Gas Separation Membranes Research Articles

Computational Analysis of Permeance Prediction for Gas Separation Membrane Using Countercurrent Flow Model

Gas separation polymeric membranes have gained significant attention in various industries, including gas processing, petrochemicals, and environmental applications. Accurately predicting the permeance of these membranes is crucial for optimizing their design and performance. This paper presented a numerical prediction model for the permeance of a binary gas mixture under various process conditions, using only algebraic equations to minimize the calculation effort. The model considers input parameters such as feed and permeate flow rates, feed pressure, feed and permeate mole fraction, and membrane area. It demonstrates the behavior of permeance and selectivity in this study. The study also presented two case studies that utilize predicted selectivity to model counter-current flow and compare the results with experimental data from the literature. The first case study involves recovering helium from natural gas using an asymmetric high-flux membrane. The second case study separates air using nano-porous carbon as the membrane material. The numerical analysis successfully accurately predicted the permeance of gas separation polymeric membranes. The model accounted for variations in membrane thickness, feed composition, and operating conditions, providing valuable insights into the overall performance of the membrane system. It also allowed for the optimization of membrane design and operational parameters to enhance separation efficiency and productivity. The study showcased the effectiveness of numerical techniques in accurately predicting membrane performance. The developed model can be a valuable tool for membrane designers and engineers, enabling them to optimize the design and operation of gas separation systems.

Carbon molecular sieve gas separation membranes derived from in-situ crosslinked polyimide containing bromine groups

The network polyimide (PI) precursor is a promising candidate to fabricate the advanced carbon molecular sieve (CMS) membrane since its robust crosslinked structure could prevent pore structure collapse during CMS formation. Exploring the relationship between network PI precursor structure and CMS membrane is highly significant. Herein, a novel brominated network PI precursor was fabricated via in-situ cross-linking technique firstly, and then the thermally induced bromination/debromination process was applied to tune the microporosity and gas transport properties of network PI precursor and its derived CMS membranes. After the bromination/debromination and carbonization at 550 °C, the resultant CMS membrane exhibited both high gas permeability and gas selectivities compared to its un-brominated network PI precursor derived CMS membrane. This is due to the bromination/debromination and carbonization enable the CMS membrane possesses large d-spacing values (11.06 Å), high BET surface areas (538.57 m2/g), and high percentage (∼71 %) of ultra-micropores. For instance, 6F-TA/TA-Br-550/30 membrane exhibits great CO2/CH4 separation ability, with a CO2 permeability of 6300 Barrer and a CO2/CH4 selectivity of 46.11, which is suppressed the latest 2019 trade-off curves. We anticipate that the present bromination/debromination and carbonization could provide a fresh perspective on the rational design of network PI precursor and its derived CMS membrane with excellent gas separation performance.

Tertiary-Amine-Functional Poly(arylene ether)s for Acid-Gas Separations.

Competitive sorption enables the emergent phenomenon of enhanced CO2-based selectivities for gas separation membranes when using microporous polymers with primary amines. However, strong secondary forces in these polymers through hydrogen bonding results in low solvent solubility, precluding standard solution processing approaches to form these polymers into membrane films. Herein, we circumvent these manufacturing constraints while maintaining competitive-sorption enhancements by synthesizing eight representative microporous poly(arylene ether)s (PAEs) with tertiary amines. High-pressure H2S, CO2, and CH4 sorption isotherms were collected for these samples to demonstrate enhanced affinity for acid gases relative to the unfunctional control polymer. Although competitive sorption was observed for all samples, improvements were less pronounced than for primary-amine-functional analogs. For H2S-based separations, the benefits of competitive sorption offset decreases in selectivity due to plasticization. This detailed study helps to elucidate the role of tertiary amines for acid gas separations in solution-processable microporous PAEs.

Hydrogen Separation Membranes: A Material Perspective

The global energy market is shifting toward renewable, sustainable, and low-carbon hydrogen energy due to global environmental issues, such as rising carbon dioxide emissions, climate change, and global warming. Currently, a majority of hydrogen demands are achieved by steam methane reforming and other conventional processes, which, again, are very carbon-intensive methods, and the hydrogen produced by them needs to be purified prior to their application. Hence, researchers are continuously endeavoring to develop sustainable and efficient methods for hydrogen generation and purification. Membrane-based gas-separation technologies were proven to be more efficient than conventional technologies. This review explores the transition from conventional separation techniques, such as pressure swing adsorption and cryogenic distillation, to advanced membrane-based technologies with high selectivity and efficiency for hydrogen purification. Major emphasis is placed on various membrane materials and their corresponding membrane performance. First, we discuss various metal membranes, including dense, alloyed, and amorphous metal membranes, which exhibit high hydrogen solubility and selectivity. Further, various inorganic membranes, such as zeolites, silica, and CMSMs, are also discussed. Major emphasis is placed on the development of polymeric materials and membranes for the selective separation of hydrogen from CH4, CO2, and N2. In addition, cutting-edge mixed-matrix membranes are also delineated, which involve the incorporation of inorganic fillers to improve performance. This review provides a comprehensive overview of advancements in gas-separation membranes and membrane materials in terms of hydrogen selectivity, permeability, and durability in practical applications. By analyzing various conventional and advanced technologies, this review provides a comprehensive material perspective on hydrogen separation membranes, thereby endorsing hydrogen energy for a sustainable future.

Tartaric acid anion intercalated layered double hydroxides constructed a CO2-favorable transport channel in PIM-1 mixed matrix membranes for CO2/N2 separation

As a unique polymer of intrinsic microporosity, PIM-1 has great potential in mixed matrix membranes (MMMs) for gas separation with ultra-high permeability. However, the separation performance, e.g. CO2/N2 separation, is still limited by its poor selectivity. In this work, PIM-1-based MMMs containing tartaric acid anion intercalated layered double hydroxides (TA-LDH) nanofillers are successfully prepared. The uniformly dispersed TA-LDH in the PIM-1 matrix contributes to optimizing the stacking of polymer chains for accessible pores, which is conducive to CO2 diffusion. In addition, the assistant experiment and theoretical calculation point out that the -COO-, –OH between the TA-LDH layers had an affinity for CO2, creating a two-dimensional fast transport channel between the enlarged interlayer space. Compared with the pristine PIM-1 membrane, the CO2 permeability and CO2/N2 selectivity of 0.7 % TA-LDH/PIM-1 membrane is increased by 43.19 % and 105.48 %. The excellent gas separation performance with CO2 permeability of 8343.18 Barrer and CO2/N2 selectivity of 39.02 exceeds the 2019 Robson upper limit. This paves a new way to improve the gas separation performance of PIM-1-based MMMs and suggests high perspectives for real applications.

Diffusive nature of different gases in graphite: Implications for gas separation membrane technology

Graphite membranes have gained attention in membrane technology for gas separation due to their high stiffness, strength, and stability in corrosive and high-temperature environments. Graphite exists naturally in geologic media and can also be prepared by synthetic processes. In- situ hydrogen (H2) production in petroleum reservoirs or H2 storage in depleted gas reservoirs results in contamination with CH4 and CO2. To address the separation challenge at the surface, a sustainable downhole wellbore membrane must be available to separate H2, leaving behind CO2 and other gases to improve operational economics. Currently, there are no studies in the literature on the self-diffusivity of gases (CO2, H2, and CH4) in graphite as a function of pore size. To overcome time constraints in diffusivity experiments, this work utilized mathematical models and molecular simulations to delineate the self-diffusivity of gases in graphite of different pore sizes.To acknowledge subsurface operational conditions during in situ hydrogen production, we considered a temperature of 360 K and a wide pressure spectrum from 2 MPa to 20 MPa. In this study, we explored the diffusive nature of H2, CH4, and CO2 gases in different nanopore-sized graphite using analytical and molecular simulation approaches. We validated the results by presenting unrestricted case density calculations. First, effective diffusivity was calculated using the mean free pore path, followed by gas adsorption at high pressures (10–20 MPa) and a temperature of 350 K. The study utilized theoretical models and molecular dynamics (MD) simulations to determine the self-diffusivity of gases in graphite systems with various structures.

Applying membrane techniques to separate hydrogen from natural gas for hydrogen technologies

Today, the world's energy industry is grappling with several issues arising from restrictions on harmful greenhouse gas emissions, related to climate and environmental protection. Methods of storing energy in various types of carriers are becoming increasingly important, the most promising of which is hydrogen. Various methods of storing it are being considered, where an interesting option is to store it temporarily, under certain conditions, in existing natural gas networks. The challenge, however, is its subsequent recovery. Membrane technology is receiving special attention among recovery methods because of its potential efficiency in hydrogen separation, safety and ease of operation, low energy requirements and environmental friendliness. The paper presents a literature study of the issue, preliminary analyses of membrane gas separation and the advisability of the direction of the new research that has been undertaken. The possibility of applying membrane techniques to the process of separating hydrogen from natural gas in transmission and distribution networks for hydrogen storage technology is presented. Preliminary research is geared toward exploring the scope of applicability of this method of membrane separation, in which the primary goal is to determine the possibility of separating hydrogen from methane. In this paper, within the framework of the adopted simplified structure of the polymeric membrane module, the basic parameters of the process (share of hydrogen in the feed, membrane surface area) were determined, as well as the influence and dependence of transmembrane pressure (permeate) on the values of membrane selectivity, recovery rate and purity of the obtained hydrogen.

Modelling across Multiple Scales to Design Biopolymer Membranes for Sustainable Gas Separations: 2-Multiscale Approach

The majority of materials used for membrane-based separation of gas mixtures are non-renewable and non-biodegradable, and the assessment of alternative bio-based polymers requires expensive and time-consuming experimental campaigns. This effort can be reduced by adopting suitable modelling approaches. In this series of works, we propose various modelling approaches to assess the CO2/CH4 separation performance of eight different copolymers of 3-hydroxybutyrate and 3-hydroxyvalerate (PHBV) using a limited amount of experimental data for model calibration. In part 1, we adopted a fully atomistic approach based on Molecular Dynamics (MD), while, in this work, we propose a multiscale methodology where a molecular description of the polymers is bridged to a macroscopic prediction of its gas sorption behaviour. PHBV structures were simulated using MD to obtain pressure–volume–temperature data, which were used to parametrise the Sanchez–Lacombe Equation of State. This, in turn, allows for the evaluation of the CO2 and CH4 solubility in the copolymers at various pressures and compositions with little computational effort, enabling the estimate of the sorption-based selectivity. The gas separation performance obtained with this multiscale technique was compared to results obtained with a fully atomistic model and experimental data. The solubility–selectivity for the CO2/CH4 mixture is in reasonable agreement between the two models and the experimental data. The multiscale method presented is a time-efficient alternative to fully atomistic methods and detailed experimental campaigns and can accelerate the introduction of renewable materials in different applications.

Highly plasticization-resistant Tröger’s base polymer–MOF mixed-matrix membranes for stable gas separation

The plasticization phenomenon, characterized by polymer chain swelling when exposed to highly soluble gas at high feed pressure, has been a major problem for polymer membranes. To address this, we prepared Tröger’s base (TB) polymer-based metal–organic framework (MOF) mixed-matrix membranes (MMMs), which are highly plasticization-resistant membranes. Three different highly porous MOF nanoparticles (ZIF-8, UiO-66-NH2, and MIL-101(Cr)) were prepared in small particle sizes. The corresponding MMMs were formed with a 30 wt% MOF loading to study the effects of MOF species, particle size, chemical functionality, and MOF–polymer interfacial interaction on gas-transport properties and plasticization behavior. The TB polymer-based MOF MMMs exhibited significantly higher H2 and CO2 permeabilities (fourfold to eightfold) than the TB polymer membrane owing to the porous nature and high gas adsorption capacity of the MOF. Additionally, the TB-based MOF MMMs, particularly the UiO-66-NH2 MMM, displayed exceptional CO2 plasticization resistance. This is attributed to the interfacial interaction of the strong hydrogen bonding between the amine group of the TB polymer and the amino group of UiO-66-NH2, as confirmed by small-angle X-ray scattering characterization. This TB polymer-based MOF MMM approach provides a feasible and effective route for enhancing gas-transport properties and CO2 plasticization resistance to achieve energy-efficient and stable gas separation.

Heterostructure ZIF-8@MXene with sieving effect in mixed matrix membranes for CO2 separation

At the present time, the realm of gas separation membranes is predominantly centered on the development of membrane materials that simultaneously exhibit exceptional gas permeability and selectivity. Mixed Matrix Membranes (MMMs) hold significant promise in addressing the long-standing challenge of balancing membrane selectivity with permeability. By incorporating a molecular sieving mechanism, an enhanced gas separation process is achieved. In this research endeavor, single-layer MXene nanosheets serve as an ideal substrate for the direct growth of ZIF-8 nanoparticles, yielding heterogeneously structured nanofillers, coined ZIF-8@MXene. The composite filler, ZIF-8@MXene, maintains a two-dimensional lamellar structure reminiscent of MXene, while also integrating ZIF-8 nanoparticles with their optimal pore size of 3.4 Å. This optimal pore size efficiently facilitates the unimpeded passage of CO2 molecules through the channels, creating a preferential pathway. Conversely, N2 molecules encounter greater difficulty traversing these channels, effectively coupling the solubility-diffusion mechanism with the molecular sieving mechanism to enhance selectivity in gas separation processes. Consequently, the mixed matrix membrane exhibits a remarkable enhancement in gas selectivity. Particularly, under sub-ambient conditions (−20 °C), the PEO/ZIF-8@MXene-0.3 % membrane achieves an exceptional CO2/N2 selectivity of 294.1, accompanied by a high CO2 permeability of 115.58 Barrer. This outstanding performance surpasses the 2019 upper bound, underscoring the significant potential of heterostructured nanofillers to be harnessed in advancing various mixed matrix membranes (MMMs) towards high-performance gas separation capabilities.

Impact of concentration polarization on the performance of membrane gas separation processes: application to biogas upgrading

Through the illustrative application of biogas treatment, this paper investigates the impact of concentration polarization on the separation performance of emerging inorganic membranes in membrane gas separation processes. The results show that polarization may significantly reduce the biogas purification rate, although its effects on methane recovery remain moderate. Contrary to previous assumptions, the impact of polarization does not monotonously increase with increasing permeance to CO2 and selectivity. Material selectivity is shown to not significantly influence the polarization intensity, and the CO2 permeance at which peak polarization conditions occur is not constant but varies depending on the operating and geometric conditions considered. The impact of polarization impact intensifies with increasing fiber diameter and operating pressure, preventing taking full advantage of the exceptional permeances of inorganic membranes, and therefore, constitutes a major obstacle to their use as an alternative to conventional polymeric fibers.

Diffusion-Regulated Interfacial Polymerization of Hierarchically Microporous Polyamide Membranes for Permselective Gas Separations.

Interfacial polymerization has emerged as a robust method for fabricating task-specific polyamide (PA) membranes. However, the limited microporosity of highly cross-linked PA membranes constrains their effectiveness in gas separation applications. Herein, we introduce an ionic liquid (IL)-regulated interfacial polymerization process to fabricate polyamide nanofilms incorporating kinked tetrakis (4-aminophenyl) methane monomers. In situ ultraviolet-visible spectroscopy demonstrates that the diffusion of 1,3,5-benzenetricarbonyl trichloride (TMC) toward the interface increases with the IL/H2O ratio, leading to the formation of a more compact membrane with a higher cross-linking degree. The PA-TAM7/3-60 min membrane exhibits a CO2 permeance of 29.8 GPU and a CO2/CH4 selectivity of 109, exceeding the 2008 Robeson upper bond. Additionally, the highly cross-linked structure imparts the membranes with notable plasticization resistance. Mixed-gas tests (CO2/CH4 = 50/50, v/v) reveal that the PA-TAM7/3-60 min membrane experiences only a 2% reduction in CO2 permeance and a 10% decrease in CO2/CH4 selectivity at a CO2 partial pressure of 300 PSIG, compared to its performance at 30 PSIG. The ease of tuning membrane structure and gas separation performance, along with its excellent plasticization resistance, underscores the potential of these PA membranes for task-specific gas separations.

Healable, Recyclable, and Upcyclable Gel Membranes for Efficient Carbon Dioxide Separation.

Ionic liquids (ILs) are prized for their selective dissolution of carbon dioxide (CO2), leading to their widespread use in ionogel membranes for gas separation. Despite their advantages, creating sustainable ionogel membranes with high IL contents poses challenges due to limited mechanical strength, leakage risks, and poor recyclability. Herein, we leverage copolymerized and supramolecularly bound ILs to develop ionogel membranes with high mechanical strength, zero leakage, and excellent self-healing and recycling capabilities. These membranes exhibit superior ideal selectivity for gas separation compared to other reported ionogel membranes, achieving a CO2/nitrogen selectivity of 61.7 and a CO2/methane selectivity of 24.6, coupled with an acceptable CO2 permeability of 186.4 Barrer. Additionally, these gas separation ionogel membranes can be upcycled into ionic skins for sensing applications, further enhancing their utility. This research outlines a strategic approach to molecularly engineer ionogel membranes, offering a promising pathway for developing sustainable, high-performance materials for advanced gas separation technologies.

Novel mixed matrix membranes containing calixarene for enhanced CO2/N2 separation

Calixarene possessing advantages of small intrinsic cavity size, small particle dimension, and facile dispersion or dissolution in common solvents, is promising filler candidates in mixed matrix membranes (MMMs) for gas separation. Herein, the supramolecular C-propylpyrogallol[4]arene (PgC3) was successfully synthesized via a phenolic resin condensation reaction. It was then incorporated into a polyether-polyamide block copolymer (Pebax-1657) to prepare mixed matrix membranes (MMMs) with varying PgC3 loadings for CO2/N2 separation. The formation of hydrogen bonds between PgC3 and Pebax facilitated the uniform dispersion of PgC3 within the polymer matrix, enhancing interfacial compatibility. The incorporation of PgC3 significantly enhanced the CO2/N2 selectivity of the Pebax while preserving its permeability. The 1 wt% PgC3/Pebax MMMs performs the best CO2/N2 selectivity of 58.6, substantially higher than that of pristine Pebax membrane (32.4). This improvement can be attributed to the strong affinity between the abundant –OH groups in PgC3 and CO2 molecules. Moreover, the PgC3/Pebax MMMs exhibited exceptional stability under operating pressures of up to 5 bar, maintaining its performance consistently throughout a demanding100-hour test. The good separation performance together with the excellent stability enables PgC3 a promising filler in MMMs for CO2/N2 or other gas separations.

Binary-Cooperative Ultrathin Porous Membrane for Gas Separation.

The construction of ultrathin porous membranes with stable structures is critical for achieving efficient gas separation. Inspired by the binary-cooperative structural features of bones and teeth-composed of rigid hydroxyapatite and flexible collagen, which confer excellent mechanical strength-a binary-cooperative porous membrane constructed with gel-state zeolitic imidazolate frameworks (g-ZIFs), synthesized using a metal-gel-induced strategy, is proposed. The enlarged cavity size and flexible frameworks of the g-ZIF nanoparticles significantly improve gas adsorption and diffusion, respectively. After thermal treatment, the coordination structures forming rigid segments in the g-ZIF membranes appear at the stacked g-ZIF boundaries, exhibiting a higher Young's modulus than the g-ZIF nanoparticles, denoted as the flexible segments. The g-ZIF membranes demonstrate excellent tensile and compression resistances, attributed to the effective translation of binary-cooperative effects of rigidity and flexibility into the membranes. The resulting dual-aperture structure, composed of g-ZIF nanoparticles surrounded by nanoscale apertures at the boundaries, yields a membrane with a stable CO2 permeance of 4834 GPU and CO2/CH4 selectivity of 90 within 3.0MPa.

Polydopamine-assisted minute fabrication of vertically aligned ZIF-L membranes for CO2/N2 separations

Metal-organic framework (MOF) membranes demonstrate significant potential for industrial gas separations, yet fabricating vertically aligned two-dimensional (2D) MOF membranes on polymer substrates remains a considerable challenge due to the weak adhesion and difficulty in achieving precise alignment. This study presents a one-step method for the rapid synthesis of vertically aligned ZIF-L membranes, using a polydopamine (PDA)-assisted approach. This method enables the rapid formation of homogenous, thin but defect-free membranes on polymer substrates within 5 min, drastically reducing the preparation time by over 200 times. The vertically aligned ZIF-L membranes, characterized by the interlayer galleries facing the gas mixtures, exhibit an excellent CO2 permeance exceeding 1200 GPU and a CO2/N2 selectivity of 14.5. The versatility of this approach allows for its application on various flat-sheet or even hollow fiber supports. Our findings highlight the potential of PDA-assisted synthesis for efficient, scalable preparation of high-performance MOF membranes for industrial gas separations.

Low-crystallinity ZIF-8/dcIm membranes for CO2 sieving

Metal-organic framework membranes have shown great potential for efficient separation applications. However, unavoidable grain boundary defects and the inconstant pore size due to framework flexibility hinder the application of MOF membranes for gas separation. Herein, low-crystallinity ZIF-8/dcIm-x membranes are synthesized via fast current-driven synthesis. The competitive coordination of 4,5-dichloroimidazole (dcIm) linker leads to the formation of a low-crystallinity MOF with reduced grain boundary defects for better gas separation performance. Meanwhile, the in-situ substitution of the mIm linker by dcIm narrows the pore size and transforms the ZIF-8 structure in the stiffened Cm polymorph, allowing precise CO2 sieving. Consequently, the low-crystallinity ZIF-8/dcIm-14 membrane exhibits a promising CO2/CH4 selectivity of 32.4 with a CO2 permeance of 1.73 × 10−8 mol m−2 s−1 Pa−1, which surpasses other membranes. Furthermore, the membrane presents a remarkable pressure resistance up to 3 bar and stable temperature-swing operation between room temperature and 125 °C over 120 h.

Investigation on the synthesis conditions of poly(4-methyl-1-pentene) hollow fiber membrane with high gas permeability and strong tensile strength

Investigation on the synthesis conditions of poly(4-methyl-1-pentene) hollow fiber membrane with high gas permeability and strong tensile strength

Porous organic cage induced high CO2/CH4 separation efficiency of carbon molecular sieve membranes

Porous fillers have been incorporated into precursor membranes to fabricate carbon molecular sieve (CMS) membranes, where the interfacial compatibility between fillers and matrix significantly influences the separation performance of converted CMS membranes. To address this issue, we introduce molecular fillers of porous organic cages (POCs) into precursor membranes, the organic composition and discrete molecular characteristics of which can enhance the compatibility with the polymer matrix (PIM-1), bringing enhanced porosity and narrowed pore size distribution, especially in the ultra-micropore region, to the CMS membranes. The obtained CMS membrane shows simultaneously improved gas permeability and selectivity wherein the optimized CMS membrane pyrolyzed at 600 °C with 10 wt% POCs loading (CMS-10%-600) exhibits a CO2 permeability of 2002 Barrer and a CO2/CH4 selectivity of 221.6, surpassing the 2019 CO2/CH4 upper bound and superior to most of reported CMS membranes. Due to the incorporation of POC, the CMS-10%-600 membrane demonstrates a 147.5 % increase in CO2 permeability and a 297.1 % increase in CO2/CH4 selectivity compared to pristine PIM-1-based CMS membrane, along with the excellent aging resistance for over 30 days. This study provides new insights into the design of high-performance CMS membranes for gas separation.

Assembly-Dissociation-Reconstruction Synthesis of Covalent Organic Framework Membranes with High Continuity for Efficient CO2 Separation.

Covalent organic frameworks (COFs), at the forefront of porous materials, hold tremendous potential in membrane separation; however, achieving high continuity in COF membranes remains crucial for efficient gas separation. Here, we present a unique approach termed assembly-dissociation-reconstruction for fabricating COF membranes tailored for CO2/N2 separation. A parent COF is designed from two-node aldehyde and three-node amine monomers and dissociated to high-aspect-ratio nanosheets. Subsequently, COF nanosheets are orderly reconstructed into a crack-free membrane by surface reaction under water evaporation. The membrane exhibits high crystallinity, open pores and a strong affinity for CO2 adsorption over N2, resulting in CO2 permeance exceeding 1060 GPU and CO2/N2 selectivity surpassing 30.6. The efficacy of this strategy offers valuable guidance for the precise fabrication of gas-separation membranes.