The effect of ethane on the performance of commercial polyphenylene oxide and Cardo-type polyimide hollow fiber membranes in CO<Subscript>2</Subscript>/CH<Subscript>4</Subscript> separation applications

Impurities such as hydrogen sulfide, water vapor and heavy hydrocarbons in natural gas have considerable effects on the membrane performance. Small amounts of condensable and polymer soluble components in the feed gas cause swelling or plasticization of glassy membranes, leading to a reduction in membrane selectivity. In the present research the influence of ethane was investigated on the permeance and selectivity of two commercially available hollow fiber membranes, namely Cardo-type polyimide and PPO hollow fibers for CO₂/CH₄ separations. It was concluded that the gas mixture permeation rate was increased in the presence of C₂H₆. However, the CO₂/CH₄ separation factors remained almost the same in the presence and absence of the C₂H₆.     

Polydimethylsiloxane/postmodified MIL‐53 composite layer coated on asymmetric hollow fiber membrane for improving gas separation performance

Composite layer containing postmodified MIL‐53 (P‐MIL‐53) was exploited to be coated on as‐fabricated asymmetric hollow fiber membrane for improving gas separation performance. The morphology and pore size distribution of P‐MIL‐53 particles were characterized by SEM and N₂ adsorption isotherm. The EDX mapping and FTIR spectra were performed to confirm the presence of P‐MIL‐53 deposited on the outer surface of hollow fiber membranes. The results of pure gas permeation measurement indicated that incorporation of P‐MIL‐53 particles in coating layer could improve permeation properties of hollow fiber membranes. By varying coating times and P‐MIL‐53 content, the membrane coated with PDMS/15%P‐MIL‐53 composite by three times achieved best performance. Compared to pure PDMS coated membrane, CO₂ permeance was enhanced from 29.96 GPU to 40.24 GPU and ideal selectivity of CO₂/N₂ and CO₂/CH₄ also increased from 23.28 and 26.95 to 28.08 and 32.03, respectively. The gas transport through composite membrane was governed by solution‐diffusion mechanism and CO₂ preferential adsorption of P‐MIL‐53 contributed to considerable increase of CO₂ solubility resulting in accelerated permeation rate. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44999.     

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     

Effects of halloysite nanotubes on the morphology and CO2/CH4 separation performance of Pebax/polyetherimide thin-film composite membranes

Enhancing the performance of gas separation membranes is one of the major concerns of membrane researchers. Thus, in this study, poly(ether-block-amide) (Pebax)/polyetherimide (PEI) thin-film composite membranes were prepared and their CO₂/CH₄ gas separation performance was investigated by means of pure and mixed gases permeation tests. To improve the properties of these membranes, halloysite nanotubes (HNT) were added to Pebax layer at different loadings of 0.5, 1, 2, and 5 wt % to form Pebax-HNT/PEI membranes. Scanning electron microscopy, gas sorption, X-ray diffraction, Fourier-transform infrared, and differential scanning calorimetry tests were also performed to investigate the impact of HNT on structure and properties of prepared membranes. Results showed that both CO₂/CH₄ selectivity and CO₂ permeance increased by adding HNT to Pebax layer up to 2 wt %. By increasing HNT loading to 5 wt %, the CO₂/CH₄ selectivity decreased from 32 to 18, while CO₂ permeance increased from 3.25 to 4.2 GPU. Pebax/PEI and Pebax-HNT/PEI membranes containing 2 wt % of HNT were tested using CO₂/CH₄ gas mixtures at different feed CO₂ concentrations and feed pressure of 4 bar. The results showed that with increasing CO₂ concentration from 20 to 80 vol %, CO₂/CH₄ selectivity of Pebax/PEI composite membranes increased by 19%, while, in Pebax-HNT/PEI membrane, CO₂/CH₄ selectivity decreased by 40%. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48860.     

Gas separation performance of 6FDA-based polyimides with different chemical structures

This work reports the gas separation performance of several 6FDA-based polyimides with different chemical structures, to correlate chemical structure with gas transport properties with a special focus on CO₂ and CH₄ transport and plasticization stability of the polyimides membranes relevant to natural gas purification. The consideration of the other gases (He, O₂ and N₂) provided additional insights regarding effects of backbone structure on detailed penetrant properties. The polyimides studied include 6FDA-DAM, 6FDA-mPDA, 6FDA-DABA, 6FDA-DAM:DABA (3:2), 6FDA-DAM:mPDA (3:2) and 6FDA-mPDA:DABA (3:2). Both pure and binary gas permeation were investigated. The packing density, which is tunable by adjusting monomer type and composition of the various samples, correlated with transport permeability and selectivity. The separation performance of the polyimides for various gas pairs were also plotted for comparison to the upper bound curves, and it was found that this family of materials shows attractive performance. The CO₂ plasticization responses for the un-cross-linked polyimides showed good plasticization resistance to CO₂/CH₄ mixed gas with 10% CO₂; however, only the cross-linked polyimides showed good plasticization resistance under aggressive gas feed conditions (CO₂/CH₄ mixed gas with 50% CO₂ or pure CO₂). For future work, asymmetric hollow fibers and carbon molecular sieve membranes based on the most attractive members of the family will be considered.     

Effect of Raw Multi-Wall Carbon Nanotubes on Morphology and Separation Properties of Polyimide Membranes

Raw multi wall carbon nanotubes (r-MWCNTs) were embedded as fillers inside the polyimide (PI) matrix and PI/r-MWCNTs mixed matrix membranes were fabricated by the phase inversion method. The TEM images and permeation results using helium as test gas showed that r-MWCNTs were generally closed ended and acted as impermeable nano particles. Gas permeation tests using CO₂ and CH₄ showed that the addition of r-MWCNTs into the dope solution increased the CO₂/CH₄ separation factor while decreasing the carbon dioxide and methane permeances. When the r-MWCNTs content was increased from 0% to 6 wt.%, permeance of CO₂ in the flat sheet mixed matrix membranes decreased from 9.15 GPU to 5.49 GPU and CO₂/CH₄ separation factor increased from 19.05 to 45.75. Identical to flat sheet mixed matrix membranes, the addition of 2 wt.% r-MWCNTs into a spinning dope increased the CO₂/CH₄ separation factor from 46.61 to 72.20. The glass transition temperature of the mixed matrix flat sheet membranes increased with an increase in the r-MWCNTs content. This implies a good segmental-level attachment between the two phases that forms a rigidified polymer region at the polymer/r–MWCNTs interface. FESEM images showed well dispersed r-MWCNTs in the polymer matrix at a loading of 2 wt% r-MWCNTs.     

Preparation of polyvinylamine/polysulfone composite hollow‐fiber membranes and their CO2/CH4 separation performance

Fixed‐carrier composite hollow‐fiber membranes were prepared with polyvinylamine (PVAm) as the selective layer and a polysulfone ultrafiltration membrane as the substrate. The effects of the PVAm concentration in the coating solution, the number of coatings, and the crosslinking of glutaraldehyde and sulfuric acid on the CO₂ permeation rate and CO₂/CH₄ selectivity of the composite membranes were investigated. As the PVAm concentration and the number of coatings increased, the CO₂/CH₄ selectivity increased, but the CO₂ permeation rate decreased. The membranes crosslinked by glutaraldehyde or sulfuric acid possessed higher CO₂/CH₄ selectivities but lower CO₂ permeation rates. For the pure feed gas, a composite hollow‐fiber membrane coated with a 2 wt % PVAm solution two times and then crosslinked with glutaraldehyde and an acid solution in sequence had a CO₂ permeation rate of 3.99 × 10^?6 cm³ cm^?2 s^?1 cmHg^?1 and an ideal CO₂/CH₄ selectivity of 206 at a feed gas pressure of 96 cmHg and 298 K. The effect of time on the performance of the membranes was also investigated. The performance stability of the membranes was good during 6 days of testing. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 1885–1891, 2006     

Ultra-thin skin carbon hollow fiber membranes for sustainable molecular separations

A transformative platform is reported to derive ultra-thin carbon molecular sieve (CMS) hollow fiber membranes from dual-layer precursor hollow fibers with independently tuned skin layer and substrate properties. These ultra-thin CMS hollow fiber membranes show attractive CO₂/CH₄ separation factors and excellent CO₂ permeances up to ~1,400% higher than state-of-the-art asymmetric CMS hollow fiber membranes. They provide a unique combination of permeance and selectivity competitive with zeolite membranes, but with much higher membrane packing density and potentially much lower costs.     

Transport and Permeation Properties of a Ternary Gas Mixture in a Medium-Size Polysulfone Hollow Fiber Permeator

Abstract

Permeation properties were analyzed for a mixture of CO₂, O₂, and N₂ in a medium-size polysulfone hollow fiber permeator with a net permeation area of 4.22 m². Measurements were conducted as a function of feed composition, reject flow rate, and feed pressure. Results included variations in species permeability, separation factor, permeate enrichment, reject depletion, and stage cut as a function of system parameters. Variations in permeation properties show strong dependence on feed composition, reject flow rate, and feed pressure. Permeability of carbon dioxide was higher at larger feed pressures and higher carbon dioxide content in the feed stream. Effect of increasing the reject flow rates on the permeability of carbon dioxide was affected by the system pressure and the carbon dioxide content in the feed stream. At low pressures, increase of the reject flow rate resulted in a decrease of carbon dioxide permeability. The opposite behavior was obtained at higher feed pressures. Increase of the reject flow rate reduced the gas residence time within the permeator. Increase of reject flow rate reduced species residence within the permeator and in turn increased resistance to species transport within the permeator. However, higher system pressures and carbon dioxide content in the feed stream resulted in larger levels of membrane plasticization, which increased the permeation rates of all species. The combined efféct of reducing the species residence time within the permeator and the level of membrane plasticization favored the permeation of carbon dioxide versus the other two species. Variations in other permeation properties, which include oxygen and nitrogen permeabilities, stage cut, permeate enrichment in carbon dioxide, and reject depletion in carbon dioxide, were also explained in terms of resistances encountered within the permeator and the membrane.     

Fabrication and characterization of PEI/PVP‐based carbon hollow fiber membranes for CO2/CH4 and CO2/N2 separation

Carbon hollow fiber membranes derived from polymer blend of polyetherimide and polyvinylpyrrolidone (PVP) were extensively prepared through stabilization under air atmosphere followed by carbonization under N₂ atmosphere. The effects of the PVP compositions on the thermal behavior, structure, and gas permeation properties were investigated thoroughly by means of differential scanning calorimetry, thermogravimetric analysis, X‐ray diffraction, and pure gas permeation apparatus. The experimental results indicate that the transport mechanism of small gas molecules of N₂, CO₂, and CH₄ is dominated by the molecular sieving effect. The gas permeation properties of the prepared carbon membranes have a strong dependency on PVP composition. The carbon membranes prepared from polymer blends with 6 wt % PVP demonstrated the highest CO₂/CH₄ and CO₂/N₂ selectivities of 55.33 and 41.50, respectively. © 2011 American Institute of Chemical Engineers AIChE J, 58: 3167–3175, 2012     

Characterization of crosslinked hollow fiber membranes

As the applications for polymeric membranes expand, new challenges arise. One of the largest of these challenges is the plasticization caused by strongly swelling penetrants such as carbon dioxide at elevated pressures. A considerable amount of material research has investigated crosslinking of dense film membranes to increase plasticization resistance. This paper extends such materials research to include more practically relevant asymmetric hollow fibers. Crosslinkable polyimide fibers were spun and an ester crosslinking reaction was studied using chemical and spectroscopic techniques to characterize the extent of crosslinking and to relate the effect of the reaction on fiber stability. CO₂ permeance and CO₂/CH₄ selectivity were studied at a variety of pressures and temperatures over time to yield indications of real-world separation performance.     

Optimization of CO2–CH4 separation performance of integrally skinned asymmetric membranes prepared from poly(2,6‐dimethyl‐1,4‐phenylene oxide) by factorial design

Integrally skinned asymmetric flat sheet membranes were prepared from poly(2,6‐dimethyl 1,4‐phenylene oxide)(PPO) for CO₂–CH₄ separation. Various experiments were carried out to identify PPO membranes, which have good mechanical strength and gas separation abilities. Membrane strength and selectivity depend on the interplay of the rate of precipitation and the rate of crystallization of the PPO. The effects of major variables involved in the membrane formation and performance, including the concentration of the polymer, solvent, and additive, the casting thickness, the evaporation time before gelation, and the temperature of the polymer solution, were investigated. Factorial design experiments were carried out to identify the factor effects. The membrane performance was modelled and optimized to approach preset values for high CO₂ permeance and a high CO₂ : CH₄ permeance ratio. Membranes were prepared based on the optimum conditions identified by the model. Essentially, defect‐free membranes were prepared at these conditions, which resulted in a pure gas permeance of 9.2 × 10⁻⁹ mol/m² s Pa for CO₂ and a permeance ratio of 19.2 for CO₂ : CH₄. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 1601–1610, 1999     

CHARACTERIZATION OF MIXED GAS PERMEATION THROUGH HOLLOW FIBERS

We have studied the mixed gas permeation in hollow fiber membrane modules using two approaches: namely, the co- current plug flow model and the complete mixing model with the combination of experimental data. Elucidation was made to determine the permeance of CO2 and CH4 and the selectivity of CO2/CH4 in a polyimide hollow fiber membrane permeator It is found that the intrinsic gas separation properties of hollow fibers for mixed gases can be accurately determined based on (1) the cocurrent plug now model, and (2) the complete mixing model with the assumption of averaged retentate concentration of the feed and the retentate outlet.     

Mixed gas and pure gas transport properties of copolyimide membranes

Copolyimides were synthesized from dianhydride of 4,4′‐(hexafluoroisopropylidene)diphthalic anhydride (6FDA) with various diamine contents of 4,4′‐oxydianiline (ODA) and 2,3,5,6‐tetramethyl‐1,4‐phenylenediamine (TeMPD) by chemical imidization in a two‐step procedure. Polyimides (PIs) were characterized using thermogravimetric analysis, Fourier transform infrared spectroscopy, differential scanning calorimetry, as well as specific volume and free volume. The gas transport properties for pure gas and blends of CO₂ and CH₄ for the homopolymers and 6FDA‐ODA/TeMPD copolymers were investigated at 35°C and 150 psi pressure. In pure gas permeation, permeability of CO₂ and CH₄ increased with increasing TeMPD content in the diamine moiety, whereas the ideal selectivity decreased with increasing TeMPD content. In the mixed gas permeation, permeabilities and separation factor were measured as a function of CO₂ feed molar fraction for five PI membranes. The behavior of pure gas and mixed gas permeabilities and separation factor of CO₂/CH₄ mixtures as the chemical nature of the diamine and the CO₂ molar fraction in the feed gas were varied and are discussed in detail. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2013     

Dual-layer hollow carbon fiber membranes for gas separation consisting of carbon and mixed matrix layers

Hollow carbon fiber membranes for gas separation have been successfully fabricated for the first time by a special type of precursor. This precursor is dual-layer hollow fiber composed of a polysulfone-beta zeolite (PSF-beta) mixed matrix outer layer and a Matrimid inner layer. Pure gas permeation measurements show that the resultant hollow carbon fiber has O₂/N₂ and CO₂/CH₄ selectivities of 9.3 and 150, respectively; this performance is much better than that of the hollow carbon fiber derived from single-layer Matrimid hollow fiber. Mixed gas measurements show the CO₂/CH₄ selectivity of 128. After pyrolysis, the PSF-beta layer in the dual-layer precursor evolves into a continuous structure of closely packed zeolite particles embedded in the PSF carbon residue. TGA spectra suggest that the possible reason for the above observation is that the PSF-beta outer layer and Matrimid inner layer has significantly changed each other’s pyrolysis dynamics and thermal degradation process.     

Novel fixed‐carrier membranes for CO2 separation

A new membrane material having two kinds of CO₂ carriers was obtained. Composite membranes were prepared with the material and support membranes. The facilitated transport of CO₂ through these membranes was performed with pure CH₄ and CO₂ as well as CH₄/CO₂ mixtures containing 50 vol % CO₂. The results show that the membranes possess better CO₂ permeance than that of other fixed carrier membranes reported in the literature. In the measurements with pure gases, at 26°C, 0.013 atm of CO₂ pressure, the membrane with polysulfone support displays a CO₂ permeance of 7.93 × 10^?4 cm³ /cm² s cmHg and CO₂/CH₄ ideal selectivity of 212.1. In the measurements with mixed gases, at 26°C, 0.016 atm of CO₂ partial pressure, the membrane displays a CO₂ permeance of 1.69 × 10^?4 cm³ /cm² s cmHg and CO₂/CH₄ selectivity of 48.1. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 2222–2226, 2002     

Separation of CO2/CH4 through carbon molecular sieve membranes derived from P84 polyimide

The separation of CO₂/CH₄ separation is industrially important especially for natural gas processing. In the past decades, polymeric membranes separation technology has been widely adopted for CO₂/CH₄ separation. However, polymeric membranes are suffering from plasticization by condensable CO₂ molecules. Thus, carbon molecular sieve membranes (CMSMs) with excellent separation performance and stability appear to be a promising candidate for CO₂/CH₄ separation. A commercially available polyimide, P84 has been chosen as a precursor in preparing carbon membranes for this study. P84 displays a very high selectivity among the polyimides. The carbonization process was carried out at 550–800 °C under vacuum environment. WAXD and density measurements were performed to characterize the morphology of carbon membranes. The permeation properties of single and equimolar binary gas mixture through carbon membranes were measured and analyzed. The highest selectivity was attained by carbon membranes pyrolyzed at 800 °C, where the pyrolysis temperatures significantly affected the permeation properties of carbon membranes. A comparison of permeation properties among carbon membranes derived from four commercially available polyimides showed that the P84 carbon membranes exhibited the highest separation efficiency for CO₂/CH₄ separation. The pure gas measurement underestimated the separation efficiency of carbon membranes, due to the restricted diffusion of non-adsorbable gas by adsorbable component in binary mixture.     

High speed spin coating in fabrication of Pebax 1657 based mixed matrix membrane filled with ultra-porous ZIF-8 particles for CO<Subscript>2</Subscript>/CH<Subscript>4</Subscript> separation

Modified ultra-porous ZIF-8 particles were used to prepare novel ZIF-8/Pebax 1657 mixed matrix membranes (MMMs) on PES support for separation of CO₂ from CH₄ using spin coating method. TEM and SEM were used to characterize modified ZIF-8 particles. SEM was also used to investigate the morphology of synthesized MMMs. The MMMs with thinner selective layer showed higher CO₂ permeability and lower CO₂/CH₄ selectivity in permeation tests compared to MMMs with thicker selective layer. The plasticization was recognized as the main reason for rise in CO₂ permeability and drop in CO₂/CH₄ selectivity of thinner MMMs. The gas sorption results showed that the high permeability of CO₂ in MMMs is mainly due to the high solubility of this gas in MMMs, leading to high CO₂/CH₄ solubility selectivity for MMMs. The fractional free volume and void volume fraction of MMMs increased as the thickness of membrane decreased. Applying higher mixed feed pressures and permeation tests temperatures resulted in increase in CO₂ permeability and decrease in CO₂/CH₄ selectivity. At highest testing temperature (60 °C), the CO₂ permeability of synthesized MMMs with thinner selective layer remarkably increased.     

A study of gas transport through interfacially formed poly(N,N-dimethylaminoethyl methacrylate) membranes

Gas transport through interfacially formed poly(N,N-dimethylaminoethyl methacrylate) membranes was investigated. The membrane performance for the separation of binary CO₂/N₂, CO₂/CH₄ and CO₂/H₂ mixtures was studied, and the coupling effects between the permeating species were evaluated by comparing the permeance of individual components in the mixture with their pure gas permeance. For the permeation of these binary gas mixtures, the presence of CO₂ was shown to influence the permeation of the other components (i.e., N₂, H₂ and CH₄), whereas the permeation of CO₂ was not affected by these components. In consideration that water vapor is often encountered in applications involving CO₂ separation, the presence of water vapor on the membrane permselectivity was also studied. When hydrated, the membrane was shown to be more permeable to CO₂, while the membrane selectivity did not change significantly. Unlike membranes based on size-sieving of penetrant molecules, the present membranes exploit the favorable interactions between the hydrophilic quaternary amines in the membrane and CO₂, especially in the presence of water vapor in the feed.     

Effect of stabilization temperature on gas permeation properties of carbon hollow fiber membrane

Carbon hollow fiber membranes (CHFMs) derived from polymer blend of polyetherimide (PEI) and polyvinylpyrrolidone (PVP) were extensively prepared through stabilization under air atmosphere followed by carbonization under N₂ atmosphere. The effects of the stabilization temperature on the morphological structure, chemical structure, and gas permeation properties were investigated thoroughly by means of scanning electron microscopy, Fourier transform infrared spectroscopy, and single gas permeation system. The experiment results indicate that the transport mechanism of small gas molecules of N₂, CO₂, and CH₄ is dominated by the molecular sieving effect. Based on morphological structure and gas permeation properties, an optimum stabilization condition for the preparation of CHFM derived from PEI/PVP was found at 300°C under air atmosphere. The selectivity of ?55 and 41 for CO₂/CH₄ and CO₂/N₂, respectively, were obtained for CHFMs prepared at stabilization temperature of 300°C. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013