Permeation of carbon dioxide through homogeneous and asymmetric polysulfone membranes

To confirm the validity of the working assumption that a thin dense skin layer in an asymmetric membrane can be essentially replaced by a thick homogeneous dense membrane, both homogeneous and asymmetric polysulfone membranes were prepared by solvent casting, and the permeation behavior of carbon dioxide through these two types of membranes was investigated. The pressure dependence of the mean permeability coefficient through an asymmetric polysulfone membrane is apparently very similar to that through a homogeneous dense membrane, following the dual mode mobility model driven by gradients of chemical potential. The dense skin layer in the asymmetric membrane can be simulated approximately by a homogeneous dense membrane from the point of view of gas sorption and diffusion.     

Correlation between Performances of Hollow Fibers and Flat Membranes for Gas Separation

This work presents an attempt at correlating the available permeability/selectivity literature data for hollow fibers and flat membranes. Therefore, this paper gathers the information pertaining to membrane materials for which membrane properties of flat membranes and hollow fibers have both been reported. An overview of the relations between selectivity and permeance of hollow fiber membranes for various gas pairs (O₂/N₂, CO₂/CH₄, CO₂/N₂, H₂/N₂, H₂/CO₂, H₂/CH₄ and He/N₂) is presented first. The upper bound lines are the ones proposed by Robeson, which were calculated by assuming a one-micron-thick skin layer as proposed by Robeson in 2008. From the results obtained, a relation between the selectivity ratio in both kinds of membranes (α_H/α_f) and skin layer thickness (l) calculated from flat membranes and hollow fibers gas permeation data for these pairs of gases is also presented. The skin layer thicknesses measured using seven different experimental techniques for six commercial membranes are compared. The influences of spinning parameters on the morphology and performance of hollow fiber membrane gas separation are discussed. Finally, an analysis is made of the reasons why the dense skin layer thicknesses of a hollow fiber calculated using permeance and permeability data vary for different gases and also differ from direct experimental measurements.     

Formation of integrally skinned asymmetric polysulfone gas separation membranes by supercritical CO2

Integrally skinned asymmetric polysulfone membranes were prepared from originally dense films inducing asymmetry by the formation of the porous layer adding to one side of the membranes chloroform and supercritical CO₂ (SCCO₂), and then allowing the SCCO₂ expansion to occur. The influence of the chloroform/polysulfone mass ratio (g CH₃Cl/g PSF), SCCO₂ density and depressurization rate over the thickness of both the porous and the dense skin layers, the morphology of the porous support and the pure O₂ and N₂ permeability and selectivity performance were studied.The results show that it is possible to induce a very-controlled asymmetry in a dense film following the procedure described in this work and as expected, the thickness of the porous layer increases while the dense skin layer decreases as the chloroform/polysulfone mass ratio increases. Images of the porous layer show that the average-pore size decreases at high SCCO₂ densities and slightly decreases with increasing the CO₂ depressurization rates. The O₂ and N₂ permeability coefficients, measured at 35 °C and 2 bar, for the polysulfone asymmetric membranes are practically the same of those determined in dense films, suggesting that the dense skins are essentially defect-free of pinholes.     

High performance of PES-GNs MMMs for gas separation and selectivity

In this study, graphene nanosheets (GNs) were incorporated into polyethersulfone (PES) by phase inversion approach for preparing PES-GNs mixed matrix membranes (MMMs). To investigate the impact of filler content on membrane surface morphology, thermal stability, chemical composition, porosity and mechanical properties, MMMs were constructed with various GNs loadings (0.01, 0.02, 0.03, and 0.04 wt%). ?The performance of prepared MMMs was tested for separation and selectivity of CO₂, N₂, H₂ and CH₄ gases at various pressures from 1 to 6 bar and temperature varying from 20 to 60 °C. It was observed that, compared to the pristine PES membrane, the prepared MMMs significantly improved the gas separation and selectivity performance with adequate mechanical stability. The permeability of CO₂, N₂, H₂ and CH₄ for the PES + 0.04 wt% GNs increases from 9 to 2246, 11 to 2235, 9 to 7151, and 3 to 4176 Barrer respectively, as compared with pure PES membrane at 1 bar and 20 °C due to improving the membrane absorption and porosity. In addition, by increasing the pressure, the permeability and selectivity of CO₂, N₂, H₂ and CH₄ are increased due to the increased driving force for the transport of gas via membranes. Furthermore, the permeability of CO₂, N₂, H₂ and CH₄ increased by increasing the temperature from 20 to 60 °C due to the plasticization in the membranes and the improvement in polymer chain movement. This result proved that the prepared membranes can be used for gas separation applications.     

Effect of the preparation conditions on the formation of asymmetric poly(vinylidene fluoride) hollow fibre membranes with a dense skin

Integrally skinned asymmetric poly(vinylidene fluoride) hollow fibre membranes were prepared and characterized. The effects of phase inversion methods (dry-wet or wet) and spinning conditions, such as the type of solvent (NMP, DMAc), the concentration of polymer in dope solution, temperature of the external coagulation bath and the composition of the inner coagulant on the morphology and on the formation of a dense skin layer were investigated. The structure of the membranes was analyzed by scanning electron microscopy and the gas permeation properties with six different gases (He, H₂, N₂, O₂, CH₄ and CO₂) were measured at 25 °C to confirm the integrity of the selective skin layer. Under the proper conditions highly selective and permeable PVDF hollow fibre membranes were thus obtained by dry-wet spinning of a 30 wt.% PVDF solution in DMAc, using hot water (50 °C) as the external coagulant and a bore fluid of pure water as the internal coagulant. The best membrane had a selective outer skin with an effective thickness of approximately 0.2 μm. The ideal selectivity of the hollow fibres approached or even exceeded the intrinsic ideal selectivity of a dense PVDF film, for instance the selectivity for He over N₂ was 86.2 for the hollow fibre, whereas it was 83.5 for a dense PVDF reference film. DSC and FT-IR/ATR analysis indicated a higher fraction of the β-crystal phase in the selective skin and a high overall crystallinity than in the melt-processed film. The latter explains the relatively high selectivity and low permeability of the membranes. Intrinsic polymer properties make the membranes also suitable for vapour transport than for gas separation.     

Etched ZIF-8 as a Filler in Mixed-Matrix Membranes for Enhanced CO2/N2 Separation

Zeolite ZIF-8 has been etched with acid to form microporous ZIF-8-E crystals. These were then introduced into a polyethersulfone (PES) membrane matrix to enhance its CO₂/N₂ separation performance. Open through pores of size about 100 nm formed in the ZIF-8 crystals allow the ingrowth of polyethersulfone chains, ensuring a reduction in the number of nonselective voids, thereby achieving better interaction between ZIF-8-E and PES. As a result, the CO₂/N₂ separation performance of the ZIF-8-E/PES membrane increased significantly, showing a CO₂ permeability of 15.7 Barrer and a CO₂/N₂ ideal selectivity of 6.5.     

Preparation and characterization of (Pebax 1657 + silica nanoparticle)/PVC mixed matrix composite membrane for CO<Subscript>2</Subscript>/N<Subscript>2</Subscript> separation

In this work, the films of poly(ether-block-amide) (Pebax 1657) and hydrophilic/hydrophobic silica nanoparticles (0–10 wt%) were coated on a poly(vinyl chloride) (PVC) ultrafiltration membrane to form new mixed matrix composite membranes (MMCMs) for CO₂/N₂ separation. The membranes were characterized by SEM, FTIR, DSC and XRD. Successful formation of a non-porous defect-free dense top layer with ~4 μm of thickness and also uniform dispersion of silica nanoparticles up to 8 wt% loading in Pebax matrix were confirmed by SEM images. The gas permeation results showed an increase in the permeance of all gases and an increase in ideal CO₂/N₂ selectivity with the increase in silica nanoparticle contents. Comparison between the incorporation of hydrophilic and hydrophobic silica nanoparticle into Pebax matrix revealed that the great enhancement of CO₂ solubility is the key factor for the performance improvement of Pebax + silica nanoparticle membranes. The best separation performance of the hydrophilic silica nanoparticle-incorporated Pebax/PVC membrane for pure gases (at 1 bar and 25 °C) was obtained with a CO₂ permeability of 124 barrer and an ideal CO₂/N₂ selectivity of 76, i.e., 63 and 35% higher than those of neat Pebax membrane, respectively. The corresponding values for hydrophobic silica nanoparticle-incorporated Pebax/PVC membrane were 107 barrer for CO₂ permeability and 61 for ideal CO₂/N₂ selectivity. Also the performances of MMCMs improved upon pressure increase (1–10 bar) owing to the shift in plasticizing effect of CO₂ towards the higher pressures. In addition, an increase in permeabilities with a decrease in ideal selectivity was observed upon temperature increase (25–50 °C) due to the intensification of chain mobility.     

Permeation of pure carbon dioxide and methane and binary mixtures through cellulose acetate membranes

Steady-state permeation rates for pure CO₂ and CH₄ and their binary mixtures through homogeneous dense cellulose triacetate membranes have been measured at three temperatures between 20 and 40°C and pressures up to 2.8 MPa. The pressure dependence of the mean permeability coefficient for CO₂ can be described by the total immobilization model in conjunction with a modified free-volume model. No appreciable pressure dependence of the permeability coefficient for CH₄ is observed, while the permeability coefficients for CH₄ in binary mixture of CO₂ and CH₄ depend on applied gas pressure. The pressure dependences of the mean permeability coefficients for the components in the binary mixture are discussed in terms of the above mobility model. Membrane plasticization induced by CO₂ affects permeation by both gases.     

Gas transport properties of asymmetric polyimide membranes prepared by plasma‐based ion implantation

In this study, we report the gas permeance and selectivity of the asymmetric polyimide membrane prepared by plasma‐based ion implantation (PBII). The asymmetric polyimide membranes were prepared using a dry–wet phase inversion process, and the surface skin layer on the membrane was implantated by He ions at 2.5 keV. The asymmetric membranes treated by PBII were measured using a high vacuum apparatus with a Baratron absolute pressure gauge at 76 cmHg and 35°C. The (O₂/N₂) and (CO₂/CH₄) selectivities in the He⁺‐implanted asymmetric membrane at 60 sec resulted in 1.5 and 1.8 time increases, respectively, when compared to those of the asymmetric membrane before PBII. On the other hand, the O₂ and CO₂ permeances in the asymmetric membrane after PBII decreased with an increase in the He⁺ treatment time. In this paper, we addressed, for the first time, the gas permeation behavior of the asymmetric polyimide membranes prepared by PBII. Copyright © 2009 John Wiley & Sons, Ltd.     

Preparation and characterisation of polyaniline based membranes for gas separation

Transport rates (permeability) and ideal separation factors for several gas pairs through dense polyaniline membranes are reported. The ideal separation factors for all gas pairs tested were found to be independent of the polyaniline membrane thickness whereas the permeability of the single gases showed significant variations. Both dedoped and redoped films (film thickness between 9 and 67 μm) were studied. The highest selectivities α_(A/B) found were 7.6 for the gas pair H₂/CO₂ in the case of the dedoped membrane and 10 for the gas pair H₂/CO₂, 6 for O₂/N₂ and 200 for H₂/N₂ in the case of the redoped membrane. Statistical analysis of a large number of membranes allowed the critical comparison with results obtained by other groups.Comparison with other membrane materials shows that an approximately sixfold enhancement of the respective separation factors is possible for gas pairs containing hydrogen. Similar separation factors are observed for the gas pairs CO₂/O₂, CO₂/N₂ and N₂/O₂.Membranes for which Knudsen diffusion was observed exhibited regularly distributed micropores (400 nm diameter).     

Permeation of carbon dioxide through homogeneous dense and asymmetric cellulose acetate membranes

Sorption isotherms for carbon dioxide in a homogeneous dense cellulose acetate membrane were measured by the pressure decay method at three temperatures between 20 and 40°C and gas pressures up to 1.7 MPa. Steady-state permeation rates for the same system at three temperatures between 24 and 40°C and gas pressures up to 2.2 MPa were measured by the variable volume method. The equilibrium sorption was described well in terms of the dual-mode sorption model. The pressure dependence of the mean permeability coefficient was interpreted by the total immobilization model, i.e., a limiting case of the dual-mode mobility model, where the diffusion coefficient for the Henry's law mode is not assumed to be constant but depends upon gas pressure via a modified free-volume theory. The observed pressure dependence of the mean permeability coefficient through an asymmetric cellulose acetate membrane was very similar to that through a homogeneous dense membrane. The thin skin layer in the asymmetric membrane can be simulated by a homogeneous dense membrane from the point of view of gas sorption and diffusion.     

Gas–liquid membrane contactors for CO2 removal

In the present work we use a membrane contactor for the separation of CO₂ from CH₄ and we systematically investigate the influence of both the type of membrane and the different process parameters on the overall process performance (permeability and selectivity). This work is important because it reports real process performance data (permeances and selectivities) for the total process consisting of absorption and desorption under practical conditions using feed mixtures. Commercially available porous PP hollow fiber membranes and asymmetric PPO hollow fiber membranes have been applied and MEA was used as absorption liquid in the membrane contactor. The proposed approach allows us to identify the operating window and potential of the process. Although the performance of the PP membranes outperforms the performance of the PPO membranes in terms of productivity and selectivity, the PP fibers are extremely sensitive to only small variations in the feed pressure, resulting in severe performance loss. In addition to that, extremely high liquid losses are observed for the PP fibers especially at elevated temperatures. Factors that are significantly reduced when asymmetric PPO membranes with a dense, ultrathin top layer are used, which thus improves the performance and significantly increases the operating window and potential of the membrane contactor process.     

Highly selective multilayer polymer thin films for CO2/N2 separation

Multilayer thin films of poly(ethylene oxide) (PEO) and poly(methacrylic acid) (PMAA), deposited via layer‐by‐layer (LbL) assembly from aqueous solutions, are investigated for CO₂/N₂ separation. Eight and ten bilayer (217 and 389 nm thick, respectively) PEO/PMAA thin films deposited on a 25 μm polystyrene substrate exhibit CO₂/N₂ selectivities of 142 and 136, respectively. These are the highest reported to‐date for this gas pair separation using a homogeneous polymer film. While further work remains to improve CO₂ permeability, these results indicate the potential of LbL assemblies as standalone CO₂ separation membranes for low‐flux/high‐purity applications, or as part of a composite and/or mixed‐matrix membrane for high‐flux applications. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1730–1737     

Carbon Molecular Sieve Membrane Preparation by Economical Coating and Pyrolysis of Porous Polymer Hollow Fibers

Dip coating and pyrolysis processes are used to create multi‐layer asymmetric carbon molecular sieve (CMS) hollow fiber membranes with excellent gas separation properties. Coating of an economical engineered support with a high‐performance polyimide to create precursor fibers with a dense skin layer reduces material cost by 25‐fold compared to monolithic precursors or ceramic supports. CMS permeation results with CO₂/CH₄ (50:50) mixed gas feed show attractive CO₂/CH₄ selectivity of 58.8 and CO₂ permeance of 310 GPU at 35 °C.     

Plasticization of ultra-thin polysulfone membranes by carbon dioxide

Plasticization of gas separation membranes by carbon dioxide permanently alters their performance and increases the possibility of membrane failure. This is amplified in ultra-thin composite membranes, where the active polymeric layer is less than 2 μm. Here, the plasticization influence of CO₂ is measured on ultra-thin polysulfone composite membranes for a range of active layer thicknesses, at four temperatures. The resulting permeability–pressure isotherms demonstrate plasticization occurs for all thicknesses at pressures lower than has been reported for dense membranes. These isotherms were quantitatively fitted with an expanded dual-sorption model that takes into account plasticization of the membrane. The plasticization potential of CO₂ for polysulfone was found to increase with reduced active layer thickness. Similarly, the plasticization potential of CO₂ was found to decrease with temperature. These results are consistent with similar research that shows that thin films behave differently to dense membranes.     

Effect of polyethyleneglycol (PEG) on gas permeabilities and permselectivities in its cellulose acetate (CA) blend membranes

The effect of polyethyleneglycol (PEG) on gas permeabilities and selectivities was investigated in a series of miscible cellulose acetate (CA) blend membranes. The permeabilities of CO₂, H₂, O₂, CH₄, N₂ were measured at temperatures from 30 to 80°C and pressures from 20 to 76 cmHg using a manometric permeation apparatus. It was determined that the blend membrane having 10 wt% PEG20000 exhibited higher permeability for CO₂ and higher permselectivity for CO₂ over N₂ and CH₄ than those of the membranes which contained 10% PEG of the molecular weight in the range 200–6000. The CA blend containing 60 wt% PEG20000 showed that its permeability coefficients of CO₂ and ideal separation factors for CO₂ over N₂ reached above 2 × 10⁻⁸ [cm³ (STP) cm/cm² s cmHg] and 22, respectively, at 70°C and 20 cmHg. Based on the data of gas permeability coefficients, time lags and characterization of the membranes, it is proposed that the apparent solubility coefficients of all CA and PEG blend membranes for CO₂ were lower than those of the CA membrane. However, almost all the blend membranes containing PEG20000 showed higher apparent diffusivity coefficients for CO₂, resulting in higher permeability coefficients of CO₂ with relation to those of the CA membrane. It is attributed to the high diffusivity selectivities of CA and PEG20000 blend membranes that their ideal separation factors for CO₂ over N₂ were higher than those of the CA membrane in the range 50–80°C, even though the ideal separation factors of almost all PEG blend membranes for CO₂ over CH₄ became lower than those of the CA membrane over nearly the full range from 30° to 80°C.     

Effect of molecular weight on gas selectivity of oriented thin polyimide membrane

In this study, we focused on effect of the molecular weight of polyimide on the gas selectivity of the asymmetric membrane with an oriented surface skin layer prepared at different shear stresses. Asymmetric polyimide membranes, which have a defect‐free surface skin layer supported by a porous substructure, were prepared by a dry/wet phase inversion process. The structures of the asymmetric polyimides consisted of a thin skin layer and a porous substructure characterized by the presence of finger‐voids. The gas selectivities of the asymmetric polyimide membranes increased with an increase in the shear rate or a decrease in the molecular weight, indicating that the oriented polyimide structure in the surface skin layer provided a high size and shape discrimination between the gas molecules. The selectivity values of (O₂/N₂) and (CO₂/CH₄) in the asymmetric polyimide membrane prepared from the 7.2 × 10⁴ molecular weight material at 1000 sec^?1 shear rate were 12 and 143, respectively. Copyright © 2003 John Wiley & Sons, Ltd.     

Structure-property relationships for novel wholly aromatic polyamide-hydrazides containing various proportions of para-phenylene and meta-phenylene units III. Preparation and properties of semi-permeable membranes for water desalination by reverse osmosis separation performance

Flat sheet asymmetric reverse osmosis membranes were successfully prepared from N,N-dimethylacetamide (DMAc) solutions of a series of novel wholly aromatic polyamide-hydrazides that contained different amounts of para- and meta-phenylene rings. These polyamide-hydrazides were synthesized by a low temperature solution polycondensation reactions of either 4-amino-3-hydroxybenzhydrazide or 3-amino-4-hydroxybenzhydrazide with an equimolar amount of either terephthaloyl dichloride [TCl], isophthaloyl dichloride [ICl] or mixtures of various molar ratios of TCl and ICl in anhydrous DMAc as a solvent. All the polymers have the same structural formula except of the way of linking phenylene units inside the polymer chains. The content of para- to meta-phenylene moieties was varied within these polymers so that the changes in the latter were 10 mol% from polymer to polymer, starting from an overall content of 0-100 mol%. All the membranes were characterized for their salt rejection (%) and water permeability (cm³ cm⁻² day⁻¹) of 0.5 N aqueous sodium chloride feed solution at 3924 kPa operating pressure. The effects of polymers structural variations together with several processing parameters to achieve the best combination of high selectivity and permeability were studied. Effects of various processing parameters of the membranes on their transport properties were investigated by varying the temperature and period of the solvent evaporation of the cast membranes, coagulation temperature of the thermally treated membranes, annealing of the coagulated membranes, casting solution composition, membrane thickness and the operating pressure. During the thermal treatment step, the asymmetric structure of the membranes with a thin dense skin surface layer supported on a more porous layer was established. The former layer seems to be responsible for the separation performance. The results obtained showed that membrane performance was very much influenced by all of the examined processing variables and that membranes with considerably different properties could be obtained from the same polymer sample by using different processing parameters. Thus, the use of higher temperatures and longer exposure times in the protomembrane forming thermal treatment step would result in a membrane of lower solvent content and with a thicker skin layer and consequently led to higher salt rejection at lower water permeability. Most significantly, the membrane properties clearly depended on the polymer structure. Under identical processing condition, substitution para-phenylene rings for meta-phenylene ones within the polymer series resulted in an increase in salt rejection capability of the membranes. This may be attributed to an increase in their chain symmetry associated with increased molecular packing and rigidity through enhanced intermolecular hydrogen bonding. This produces a barrier with much smaller pores that would efficiently prevent the solute particles from penetration. Coagulation temperature controls the structure (porosity) of the membrane particularly its supported layer and consequently its water permeability. Moreover, annealing of the prepared membranes in deionized water at 100 °C was found essential for useful properties in the single-stage separation applications, which required optimum membrane selectivity. Upon annealing, the membrane shrinks resulting in reducing its pore size particularly in the skin layer and consequently improving the salt rejection. Addition of lithium chloride to the casting solution produced a membrane with increased porosity and improved water permeability. Salt rejection capability of the membranes is clearly affected by the applied pressure, reaching its maximum at nearly 4000 kPa. Furthermore, the water permeability is inversely proportional to the membrane thickness, while the salt rejection is not substantially influenced.     

Poly(ether urethane) and poly(ether urethane urea) membranes with high H2S/CH4 selectivity

The objective of this study was to synthesize rubbery polymers with a high H₂S/CH₄ selectivity for possible use as membrane materials for the separation of H₂S from ‘low-quality’ natural gas. Two poly(ether urethanes), designated hereafter PU1 and PU3, and two poly(ether urethane ureas), designated PU2 and PU4, were synthesized and cast in the form of ‘dense’ (homogeneous) membranes. PU1 and PU2 contained poly(propylene oxide) whereas PU3 and PU4 contained poly(ethylene oxide) as the polyether component. The permeability of these membranes to two ternary mixtures of CH₄, CO₂, and H₂S was measured at 35°C, and for a PU4 membrane also at 20°C, in the pressure range from 4 to 13.6 atm (4.05–13.78×10⁵ Pa). PU4 is a very promising membrane material for H₂S separation from mixtures with CH₄ and CO₂, having a H₂S/CH₄ selectivity greater than 100 at 20°C as well as a very high permeability to H₂S. Permeability measurements were also made with commercial PEBAX^TM membranes for comparison. The possibility of upgrading low-quality natural gas to US pipeline specifications for H₂S and CO₂ by means of membrane processes utilizing both highly H₂S-selective and CO₂-selective polymer membranes is discussed.     

Evaluating the impact of functional groups on membrane-mediated CO2/N2 gas separations using a common polymer backbone

Polymeric membranes have shown tremendous promise for the separation of CO₂ from flue gas streams. However, few systematic studies have been conducted to better understand the impact that chemical functionalities have on membrane-based gas separation performance. To address this gap, we herein describe the synthesis and gas separation performance of a series of vinyl-addition polynorbornenes bearing various CO₂-philic functional groups. To facilitate direct comparison between functional groups, each material was designed to maintain a common polymer backbone. Though the incorporation of CO₂-philic moieties within a dense polymeric membrane is frequently hypothesized to enhance CO₂ solubility, and thereby increase CO₂/N₂ selectivity, our results demonstrate that the incorporation of CO₂-philic groups onto a common polymer backbone do not necessarily result in increased gas separation performance. Experimental and computational results demonstrate that the incorporation of amidoxime groups onto a polynorbornene backbone increase CO₂/N₂ selectivity, whereas commonly employed ethereal side chains only increased permeability.