membranes made of poly(2,6-dimethylphenylene oxide-1,4) for gas separation were studied. The phase inversion method was used to manufacture the membranes. The dependence of the separation characteristics of the membrane on such spinning parameters as the type of solvent, the exposure time of the polymer solution in the “air” gap, and the type of non-solvents (coagulants) has been studied. The characteristics of the membrane were obtained by determining their gas permeability. It is shown that higher separation and gas transport characteristics of the PPO membrane are obtained using the wet spinning method. An intrinsic selectivity of 4.8 ± 0.4 was obtained at a specific oxygen permeability (20°C) – (P/l) 790 ± 82 [m3 (s.t.p.) m–2 s–1 kPa] for oxygen–nitrogen system. The developed membranes are promising for use in case for producing nitrogen and oxygen-enriched air.

Development of cellulose triacetate asymmetric hollow fiber membranes with highly enhanced compaction resistance for osmotically assisted reverse osmosis operation applicable to brine concentration

Osmotically assisted reverse osmosis (OARO) has been recently proposed for concentrating high-salinity brines. The long-term performance stability of the membrane is essential for OARO, considering the long-term commercial operation. In this study, we developed hollow fiber membranes for OARO operation made of cellulose triacetate, and elucidated the relationship between the membrane structure and resistance to compaction against long-term operation under high pressure at 7.0 MPa. The mechanical properties and the performance of membranes operated under reverse osmosis (RO) conditions were very well correlated with the long-term performances and dimensional stabilities of membranes operated under OARO conditions. Raman spectroscopy revealed the asymmetricity difference of the membranes before and after long-term operation. Finally, operation for more than 700 h under OARO conditions was performed using commercial-sized membrane modules, and this result was as expected based on results from laboratory-scale membrane modules. The membrane developed for higher compaction resistance retained 95% water permeance even after operation for 700 h. This study clearly showed a relationship between the resistance to compaction and the membrane structure, both of which are important for practical OARO operation.

The role of skin layer defects in organic solvent reverse osmosis membranes

The fractionation of complex liquid hydrocarbon mixtures is an important and emerging area of membrane science. Polymeric asymmetric hollow fiber membranes have the potential to be used for this purpose, especially if the size and number of defects in the membrane skin layer can be precisely engineered. Here, we fabricated various “defect-engineered” Torlon hollow fiber membranes by modifying hollow fiber spinning conditions and spin dopes to study the role of skin layer defects in the organic solvent reverse osmosis (OSRO) membranes. The quality of the membranes was investigated using several sets of pure gas permeation experiments, which provided input data for a permeation resistance model that estimates the pore size and surface porosity of the asymmetric hollow fiber membrane. We develop and experimentally validate a resistance permeation model for solvent permeation and utilize the surface properties derived from the gas permeation experiments to estimate the relative permeation rates of solvents in a mixture. The approach outlined here highlights the interconnection between gas permeation analysis and OSRO separation performance using Torlon hollow fiber membranes as an exemplar test case. The solvent permeation model is then utilized to provide quantitative insight on the differences between OSRO and organic solvent nanofiltration (OSN), and highlight the important transition region between these two modalities.

Assessing the potential of integrally skinned asymmetric hollow fiber membranes for addressing membrane fouling in pressure retarded osmosis process

Thin-film composite (TFC) membranes are generally preferred over integrally skinned asymmetric (ISA) membranes in pressure retarded osmosis (PRO) process due to their much higher water permeability, but they cannot operate in the active layer facing feed solution (AL-FS) orientation because of potential delamination of the rejection layer under the high applied pressure. However, operating under AL-FS orientation is preferred as a strategy for better fouling control. Hence, in this study, three ISA hollow fiber membranes were fabricated using a dry-jet wet-spinning method, followed by chemical cross-linking to obtain membranes for PRO application. The best-performing membrane was subjected to fouling test in two orientations. Our results demonstrate that the AL-FS orientation is better for a sustainable long-term PRO operation, given the stable performance throughout the two-day testing. Also, the current use of power density per membrane area as a performance benchmark may not be practical for large-scale PRO operation, and the volumetric power density per module is more relevant. In terms of volumetric power density for a hypothetical 5-in. module, our best-performing membrane is predicted to generate a 3.6 to 10 times better power density than commercial modules, rendering our ISA hollow fiber membranes promising for scaling-up to bigger modules.

Scaling-up defect-free asymmetric hollow fiber membranes to produce oxygen-enriched gas for integration into municipal solid waste gasification process

Municipal solid waste (MSW) gasification is an attractive waste-to-energy (WTE) technology with promise for future solid waste management, owing to the production of syngas which can be used as an alternative energy resource. However, quality of produced syngas is not meeting expectation. One promising solution is to use oxygen-enriched gas (OEG) to elevate produced syngas quality but producing OEG is costly. Herein, we demonstrated an asymmetric defect-free hollow fiber membrane made up of Matrimid® 5218 polyimide for scaling-up to 2-inch membrane modules. Our modules have an effective surface area up to 2.6 m2 with a packing density of up to 44% to produce OEG of 45% O2 purity. The 2-inch membrane module was integrated into a lab-scale gasification process to produce syngas using refuse derived fuel (RDF) generated from MSWs collected from the university campus. Prior to this, the lab-scale gasification was first optimized by tuning the gasification temperature, O2 purity and equivalence ratio (ER). Then, MSW gasification using membrane-based OEG was carried out, and compared against air gasification and synthetic OEG of the same O2 purity. Our results showed that, at O2 purity of 45%, gasification temperature of 900 °C and ER of 0.15, the quality of produced syngas was elevated with a lower heating value (LHV) of 9.15 MJ/m3 and H2/CO ratio of 1.43, which was substantially higher than air gasification with LHV and H2/CO ratio reported at 5.38 MJ/m3 and 0.81, respectively. Gasification results for membrane-based OEG and synthetic OEG were also comparable, while process simulation evidence suggested that membrane-based separation was at least 2-fold less energy-intensive than cryogenic distillation and pressure swing adsorption. Overall, this proof-of-concept successfully demonstrates the viability and competitiveness of membrane process for producing OEG at medium O2 purity (i.e., 40–50%) to support MSW gasification for WTE conversion.

Physical and Mechanical Properties of Hollow Fiber Membranes and Technological Parameters of the Gas Separation Process.

The porous layer of composite and asymmetric hollow fiber membranes acts as a support and is exposed to strong mechanical stresses. The effect of external pressure on the polymer structure and, as a consequence, the separation characteristics of the membrane remains unsolved. Based on the solution of the Lamé approach to the calculation of the stress state of a hollow cylinder, a method of calculation was proposed for hollow fiber membranes. Calculations were based on the approximation of the isotropic nature of the physical and mechanical characteristics of the selective layer and substrate. Permissible deformation of the membrane’s selective layer was determined from the linear sector of strain-on-stress dependence, where Hooke’s law was performed. For these calculations, commercial polyethersulfone membranes were chosen with an inner and/or outer selective layer and with the following values of Young’s modulus of 2650 and 72 MPa for the selective and porous layers, respectively. The results obtained indicate that the dependence of the maximum allowable operating pressure on the substrate thickness asymptotically trends to a certain maximum value for a given membrane. Presented data showed that membranes with outer selective layer can be operated at higher working pressure. Optimal parameters for hollow fiber gas separation membrane systems should be realized, solving the optimization problem and taking into account the influence of operating, physicochemical and physicomechanical parameters on each other.

Development of an extended model for the permeation of environmentally hazardous CO2 gas across asymmetric hollow fiber composite membranes

This study presents an extended thermodynamic and phenomenological combined model to mitigate the environmental hazardous acid gas over composite membranes. The model has been applied to an acid gas such as carbon dioxide (CO2) for its permeation through polyetherimide incorporated montmorillonite (Mt) nanoparticles hollow fiber asymmetric composite membranes. The well-established non-equilibrium lattice fluid (NELF) model for penetrating low molecular weight penetrant in a glassy polyetherimide (PEI) was extended to incorporate the other important polymer/filler system features such as tortuosity in acid gas diffusion pathways resulted from layered filler aspect ratio and concentration. The model mentioned above predicts the behavior of acid gas in PEI-Mt composite membranes based on thermodynamic characteristics of CO2 and PEI and tortuosity due to Mt. The calculated results are compared to experimentally determined values of CO2 permeability through PEI-Mt composite asymmetric hollow fiber membranes at varying transmembrane pressures and Mt concentrations. A reasonable agreement was found between the model predicted behavior and experimentally determined data in terms of CO2 solubility, Mt concentration and aspect ratio were calculated based on average absolute relative error (%AARE). The proposed modified model efficiently predicts the CO2 permeance across MMMs up to 3 wt% Mt loadings and 6 bar pressure with ± 10%AARE.

In situ nanoporous structural characterization of asymmetric hollow fiber membranes for desalination using Raman spectroscopy

Observing the nanoporous structure inside membranes has been one of the greatest challenges in membrane research, and many recent studies have employed positron annihilation lifetime spectroscopy, among other methods, to gain structural insights. In this report, we demonstrate the significant potential of Raman spectroscopy for investigating the nanoporous structure inside membranes under in situ conditions. Sulfonated poly(arylene ether sulfone) and cellulose triacetate asymmetric hollow fiber membranes were fabricated and their nanoporous structures were examined using Raman spectroscopy. The membrane dimensions, mechanical properties, and reverse osmosis (RO) performances were also measured. The membrane structures estimated by Raman spectroscopy correlated very well with the mechanical properties and RO performances. This study indicated that Raman spectroscopy represents a promising method for detecting the nanoporous structures inside membranes for desalination applications, and furthermore, that it would be a good tool for developing membranes with optimized nanoporous structures.

Modeling flux in tangential flow filtration using a reverse asymmetric membrane for Chinese hamster ovary cell clarification.

Tangential flow filtration is advantageous for bioreactor clarification as the permeate stream could be introduced directly to the subsequent product capture step. However, membrane fouling coupled with high product rejection has limited its use. Here, the performance of a reverse asymmetric hollow fiber membrane where the more open pore structure faces the feed stream and the barrier layer faces the permeate stream has been investigated. The open surface contains pores up to 40 μm in diameter while the tighter barrier layer has an average pore size of 0.4μm. Filtration of Chinese hamster ovary cell feed streams has been investigated under conditions that could be expected in fed batch operations. The performance of the reverse asymmetric membrane is compared to that of symmetric hollow fiber membranes with nominal pore sizes of 0.2 and 0.65 μm. Laser scanning confocal microscopy was used to observe the locations of particle entrapment. The throughput of the reverse asymmetric membrane is significantly greater than the symmetric membranes. The membrane stabilizes an internal high permeability cake that acts like a depth filter. This stabilized cake can remove particulate matter that would foul the barrier layer if it faced the feed stream. An empirical model has been developed to describe the variation of flux and transmembrane pressure drop during filtration using reverse asymmetric membranes. Our results suggest that using a reverse asymmetric membrane could avoid severe flux decline associated with fouling of the barrier layer during bioreactor clarification.

Asymmetric hollow-fiber filtration membranes based on insoluble polyimide (R-BAPB): Influence of coagulation bath on porous structure

The insoluble polyimides are the most promising group of polymer materials for fabrication of solvent stable filtration membranes suitable for operation at elevated temperatures. In order to synthesize asymmetric membranes from insoluble polyimide, it is proposed to fabricate the membranes from a pre-polymer solution (polyamide acid – PAA) by non-solvent induced phase separation method followed by imidization to form non-soluble porous polyimide membrane. The thermoplastic crystallizable polyimide R-BAPB, which is resistant to a number of known organic solvents, was chosen as a membrane material. For the first time, hollow fiber membranes based on imidized PAA (R-BAPB) with a controlled distribution of pores on the inner side of the hollow fiber were formed. It has been established that the use of “hard” non-solvents, such as water or aqueous-organic solutions, is preferable for the formation of a porous structure in the membranes based on PAA (R-BAPB). Synthesized PAA membranes were used to prepare porous membranes based on the thermoplastic polyimide R-BAPB by thermal imidization. Imidization process was confirmed by an increase in the glass transition temperature of the material to 220 °C (corresponds to the glass transition temperature of polyimide (R-BAPB)) and is accompanied by a significant increase in its elastic modulus. The results of the measurements of the transport properties of polyimide membranes for gases and liquids indicate that microfiltration transport pores are present in the membranes.

Fabrication of Defect-Free P84® Polyimide Hollow Fiber for Gas Separation: Pathway to Formation of Optimized Structure.

The elimination of the additional defect healing post-treatment step in asymmetric hollow fiber manufacturing would result in a significant reduction in membrane production cost. However, obtaining integrally skinned polymeric asymmetric hollow fiber membranes with an ultrathin and defect-free selective layer is quite challenging. In this study, P84® asymmetric hollow fiber membranes with a highly thin (~56 nm) defect-free skin were successfully fabricated by fine tuning the dope composition and spinning parameters using volatile additive (tetrahydrofuran, THF) as key parameters. An extensive experimental and theoretical study of the influence of volatile THF addition on the solubility parameter of the N-methylpyrrolidone/THF solvent mixture was performed. Although THF itself is not a solvent for P84®, in a mixture with a good solvent for the polymer, like N-Methyl-2-pyrrolidone (NMP), it can be dissolved at high THF concentrations (NMP/THF ratio > 0.52). The as-spun fibers had a reproducible ideal CO2/N2 selectivity of 40, and a CO2 permeance of 23 GPU at 35 °C. The fiber production can be scaled-up with retention of the selectivity.

A Rational Asymmetric Hollow Fiber Membrane for Oxygen Permeation

A typical oxygen permeation hollow fiber membrane fabricated by phase inversion‐based extrusion process demonstrates heterogeneous porous microstructures, in which the surface layer with relatively low porosity is used as a separation layer after sintering. It is usually not convenient to control the thickness of separation layer. And a high sintering temperature is needed to densify the separation layer, which in turn could destroy the desired porous microstructures in other portion. This paper studies a novel process to fabricate multilayer asymmetric hollow fiber membrane with a rational design using 67 vol. % Gd0.2Ce0.8O2−δ−33 vol. % La0.6Sr0.4Co0.2Fe0.8O3−δ (GDC‐LSCF) as a model material system. The phase inversion‐based extrusion process in open literature is employed to fabricate a hollow fiber substrate featuring radially well‐aligned microchannels open at the inner surface. Built upon the hollow fiber substrate, a thin dense separation layer and porous surface catalyst layer at shell side are then fabricated through dip‐coating and sintering process alternatively. The oxygen permeation flux of the fabricated hollow fiber membrane reaches 2.68 mL/cm2/min at 900°C under Ar/air gradient, the highest performance of the membranes with GDC‐LSCF material system in open literature. The innovative fabrication process is able to readily control the thickness of functional layers while decreasing sintering temperatures.

Incorporation of CoIII acetylacetonate and SNW-1 nanoparticles to tailor O2/N2 separation performance of mixed-matrix membrane

Mixed-matrix membranes containing a porous organic framework (SNW-1 nanoparticles) and a cobalt-based complex (Co(acac)3) were fabricated using the solution-casting technique for potential applications in O2/N2 separation. The mixed-matrix membranes did not show any appreciable defects. In the following mixture gas permeation testing conducted with a binary mixture composed of 21% O2 and 79% N2 at 1 bar feed pressure and 35 °C, the mixed-matrix membranes containing SNW-1 nanoparticles showed an enhanced O2 permeability, whereas Co(acac)3 improved the O2/N2 selectivity. Hence, with a combination of both nanoparticles and a Cobalt complex in a polymer matrix, a clear synergistic enhancement in O2/N2 separation properties was reported, in which O2 permeability and O2/N2 selectivity were escalated by 33% and 18%, respectively. Solubility-diffusivity analysis in the mixed-matrix membranes revealed that SNW-1 and Co(acac)3 can improve O2 diffusivity and O2 solubility, respectively, resulting in an enhancement of both O2 permeability and O2/N2 selectivity for membranes containing both the nanoparticles and the cobalt complex. Future efforts will be focused on developing asymmetric hollow fiber membranes with thin skin layer comprising both SNW-1 and Co(acac)3.

Simultaneously tuning dense skin and porous substrate of asymmetric hollow fiber membranes for efficient purification of aggressive natural gas

Highly permeable, selective, and stable asymmetric membranes are required to replace the traditional separation approaches for natural gas purification with higher energy efficiency and smaller footprints. Herein, we report on the design and engineering of defect‐free asymmetric hollow fiber membranes with a thin dense skin and highly porous substrate to effectively deal with aggressive natural gas. A crosslinkable polymer with rigid molecular structure and high molecular weight was synthesized for developing spinning dope with desirable solution properties. Phase separation behavior of the polymer was carefully controlled by systematic formulation of the dope composition and optimizing spinning conditions, thereby realizing simultaneously tuning dense skins and porous substrates of the spun asymmetric hollow fiber membranes. The crosslinked hollow fiber membrane, with well‐preserved delicate asymmetric nanostructures, exhibited unprecedentedly high and stable separation performance for long‐term processing extremely aggressive CO2/CH4 mixtures (with pressure up to 820 psi containing C6+ hydrocarbons), thereby showing great potential for practical application of natural gas purification. This work offers a new platform to create hollow fiber membranes with both high permeance and plasticization resistance in natural gas service. © 2019 American Institute of Chemical Engineers AIChE J, 65: 1269–1280, 2019

High-Pressure Aging of Asymmetric Torlon® Hollow Fibers for Helium Separation from Natural Gas

Membrane separation for helium extraction from natural gas gained increased interest recently. Several vendors offer membrane elements for helium extraction, although data on their performance and operating experience are unpublished. The aim of this work was to obtain and study the separation performance of asymmetric hollow-fiber membrane element from commercial polyamide-imide Torlon®, in conditions close to the industrial process of helium extraction from natural gas. A membrane element with an active area of 0.177 m2, a helium permeance of 100 l(STP)/(m2·h·bar), and a selectivity α(He/CH4) = 340 was produced. This corresponds to a selective layer thickness of 82.3 nm, which was confirmed by SEM and resistance model calculations. The obtained membrane element was employed to decrease the concentration of helium in its binary mixture with methane from 0.4% to 0.05%. A relationship of separation characteristics from transmembrane pressure is also presented. At 70 bar and a stage cut of 2.7%, the feed flow rate was 0.16 m3(STP)/h, which yielded a helium permeate concentration of 14.7%. At 80 bar, a decrease in permeance to 60 l(STP)/(m2·h·bar) and in selectivity to 240 was observed. It was shown that the main reason for aging was the increased support resistance, due to a partial compaction of pores with a radius of less than 15 nm.

A rational asymmetric hollow fiber membrane for oxygen permeation

AbstractA typical oxygen permeation hollow fiber membrane fabricated by phase inversion‐based extrusion process demonstrates heterogeneous porous microstructures, in which the surface layer with relatively low porosity is used as a separation layer after sintering. It is usually not convenient to control the thickness of separation layer. And a high sintering temperature is needed to densify the separation layer, which in turn could destroy the desired porous microstructures in other portion. This paper studies a novel process to fabricate multilayer asymmetric hollow fiber membrane with a rational design using 67 vol. % Gd0.2Ce0.8O2−δ−33 vol. % La0.6Sr0.4Co0.2Fe0.8O3−δ (GDC‐LSCF) as a model material system. The phase inversion‐based extrusion process in open literature is employed to fabricate a hollow fiber substrate featuring radially well‐aligned microchannels open at the inner surface. Built upon the hollow fiber substrate, a thin dense separation layer and porous surface catalyst layer at shell side are then fabricated through dip‐coating and sintering process alternatively. The oxygen permeation flux of the fabricated hollow fiber membrane reaches 2.68 mL/cm2/min at 900°C under Ar/air gradient, the highest performance of the membranes with GDC‐LSCF material system in open literature. The innovative fabrication process is able to readily control the thickness of functional layers while decreasing sintering temperatures.