Gas Permeation Data Research Articles

Description of non-ideal effects in composite membranes by a new theoretical approach

In present study, a theoretical model is introduced to calculate the gas permeability of mixed matrix membranes (MMMs) containing porous fillers. This model is based on analogy of gas permeation in MMMs with tensile modulus in composites. The representative parameters of MMM morphologies such as polymer chains tightening around the filler (β<1), unselective channel formation (β>1) and polymer chains penetrating into filler pores (0≤α≤1) which leads to the filler pore blocking are all included in the model. Furthermore, the interphase thickness (t) which is a critical parameter of MMMs participates in calculation equation along with the other parameters. To validate the proposed model, an experimental study with poly(vinylidene fluoride) (PVDF)/poly(methyl methacrylate) (PMMA) blend as polymer matrix and zeolite 4A and NH2-MOF-199 as fillers is performed. The obtained gas permeability data of constructed MMMs and some of the literature data are employed to be fitted with the proposed model. The details of the existence morphologies of MMMs can be described by earning the mentioned parameters ranges. Therefore, by this model, by having a measure of t, the gas permeation of MMM can be achieved accompany with details of the related morphologies. All the theoretical investigations are done utilizing one equation which contains all parameters simultaneously.

Machine learning-based discovery of molecular descriptors that control polymer gas permeation

While machine learning has found increasing use in predicting the properties of polymeric materials with only a knowledge of chain architecture, determining the molecular factors underpinning properties (“interpretable AI”) has remained less well explored. We show that encoding chain chemistry in commonly employed formats, e.g., binary-valued fingerprints, leads to uniqueness issues during the hashing process to save storage space. This is because the hashing algorithm can map several chemical moieties into the same bit. These issues carry over into the ML algorithms, especially for “inverse” design and interpretable AI, and cannot be avoided by changing the length of the fingerprint. Using MACCS key featurizations of monomer repeats resolves some of these issues, and we show that a few substructures consistently appear in top features for maximizing permeability across several gases and ML models. These are carbon–carbon double bonds (as in polyacetylenes) especially when they are associated with methyl groups (found in branching architectures). These results, derived from the limited data set of ∼500 polymers with experimental gas permeation data, are in agreement with physical insight and thus provide a robust foundation which could further enable study of these material classes through detailed experiments and simulations.

Effect of the annealing temperature of polybenzimidazole membranes in high pressure and high temperature H2/CO2 gas separations

The effect of the annealing temperature of polybenzimidazole (PBI) membranes on H2/CO2 gas separations was investigated. Membranes annealed from 250 °C to 400 °C were tested for gas permeation with pure H2, CO2, and N2 gases and a H2:CO2 (1:1) mixture at 35 °C, 100 °C, 200 °C, and 300 °C and at pressures up to 45 bar. Gas permeation data show that permeability and selectivity of the membranes is significantly impacted by the annealing temperature, the presence of adsorbed water, and remaining casting solvent (DMAc). At a testing temperature of 35 °C, ideal H2/CO2 selectivities of 50, 49, and 66 with pure H2 permeabilities of 1.5, 0.8, and 1.5 Barrer were obtained for membranes annealed at 250 °C, 300 °C, and 400 °C, respectively. At this temperature, high gas mixture H2/CO2 selectivities of 41, 73, and 47 with H2 permeabilities of 1.03, 0.26, and 0.50 Barrer were also obtained for these membranes. At testing temperatures of 300 °C, both the ideal and gas mixture H2/CO2 selectivities dropped to 44, 28, and 30 (ideal, H2 = 45, 45, 44 Barrer) and to 19, 22, and 23 (mixture, H2 = 41, 43, and 44 Barrer) for membranes annealed at 250 °C, 300 °C, and 400 °C, respectively. As water was removed from the membranes at temperatures greater than 100 °C during permeation cycles, where the testing temperature was increased from 35 °C to 300 °C, the permselectivity properties of the membranes annealed at 400 °C became more reproducible. Permeabilities at 35 °C from a second permeability cycle increased, but H2/CO2 selectivities decreased to 21 for gas mixtures (H2 = 1.4 Barrer) and to 34 for pure gases (H2 = 2.2 Barrer). The results suggest that high annealing temperatures may induce changes in the configuration and conformation of the polymer chains, imparting distinctive permselectivity properties to the membranes. Activation energies of permeability for H2, CO2, and N2 from pure gases and H2:CO2 mixtures correlated with these changes as well.

Gas permeation model for mixed matrix membranes: the new renovated Maxwell model

ABSTRACT The accurate prediction of gas separation properties of mixed matrix membranes (MMMs) is essential to eliminate the tedious experimental work. In this work, a new model is proposed to predict the gas permeability of MMMs, considering the role of interface voids between the polymer matrix and inorganic fillers. The new model is developed based on an analogy with a model derived for thermal conduction through the particulate composites using effective medium theory. The new model is validated using the gas permeability data of four sets of MMMs containing spherical micro- or nano-fillers reported in the literature. The results show an excellent agreement between new model predictions and gas permeability experimental data with deviations lower than 10% in comparison with the other models (29–43%). In addition, the value of the thickness of the interface voids layer for MMMs, which is difficult to directly determine by experimental methods, is correctly predicted using the new model. Therefore, a new theoretical model named as renovated Maxwell model considering the effect of filler/polymer interface voids is developed to accurately predict the gas permeability of MMMs.

Nematic liquid crystalline polymer films for gas separation

ABSTRACT The gas separation performances of free-standing planar aligned nematic LC polymer films were investigated for gas separations of He, CO2, CH4 and Xe. The films consist of derivatives of 1,4-phenylene bis(4-((6-(acryloyloxy)hexyl)oxy)benzoate)s with respective cyano, chloro, methyl and phenyl substituents on the central aromatic cores. Two new LC derivatives of 1,4-phenylene bis(4-((6-(acryloyloxy)hexyl)oxy)benzoate)s were successfully synthesised and fully characterised. Single gas permeation and sorption data show increasing gas permeabilities with increasing steric size of the substituents while the ideal gas selectivity of He over CH4 and He over CO2 decreases. The sorption coefficient of all films is independent of the LC substituents, while the subsequently extracted diffusion coefficient for the films with a phenyl substituent is three times higher compared to the films with a cyano substituent, demonstrating that the steric size of the LC substituents mainly affects the diffusion of gasses rather than the solubility of the gases. Irrespective of a methyl or a phenyl substituent, a larger kinetic diameter of Xe gives a 20 times lower diffusion coefficient compared to the smaller species (CO2).

Dibenzomethanopentacene-Based Polymers of Intrinsic Microporosity for Use in Gas-Separation Membranes.

Dibenzomethanopentacene (DBMP) is shown to be a useful structural component for making Polymers of Intrinsic Microporosity (PIMs) with promise for making efficient membranes for gas separations. DBMP-based monomers for PIMs are readily prepared using a Diels-Alder reaction between 2,3-dimethoxyanthracene and norbornadiene as the key synthetic step. Compared to date for the archetypal PIM-1, the incorporation of DBMP simultaneously enhances both gas permeability and the ideal selectivity for one gas over another. Hence, both ideal and mixed gas permeability data for DBMP-rich co-polymers and an amidoxime modified PIM are close to the current Robeson upper bounds, which define the state-of-the-art for the trade-off between permeability and selectivity, for several important gas pairs. Furthermore, long-term studies (over ≈3 years) reveal that the reduction in gas permeabilities on ageing is less for DBMP-containing PIMs relative to that for other high performing PIMs, which is an attractive property for the fabrication of membranes for efficient gas separations.

Dibenzomethanopentacene‐Based Polymers of Intrinsic Microporosity for Use in Gas‐Separation Membranes

AbstractDibenzomethanopentacene (DBMP) is shown to be a useful structural component for making Polymers of Intrinsic Microporosity (PIMs) with promise for making efficient membranes for gas separations. DBMP‐based monomers for PIMs are readily prepared using a Diels–Alder reaction between 2,3‐dimethoxyanthracene and norbornadiene as the key synthetic step. Compared to date for the archetypal PIM‐1, the incorporation of DBMP simultaneously enhances both gas permeability and the ideal selectivity for one gas over another. Hence, both ideal and mixed gas permeability data for DBMP‐rich co‐polymers and an amidoxime modified PIM are close to the current Robeson upper bounds, which define the state‐of‐the‐art for the trade‐off between permeability and selectivity, for several important gas pairs. Furthermore, long‐term studies (over ≈3 years) reveal that the reduction in gas permeabilities on ageing is less for DBMP‐containing PIMs relative to that for other high performing PIMs, which is an attractive property for the fabrication of membranes for efficient gas separations.

Preparation and oxygen permeability of pure supramolecular polymer membranes synthesized from poly (substituted phenylacetylene)

We previously reported a new preparation method for supramolecular polymers having self-membrane forming ability from poly (substituted phenylacetylene)s using highly selective photocyclic aromatization (the SCAT reaction). However, since mechanical strengths of their membranes were not enough for oxygen permeation process, we could not estimate their oxygen permselectivities. In this study, we successfully obtained oxygen permselectivities from the experimental data for their pure supramolecular polymer membranes prepared on porous supports. This is the first example of gas permeability data for pure supramolecular polymer membranes. Surprisingly, the oxygen permselectivities were higher than those for their corresponding conventional polymer membranes. It may be because the supramolecular polymer membranes had denser and less defected structures than their corresponding conventional polymer membranes. It was thought that the supramolecularly-bonded main chains in these pure supramolecular polymers worked like the covalently-bonded main chains in the conventional polymers in the permeation.

Molecular Order Determines Gas Transport through Smectic Liquid Crystalline Polymer Membranes with Different Chemical Compositions

Amorphous polymers are often used for gas separation but have a trade-off between gas permeability and selectivity. Here, the effect of chemical composition and temperature on gas permeability and solubility in well-ordered LC polymer membranes is investigated. Membranes with various compositions of a monomethacrylate LC (M1) with a crown ether functionality to enhance CO2 solubility and a smectic diacrylate (M2) cross-linker were fabricated, while all having the same order (smectic C) and alignment (planar). Single gas sorption and permeation data show for the membranes with 30 wt % M1 a higher CO2 solubility coefficient compared to membranes without M1, which results in a higher CO2 permeability and selectivity. For membranes that contain more than 30 wt % M1 decreasing layer spacings lead to reduced gas solubilities that result in lower gas permeabilities without additional selectivity gain toward CO2. The effect of temperature is demonstrated by comparing single gas sorption and permeation data below and above the Tg of the membranes. The diffusion coefficient increases above the Tg of the membranes with increasing M1 content leading to higher CO2 permeabilities and selectivities. These results show that not only the chemical composition but also the layer spacing of the smectic structures determines the gas separation performance of smectic LC polymer membranes.

Toward the truth of condensing-water membrane for efficient biogas purification: Experimental and modeling analyses

Facilitated transport membranes with high CO2/CH4 separation performance under humidified state have shown great prospects for biogas purification. The role of water might include the formation of condensed water film on membrane surface, but it was seldom verified and investigated. In this study, the condensation of water film on polyamide (PA) thin-film composite membrane was performed and regulated. Thanks to the absence of active carrier groups that can reversibly with CO2, the contribution of water to facilitated transport can be neglected, and the truth of condensing-water membrane was allowed to be explored. The PA layer was fabricated by interfacial polymerization with its hydrophilicity tuned by polyvinyl alcohol. The regulation of water film is based on three operational approaches: simple humidifying the feed gas, pre-wetting the membrane, or heating the humidifier. The remarkable increment of selectivity along with the improvement of water-condensing conditions implied the formation of condensed water film. Fortunately, the series of gas permeation data allowed the estimation of water film thickness according to the mass transfer resistance model for multilayer composite membranes. The decline of selectivity over time under elevated pressure was found effective to evaluate the stability of such condensing water membranes. The results show that a polyamide thin-film composite membrane with poor CO2/CH4 selectivity under dry state can achieve rather high performance (CO2 permeance 374 GPU and CO2/CH4 selectivity 31 for 16 days under 3.25 bar) with the aid of a simple condensed water layer. Hopefully, it provides a method to re-evaluate the effect of water on a given facilitated transport membrane, and possibilities of further enhancing its separation performance by fine tuning the condensed water film.

Performance evaluation of biopolymer mixed matrix membrane for CO2/H2 separation

Hydrogen (H2) energy meets the demand for green and efficient energy; therefore, hydrogen generation, purification, and separation are required. Membrane technology produce ultrapure hydrogen gas using less energy, allowing the process to run continuously. For preparing H2 selective membranes, palladium (Pd) is a potential material because of its high affinity toward H2 absorption. This work modified natural polysaccharide guar gum matric with natural chitosan polymer. The crosslinking of both the polymer improves the tensile strength and elongation at break (%). The composite polymer membrane enhances the separation performance of CO2/N2 by 95.35% and CO2/H2 by 34.29%. Further improvements in CO2/H2, the Pd nanoparticles are incorporated into the composite polymer matrix (GG/CH). The gas permeability data shows a drastic reduction in H2 permeability by incorporating Pd-nanoparticles. In-room temperature, the selectivity of CO2/H2 was obtained 13.06. The morphology of membranes and the presence of Pd nanoparticles in the membrane is characterized by SEM-EDS for confirmation of Pd nanoparticle.

K2O-Metakaolin-Based Geopolymer Foams: Production, Porosity Characterization and Permeability Test.

In this paper, four near-net shaped foams were produced via direct foaming, starting from a benchmark metakaolin-based geopolymer formulation. Hydrogen peroxide and metallic silicon were used in different amounts as blowing agents to change the porosity from meso- to ultra-macro-porosity. Foams were characterized by bulk densities ranging from 0.34 to 0.66 g cm−3, total porosity from 70% to 84%, accessible porosity from 41% to 52% and specific surface area from 47 to 94 m2 g−1. Gas permeability tests were performed, showing a correlation between the pore features and the processing methods applied. The permeability coefficients k1 (Darcian) and k2 (non-Darcian), calculated applying Forchheimer’s equation, were higher by a few orders of magnitude for the foams made using H2O2 than those made with metallic silicon, highlighting the differing flow resistance according to the interconnected porosity. The gas permeability data indicated that the different geopolymer foams, obtained via direct foaming, performed similarly to other porous materials such as granular beds, fibrous filters and gel-cast foams, indicating the possibility of their use in a broad spectrum of applications.

Mass Transfer Characteristics through Alumina Membranes with Different Pores Sizes and Porosity

Different membranes covering the macroporous to nano-pororous range and having different porosities have been used to study the mass transfer of methane and carbon dioxide single gases. The effect of flow parameters on the transport mechanisms through porous membranes were reviewed in detail. The characteristics of gas transport through the macroporous, microporous, and nano-porous membranes were investigated with several gas diffusion models in the range of 20–100 ◦C and at pressure differences ranging from 0.2 to 3 bar. The experimental gas permeation data of the membranes were analyzed using the Darcy flow model. The results clearly showed good agreement between the model analysis and the experimental data. The experimental data showed that the permeation followed a parallel flow model in which the behavior of gases was governed by viscous and Knudsen diffusion, although to varied degrees. Permeation of the gases through each membrane varies considering the viscosity of the gases at the same temperature. Furthermore, the membranes followed the configurational diffusion model in which the permeance increased with increasing pressure and decreasing temperature. For the gas flow measurements through macroporous and nano-porous membranes with diameters ranging from 6000nm to 15nm, the results indicate that the experimental flux agrees well with the calculated (model) flux through which gas flows from the bulk stream in the shell side to the membrane outer surface where viscous flow and Knudsen diffusion coexist. The study shows that experimental flux is larger than Knudsen diffusion, and the contribution of Knudsen diffusion to the experimental flux increases with the decrease in the diameter. On the other hand, the effects of gas slippage are considerable as gas velocity near the wall is higher than zero. The slip length effects are inversely proportional to pore size and with driving pressure.

Numerical Simulation and Optimization of 4-Component LDG Separation in the Steelmaking Industry Using Polysulfone Hollow Fiber Membranes.

A general finite element model and a new solution method were developed to simulate the permeances of Lintz Donawiz converter gas (LDG) components and the performance of a polysulfone membrane separation unit. The permeances at eight bars of CO, N2, and H2 in LDG simulated using the developed model equations employing the experimental mixed gas data were obtained by controlling the finite element numbers and comparing them with pure gas permeation data. At the optimal finite element numbers (s = 15, n = 1), the gas permeances under the mixed-gas condition were 6.3% (CO), 3.9% (N2), and 7.2% (H2) larger than those of the pure gases, On the other hand, the mixed-gas permeance of CO2 was 4.5% smaller than that of pure gas. These differences were attributed to the plasticization phenomenon of the polysulfone membrane used by CO2. The newly adopted solution method for the stiff nonlinear model functions enabled the simulation of the performance (in terms of gas recovery, concentration, and flow rate) of the first-stage membrane within two seconds under most gas flow conditions. The performance of a first-stage membrane unit separating LDG could be predicted by the developed model with a small error of <2.1%. These model and solution methods could be utilized effectively for simulating gas permeances of the membrane that is plasticized severely by the permeating gas and the separation performance of two- or multi-stage membrane processes.

Polymer Morphological Effect on Gas Transport within Triptycene-Based Polysulfones

In this study, the effect of polymer morphology on chain packing and physical and gas transport properties was explored via the creation and study of three polysulfone copolymer membranes─a random copolymer, 15k–15k (g/mol) multiblock copolymer, and polymer physical blend─each containing a 50:50 mol equiv of triptycene and phenolphthalein bridging units. These polysulfones with varied morphologies were directly compared to each other, as well as to their relevant triptycene- and phenolphthalein-based polysulfone homopolymer counterparts. Each polysulfone containing equivalent amounts of triptycene and phenolphthalein exhibited similar physical properties, densities, and fractional free volume results regardless of combination method. Similarly, gas permeation data followed an expected trend of performance between that of the two homopolymers for H2/CH4 and O2/N2 separations. Uniquely, the random copolymer exhibited unexpected suppression of methane permeability and, to a lesser extent, nitrogen permeability, leading to the highest selectivities for CO2/CH4 and CO2/N2 among the series. This can likely be attributed to the greater abundance of triptycene and phenolphthalein units in close proximity along the polymer backbone in the random copolymer, leading to synergistic performance effects from different inter- and intrachain interactions unattainable in the multiblock, physical blend, and homopolymers.

Interfacial layer thickness is a key parameter in determining the gas separation properties of spherical nanoparticles-mixed matrix membranes: A modeling perspective

The existing traditional models cannot accurately predict the gas permeability of spherical nanoparticle-mixed matrix membranes (SP-MMMs) due to ignoring the physical and chemical characteristics of the SP/matrix interface. In this paper, first, a new model is derived for the prediction of thermal conductivity of SP-polymer composites according to multiple scattering theory. Then, a new theoretical model for gas permeability of SP-MMMs is developed based on the analogy with the derived model for the prediction of thermal conductivity. The significant feature of the new model is its ability to quantify the crucial role of SPs/matrix interface in gas permeability by introducing a new dense interfacial layer thickness (aint) parameter, which increases with increasing the strength of interfacial interactions. It is demonstrated that the value of aint is independent of the nature of gas molecules and mathematically correlated to the strength of SPs/matrix interfacial interactions. Finally, the gas permeability of SP-MMMs is accurately predicted by inserting the values of non-adjustable aint parameter, obtained from the correlation, diameter of SPs as well as the gas permeability of matrix into the new model, without using any adjustable parameter. Moreover, this technique can be utilized to determine the dense interfacial layer thickness in SP-polymer composites using gas permeation data.

Helium separation using membrane technology: Recent advances and perspectives

• An overview of recent advances in helium separation membranes and processes is presented. • Helium separation performances of more than 1000 membrane materials are summarized and analyzed. • Strengths and challenges of membrane processes for helium recovery and purification from natural gas are critically analyzed. • Future research directions in membrane materials and processes for helium separation are proposed. Helium is an unrenewable noble gas produced from natural gas with a wide range of scientific, medical, and industrial applications. Due to the large differences in the kinetic diameters between helium (0.26 nm) and nitrogen (0.364 nm) or methane (0.38 nm), membrane technology has been considered a promising alternative to traditional technologies for helium recovery and purification. This paper systematically reviews the advances in membrane material development for helium separation in recent years. Gas permeation data presented in this work were collected from over 1000 membrane materials, including polymeric, inorganic, and mixed matrix membranes. Moreover, membrane processes for helium recovery and purification from natural gas were critically analyzed and discussed concerning technical feasibility, energy consumption, and separation costs. Challenges in helium purification using membrane technology were also discussed, and potential solutions have been suggested. Lastly, future perspectives on research directions on membrane material development and hybrid helium purification process design and optimization are proposed.

On the Order and Orientation in Liquid Crystalline Polymer Membranes for Gas Separation.

To prevent greenhouse emissions into the atmosphere, separations like CO2/CH4 and CO2/N2 from natural gas, biogas, and flue gasses are crucial. Polymer membranes gained a key role in gas separations over the past decades, but these polymers are often not organized at a molecular level, which results in a trade-off between permeability and selectivity. In this work, the effect of molecular order and orientation in liquid crystals (LCs) polymer membranes for gas permeation is demonstrated. Using the self-assembly of polymerizable LCs to prepare membranes ensures control over the supramolecular organization and alignment of the building blocks at a molecular level. Robust freestanding LC membranes were fabricated that have various, distinct morphologies (isotropic, nematic cybotactic, and smectic C) and alignment (planar and homeotropic), while using the same chemical composition. Single gas permeation data show that the permeability decreases with increasing molecular order while the ideal gas selectivity of He and CO2 over N2 increases tremendously (36-fold for He/N2 and 21-fold for CO2/N2) when going from randomly ordered to the highly ordered smectic C morphology. The calculated diffusion coefficients showed a 10-fold decrease when going from randomly ordered membranes to ordered smectic C membranes. It is proposed that with increasing molecular order, the free volume elements in the membrane become smaller, which hinders gasses with larger kinetic diameters (Ar, N2) more than gasses with smaller kinetic diameters (He, CO2), inducing selectivity. Comparison of gas sorption and permeation performances of planar and homeotropic aligned smectic C membranes shows the effect of molecular orientation by a 3-fold decrease of the diffusion coefficient of homeotropic aligned smectic C membranes resulting in a diminished gas permeation and increased ideal gas selectivities. These results strongly highlight the importance of molecular order and orientation in LC polymer membranes for gas separation.

Determination of the Thickness of Interfacial Voids in a Spherical Nanoparticles - Polymer Membrane: Fundamental Insight from the Gas Permeation Modeling

In this work, a new approach is proposed to determine the thickness of interfacial voids in impermeable spherical nanoparticles-containing mixed matrix membranes (SP-MMMs) based on the gas permeation data. The interfacial void thickness, lv, between SPs and polymer matrix is firstly obtained as an adjustable parameter by fitting the modified Maxwell model to the experimental gas permeability data reported in the literature. It is shown that the value of lv parameter is independent of the nature of gas molecules and mathematically related to the strength of SP/matrix interfacial interactions. Then, for the first time in this paper, a universal relationship is established between the SPs/polymer interfacial void thickness and the interfacial interactions at the SPs/polymer interface, which is described by the Flory–Huggins interaction parameter, χ. Finally, the gas permeability of SP-MMMs is predicted using the values of lv parameter, diameter and volume fraction of SPs as well as the gas permeability of polymer matrix. Moreover, according to the obtained relationship, the interfacial void thickness in SP-polymer composites can be directly calculated for a wide range of interaction parameter values.

Impact of humidity on gas transport in polybenzimidazole membranes

Polybenzimidazoles (PBIs) are promising materials for high temperature H2/CO2 separation in applications such as steam reforming and pre-combustion carbon capture where significant amounts of water are often present. However, PBIs are hydrophilic, and the impact of humidity on PBI gas separation properties is relatively unexplored. Furthermore, opportunity exists to elucidate the interplay between plasticization, free volume, and gas transport in glassy polymer membranes such as PBIs. This study investigates the effect of humidity on H2, O2, and CO2 permeabilities at 35 °C in a commercial PBI and two sulfone-containing PBIs. Water uptake significantly reduces PBI gas permeabilities at low humidities due to competitive sorption and antiplasticization. At high humidities, plasticization increases the permeabilities of larger gases in more hydrophilic PBIs. Effective fractional free volumes evaluated from gas permeation data and previously reported water sorption and dilation data suggest water plasticizes PBIs by increasing accessible free volume via enhanced molecular dynamics rather than by creating new free volume cavities.