Asymmetric Porous Membranes Research Articles

An experimental study on absorption/stripping CO 2 using mono-ethanol amine hollow fiber membrane contactor

Abstract The present article focuses on making asymmetric porous hollow fiber membranes of polysulfone (PSF) through alternating phases of process using ethanol (2 wt.%) as a non-soluble additive in polymer solution. An aqueous solution of 1-methyl-2-pyrrolidone (NMP) was used as the splitter fluid to prevent the formation of internal dense crust and water was used as an external coagulant. The surface porosity, the pore size and the effective porosity of membrane were examined through characteristic tests. The structure of membrane was studied by scanning electron microscopy (SEM). The performance of the produced membranes was examined to absorb and excrete carbon dioxide in a gas–liquid contactor membrane through the solution of mono-ethanolamine (MEA). The results of gas absorption and stripping test showed that the liquid phase is enhanced by increasing the flow rate. Membranes produced by ethanol indicated that the absorption and excretion of carbon dioxide hollow fiber membrane made of PSF were more than other membranes. The highest absorption of carbon dioxide (3.9 × 10 −3 mol/m 2 /s) and stripping (2.00 × 10 −4 mol/m 2 /s) could be obtained at the liquid flow rate of 300 and 200 ml/min, respectively.

Polysulfone porous hollow fiber membranes for ethylene-ethane separation in gas-liquid membrane contactor

Abstract Separation of olefin from paraffin having same carbon number is one of the most energy intensive separations in petrochemical processes. Employing gas-liquid membrane contactors for olefin/paraffin separation is very attractive. In the present work, the membrane contactor ethylene/ethane separation was investigated based on porous asymmetric polysulfone hollow fiber membranes having mesoporous (dav ∼ 2 nm) skin layer structure in contrast to conventional ultrafiltration membranes (dav ∼ 50–100 nm). Also, the membranes selective layer surface was modified to increase its hydrophobic properties expecting that pore size and surface properties tailoring prevents liquid absorbent penetration into membrane pores. The membrane contactor performance was studied under various operating parameters such as absorbent (aqueous AgNO3) solution concentration and gas and liquid flow rates. Ethylene permeance value was 185 L/(m2 h bar), which is at the level of the best results available in literature for porous membranes, at ethylene recovery rate up to 39%, which is three times higher than that in literature. Modified membranes were characterized after membrane contactor experiments. SEM, EDX and XRD analysis as well as gas permeance measurements showed that silver nitrate crystals deposition in the membranes pore space occurs. However, no noticeable change of membrane contactor performance was observed during two months of experiments on ethylene/ethane separation.

Porous hollow fiber membranes with varying hydrophobic–hydrophilic surface properties for gas–liquid membrane contactors

In relation to the demand for asymmetric porous hollow fiber membranes to be used in gas–liquid membrane contactors designed for operation in organic media, polysulfone membranes of this type have been prepared and subsequently modified to impart oleophobic properties to their surface. The structure and properties of the membranes have been characterized using various techniques, such as optical and scanning electron microscopy, and by measuring contact angles and the permeability of helium, carbon dioxide, and hexane. The surface properties of the membranes have been modified by etching with a mixture of hydrogen peroxide and sulfuric acid or coating with a perfluorinated acrylic copolymer. In the latter case, modified membrane samples have shown a significant reduction in wettability with both water and organic liquids. The hexane permeability data indicate the absence of hexane flow through the membrane modified with perfluorinated acrylic copolymer until a gauge pressure of about 1 atm. The results of the study lead to the conclusion that these membranes can find use in gas–liquid membrane contactors, e.g., for the removal of dissolved gases from liquid hydrocarbons.

Sulfonated polyphenylsulfone asymmetric membranes: Effect of coagulation bath (acetic acid‐NaHCO3/isopropanol) on morphology and antifouling properties

ABSTRACTSulfonated polyphenylsulfone porous asymmetric membranes, S‐PPSU with different sulfonation degrees, 21, 33, 50 wt %, were prepared by phase inversion. Two different coagulation baths were explored for asymmetric membrane preparation: acetone/isopropanol and acetic acid (AA)‐NaHCO3/isopropanol. The latter bath allows better morphology control for the nucleation and pore formation of the membrane. Scanning electron microscopy of membranes shows that pore interconnectivity is improved, when the mixture of AA‐NaHCO3/isopropanol was used for asymmetric S‐PPSU ultrafiltration membranes preparation. S‐PPSU asymmetric membranes show an increasing hydrophilicity with increasing sulfonation degree. Asymmetric membrane antifouling properties improve as the concentration of sulfonic groups increases in the membrane showing twice the flux recovery ratio and lower BSA protein absorption in static and dynamic flux tests. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44502.

Comparison of the Flow of Permanent and Condensable Gases through an Asymmetric Porous Membrane

AbstractPorous inorganic membranes are usually composed of several layers, which are asymmetric with respect to the flow direction. For permanent gases, investigations about the influence of the flow direction on the mass flux through an asymmetric ceramic membrane were previously performed and can be found in literature. Mass flow differences between the two flow directions were calculated. Here the flow of condensable gases, i.e., of vapors, through the same membrane is analyzed and it is shown that the mass flow might be up to 40 % larger in one direction than in the other direction.

Development of High-Antifouling PPSU Ultrafiltration Membrane by Using Compound Additives: Preparation, Morphologies, and Filtration Resistant Properties.

In this study, flat sheet asymmetric polyphenylsulfone (PPSU) ultrafiltration membranes with enhanced antifouling properties were prepared with a non-solvent induced phase separation (NIPS) method through compound additives containing a polymeric pore-forming agent, a small molecular non-solvent and a surfactant. The formation processes of the porous asymmetric membranes with different kinds of additives were studied in detail, and the microstructure controllable preparation of membrane was achieved by establishing a bridge between the membrane preparation parameters and separation performances. All prepared membranes were characterized by using a scanning electron microscope (SEM), contact angle analysis, porosity, maximum pore size, water and BSA solution permeability studies. The performance efficiency of the membrane was evaluated by using BSA as a model foulant in terms of permeability, solute rejection (R), Rm (membrane inherent resistance), Rc (cake layer resistance), and Rp (pore plugging resistance). The results showed that when the compound additives were used, the inter-connected pores were observed, maximum pore size, contact angle and membrane filtration resistance decreased, while the porosity increased. When PVP compound additives were added, the water flux increased from 80.4 to 148.1 L/(m2·h), the BSA rejection increased from 53.2% to 81.5%. A similar trend was observed for membranes with added PEG compound additives; the water flux and BSA rejection simultaneously increased. The filtration resistance decreased as a result of compound additives. The uniformity of membrane and the number of effective pores could be enhanced by adding compound additives through the cooperation of different additives.

3D Membrane Imaging and Porosity Visualization

Ultrafiltration asymmetric porous membranes were imaged by two microscopy methods, which allow 3D reconstruction: focused ion beam and serial block face scanning electron microscopy. A new algorithm was proposed to evaluate porosity and average pore size in different layers orthogonal and parallel to the membrane surface. The 3D reconstruction enabled additionally the visualization of pore interconnectivity in different parts of the membrane. The method was demonstrated for a block copolymer porous membrane and can be extended to other membranes with application in ultrafiltration, supports for forward osmosis, etc., offering a complete view of the transport paths in the membrane.

Asymmetric porous membranes from binary polymer solution by physical gelation induced phase separation

Phase separation induced by physical gelation has been identified, based on systematic experimental data, as the mechanism responsible for the generation of porous structures in elastomeric polydimethylsiloxane (PDMS) assisted drying of binary polymer solution. Comparative experiments of natural drying have enabled one to clarify the similarities and differences in those different drying processes. Based on experimental observations, qualitative explanatory drying models have been suggested for those drying processes. Slow, equilibrated solvent removal in natural drying has led to the formation of gel-phase skin, and this gel-phase skin undergoes wrinkling instability due to large volume shrinkage. The gelation-induced phase separation has occurred at the very last stage of natural drying, leading to nanoscale pores in very thin bottom-most layer. On the other hand, rapid, instantaneous absorption of solvent into PDMS in PDMS-assisted drying has yielded glassy skin at the initial stage of drying. Then the underlying, trapped solution has been separated, induced by physical gelation, into solvent-rich sub-phases embedded into polymer-rich matrix, leading finally to microporous support layer in asymmetric polymeric membrane structure. Physical gelation-induced phase separation, ahead of vitrification of amorphous polymer, has been found to be the responsible mechanism for the generation of porous structure in elastomeric PDMS-assisted drying process. The elucidated phase separation mechanism involved in PDMS-assisted drying, when combined with quantitative kinetic models for drying processes, can pave the way for the fabrication of asymmetric polymeric membranes using other amorphous polymers.

Engineered Asymmetric Heterogeneous Membrane: A Concentration-Gradient-Driven Energy Harvesting Device.

Engineered asymmetric membranes for intelligent molecular and ionic transport control at the nanoscale have gained significant attention and offer prospects for broad application in nanofluidics, energy conversion, and biosensors. Therefore, it is desirable to construct a high-performance heterogeneous membrane capable of coordinating highly selective and rectified ionic transport with a simple, versatile, engineered method to mimic the delicate functionality of biological channels. Here, we demonstrate an engineered asymmetric heterogeneous membrane by combining a porous block copolymer (BCP) membrane, polystyrene-b-poly(4-vinylpyridine) (PS48400-b-P4VP21300), with a track-etched asymmetric porous polyethylene terephthalate membrane. The introduction of chemical, geometrical, and electrostatic heterostructures provides our heterogeneous membrane with excellent anion selectivity and ultrahigh ionic rectification with a ratio of ca. 1075, which is considerably higher than that of existing ionic rectifying systems. This anion-selective heterogeneous membrane was further developed into an energy conversion device to harvest the energy stored in an electrochemical concentration gradient. The concentration polarization phenomenon that commonly exists in traditional reverse electrodialysis can be eliminated with an asymmetric bipolar structure, which considerably increases the output power density. This work presents an important paradigm for the use of versatile BCPs in nanofluidic systems and opens new and promising routes to various breakthroughs in the fields of chemistry, materials science, bioscience, and nanotechnology.

Silicon Asymmetric Membranes for Efficient Lithium Storage: A Scalable Method

AbstractIn this study, scalable membrane technologies are adapted to obtain silicon asymmetric membranes for lithium‐ion battery anodes. The unique asymmetric porous structure can provide both mechanical support and free volume to accommodate the large volume expansion during silicon lithiation, thus leading to excellent rate and cycling performance. An overall specific capacity as high as 1500 mAh g−1 was achieved at 100 mA g−1. Even at 1000 mA g−1, the capacity was still above 800 mAh g−1. More than 90 % of the initial capacity was retained after 200 cycles. It was also observed that a lower Si content and higher carbonization temperature can help achieve stable cycling performance in general. This report is significant in terms of demonstrating a simplistic, generic, and scalable method to create a robust, porous asymmetric membrane structure for efficient lithium‐ion storage.

Moisture transport through asymmetric porous membranes with finger-like holes for indoor humidity control: A lattice Boltzmann simulation approach

Asymmetric porous membranes with finger-like holes are one sort of promising membrane materials for indoor humidity control. Moisture transport in these materials is the key factor influencing humidity control performance. To overcome the difficulties in modelling meso-scale mass transfer in these materials, a lattice Boltzmann simulation (LBM) methodology has been proposed to model the pore-scale gas flow and mass transfer in the asymmetric membranes with finger-like holes. A typical membrane is classified into three sub-layers: a porous support layer, a layer with finger holes, and a denser skin layer. Simulated annealing technique was used in our study to reconstruct the calculating domain. Then fluid flow and mass transfer in the membrane were predicted with LBM, and permeability and effective diffusivity were evaluated. The existence of finger holes in the matrix could dramatically enhance the overall mass transfer in the membranes. Besides, the inhomogeneity in membrane structures would make the macro-scale lumped parameter prediction of membrane performance questionable. Our findings show differences in comparison to previous macro-scale multi-layer analysis. The dominant resistance is in the skin layer. There should be a change in emphasis for membrane optimization with a focus on the skin layer rather than the porous layer.

Natural gas purification using supported ionic liquid membrane

This paper examines the possibility of the application of a supported ionic liquid membrane (SILM) for natural gas purification. The ionic liquid (IL) 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([emim][FAP]) was impregnated successfully in the γ-alumina layer of a tubular porous asymmetric membrane. The pure gas permeability of natural gas components, such as carbon dioxide (CO2), methane (CH4), ethane (C2H6) and propane (C3H8) were tested through the SILM at a trans-membrane pressure of 0.7MPa and temperature of 313K. The following trend of pure gas permeability was observed for the SILM in this study: PCO2>PCH4>PC2H6>PC3H8. Moreover, the CO2/CH4 ideal permselectivity was calculated. Mixed gas permeability and permselectivity for the binary mixture of CO2/CH4 (50/50%, v/v) was also measured. The mixed gas permselectivity (α=1.15) was found to be much lower than the ideal permselectivity (α=3.12). The performance of the SILM was significantly affected by the presence of water, which is also generally present in natural gas. Even though [emim][FAP] is an excellent alternative absorbent with high CO2 absorptive capacity and ideal solubility selectivity for CO2/CH4 (S=9.69), the incorporation of this IL in a SILM is less promising for the removal of CO2 from natural gas streams, because the permselectivity for CO2/CH4 is low.

PVDF-MFI mixed matrix membranes as VOCs adsorbers

This work focuses on the preparation of porous mixed matrix membranes (MMMs) by non-solvent induced phase separation (NIPS) using poly(vinylidenefluoride) (PVDF) as polymer and nano-sized silicalite-1 (S-1) and Ti-silicalite-1 (TS-1) seeds of MFI zeolite structure as inorganic fillers. These MMMs have been used as VOCs adsorbers. Different zeolite seed concentrations (up to 22 wt.%) were homogeneously dispersed into the polymeric matrix to evaluate the influence of the amount of zeolites on the membrane characteristics. In all cases, asymmetric porous membranes, made up of two layers were obtained: the bottom side exhibited spherulitic structure; whereas, at the top side, a smooth layer was observed. With respect to the neat PVDF membranes, the hydrophobic character was found to be improved by the zeolite addition; in particular S-1 nano-seeds. The mechanical properties of the membranes were reduced by the addition of the filler, although the mechanical resistance of the MMMs was still sufficiently preserved. Experiments of hexane adsorption were carried out in order to evaluate the performance of the adsorbers in Volatile Organic Compounds (VOCs) removal. The as-prepared composite porous membranes revealed high adsorption capacity, confirming their potential as adsorbers for removing VOCs traces from waste air environment.

Taming the polysulphide shuttle in Li–S batteries by plasma-induced asymmetric functionalisation of the separator

A novel polymer separator in the form of an asymmetric porous membrane with grafted Li+ exchange groups demonstrated superior polysulphide rejection ability and improved coulombic efficiency in comparison to the standard Celgard separator.

Asymmetric porous membranes formed by coagulation-induced phase separation in poly(ether sulfone)/poly(vinyl pyrrolidone)/genistein blends

Development of asymmetric channel morphology driven by coagulation-induced phase separation of genistein (G) modified poly(ether sulfone)/poly(vinyl pyrrolidone) (PES/PVP) blends has been examined in relation to their miscibility phase diagram. PES/G pairs turned out to be miscible in the amorphous state, whereas solid–liquid phase separation occurred at high genistein compositions. The solid–liquid phase diagram involving the liquidus and solidus lines were computed self-consistently in the framework of the combined free energy of Flory-Huggins for liquid–liquid phase separation and phase field free energy for crystal solidification. The ternary phase diagram of PES/PVP/G blends was subsequently established that consisted of various coexistence regions. The actual amounts of genistein incorporated in the PES/PVP membranes were determined as a function of weight percent of genistein in feed. On the basis of UV-vis spectroscopy, the extent of genistein leaching during incubation in human blood was evaluated in conjunction with the PVP leaching from the blend membrane.

Membrane Design for Water Separation Technologies

Membranes are the core of efficient technologies for water desalination (reverse osmosis) and reuse (ultrafiltration and membrane bioreactor), biomedical processes (hemodialysis) and energy conversion (fuel cell and pressure retarded osmosis). We have been designing multifunctional polymers and copolymers and manufacturing membranes with tailored morphologies for different membrane applications. We have been synthesizing polyazoles with controlled hydrophilicity and porosity and testing for different applications. We explored dense films of polyoxadiazoles and polytriazoles with ionic charge for fuel cell. Hydrophobic fluorinated polyazoles were tested for membrane distillation. The exceptionally high oxidative and temperature resistance as well as the possibility of preparing membranes insoluble in organic solvents have extended the applications to harsh conditions, usually addressed only by ceramic membranes. The biocide function of triazole has been also investigated to reduce biofilm formation under operation for wastewater treatment. Triazole modified polysulfone membranes were synthesized for manufacture of ultrafiltration membranes with low fouling susceptibility. Porous asymmetric polyazole membranes have been prepared as flat-sheet and hollow fiber membranes. Random, graft and block coopolymers based on triazole are currently being explored for membrane manufacture in our group.

Large mass flux differences for opposite flow directions of a condensable gas through an asymmetric porous membrane

The one-dimensional, adiabatic flow of vapors through porous ceramic membranes consisting of three different layers is theoretically investigated. The individual layers have different thicknesses with pore sizes of 10nm for the separation layer, 100nm for the middle layer and 6μm for the support layer. The mass flow is calculated for values of the upstream relative pressure varying between 0.7 and 1. The relative pressure is the ratio of the actual pressure to the saturation pressure. For upstream relative pressures smaller than about 0.77, the fluid does not condense within the porous membrane, and the mass flux is a few percent larger for the flow direction from the support to the separation layer than for the other flow direction. For larger upstream relative pressures, the fluid may condense and liquid or a two-phase mixture may be transported through parts of the membrane. The mass flow can become about 10 times larger for one flow direction than for the opposite flow direction. The flow is modeled using the energy balance and accounting for capillary condensation, for the transport of the enthalpy of vaporization, and for the temperature variation due to the Joule–Thomson effect.

Porous asymmetric SiO2-g-PMMA nanoparticles produced by phase inversion

A new kind of asymmetric organic–inorganic porous structure has been proposed. Asymmetric lattices of polymer grafted silica nanoparticles were manufactured by casting and phase inversion in water. Silica nanoparticles were first functionalized with 3-(dimethylethoxysilyl)propyl-2-bromoisobutyrate, followed by grafting of poly(methylmethacrylate) (PMMA) segments, performed by atom-transfer radical polymerization. Mechanically stable self-standing films were prepared by casting a dispersion of functionalized nanoparticles in different solvents and immersion in water. The resulting asymmetrically porous morphology and nanoparticle assembly was characterized by scanning electron and atomic force microscopy. The PMMA functionalized SiO2 hybrid material in acetone or acetone/dioxane led to the best-assembled structures. Porous asymmetric membranes were prepared by adding free PMMA and PMMA terminated with hydrophilic hydroxyl group. Nitrogen flow of 2800 L m−2 h−1 was measured at 1.3 bar demonstrating the porosity and potential application for membrane technology.

Polymeric and mixed matrix polyimide membranes

Three different polyimide type polymers were used to prepare porous asymmetric membranes by non-solvent induced phase separation (NIPS): a homopolymer (Matrimid) and two co-polymers (P84 and Torlon). The effect of the casting solvent on membrane morphology and transport properties was studied. Moreover, the co-polyimide P84 was selected for further investigations in the form of mixed matrix membranes (MMMs) containing functionalized multiwalled carbon nanotubes (oxidized or aminated MWCNTs). The introduction of polar functional groups was found to be fundamental in order to have a good dispersion of the MWCNTs in the casting solution before, and in the formed membrane, after NIPS, thanks to the formation of hydrogen bonds between the solvents, the MWCNTs and the polymer. The presence of the MWCNTs in the membrane matrix improved the performance in the filtration of dyes solutions in water or ethanol (enhanced permeance and reduced fouling, with similar or higher rejection), with respect to reference polymeric membranes (without MWCNTs). The separation properties of the membranes produced were also compared with literature data obtained with commercial and other laboratory made mixed matrix membranes.

Preparation and Properties of Asymmetric Porous Aluminium-Oxide Ceramic Hollow Fibre Membranes

Asymmetric porous aluminium-oxide ceramic hollow fibre membranes have been prepared by the phase inversion / sintering technique. The effect of non-solvent such as ethanol, isopropanol and ethylene glycol monomethylether(2-methoxyethanol) on the geometry and performance of hollow fibres was investigated. Morphologies of Al2O3 ceramic hollow fibre membranes were characterized using a scanning electron microscope (SEM). The effective porosity and the mechanical strength were determined by Archimedes method, and three point method, respectively. The prepared Al2O3 hollow fibre membranes show the asymmetric structure with a finger-like layer and a sponge-like layer. The effective porosity of the prepared hollow fibre membranes exceeds 47%, and the bending strength of the hollow membranes exceeds 63 MPa. The Al2O3 hollow fibre membranes with moderate permeation characteristics for gas and pure water are prepared by the introduction of nonsolvent in membrane casting solution. The separation factors of H2 to N2 or CO2 of the hollow fibers with nonsolvent are over 2.0.