Gas Permeation Rate Research Articles

Mixed matrix membrane for hydrogen separation enhanced by Oxacalix[4]arene-Containing porous organic polymers

The mixed matrix membranes (MMMs) have great potential for industrial H2 enhancement applications. However, some challenges remain to incorporate the nanoporous architectures with industrial processable polymers. In this work, a macrocyclic cavitand from oxacalix[4]arene was integrated into porous polymers via a facile ring-opening-metathesis polymerization (ROMP), the resulting BIT-POP-20 was added to polysulfone matrix to make novel MMMs with different mass ratios. These membranes improved gas permeation rate for CO2, H2 and He, compared with pristine polysulfone, and high H2/N2 selectivity of 105 was achieved. This huge improved H2/N2 separation performances were due to the nanoporous gas passages through the open supramolecular cavities. This work presents an interesting example in designing MMMs using intrinsic macrocyclic cavity in porous polymer backbone that helps increase their performances in gas separations.

Hyper-Cross-Linked Polymers with Sulfur-Based Functionalities for the Prevention of Aging Effects in PIM-1 Mixed Matrix Membranes

The instability of gas permeation rates over time in PIM-1-based membranes is one of the obstacles preventing the implementation of this technology on larger scales. The addition of porous materials to the PIM-1 matrix is a known process to reduce aging effects in PIM-1-based membranes. Many porous solids of various chemical natures have been tested, ranging from inorganic to inorganic–organic hybrids to completely organic materials. Here, we report on the synthesis of two hyper-cross-linked polymers (HCPs) based on the diphenyl sulfide monomer, synthesized in either dichloromethane at 35 °C or dichloroethane at 78 °C. Two mixed matrix membranes (MMMs) were obtained from the addition of 3 wt % of either filler to PIM-1. Both the CO2 permeation rates and the CO2/N2 selectivity were measured for the pure PIM-1 membrane and for the two MMMs over a period of 400 days. PIM-1 exhibits a classic aging behavior with an 85% drop in the CO2 permeation rate with respect to the initial value. For the two MMMs, a retention of 50% of the initial CO2 permeability was instead observed after more than 400 days of aging. Despite the low surface area and low pore volume of the fillers, we show that diphenyl sulfide-based HCPs can reduce the aging rate in PIM-1 membranes.

Investigation of mechanical damage in PFSA membrane via gas permeation rate test of membrane blister

The perfluorinated sulfonic-acid (PFSA) membrane is a key component in proton exchange membrane (PEM) fuel cells. The gas permeability of PFSA membrane, related to its deformation and damage, affects the performance and safety of the fuel cell. On the basis of constitutive model and solution-diffusion model of PFSA membrane, a numerical model was built to predict the deformation and gas permeation rate of membrane blister. The result indicates that damage occurs during the blister and accelerate the gas permeation.

Preparation of nano-porous poly(ether ether ketone) hollow fiber membrane and its performance for desalination in vacuum membrane distillation

Nano-porous poly(ether ether ketone) (PEEK) hollow fiber membranes were successfully fabricated from PEEK/polyetherimide (PEI) blends via thermally-induced phase separation (TIPS) followed by chemical extraction of PEI method. The gas permeation rates, morphologies and mechanical properties of membranes were systematically investigated under different spinning conditions. The content of PEEK in blends substantially determined the gas permeation rates following the order: PEEK30/PEI70 (~9000 GPU) > PEEK40/PEI60 (~5100 GPU) > PEEK50/PEI50 (~1700 GPU) > PEEK60/PEI40 (~300 GPU) > PEEK70/PEI30 (below 1.0 GPU). Faster winding rates and bigger N2 bore fluid flow rates would result in bigger pores, higher porosity and larger gas permeation rates, while would cause lower mechanical strength. Furthermore, the silane-grafted hydrophobic PEEK hollow fiber membranes were successfully prepared via one-step reduction followed by silane modification. The performance of virgin and modified membranes was evaluated by treating saline feed containing 3.5 wt% sodium chloride in vacuum membrane distillation (VMD) process. Compared with virgin PEEK membrane, the modified membranes exhibited a more stable flux and higher salt rejection over 99.9 % for 33 h, indicating the significant improvement in the anti-wetting property caused by silane modification. The application attempt expands the research direction of PEEK membranes and promotes its further application potential in VMD process.

A study of the permeation barrier of nanoporous Graphene

Nanoporous Graphene, due to its single atom thickness and good mechanical strength is an emerging candidate for efficient and reliable gas separation applications. The permeation of the gas molecules through the pores can be seen as an activated process, whereby the gas molecules have to overcome a threshold barrier before it can pass through the pore. Here, we study and compare the Single Point Energy of the CO2 molecule as it approaches a nanoporous Graphene sheet comprising of 6 Carbon atoms removed contiguously to create what is known as the Type A nanopore. The distance of the single molecule from the pore is varied continuously for 2 configurations of the gas molecule, one parallel and other perpendicular to the nanopore, positioned vertically above the centre of the 6-pore, by means of ab-initio simulations based on Density Functional Theory. The permeation barrier of the CO2 molecule is estimated. It is found that orientation of incident molecule along with the pore geometry, plays an important role in gas permeation and penetration barriers, and thereby in all applications that crucially depend upon the rate of gas permeation, like gas separation for example.

Numerical and experimental characterization of gas permeation through membranes with consideration of mechanical degradation in proton exchange membrane fuel cells

Exposed to severe loading condition in fuel cells, the perfluorinated sulfonic-acid (PFSA) membrane degrades over time, which becomes the primary cause of the increasing transmembrane gas permeation during the service. As a result, the hydrogen crossover rate increases with the damage propagation, leading to loss in power and life. However, the effect of mechanical degradation on gas permeability of membrane cannot be quantitatively estimated due to difficulties in damage observation and modeling. This paper first establishes a numerical technique to predict the gas permeation through membrane with consideration of mechanical degradation. Blister test as well as gas permeation test are conducted for the verification of the numerical model. The model shows great prediction accuracy, based on which the uneven distributed damage and its effect on gas permeability are analyzed. Result shows that the gas permeation rate grows with pressure and mechanical degradation has a remarkable contribution, accounting for about 7.3% of total permeation rate at testing pressure of 130 kPa. Finally, the varied membrane shape and gas permeability, which all accelerate the gas permeation, are further analyzed based on the numerical model.

Gas permeation rates of ultrathin graphite sealed SiO2 cavities.

Despite the proven impermeability of graphene toward most standard gases, graphene/graphite sealed SiO2 cavities always exhibit a nonzero leak rate, and the physical leakage mechanism is still unclear. By measuring leak rates of different gases for the same cavities sealed by ultrathin graphite under identical conditions, we find that the leak rates generally depend on the kinetic diameter of the gas molecules, which implies that the leakage is caused by a molecular sieving mechanism. By comparing different samples, we find that the leak rate of any gas in a particular sample is well predicted by the leak rate of N2 in that sample. In addition, we observe enhanced leak rates of water-soluble molecules. We infer that the leakage path (i.e., the graphene/graphite-SiO2 interface) favors hydrophilic species.

Understanding Gas Permeation during High Pressure Operation of PEM Water Electrolyzers

Governments and industries across the world have initiated implementation of clean hydrogen (H2) to achieve zero emissions, yet significant challenges remain for large scale adoption. With the decreasing renewable electricity cost PEM water electrolyzers (PEMWEs) show significant potential for at scale deployment due to their high faradaic efficiency and high operating current density. To reduce the per kilogram price of hydrogen and increase the overall energy efficiency, PEMWEs need to be operated at high differential pressures (30 to 50 bar) to eliminate the need for compression during storage and transportation1. To achieve high faradaic efficiency, thinner membranes are required to reduce voltage losses within the cell. However, thinner membranes exhibit a higher gas permeation rate which leads to increased crossover of hydrogen from the cathode through the membrane to the anode. Such increased crossover of H2 not only reduces the efficiency but can also result in flammable gas mixtures (lower flammability limit for H2 in O2 is 4 %) as Iridium/Iridium oxide (anode catalyst) is inefficient in oxidizing H2 1. The addition of a gas recombination catalyst (GRC) to the membrane is an useful strategy to oxidize permeating H2 and minimize the H2:O2 ratio at the anode.In this study, a high pressure (up to 30 bar) electrolyzer setup is coupled with online gas chromatography-mass spectrometry to understand the H2 gas permeation in membranes as a function of H2 partial pressure and operating current density (simulated by varying the flow rates) as shown in Figure 1 . Effect of the GRC, its loading, and its location on the H2 permeation will be elucidated. Differences induced by the addition of anode and cathode catalyst layers on H2 permeation rates will also be presented.Figure 1. Ex-situ H2 permeation rate at different cathode operating pressure at 80 °C. The anode was maintained at 1 bar of O2 partial pressure Acknowledgement This research is supported by the U.S. Department of Energy (DOE) through the Hydrogen and Fuel Cell Technologies Office, program manager Dave Peterson Reference Bernt, M., Schröter, J., Möckl, M. & Gasteiger, H. A. Analysis of Gas Permeation Phenomena in a PEM Water Electrolyzer Operated at High Pressure and High Current Density. J. Electrochem. Soc. 167, 124502 (2020). Figure 1

Gas Identification by Simultaneous Permeation through Parallel Membranes: Proof of Concept

This paper describes an experimental system for simultaneous permeation of a pressurized test gas through different gas permeable membranes and provides a proof of concept for a novel approach for gas identification/fingerprinting for potential construction of electronic noses. The design, construction, and use of a six-channel system which allows simultaneous gas permeation from a single pressurized gas compartment through six different parallel membranes are presented. The permeated gas is accumulated in confined spaces behind the respective membranes. The rate of gas pressure accumulation behind each membrane is recorded and used as a measure of the gas permeation rate through the membrane. The utilized gas permeable membranes include Teflon AF, silicone rubber, track-etch hydrophilic polycarbonate, track-etch hydrophobic polycarbonate, track-etch polyimide, nanoporous anodic aluminum oxide, zeolite ZSM-5, and zeolite NaY. An analogy between the rate of pressure accumulation of the permeating gas behind the membrane and the charging of an electric capacitor in a single series RC circuit is proposed and thoroughly validated. The simultaneous permeation rates through different membranes demonstrated a very promising potential as characteristic fingerprints for 10 test gases, that is, helium, neon, argon, hydrogen, nitrogen, carbon dioxide, methane, ethane, propane, and ethylene, which are selected as representative examples of mono-, di-, tri-, and polyatomic gases and to include some homologous series as well as to allow testing the potential of the proposed system to discriminate between closely related gases such as ethane and ethylene or carbon dioxide and propane which have almost identical molecular masses. Finally, a preliminary investigation of the possibility of applying the developed gas permeation system for semiquantitative analysis of the CO2-N2 binary mixture is also presented.

Molecular dynamics study on electric field-facilitated separation of H2O/O2 through nanoporous graphene oxide membrane

The application of external stimuli, such as electric fields, could improve gas separation. Molecular dynamics (MD) simulations were conducted to analyze the dynamic electric field-facilitated separation process of H2O/O2 gas molecules through a double-layered nanoporous graphene oxide membrane. The computational results illustrated an enhanced gas permeation rate of 3.26 × 103 mol·m−2·s−1 for H2O molecules with an external electric field of 10−4 V·Å−1, and an enhanced selectivity of 1.44 over O2 molecules, which was 0.33 in the absence of the external electric field. The mechanism of enhanced H2O/O2 separation has also been illuminated by examining the variation in interaction energy with electric field strength. The electric field inhibits the hydrogen bond interactions between H2O molecules and H2O membranes. Accelerating desorption under electric fields would enable more adsorption sites on the membrane, thereby facilitating H2O molecules permeation.

Mixed‐matrix membranes based on Li1.6Mn1.6O4 (LMO) ultrathin nanosheet for high‐performance CO2 separation

AbstractTwo‐dimensional flake Li1.6Mn1.6O4 (LMO) was added to the Pebax®MH 1657 (Pebax) matrix to prepare mixed‐matrix membranes (MMMs) for CO2/CH4 mixed gas separation. The average pore diameter of the LMO nanosheet was about 10.79 nm: a fast CO2 transport channel could be established in the membrane. In addition, the Li and oxygen defect site (Oδ−) in the LMO had an affinity for CO2, which improved the permeability of the membrane and the selectivity for CO2 separation. Furthermore, the ultrathin structure of LMO with large aspect ratio was also conducive to gas separation. When the LMO filling amount was 2.0 wt%, the permeation rate of pure CO2 gas of Pebax/LMO‐2.0 membrane was 117.9 Barrer, and the CO2/CH4 ideal selectivity was 18.74, which were 119.7 and 35.8% higher than Pebax membrane, respectively. The CO2/CH4 mixed gas separation results of Pebax/LMO‐2.0 membrane showed that the CO2 permeability and CO2/CH4 selectivity were 158.54 Barrer and 33.41, respectively, and were increased by 83.24 and 23.15%, compared with Pebax membrane.

MIL-101(Cr) Microporous Nanocrystals Intercalating Graphene Oxide Membrane for Efficient Hydrogen Purification.

Graphene oxide (GO) is a promising two-dimensional building block for fabricating high-performance gas separation membranes. Whereas the tortuous transport pathway may increase the transport distance and lead to a low gas permeation rate, introducing spacers into GO laminates is an effective strategy to enlarge the interlayer channel for enhanced gas permeance. Herein, we propose to intercalate CO2 -philic MIL-101(Cr) metal-organic framework nanocrystals into the GO laminates to construct a 2D/3D hybrid structure for gas separation. The interlayer channels were partially opened up to accelerate gas permeation. Meanwhile, the intrinsic pores of MIL-101 provided additional transport pathways, and the affinity of MIL-101 to CO2 molecules resulted in higher H2 /CO2 diffusion selectivity, leading to a simultaneous enhancement in gas permeance and separation selectivity. The MIL-101(Cr)/GO membrane with optimal structures exhibited outstanding and stable mixed-gas separation performance with H2 permeance of 67.5 GPU and H2 /CO2 selectivity of 30.3 during the 120-h continuous test, demonstrating its potential in H2 purification application.

Hollow Fiber Polyimide Membranes Prepared in a Triple Orifice Spinneret: Effect of a Reduced Water Activity in the Bore Fluid on the Gas Separation Performance.

Polyimide-based hollow fibers were spun using a triple orifice spinneret in order to apply them in gas separation. The membrane structure was tailored producing a porous external layer and a thin internal skin layer, that controlled the gas transport. The measurement of gas permeation rates and the morphological analysis were combined to obtain information on the performance of the membranes. The aim was to tune the inner top layer and investigate the role of the bore fluid on the gas permeation properties of the membranes. The bore fluid composition was explored by using water mixtures containing the solvent used for preparing the dope solution or a salt in order to reduce the water activity in the inner coagulant, but also a low amount of a crosslinker for improving the gas selectivity. The change of the dope flow-rate was also analyzed. At moderate dope flow-rates, the use of a saline water solution as bore fluid is more effective in enhancing the membrane gas selectivity with respect to a bore fluid containing certain amounts of solvent. This option represents a green approach for the preparation of the membrane. The behavior of the prepared hollow fibers over time (physical aging) in gas permeation was discussed.

Investigation of the silica pore size effect on the performance of polysulfone (PSf) mixed matrix membranes (MMMs) for gas separation

Abstract MCM-41 and SBA-15 mesoporous silica materials with different pore sizes (3.08 nm for small pore size MCM-41 (P 1), 5.89 nm for medium pore size SBA-15 (P 2), and 7.81 nm for large pore size SBA-15 (P 3)) were synthesized by the hydrothermal method and then functionalized with 3-aminopropyltrietoxysilane by postsynthesis treatments. Next, polysulfone-mesoporous silica mixed matrix membranes (MMMs) were prepared by the solution casting method. The obtained materials and MMMs were characterized by various techniques including X-ray diffraction, scanning electron microscopy, and N2 adsorption-desorption, and Brunauer-Emmett-Teller method to examine the crystallinity, morphology, and particle size, pore volume, specific surface area, and pore size distribution, respectively. Finally, the gas permeation rates of prepared MMMs were measured in 8 bar and 25 °C and the effect of pore size of modified and unmodified mesoporous silica on the gas separation performance of these MMMs were investigated. The experimental results indicate that the carbon dioxide (CO2) and methane (CH4) permeability and CO2/CH4 selectivity were increased with an enhancement in the particle pore size.

The Effect of Glycerol on Alginate/Zeolite Membranes for Selectivity of CH<sub>4</sub>/CO<sub>2</sub> Gas

Synthesis of alginate/zeolite membrane with the glycerol plasticizer for separation of CH4 towards CO2 has been carried out. This study aimed to investigate the effect of glycerol addition towards physical and mechanical properties of membrane on the permeability and selectivity of CH4/CO2. Membrane fabrication was done with mixing of alginate and activated zeolite in the aquabidest with glycerol addition as plasticizer with mass ratio of alginate:glycerol 1:0, 1:0.25, 1:0.5, 1:1 and stirring for 24 h and templeting at room temperature. Membrane formed was tested with FTIR, XRD, SEM, Texture Analyzer and Permeation gas cell. Result showed that glycerol addition into alginate can decrease tensile strength and increase elongation value for about 20% on the mass ratio alginate:glycerol 1:1. Besides, gas permeation rate was also influence by the glycerol addition. The highest permeability of CH4 was obtained with alginate:glycerol 1:0,25 and with permeation rate 1.03 × 10-4 cm3 (STP) cm-2 s-1 cmHg-1. Meanwhile, the highest permeability of CO2 was obtained with the mass ratio alginate:glycerol 1:0,5 with permeation rate about 1,19 × 10-5 cm3 (STP) cm-2 s-1 cmHg-1. The highest selectivity indicated that separation of CH4/CO2 occurred well about 49.2 for alginate:glycerol 1:1.

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.

A versatile technique for in situ investigation of the effect of thin film cracking on gas permeation of coated flexible polymers.

In this work, we report the development of a measurement chamber linked with a quadrupole mass spectrometer (QMS) for in situ investigation of the effect of thin film cracking on the gas permeation of coated flexible polymeric substrates. The chamber enables the establishment of a bulged state of the substrate/coating system, which causes the cracking of the coating layer. The increase in the gas permeation rate due to the presence of cracks can be monitored precisely using the QMS without movement or re-clamping of the samples between each measurement step. This method eliminates the probability of uncontrollable mechanical changes in the sample, and with the mass spectrometer, high sensitivity, reliability, and reproducibility of the experimental data become available.

Effects of Coagulation Residence Time on the Morphology and Properties of Poly (vinyl) Alcohol (PVA) Asymmetric Membrane via NIPS Method for O2/N2 Separation

This study revealed, a thorough investigation of coagulation residence time effects on the morphology, crystallinity, mechanical properties and oxygen (O2), nitrogen (N2) gas transport behaviour of asymmetric membranes prepared through PVA/water/THF ternary system using non-solvent induced phase separation method. For this purpose PVA asymmetric membranes were fabricated at different coagulation residence of 3–24 h and then evaluated using various analytical techniques. The scanning electron microscopy was utilised to observe membrane morphologies to determine the membrane matrix structure, pore size distribution and bi-layer thickness ratio. X-ray diffraction was employed to examine the crystallinity of membranes. The variation in overall porosity and mechanical characteristics were investigated through gravimetric based analysis and universal testing machine respectively. Whereas, O2 and N2 gas transport behaviour of asymmetric membranes was recorded using gas permeation testing unit. The results showed that different sponge type, interconnected porous membranes, having thin durable dense layer morphology were obtained by varying residence time of coagulation process at constant temperature and concentration. A shorter residence time favoured the formation of single chain porous network having small pores size (1–2 μm) with porosity of 56.31% and thicker dense layer, having low O2 gas permeation rate and high mechanical characteristics such as tensile strength and elongation at break. However, a long residence time in coagulation bath facilitated the formation of double chain more porous network having pores size distribution between 2 and 3 μm with porosity of 75.52% and thinner dense layer, having high O2 gas permeation rate. The tensile strength of membranes is high in-spite of higher porosity and lower thickness of dense layer.

Efficiency Separation Process of H2/CO2/CH4 Mixtures by a Hollow Fiber Dual Membrane Separator

Hydrogen purification and CO2 capture are of great significance in refineries and pre-combustion power plants. A dual membrane separator offers an alternative approach for improving H2/CO2 separation efficiency. In this work, H2/CO2/CH4 ternary gas mixtures separation can be achieved by a dual membrane separator with an integrated polyimide (PI) membrane and polydimethylsiloxane/polyetherimide (PDMS/PEI) composite membrane. A hollow fiber dual membrane separation equipment is designed and manufactured. Through the self-designed device, the effects of stage cut, operating temperature, operating pressure, and membrane area ratio on separation performance of dual membrane separator have been studied. The results indicate that, at a high stage cut, a dual membrane separator has obvious advantages over a single membrane separator. Operating temperature has a significant impact on gas permeation rates. At 25 °C, a dual membrane separator can obtain the highest purity of H2 and CO2. By increasing operating pressure, the purity and recovery of H2 and CO2 can be improved simultaneously. The effect of the membrane area ratio on the performance of the dual membrane separator was studied. When the permeate flows of two membranes are approximately equal by changing the membrane area ratio, the overall performance of the dual membrane separator is the best. On the basis of its synergy in promoting separation, the dual membrane separator holds great industrial application potential.

Preparation and characterization of PVDF-filled MWCNT hollow fiber mixed matrix membranes for gas absorption by Al2O3 nanofluid absorbent via gas–liquid membrane contactor

Porous polyvinylidene fluoride (PVDF)-filled MWCNT hollow fiber mixed matrix membranes were fabricated and applied for carbon dioxide absorption in a gas–liquid membrane contactor with Al2O3 water-based nanofluid absorbent. The formation of MWCNTs’ nanotubes was verified through Transmission electron microscopy (TEM) and Field emission scanning electron microscopy (FESEM) images while the resultant membranes were characterized for their morphology, gas permeation rate, pore size, porosity, surface hydrophilicity and wettability resistance. Upon the incorporation of MWCNT into the matrix, the formation of finger-like pores was facilitated and surface roughness increased. Gas permeation test results showed increasing the surface porosity and reducing the pore size. The surface contact angle also was improved from 83°±1.5 for the unfilled membrane to 103°±1.5 for the membrane containing 5wt% CNT. CO2 absorption flux also was enhanced considerably. For instance, the flux of the membrane containing 5wt% CNT as the best performing membrane at a liquid velocity of 2.5m/s and Al2O3 water-based nanofluid as absorbent was 3.85×10−3mol/m2s which was approximately 200% higher than that of the plain membrane at the same operating conditions. A long-term absorption test also revealed non-wetting tendencies of the membranes as well as the unclogged membrane pores by the nanomaterials existed in the nanofluid absorbent. Therefore, it could be hypothesized that the improved membrane structure and synthesized nanofluid absorbent considering their superior performance to several commercial and in-house made membranes can be suitable alternatives to the current polymeric membranes and commercial used absorbents, respectively.