Gas Permeation Behavior Research Articles

Effect of Temperature-Induced Aging on the Gas Permeation Behavior of Thin Film Composite Membranes of PIM-1 and Carboxylated PIM-1.

Polymers of intrinsic microporosity (PIMs) are a class of promising gas separation materials due to their high membrane permeabilities and reasonable selectivities. When processed into thin film composite (TFC) membranes, their high gas throughput aligns closely with industrial requirements, but they are prone to physical aging and plasticization effects. TFC membranes based on the prototypical PIM-1 and its carboxylated derivative cPIM-1 exhibit temperature-dependent gas permeation behavior, which has not been extensively studied before. In single CO2 permeation tests, measurable physical aging occurred when the temperature was raised to 55 °C within a period of 90 min, and the aging rate accelerated as temperature was raised further. TFC membranes prepared from cPIM-1 exhibited a faster aging rate compared to PIM-1 at the same temperature. The decreased permeance could be at least partially recovered through a 5 day methanol vapor treatment. In mixed gas experiments, all membranes showed decreased permselectivities at elevated temperatures. The plasticization pressure of TFC membranes occurred at around 1 bar of CO2 partial pressure, independent of temperature. Significant plasticization was particularly evident for cPIM-1 TFC membranes under CO2/CH4 conditions with increasing temperature, which resulted in increased gas permeance for both components.

Polyphthalonitrile as a novel precursor for carbon membranes: Tunable microstructure characteristics and gas permeation behavior

Carbon molecular sieve (CMS) membranes hold great promise for energy-efficient gas separation, contributing to mitigating greenhouse gas emissions. Exploring new types of microstructurally tunable polymeric precursors is critical for understanding the evolution of carbon microstructure arrangement and adjusting the gas permeation behavior of CMS membranes. As a precursor for CMS membranes, polyphthalonitrile (PPN) resin with both tunable intermolecular distance and π-π stacking arrangement is reported for the first time. We have demonstrated that the aforementioned two key features of the thermally crosslinked PPN network are beneficial to forming PPN-CMS membranes with enlarged intermolecular distance and small-sized, narrow-distributed ultramicropores (<7 Å), thereby improving gas permeability and ideal selectivity. This study provides new insights into the microstructure evolution of PPN-derived microporous carbon materials. Owing to its excellent thermal stability, tunable microstructure arrangement, and flexibility of chemical synthesis, PPN represents a promising class of polymeric precursor materials for fabricating CMS membranes.

Modeling of dual-skin asymmetric hollow fiber microporous membranes for predicting the skins parameters separately using gas permeation test

This study presented a comprehensive gas permeation model to predict the skin parameters of a complex system, specifically a hollow fiber membrane with an asymmetric structure consisting of two external and internal skin layers. According to the work of Haefer for n segments of pores connected in series as in hollow fiber membranes, applying the Wakao et al. model, a comprehensive model for the transition regime was derived. The model was examined against the various fabricated PES and PEI hollow fiber membranes. Unlike conventional models, the proposed method predicted the mean pore size and mean pore length independent of surface porosity for each skin layer. The study revealed valuable insights into i) the middle pressure, ii) the difference between the skin parameters of the two membrane skins, and iii) the impact of the feeding direction on gas permeation behavior. It was shown that the dependency of permeation behavior on the feeding direction is linked to the influence of middle pressure on viscous flow, which is closely related to surface porosities of each side. It is evident that even the slightest variations in the fabrication parameters can result in different structures. Furthermore, it is evident that the assumption of neglecting slip flow was reasonable within the experienced Knudsen number range. The model also highlighted that the skin parameters of the two skin layers may not be identical, a distinction not captured by the current models. The traditional approach of setting the middle pressure as the average of the upstream and downstream pressures has been improved by expressing it as a linear function of both pressures. In the feed side, higher pressures led to dominant viscous flow, whilst the permeate side was more influenced by the free molecular regime due to lower pressures. There existed a competition between the Knudsen flow and the viscous flow at a specific Knudsen number for the feeding side pores, whether it was the inner or outer surface. The finding emphasized the significance of porosity values on each side in determining the proximity of middle pressure to feed and permeate side pressures, underscoring the importance of accurate knowledge of surface porosities, mean pore sizes, and pore lengths for both skin layers in understanding processes involving porous dual-skin hollow fiber membranes.

Solvent-induced microphase separation membranes of block poly (aryl ether sulfone) copolymer with high selectivity of O2/N2 separation

Gas separation membranes have a wide range of applications in industry, and there are two important factors in judging membrane standards permeability and selectivity. How to improve the selectivity of gas separation membranes while improving permeability is a challenge. In this study, poly (aryl ether sulfone) (PES) block copolymers were synthesized from fluorine-terminated polysulfone (PSF–F) and hydroxyl-terminated cardo PES containing phenyl substituted isoindolinone side group (PES-PPI-OH). Large side groups can enlarge the free volume and hinder chain movement to improve gas permeation behavior. Solvent-induced microphase separation of block copolymers by casting membranes with solvent benzyl alcohol (BA) can improve selectivity. The PSF-b-PES-PPI (10:10) BA membrane maintain good mechanical properties and tensile strength up to 93 MPa which compared to random polymer membrane rises to 14 MPa. Gas permeation at 23 °C showed that the O2/N2 selectivity of PSF-b-PES-PPI (5:15)BA membrane was 10.6, which was the 2008 Robeson upper bound. Thus, poly (aryl ether sulfone) materials showed great potential application for O2/N2 separation.

Selective gas permeation through polymer-hybridized graphene oxide nanoribbon nanochannels: Towards enhanced H2/CO2 selectivity

Two-dimensional materials have garnered significant attention as membrane building blocks for their tunability, which enables simultaneous enhancement of permeance and selectivity. This study investigates the impact of reduced nanosheet width on H2/CO2 separation by examining the gas permeation behavior of graphene oxide nanoribbon (GONR)/polymer hybrid membranes. Polymers are incorporated into the macroporous GONR scaffold to accelerate H2 transport due to the expansion of GONR nanochannels. The GONR/polymer scaffolds can be coated on porous support by using the scalable shear coating method, resulting in micrometer-scale selective layers. The carbon layers remain intact after polymer hybridization and inhibit polymer crystallization, allowing strong polymer-CO2 interactions to hinder CO2 permeation. However, the excessive polymer composition can compromise GONR-polymer hybridization and crystallization suppression. We also explore the effect of polymer charge on gas separation performance. A relatively small amount of cationic polymer enhances H2/CO2 selectivity, but the enhancement is limited due to strong electrostatic interactions. Anionic polymer-GONR hybridization also improves gas separation performance, but is less effective compared to neutral polymer, as repulsive interactions result in a looser structure. The gas separation performance of the optimized membrane (GONR/PEO 25 wt%) shows H2 permeability of 7,108 Barrer and H2/CO2 selectivity of 10.8, surpassing the Robeson upper bound for polymeric membranes and comparable to that of previous 2D-materials-based membranes. Our finding demonstrates that GONR nanochannels and their gas transport properties can be tuned by simply adding polymers depending on the quantity, molecular weight, and charge of the polymers.

Pyrolysis temperature-regulated gas transport and aging properties in 6FDA-DAM polyimide-derived carbon molecular sieve membranes

Carbon molecular sieve (CMS) membranes with exceptional separation performance and scalable processing are promising for precise gas separation. However, their broad applicability is hampered owing to stability issues, mainly resulting from physical aging. Herein, we manipulate the pyrolysis temperature (550 °C, 650 °C and 750 °C) to regulate the gas transport properties and control the physical aging of CMS membranes derived from 6FDA-DAM polyimide. The morphology, chemical composition and pore size of the membranes were characterized using SEM, IR, XPS and XRD. Results demonstrated that the pore structure of CMS membranes shows densification with increasing the pyrolysis temperature, affording enhanced separation efficiency for gas pairs: H2/N2, H2/CH4, and CO2/CH4 pairs based on the size-sieving effect. Moreover, the effect of aging is more considerable for membranes pyrolyzed at a higher temperature. In addition, the membrane subjected to pre-aging treatment via vacuum storage exhibit better, stable gas separation performance beyond the upper-bond for polymeric membranes. The separation mechanism of the CMS membranes reveals that gas permeation behavior is dominated by diffusion. Tailoring gas permeation and physical aging can provide an alternative approach to tune the gas transport properties of CMS membranes.

Visualization of the gas permeation in core–shell MOF/Polyimide mixed matrix membranes and structural optimization based on finite element equivalent simulation

Core-shell MOFs recently have suggested the greater potential on gas separation of mixed matrix membranes (MMMs) than the single MOFs. In this work, two core–shell MOFs (ZIF-67@ZIF-8 (ZIF-67@8) and ZIF-8@ZIF-67 (ZIF-8@67)) with adverse structure were specially designed to explore what structure is more conducive to gas separation of MMMs. Moreover, a simplified equivalent simulation based on electrical conductivity of Maxwell model was adopted to predict the gas permeability and the internal gas permeation behavior for core–shell MOF based MMMs. The simulation exhibited the better predicted results than the Felske model and revealed the gas transport pathway. As a result, ZIF-67@8 based MMM with the shell-dependent transport and a thin interlayer had the highest H2 permeability of 224 Barrer and H2/CH4 separation factor of 76.7, which are 180% and 24% higher than neat PI, respectively. Finally, core–shell MOFs based MMMs also showed the lower temperature dependence of permeation for N2 and CH4 compared to pure PI. This work provides a unique method to investigate how core–shell structure affects the gas separation performance of MMMs, especially for equivalent simulation, which not only is proved to play as an efficient tool to predict the gas permeability, but also exhibits a great potential on investigation of gas transport pathway for the further structural optimization.

On the Mixed Gas Behavior of Organosilica Membranes Fabricated by Plasma-Enhanced Chemical Vapor Deposition (PECVD).

Selective, nanometer-thin organosilica layers created by plasma-enhanced chemical vapor deposition (PECVD) exhibit selective gas permeation behavior. Despite their promising pure gas performance, published data with regard to mixed gas behavior are still severely lacking. This study endeavors to close this gap by investigating the pure and mixed gas behavior depending on temperatures from 0 °C to 60 °C for four gases (helium, methane, carbon dioxide, and nitrogen) and water vapor. For the two permanent gases, helium and methane, the studied organosilica membrane shows a substantial increase in selectivity from αHe/CH4 = 9 at 0 °C to αHe/CH4 = 40 at 60 °C for pure as well as mixed gases with helium permeance of up to 300 GPU. In contrast, a condensable gas such as CO2 leads to a decrease in selectivity and an increase in permeance compared to its pure gas performance. When water vapor is present in the feed gas, the organosilica membrane shows even stronger deviations from pure gas behavior with a permeance loss of about 60 % accompanied by an increase in ideal selectivity αHe/CO2 from 8 to 13. All in all, the studied organosilica membrane shows very promising results for mixed gases. Especially for elevated temperatures, there is a high potential for separation by size exclusion.

Tunable Gas Permeation Behavior in Self-Standing Cellulose Nanocrystal-Based Membranes

Tunable Gas Permeation Behavior in Self-Standing Cellulose Nanocrystal-Based Membranes

Gas Permeability Behavior in Frozen Sand Controlled by Formation and Dissociation of Pore Gas Hydrates

Formation and dissociation of pore gas hydrates in permafrost can change its properties, including fluid flow capacity. Permeability is one of the most significant parameters in the study of hydrate-containing rocks, especially in the case of gas burial or extraction. Gas permeability variations in frozen sand partially saturated with CO2 or CH4 hydrates are studied experimentally at a constant negative temperature of −5 °C, as well as during freezing–thawing cycles. The gas permeability behavior is controlled by the formation and dissociation of pore gas hydrates in frozen sand samples. The samples with an initial ice saturation of 40 to 60% become at least half as permeable, as 40% of pore ice converts to hydrate. The dissociation process of accumulated hydrates was modeled by both depressurizing methane or CO2 to atmospheric pressure and by stepwise injection of gaseous nitrogen up to 3 MPa into a frozen sample. In sand samples, with a decrease in gas pressure and without subsequent injection of nitrogen, a decrease in pore hydrate dissociation due to self-preservation was noted, which is reflected by a deceleration of gas permeability. Nitrogen injection did not lead to a decrease in the rate of dissociation in the frozen hydrate-containing sample, respectively, as there was no decrease in the rate of gas permeability.

Preparation and gas separation properties of spirobisbenzoxazole-based polyimides

Improving the processability and permeability of aromatic polyimides is a prerequisite for promoting the application in the field of gas separation membranes. Here, a series of novel spirobisbenzoxazole-based polyimides were synthesized. The novel spirobisbenzoxazole unit showed a larger rigid step size than the reported spirobisindane unit, which more effectively destroyed the molecular packing. As a result, the obtained aromatic polyimides showed good organic solubility and significantly improved gas permeability. Such polyimides were also considered as polymers of intrinsic microporous (PIMs) with ultra-micropore distribution of 5 ∼ 7 Å and high BET surface areas ranging from 373 to 449 m2/g. Meanwhile, the obtained polyimide membranes exhibited excellent thermal properties (Tg > 400 °C) and outstanding mechanical properties (σ > 100 MPa). The effects of rotation restriction and thermal annealing process on gas permeation behavior were also studied.

Effective and efficient transport mechanism of CO2 in subnano-porous crystalline membrane of syndiotactic polystyrene

The gas permeation behavior in the single crystal of syndiotactic polystyrene (s-PS) S-I form was investigated in detail, in comparison to that in the s-PS $\varepsilon$ form. The S-I form exhibits high CO$_2$/N$_2$ and CO$_2$/CH$_4$ separation factors while preserving its high CO$_2$ permeability; a trade-off between selectivity and permeability was broken through.The mechanism of the effective and efficient transport of CO$_2$ in the S-I form was examined in relation to the cavity structure in the crystal. Because the CO$_2$ molecule is only fitted to the cavities in the S-I form, the solubility of CO$_2$ becomes high. In addition, tri-atomic molecules can more effectively diffuse in the S-I form compared with di-atomic molecules, as proved by the "long-gas" model. A novel design criterion for the CO$_2$ separation membranes is proposed based on the aspect of momentum transfer from polymer matrix to the penetrant.

Effect of Immobilization of Phenolic Antioxidant on Thermo-Oxidative Stability and Aging of Poly(1-trimethylsilyl-1-propyne) in View of Membrane Application.

The effect of phenolic antioxidant Irganox 1076 on the structure and gas permeation behavior of poly(1-trimethylsilyl-1-propyne) (PTMSP) was investigated. Isotropic films as well as thin film composite membranes (TFCM) from pure PTMSP and with added antioxidant (0.02 wt%) were prepared. PTMSP with antioxidant has a significantly higher thermal degradation stability in comparison to pure polymer. The thermal annealing of isotropic films of PTMSP with antioxidant was carried out at 140 °C. It revealed the stability of gas permeation properties for a minimum of up to 500 h of total heating time after a modest permeation values decrease in the first 48 h. X-ray diffraction data indicate a decrease in interchain distances during the heat treatment of isotropic films and indicate an increase in the packing density of macromolecules during thermally activated relaxation. Isotropic films and TFCMs from pure PTMSP and with antioxidant stabilizer were tested under conditions of constant O2 and N2 flow. The physical aging of thick and composite PTMSP membranes point out the necessity of thermal annealing for obtaining PTMSP-based membranes with predictable properties.

Fabrication of highly (110)-Oriented ZIF-8 membrane at low temperature using nanosheet seed layer

Preferred orientation represents a main concern for microstructure control and performance optimization of MOF membranes. To date, it remains a challenging task to precisely control preferred orientation of ZIF-8 membrane. Herein, we prepared highly (110)-oriented ZIF-8 membrane by oriented secondary growth. Among various elements, preparation of uniform ZIF-8 nanosheet seeds through thermal conversion of identically shaped ZIF-L precursors, spin-coating-assisted oriented deposition of (110)-oriented ZIF-8 seed layer, and controlled in-plane secondary growth at low temperature were found crucial for obtaining well-intergrown (110)-oriented ZIF-8 membrane whose H2/CO2 ideal selectivity exceeded H2/N2 and H2/CH4 under ambient conditions, owing to preferential CO2 adsorption capacity of not only PEI but also ZIF-8 nanosheets compared with their bulk counterparts. Our research highlighted the significance of preferred orientation regulation in tailoring the gas permeation behavior of MOF membranes.

Hydrogen sulfide mix gas permeation in Aquivion® perfluorosulfonic acid (PFSA) ionomer membranes for natural gas sweetening

Permeation tests of CO2/CH4 and H2S/CH4 mixtures in a commercial Perfluorosulfonic acid membrane, Aquivion® E87-12S, were carried out at different temperatures, namely 27, 35 and 50 °C, and at relative humidity (RH) ranging from 20 to 95%. A purposely modified experimental set up was used to allow mixture testing in a fixed volume variable pressure permeometer.The results confirmed the major influence of water content on the gas permeation behavior in PFSA as permeability strongly increased with RH. CO2 and CH4 results confirmed experimental data already available in the open literature, while, for H2S, permeability increased from a value of 17 Barrer measured at 27 °C and 20%RH up to 450 Barrer at the same temperature but at 95%RH. The selectivity with respect to methane also increased with RH, going from 7 to 36 at the same temperature and in the same RH range.The temperature's increase also improved H2S permeability, which reached 512 Barrer at 50 °C and 85%RH, but slightly decreased selectivity towards methane which in the same conditions was limited to 23.A simple model, already used to describe the permeation behavior of other different gases in PFSA, also proved to be able to fit experimental data of H2S with slight changes of the adjustable parameters.

Evaluation of Adsorption and Permeation Behaviors in Hydrated Nafion Membranes with Degradation

In the operation of proton exchange membrane fuel cells, its ionomeric-polymer membrane is easily attacked by free radicals, resulting in the degradation of performance. In this work, the chemical degradation effect of hydrated Nafion membranes on gas adsorption, diffusion, and permeation behaviors is evaluated by molecular dynamics and Monte Carlo simulation. The correlation of pore ratio, free volume, hydrophilic/hydrophobic interface as well as the connectivity of the hydrophilic domain of Nafion membranes with gas transport characteristics are revealed. The results demonstrate that large free volume, high large pore ratio, smooth hydrophilic/hydrophobic interface, and good connectivity of the hydrophilic domain are favorable for adsorption, diffusion, and permeability processes. The C-S bond and C-O-C bond attack of membranes can increase the gas adsorption amount, which becomes weak after the tertiary carbon is attacked.

Polyphenylsulfone (PPSU)-Based Copolymeric Membranes: Effects of Chemical Structure and Content on Gas Permeation and Separation.

Although various polymer membrane materials have been applied to gas separation, there is a trade-off relationship between permeability and selectivity, limiting their wider applications. In this paper, the relationship between the gas permeation behavior of polyphenylsulfone(PPSU)-based materials and their chemical structure for gas separation has been systematically investigated. A PPSU homopolymer and three kinds of 3,3′,5,5′-tetramethyl-4,4′-biphenol (TMBP)-based polyphenylsulfone (TMPPSf) copolymers were synthesized by controlling the TMBP content. As the TMPPSf content increases, the inter-molecular chain distance (or d-spacing value) increases. Data from positron annihilation life-time spectroscopy (PALS) indicate the copolymer with a higher TMPPSf content has a larger fractional free volume (FFV). The logarithm of their O2, N2, CO2, and CH4 permeability was found to increase linearly with an increase in TMPPSf content but decrease linearly with increasing 1/FFV. The enhanced permeability results from the increases in both sorption coefficient and gas diffusivity of copolymers. Interestingly, the gas permeability increases while the selectivity stays stable due to the presence of methyl groups in TMPPSf, which not only increases the free volume but also rigidifies the polymer chains. This study may provide a new strategy to break the trade-off law and increase the permeability of polymer materials largely.

Tailoring Crosslinked Polyether Networks for Separation of CO2 from Light Gases.

Crosslinked poly(ethylene oxide) or poly(ethylene glycol) (PEG) is an ideal membrane material for separation of CO2 from light gases (e.g., H2 , N2 , O2 , CH4 etc). In these membranes, crosslinking is used as a tool to suppress crystallinity of the PEG segments. In spite of the extensive effort to develop crosslinked PEG membranes in the last two decades, it remains a challenge to establish the structure-property relationships. This paper points out the fundamental limitations to correlate the chain topology of a network with the gas permeation mechanism. While a quantitative comparison of the molecular weight between crosslinks of networks and gas permeation mechanism reported by different research groups is challenging, effort is made to draw a qualitative picture. In this review, a focus is also put on the progress of utilization of dangling chain fractions to tailor the gas permeation behavior of PEG networks.

Pore/fracture structure and gas permeability alterations induced by ultrasound treatment in coal and its application to enhanced coalbed methane recovery

Ultrasonic treatment is a highly potential method for reservoir permeability modification in enhanced coalbed methane recovery. Quantitative evaluations of ultrasonic induced changes in structure and permeability in coal are of great interest for potential field applications of ultrasound technology. In this study the scanning electron microscope (SEM) and low-temperature liquid nitrogen (LTLN) were used to examine the ultrasound induced alterations in coal pore/fracture, and further measured gas permeability in coal by coupling the ultrasound field and external stress. The SEM images show that ultrasound treatment can extend the original pore/fracture, increase the connectivity and remove barriers in gas seepage channel in coal. The LTLN data reveals that with the ultrasonic treatment (25 kHz, 18 kW, 2 h), the maximum N 2 adsorption quantity of the tested samples increased by 106.5%–258.4%, and the average pore diameter increased by 37.06%–81.40%; the specific surface area increased by 94.80%–118.15%, and the total pore volume increased by 87.12%–252.55%. It implies that coal pores in size ranging from 1 nm to 100 nm can be remarkably improved by ultrasonic treatment, which is significant beneficial to promote gas desorption/diffusion within coal matrix. Permeability tests demonstrate that gas permeability in stressed coal increased by 29.3%–57.1% with the implement of ultrasound field (25 kHz, 18 kW, 30 min). Ultrasonic performance on permeability improvement is proportional to the treatment time, but the function of ultrasonic fracturing has time threshold effect, and after a certain period of treatment, the ultrasonic fracturing performance becomes very limited in stressed coal. • Coal pore/fracture structure alterations induced by ultrasound treatment is examined using SEM and LTLN techniques . • Ultrasound treatment significantly increases the N 2 accessible specific surface area and pore volume in coal. • External stress and ultrasound treatment time influences gas permeability behavior in coal induced by ultrasonic fracturing. • A conceptual field application scheme of ultrasonic fracturing for enhanced CBM recovery is proposed.

Numerical investigation of gas permeation and condensation behavior of flexible risers

Flexible risers are widely employed in deep-sea oil and gas development. Methane, carbon dioxide, water vapor, and other small molecular gases can permeate into the sheaths and accumulate with water condensation in the annulus, which may cause overpressure and corrosion accidents. A mechanistic model was proposed to describe the permeation of gas phase in the multiphase flow though the flexible riser inner surface and its condensation in the internal annular space. The governing equations were solved efficiently by a combination of the control volume method and the finite difference method. After validation of the models, the thermo-hydraulic characteristics, concentration fields, annulus conditions, flow patterns, and some key parameters were comprehensively investigated. The results showed that with increasing the inlet pressure and temperature, the initiation time of venting and its duration decreased exponentially. The venting frequency can be adjusted by selecting an appropriate shielding factor and venting pressure. When the shielding factor is larger than 0.75 or the venting pressure is greater than 500 kPa, the venting cycle will be significantly prolonged. The gas permeation rates varied in a wide range of 2 times with flow patterns.