Recent advances on mixed-matrix membranes for gas separation: Opportunities and engineering challenges

In the past decades, gas separation using polymeric membranes has received considerable attention and become one of the fastest growing research areas. However, existing polymeric membranes may not be able to keep up with the increasing separation needs for challenging gas mixtures such as N₂/CH₄ and light olefin/paraffin pairs on industrial scale due to their so-called permeability-selectivity bound. On the other hand, scaling-up issues poise huge challenges for highly permeable and highly selective inorganic membranes. Mixed-matrix membranes, composite membranes, provide an evolutionary solution to debottleneck the permeability-selectivity and scale-up issues currently faced by polymeric and inorganic membranes, respectively. Inorganic fillers in mixed-matrix membranes improve gas permeability and/or selectivity, outperforming polymeric membranes. Combined with relatively economical and simple scaling-up compared to inorganic membranes, mixed-matrix membranes could potentially be a next-generation membrane concept for gas separation applications. This review provides a brief summary on the recent progress in both flat sheet and hollow fiber mixed-matrix membranes with an emphasis on those made over the last five years. A separate section is dedicated to discussing engineering challenges transitioning from laboratory-scale to large-scale synthesis of mixed-matrix membranes. Finally, future prospects and perspectives in mixed-matrix membranes research are briefly outlined.     

Poly(vinyl chloride) (PVC) hollow fibre membranes for gas separation

Poly(vinyl chloride) (PVC) gas separation hollow fibre membranes were produced from multicomponent dopes using dry/wet forced convection spinning. Membranes spun from a low polymer content solution exhibited disappointing gas separation properties. Their low selectivities were indicative of thick skins and high surface porosities. In contrast, high polymer content spun fibres showed good gas separation properties. Selectivities were high, active layers relatively thin and surface porosities moderate. Coating with poly(dimethylsiloxane) nullified the surface pores. The favourable performance of the high polymer content spun fibres was also related to shear rate and forced convection residence time during spinning. To the knowledge of the authors, this work represents the first reported success in producing PVC hollow fibre membranes with morphologies suitable for gas separation. The development of PVC hollow fibres relates to the ultimate quest to produce membranes capable of reliably separating oxygen and ozone gas mixtures.     

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     

Evolution of polymeric hollow fibers as sustainable technologies: Past, present, and future

Energy, water, affordable healthcare and global warming are four major global concerns resulting from resource depletion, record high oil prices, clean water shortages, high costs of pharmaceuticals, and changing climate conditions. Among many potential solutions, advances in membrane technology afford direct, effective and feasible approaches to solve these sophisticated issues. Membrane technology encompasses numerous technology areas including materials science and engineering, chemistry and chemical engineering, separation and purification phenomena, molecular simulation, as well as process and product design. Currently, polymeric hollow fiber membranes made using a non-solvent-induced phase inversion process are the dominant products because polymers offer a broad spectrum of materials chemistry and result in membranes with desirable physicochemical properties for diverse applications. Their low cost and ease of fabrication make polymeric membranes superior to inorganic membranes. Therefore, this review focuses on state-of-the-art polymeric hollow fiber membranes made from non-solvent-induced phase inversion and the potential of membrane processes for sustainable water and energy production. The specific topics include: (i) basic principles of hollow fiber membrane formation and the phase inversion process; (ii) membranes for energy (natural gas, H₂, and biofuel) production; (iii) membranes for CO₂ capture; and (iv) emerging desalination technologies (forward osmosis and membrane distillation) for water production. Finally, future opportunities and challenges for the development of advanced membrane structures are discussed.     

PDMS/ZIF-8 coating polymeric hollow fiber substrate for alcohol permselective pervaporation membranes

In order to develop high performance composite membranes for alcohol permselective pervaporation (PV), poly (dimethylsiloxane)/ZIF-8 (PDMS/ZIF-8) coated polymeric hollow fiber membranes were studied in this research. First, PDMS was used for the active layer, and Torlon®, PVDF, Ultem®, and Matrimid® with different porosity were used as support layer for fabrication of hollow fiber composite membranes. The performance of the membranes varied with different hollow fiber substrates was investigated. Pure gas permeance of the hollow fiber was tested to investigate the pore size of all fibers. The effect of support layer on the mass transfer in hydrophobic PV composite membrane was investigated. The results show that proper porosity and pore diameter of the support are demanded to minimize the Knudsen effect. Based on the result, ZIF-8 was introduced to prepare more selective separation layer, in order to improve the PV performance. The PDMS/ZIF-8/Torlon® membrane had a separation factor of 8.9 and a total flux of 847 g·m^-2·h^-1. This hollow fiber PDMS/ZIF-8/Torlon® composite membrane has a great potential in the industrial application.     

Preparation of Hollow Fiber Membranes by Nonsolvent Induced Phase Separation along with Hydrogen Gas Formation Using a Single Orifice Spinneret

Traditional nonsolvent induced phase separation (NIPS) process for fabrication of hollow fiber membranes (HFMs) faces challenges, like design and manufacture of spinneret with two concentric orifices to provide parallel and continuous feed of polymer solution and bore fluid at specific rates. These factors limit the use of traditional technique to produce HFMs. Here, a new direct spinning method for fabricating HFMs by feeding a polymer solution, containing a gas producing agent using single orifice spinneret is reported. Polysulfone‐dimethylacetamide solution containing NaBH₄ is extruded through a stainless‐steel needle (single orifice spinneret) into HCl aqueous solution (coagulation bath) at specific rates. Effects of polysulfone concentration, temperature, and pH of coagulant bath on structure and performance of the HFMs are investigated. Synergy between hydrogen from NaBH₄ hydrolysis and NIPS process benefits fabrication of HFMs with good hollow bore structure and high porous wall. The prepared HFMs show good dye separation.     

Engineering substructure morphology of asymmetric carbon molecular sieve hollow fiber membranes

In this study, a novel pre-pyrolysis treatment is developed to restrict the morphology collapse in asymmetric carbon molecular sieve (CMS) hollow fiber membranes. The technique is referred as V-treatment, due to the use of a sol–gel crosslinking reaction between an organic-alkoxy silane (vinyltrimethoxysilane) and moisture. The V-treatment technique enables restricting the microscale morphology collapse in asymmetric CMS membranes without having a chemical reaction with the polymer precursor material. The effect of V-treatment is reported on two different polyimide precursors: Matrimid® and 6FDA:BPDA-DAM. For both the CMS V-treated Matrimid® and 6FDA:BPDA-DAM hollow fibers, a significant reduction up to 5–6-fold in apparent membrane skin thickness is observed compared to the CMS from untreated precursors. This improvement translates to an increase in gas separation productivities for both pure and mixed gas feeds in CMS V-treated Matrimid® and 6FDA:BPDA-DAM hollow fiber membranes. Moreover, several characterization analyses and transport results for V-treatment method using 100% VTMS are reported herein.     

过滤用特种纤维及其纺织品发展近况

高性能纤维滤材对节能减排有重要作用,而某些功能纤维滤材在有些应用领域起着关键或核心作用。介绍了用于水处理的中空纤维膜、纳米纤维和纳米碳管、离子交换纤维及活性碳纤维,用于海水淡化和透析的中空纤维膜,用于气体分离的中空纤维膜和纳米纤维,以及用于水环境治理的碳纤维等的应用近况。     

MATHEMATICAL AND EXPERIMENTAL ANALYSIS OF AIR SEPARATION BY HOLLOW FIBER MEMBRANES

Approximate solutions for gas separation by hollow fiber membranes have been developedby several investigators.However,there are few reports of experimental verification of the models forhigh stage cut separations.In this work,an approximate mathematical model was developed and wasexperimentally verified for high stage cut air separation.Both countercurrent and cocurrent now pat-terns were used.In addition,the applicability of feed-inside mode for low stage cut air separation byhollow fiber membrane was examined.It was found that feed-inside mods was more advantageousthan feed-outside mode when used for the generation of oxygen-enriched air.     

Graphene oxide membranes supported on the ceramic hollow fibre for efficient H2 recovery

The special channels and intrinsic defects within GO laminates make it a very potential candidate for gas separation in recent years. Herein, the gas separation performance of GO membranes prepared on the surface of ceramicα-Al_2O_3 hollow fibre was investigated systematically. The microstructures of ceramic hollow fibre supported GO membranes were optimized by adjusting operation conditions. And, the GO membrane fabricated at 30 min exhibited great promising H_2 recovery ability from H_2/CO_2 mixture. At room temperature, the H_2 permeance was over 1.00 × 10~(-7)mol·m~(-2)·s~(-1)·Pa~(-1)for both single gas and binary mixture. The corresponding ideal selectivity and mixture separation factor reached around 15 and 10, respectively. In addition, humility, operation temperature, H_2 concentration in the feed and the reproducibility were also studied in this work.     

Improvement of H2/CH4 Separation Performance of PES Hollow Fiber Membranes by Addition of MWCNTs into Polymeric Matrix

Carboxylated multiwalled carbon nanotubes (MWCNTs) were added to polyethersulfone hollow fiber membranes to improve their H₂/CH₄ separation properties. The addition of MWCNTs up to 1 wt% increased macrovoids formation in cross-section, while in 2 wt% loading, decreased due to increase in dope viscosity. The best gas separation performance for the mixed-matrix hollow fiber membranes was achieved at 1 wt% MWCNTs loading with hydrogen permeance of 69 GPU and H₂/CH₄ selectivity of 44.1 at 5 bar(g). Tensile test results showed that incorporation of MWCNTs into the polymeric matrix affected the mechanical properties of the fabricated membranes.     

An Explicit Solution for Concentration Polarization for Gas Separation in a Hollow Fiber Membrane

In the present work, a one-dimensional mathematical model is developed to analyze the concentration polarization phenomenon for the separation of gas mixtures in composite hollow fiber membranes. An analytical expression is developed for determining the interfacial concentration at the interface of dense and porous support layers. Further, the model accounts for the non-ideality of the gas mixture. Both co-current and counter-current flow configurations for the separation of hydrogen from a three-component mixture are studied. The effects of feed side pressure and velocity as well as permeability on concentration polarization are probed. It is apparent from this study that the concentration polarization phenomenon significantly affects the separation efficiency at higher permeability values.     

Asymmetric Permeators—A Conceptual Study

Abstract

The asymmetric permeator concept of Ohno et. al. utilizing two different membranes for rare gas separation has been explored in general. Various geometrical arrangements and possible applications to gas separations other than rare gas-nitrogen mixtures have been discussed. The utility of an asymmetric permeator for multicomponent gas separations has been investigated. The separation factor of a ternary system in a perfectly mixed asymmetric permeator has been obtained. The amount of separation obtained with a ternary feed in a perfect crossflow stage having no axial mixing has been analytically determined for some limiting cases with an asymmetric permeator. The asymmetric permeator concept has been extended also to a high separation factor liquid solution separation process like reverse osmosis desalination. Preliminary calculations have been carried out to show that an asymmetric desalinator with reverse osmosis (RO) and piezodialysis (PD) membranes has a lower increase in brine concentration along the module length for a given water recovery resulting in a lower operating pressure. With hollow fiber asymmetric desalinators having RO and PD membranes, the concentration polarization, if any, may be significantly reduced. Practical applications of asymmetric permeator's for phenol-water separation etc. have been discussed.     

Hydrogen separation at elevated temperatures using metallic nickel hollow fiber membranes

Nickel is a cheaper metallic material compared to palladium membranes for H₂ separation. In this work, metallic Ni hollow fiber membranes were fabricated by a combined phase inversion and atmospheric sintering method. The morphology and membrane thickness of the hollow fibers was tuned by varying the spinning parameters like bore liquid flow rate and air gap distance. H₂ permeation through the Ni hollow fibers with N₂ as the sweep gas was measured under various operating conditions. A rigorous model considering temperature profiles was developed to fit the experimental data. The results show that the hydrogen permeation flux can be well described by using the Sieverts’ equation, implying that the membrane bulk diffusion is still the rate‐limiting step. The hydrogen separation rate in the Ni hollow fiber module can be improved by 4–8% when switching the co‐current flow to the countercurrent flow operation. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3026–3034, 2017     

A Flat Polymeric Membrane Sensor for Carbon Dioxide/Nitrogen Gas Mixture

A gas sensor was developed to measure the concentration of binary gas mixtures. This sensor works based on the permeability change of different gas mixtures across the polymeric membranes. Although high values of permeability and selectivity are needed for an ideal separation, the performance of this sensor mainly depends on the permeability factor. Polysulfone and silicone rubber were applied as the membrane base and coat, respectively. Moreover, in contrast to existing polymeric sensors that use hollow fibers, the present sensor is made of flat membranes. This new design is cheaper, smaller, and easier to use in comparison to the hollow fiber polymeric sensors. In order to test the sensor applicability, nitrogen and carbon dioxide were used as model gases. The effect of pressure on the response time and sensor accuracy was studied for the aforementioned gases. The response time (T_95%) of this low price sensor was 50?s, and the tolerance of measuring concentration was approximately 1.4% at 2?bar feed pressure. Also, increasing the feed pressure can improve the response time or accuracy of the sensor.     

Preparation of poly(4‐methyl‐1‐pentene) asymmetric or microporous hollow‐fiber membranes by melt‐spun and cold‐stretch method

Poly(4‐methyl‐1‐pentene) (PMP) hollow fibers were prepared and fabricated into gas separation or microporous membranes by the melt‐spun and cold‐stretched method. PMP resin was melt‐extruded into hollow fibers with cold air as the cooling medium. The effects of take‐up speed and thermotreatment on the mechanical behavior and morphology of the fibers were investigated. Scanning electronic microscope (SEM) photos were used to reveal the geometric structure of the section and surface of the hollow fibers. It was found that the original fiber had an asymmetric structure. A “sandwich” mode was used to describe the formation of this special fine structure. And a series of PMP hollow‐fiber membranes were prepared by subsequent drawing, and it was found that there was a “skin–core” structure on the cross section of these hollow‐fiber membranes. Asymmetric or microporous PMP hollow‐fiber membranes could be obtained by controlling posttreatment conditions. The morphology of these membranes were characterized by SEM, and the gas (oxygen, nitrogen, and carbon dioxide) permeation properties of the membranes was measured. The results indicate that the annealing time of the original fiber and the stretching ratio were the key factors influencing the structure of the resulting membrane. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 2131–2141, 2006     

Characterization of hollow fiber membranes in a permeator using binary gas mixtures

We have studied the CO₂/CH₄ mixed gas permeation through hollow fiber membranes in a permeator. An approach to characterize the true separation performance of hollow fiber membranes for binary gas mixtures was provided based on experiments and simulations. Experiments were carried out to measure the retentate and permeate flow rates and compositions at each outlet. The influences of pressure drop within the hollow fibers, non-ideal gas behavior in the mixture and concentration polarization were taken into consideration in the mathematics model. The calculation results indicate that the net influence of the non-ideal gas behavior, competitive sorption and plasticization yields the calculated CO₂ permeance in a mixed gas permeator close to that obtained in pure gas tests. Whereas the CH₄ permeance is higher in the mixed gas tests than that in the pure gas tests, as the plasticization caused by CO₂ dominates the permeation process. As a result, the CO₂/CH₄ mixed gas selectivity is smaller than those obtained in pure gas tests at equivalent pressures.The calculated membrane performance shows little changes with stage cut if the effect of concentration polarization is accounted for in the calculation. The integration method developed in this study could provide more accurate characterizations of mixed gas permeance of hollow membranes than other estimation methods, as our model considers the roles of non-ideal gas behavior and concentration polarization properly.     

Polymeric hollow fiber membranes for bioartificial organs and tissue engineering applications

Polymeric hollow fiber (HF) membranes are commercially available, i.e. microfiltration and ultrafiltration cartridges or reverse osmosis and gas separation modules, to be applied for separation purposes in industry, for instance to recover valuable raw materials or products, or for the treatment of end‐of‐pipe wastes to avoid environmental impacts, to regenerate or treat waters for reuse and for the separation of key components or clarification in food and beverage industries. They have also shown important benefits as hemodialyzers, hemodiafiltration or plasma purification devices in patients with liver or kidney damage. The good mass transport properties characterizing the polymeric HFs have opened new research areas of application in the biomedical field, such as the tissue engineering (TE) and the construction of bioartificial organs (BAO). In TE, the HFs act as scaffolds or supports and/or allow high permeance of nutrients and waste removal for cell proliferation and differentiation. In BAO, HFs are used for the fabrication of bio‐hybrid constructs that replace the damaged organs of the patient or can be used as in vitro models for therapeutic studies. This review presents the state‐of‐the‐art concerning preparation and application of HFs for TE and BAO and discusses the challenges and future perspectives of the HFs in both fields. © 2014 Society of Chemical Industry     

Characterization of graft polymerization of fluoroalkyl methacrylate onto PDMS hollow‐fiber membranes and their permselectivity for volatile organic compounds

Polydimethylsiloxane (PDMS) hollow‐fiber membranes grafted with 1H,1H,9H‐hexadecafluorononyl methacrylate (HDFNMA), which is a fluoroalkyl methacrylate, using a ⁶⁰Co irradiation source, were characterized and applied to pervaporation. The PDMS hollow‐fiber membranes were filled with N₂ gas and sealed. The membranes and the HDFNMA solution were then irradiated simultaneously. In the HDFNMA solution, graft polymerization was performed. The degree of grafting of the outside surface of the hollow fiber was greater than that in the inside surface of the hollow fiber. In the grafted PDMS hollow‐fiber membranes, the best separation performance was shown due to the introduced hydrophobic polymer, poly(HDFNMA). The grafted membrane had a microphase‐separated structure, that is, a separated structure of PDMS and graft‐polymerized HDFNMA. The permeability of molecules in the poly(HDFNMA) phase was so low that the diffusion of molecules was prevented in the active layer with many poly(HDFNMA) domains, as the feed solution was introduced through the inside of the hollow fibers and the outside was vacuumed. As the feed solution was introduced through the outside of the hollow fibers and the inside was vacuumed, the diffusion of molecules was not prevented in the active layer with few poly(HDFNMA) domains. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 1573–1580, 2003     

Nickel(II) ion-intercalated MXene membranes for enhanced H2/CO2 separation

Hydrogen fuel has been embraced as a potential long-term solution to the growing demand for clean energy. A membrane-assisted separation is promising in producing high-purity H₂. Molecular sieving membranes (MSMs) are endowed with high gas selectivity and permeability because their well-defined micropores can facilitate molecular exclusion, diffusion, and adsorption. In this work, MXene nanosheets intercalated with Ni²⁺ were assembled to form an MSM supported on Al₂O₃ hollow fiber via a vacuum-assisted filtration and drying process. The prepared membranes showed excellent H₂/CO₂ mixture separation performance at room temperature. Separation factor reached 615 with a hydrogen permeance of 8.35 × 10⁻⁸ mol·m⁻²·s⁻¹·Pa⁻¹. Compared with the original Ti₃C₂T_x/Al₂O₃ hollow fiber membranes, the permeation of hydrogen through the Ni²⁺-Ti₃C₂T_x/Al₂O₃ membrane was considerably increased, stemming from the strong interaction between the negatively charged MXene nanosheets and Ni²⁺. The interlayer spacing of MSMs was tuned by Ni²⁺. During 200-hour testing, the resultant membrane maintained an excellent gas separation without any substantial performance decline. Our results indicate that the Ni²⁺ tailored Ti₃C₂T_x/Al₂O₃ hollow fiber membranes can inspire promising industrial applications.