Gas Transport Characteristics Research Articles

Practical Manufacturing of Asymmetric Hollow Fiber Membranes for Gas Separation Made of Poly(2,6-dimethylphenylenoxide-1,4)

The regularities of manufacturing of hollow fiber membranes made of poly(2,6-dimethylphenylene oxide-1,4) for gas separation were studied. The phase inversion method was used to manufacture the membranes. The dependence of the separation characteristics of the membrane on such spinning parameters as the type of solvent, the exposure time of the polymer solution in the “air” gap, and the type of non-solvents (coagulants) has been studied. The characteristics of the membrane were obtained by determining their gas permeability. It is shown that higher separation and gas transport characteristics of the PPO membrane are obtained using the wet spinning method. An intrinsic selectivity of 4.8 ± 0.4 was obtained at a specific oxygen permeability (20°C) – (P/l) 790 ± 82 [m3 (s.t.p.) m–2 s–1 kPa] for oxygen–nitrogen system. The developed membranes are promising for use in case for producing nitrogen and oxygen-enriched air.

Performance of amine‐functionalized MIL‐53 incorporated thin‐film nanocomposite Pebax membranes for CO2/CH4 mixed gas separation

AbstractIn the present paper, the gas transport characteristics of amine‐functionalized MIL‐53 metal–organic frameworks (MOFs) integrated thin‐film nanocomposite (TFN) Pebax 2533 membranes dip‐coated over a polysulfone (PSF) layer were studied. Various characterization methods were used to investigate the impact of ligand functionalization by NH2 on the characteristics of the generated TFN membranes and their gas transport properties, including TGA, FESEM, BET, gas adsorption tests, and a series of CO2/CH4 mixed gas separation tests. According to the FESEM pictures, the NH2‐MIL 53(Al) particles were well distributed in the TFN membranes, with no apparent agglomeration, The gas adsorption experiment of NH2‐MIL 53(Al) particles showed that they adsorb CO2 selectively. The incorporation of NH2‐MIL 53(Al) into TFN membranes led to improved CO2/CH4 selectivity and increased gas permeability. In addition, CO2/CH4 selectivity and CO2 permeability of the CO2/CH4 mixed gas were enhanced from 11.05 to 26.55 and from 111.6 to 260.2 Barrer, respectively, when NH2‐MIL 53(Al) 20 wt% was added to the TFN membrane. In the CO2/CH4 mixed gas experiment, increasing the temperature from 30 to 50°C increased the permeability of both CH4 and CO2 while decreasing the CO2/CH4 selectivity. Furthermore, the performance of the prepared membranes was assessed at different feed pressures ranging from 4 to 8 bar. As the feed pressure increased, the CO2/CH4 separation improved. Also, with pure gas, the increase in NH2‐MIL53(Al) loading resulted in lower diffusivity selectivity amounts for the TFN membrane than for the pristine TFN, and solubility selectivity was enhanced from 5 to 20 wt% and was reduced to 25 and 30 wt% in the TFN, respectively. In the present study, the best results were obtained at 6 bar pressure, 30°C temperature, and 20 wt% loading of NH2‐MIL53(Al).

A molecular simulation study on amine-functionalized silica/polysulfone mixed matrix membrane for mixed gas separation

Polysulfone (PSF) based mixed matrix membranes (MMMs) are one of the most broadly studied polymeric materials used for CO2/CH4 separation. The performance of existing PSF membranes encounters a bottleneck for widespread expansion in industrial applications due to the trade-off amongst permeability and selectivity. Membrane performance has been postulated to be enhanced via functionalization of filler at different weight percentages. Nonetheless, the preparation of functionalized MMMs without defects and its empirical study that exhibits improved CO2/CH4 separation performance is challenging at an experimental scale that needs prior knowledge of the compatibility between the filler and polymer. Molecular simulation approaches can be used to explore the effect of functionalization on MMM's gas transport properties at an atomic level without the challenges in the experimental study, however, they have received less scrutiny to date. In addition, most of the research has focused on pure gas studies while mixed gas transport properties that reflect real separation in functionalized silica/PSF MMMs are scarcely available. In this work, a molecular simulation computational framework has been developed to investigate the structural, physical properties and gas transport behavior of amine-functionalized silica/PSF-based MMMs. The effect of varying weight percentages (i.e., 15–30 wt.%) of amine-functionalized silica and gas concentrations (i.e., 30% CH4/CO2, 50% CH4/CO2, and 70% CH4/CO2) on physical and gas transport characteristics in amine-functionalized silica/PSF MMMs at 308.15 K and 1 atm has been investigated. Functionalization of silica nanoparticles was found to increase the diffusion and solubility coefficients, leading to an increase in the percentage enhancement of permeability and selectivity for amine-functionalized silica/PSF MMM by 566% and 56%, respectively, compared to silica/PSF-based MMMs at optimal weight percentage of 20 wt.%. The model's permeability differed by 7.1% under mixed gas conditions. The findings of this study could help to improve real CO2/CH4 separation in the future design and concept of functionalized MMMs using molecular simulation and empirical modeling strategies.

Sorption and Gas Transport Characteristics of Polyimides Based on a Mixture of Diethyl Toluene Diamine Isomers

The transport of oxygen, nitrogen, and hydrocarbons C1–C4 in polyimides based on 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and biphenyl-3,3',4,4'-tetracarboxylic acid dianhydride (BPDA) and diethyl toluene diamine (DETDA) has been studied. The dependences of the diffusion coefficient on the diffusant effective diameter and the solubility coefficients on the Lennard–Jones potential of pair interaction have been considered. It is shown that the diffusion coefficients of butane are out of the linear dependence of the logarithm of the diffusion coefficient on the square of the effective diameter of the diffusant, which may indicate the plasticization of polymers with butane at a pressure of 1 atm. The permeability of mixtures of gases O2–N2 (29 : 71 v/v) and CO2–CH4 (62 : 38 v/v) has been studied. There are no significant differences in the gas separation parameters in comparison with the experiment for individual gases; however, for a mixture of CO2–CH4, a slight increase in the separation factor has been found as compared to the ideal selectivity. The data on the measurement of sorption isotherms and solubility coefficients O2, N2, and CH4 are also presented in the article. Gas solubility coefficients have been found to be close to the values obtained indirectly as P/D.

Shale gas mass transfer characteristics in hydration-induced fracture networks

The hydration-induced fractures significantly enhance shale gas production after well shut-in, which reveals considerable gas mass transfer characteristics. However, few studies focus on multiple flow mechanisms coupling the fracture distribution and morphological properties. Therefore, a novel apparent permeability (AP) model, in which poromechanics and desorption-induced aperture evolution are captured, has been derived to precisely define gas mass transfer through fracture networks. In this study, the fracture distributions are derived by fractal law, and the morphologies are solved using the orthogonal decomposition method (ODM) and shape coefficient correction. Viscosity changes in confined channels are also considered, further upscaling volume flux, Knudsen and surface diffusion through fractal theory by discrete integrals and derivation of the AP model combined with Darcy's law. The proposed model is verified well by experiments and the literature. The results show that the viscous flow contribution ratio decreases with decreasing aperture, while the Knudsen flow ratio slightly increases, and gas desorption significantly increases permeability when pp < pL. Therefore, the viscous flow is the dominant flow regime at high pp, and Knudsen and desorption diffusion gradually dominate the transmission at low pp. The larger bmax/bmin obviously enhances AP, the more confined apertures, and the AP decreases obviously as pp decreases. The stronger desorption and diffusion capability represent that gas will be transported sufficiently, higher co and δ indicate that the aperture is close more effectively, causing the AP reduction to be fast, and hydration further lowers E and v denotes higher AP due to the aperture shrinkage being replaced by matrix parts. The real gas effect on AP reduction cannot be ignored. This study identifies the gas transport characteristics in hydration fracture networks, with the research method also being applicable to other structures.

Experimental Determination of the Effect of Temperature on the Gas Transport Characteristics of Polymeric Gas Separation Fibers Based on Polysulfone

Experimental Determination of the Effect of Temperature on the Gas Transport Characteristics of Polymeric Gas Separation Fibers Based on Polysulfone

Effect of Supercritical CO2 Treatment on Mechanical and Gas Transport Characteristics of Polyimides Based on Diethyl Toluene Diamine Isomers

Effect of Supercritical CO2 Treatment on Mechanical and Gas Transport Characteristics of Polyimides Based on Diethyl Toluene Diamine Isomers

Effect of Uniform Deformation on the Mechanical and Gas Transport Characteristics of Polyimides Based on Diethyltoluylenediamine

Effect of Uniform Deformation on the Mechanical and Gas Transport Characteristics of Polyimides Based on Diethyltoluylenediamine

Poly(2,6-Dimethyl-1,4-Phenylene Oxide) as a Polymer-Polymer Nanocomposite: Mechanical and Gas Transport Characteristics

Poly(2,6-Dimethyl-1,4-Phenylene Oxide) as a Polymer-Polymer Nanocomposite: Mechanical and Gas Transport Characteristics

Conductive Heat Transfer Through Nanoconfined Gas: From Continuum to Free-Molecular Regime

In the past few decades, great efforts have been devoted to studying heat transfer on the nanoscale due to its importance in multiple technologies such as thermal control and sensing applications. Heat conduction through the nanoconfined gas medium differs from macroscopic predictions due to several reasons. The continuum assumption is broken down; the surface forces which extend deeper through the gas medium become prominent due to the large surface-to-volume ratio, and, finally, the gas molecules are accumulated nonuniformly on the solid surfaces. In this work, to better understand the combination of these phenomena on the heat conduction through the nanoconfined gas medium, we present a series of molecular dynamics simulations of argon gas confined between either metals or silicon walls. The gas density is set so that gas experiences a wide range of Knudsen numbers from continuum to the free molecular regime. It is observed that the intrinsic characteristics of the solid determine the gas density distribution near the walls and consequently in the bulk region, and these distributions control the heat conduction through the gas medium. While the nanochannel walls have their most significant impact on the density and temperature distributions of the rarefied gas, the pressure and the heat flux across the gas domain converge toward a plateau as the gas becomes denser. We propose new analytical formulas for calculating the gas pressure, induced heat flux, and effective thermal conductivity through the strongly nanoconfined gas, which incorporates the wall force field impacts on the gas transport characteristics for the Knudsen number in the range of 0.05 to 20.

Gas-Transport Characteristics of PdCu-Nb-PdCu Membranes Modified with Nanostructured Palladium Coating.

A method for obtaining composite gas-diffusion PdCu–Nb–PdCu membranes modified with a nanostructured crystalline coating was developed to increase the performance of Nb-based membranes. A modifying functional layer with a controlled size and composition was synthesized by electrochemical deposition, which made it possible to determine a certain geometric shape for palladium nanocrystallites. Developed PdCu–Nb–PdCu membranes have demonstrated flux values up to 0.232 mmol s−1 m−2 in the processes of diffusion purification of hydrogen at 400 °C. A very significant difference in the hydrogen fluxes through the modified and non-modified composite PdCu–Nb–PdCu membranes reached 1.73 times at the lower threshold temperature of 300 °C. Cu doping of protective layer did not affect the selective properties of the membranes, which was confirmed by the obtained high selectivity values up to 1323, and made it possible to reduce the noble metal content. The research data indicate that the modification of the membrane surface significantly accelerates the hydrogen transfer process at sufficiently low temperatures due to the acceleration of dissociative–associative processes on the surface. The reported approach demonstrates new possibilities for creating productive and cost-efficient membranes based on niobium.

Effect of Branch Length on the Structural and Separation Properties of Hyperbranched Poly(1,3-dioxolane)

Polymers containing poly(ethylene oxide) (PEO) demonstrate superior membrane CO2/N2 separation properties owing to their polar ether oxygen groups exhibiting strong affinity toward CO2. Poly(1,3-dioxolane) (PDXL) shows an ether oxygen content higher than PEO and is expected to have higher CO2/N2 solubility selectivity. However, similar to PEO, the high crystallinity of PDXL greatly reduces its gas permeability. Herein, amorphous PDXL-based hyperbranched polymers were synthesized by ring opening of 1,3-dioxolane (DXL) to form poly(1,3-dioxolane) acrylate (DXLAn) followed by photopolymerization. The repeating unit of DXL (n) or branch length was systematically varied from 4 to 12 to yield amorphous polymers. The chemical and physical properties of the obtained polymers (PDXLAn) were thoroughly evaluated and used to interpret pure- and mixed-gas transport characteristics. The polymers exhibit attractive CO2/N2 and CO2/CH4 separation properties. For example, PDXLA8 exhibits a CO2 permeability of 220 Barrer and CO2/N2 selectivity of 56 at 35 °C, surpassing Robeson’s 2008 upper bound, and it shows robust separation performance when evaluated with simulated flue gas at 60 °C. This study demonstrates that hyperbranched structures are an effective route to construct amorphous yet highly polar polymers and that chain end groups are instrumental in determining the structural and gas transport characteristics.

Gas transport characteristics of supramolecular networks of metal-coordinated highly branched Poly(ethylene oxide)

Model systems are developed and investigated to better understand the effect of polyether-metal ion interactions on gas separation characteristics. These systems help answer current questions raised by the substantial body of research on metal-organic frameworks (MOFs) dispersed in polyethers to improve gas separation performance, where favorable interactions between the metal centers and polyethers are preferred to improve interfacial compatibility. Specifically, we investigate CO2/gas transport properties of supramolecular networks comprising cross-linked poly(ethylene oxide) (XLPEO) and dissociable salts, including LiClO4, Ni(BF4)2, and Cu(BF4)2. Increasing the salt content increases the glass transition temperature (Tg) and generally decreases gas diffusivity and permeability, which can be successfully described using a Tg-integrated free volume model with an expression similar to the Vogel-Tammann-Fulcher (VTF) equation. Surprisingly, low loadings of LiClO4 and Cu(BF4)2 (2 mass% or less) can increase gas permeability by 30%–70% without affecting the CO2/gas selectivity. This increase correlates with polyether-metal ion dynamics as measured by dielectric spectroscopy. Understanding how interaction-mediated dynamics affect gas transport will be instrumental to designing MOF-based mixed matrix materials for gas separations.

Effects of biochar and straw application on the soil structure and water-holding and gas transport capacities in seasonally frozen soil areas

To explore the effects of different regulation modes on the soil structure and gas transport characteristics in seasonal permafrost regions, freeze-thaw cycles (FTCs) were used as boundary conditions and three typical soils on the Songnen Plain were used: black soil, baijiang soil and meadow soil. Four treatments were established: biochar addition (B1), straw addition (S1), biochar combined with straw addition (B1S1) and an untreated control (CK). The changes in the proportion of soil water-stable aggregates, total soil porosity (TP), soil water characteristic curves (SWRCs), soil dissolved organic carbon (DOC) and soil air permeability (PL) were analyzed. The results showed that biochar and straw influenced the structure of the three soil types. The proportions of large (2–0.5 mm) and medium (0.5–0.25 mm) aggregates increased significantly. The soil aggregate stability indexes of the treated soils were better than those of the CK, and the three-phase ratios of the treated soils were closer to ideal. The different treatments had particularly obvious effects on the black soil; the generalized soil structure index (GSSI) values reached 95.59, 94.36 and 98.74 in the B1, S1 and B1S1 treatments, respectively. An interaction effect was observed between biochar and straw. B1S1 had a stronger effect than the other treatments, and the soil water holding capacity was significantly improved (FC = 0.317 cm3 cm−3). Under the B1S1 treatment, the DOC contents in black soil, baijiang soil and meadow soil were 160.78 mg/kg, 272.828 mg/kg and 271.912 mg/kg, respectively. Moreover, biochar and straw combined effectively reduced PL fluctuations under FTCs and improved the long-term stability of the soil structure. These results can aid in rational straw and biochar use to achieve comprehensive agricultural waste utilization.

Transport characteristics of dense gaseous contaminants in a large space in the presence of obstacles

In industrial buildings, large cross-sections and obstacles lead to relatively low velocity in the large space except in the vicinity of air supply jets. The dimensionless number that represents the dense gas transport characteristics is revised to effectively represent the indoor gaseous contaminant transport mode in the large space. Based on the computational fluid dynamics (CFD) model validated by experimental data, a group of orthogonal design simulations have been conducted to examine the ranges of the dimensionless number for each transport mode. The indoor gaseous transport modes in the vicinity of the source are divided into passive, transitional, and active transport modes, featured by the magnitude of the revised dimensionless number θ. When θ exceeds 29.38, the transport is in active mode, and when θ is less than 13.61, the transport is in passive mode. The critical value of θ from the passive to the transitional stage is in the range of 13.61–19.94. The critical value of θ from the transitional to the active stage is in the range of 20.68–29.38. When the gaseous contaminant is in active transport mode, the contaminants tend to move downward in the vicinity of the source, the dimensionless concentration distribution shows vertical stratification in the overall space, and the overall ventilation efficiency is notably higher than the non-active transport modes.

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.

Study on gas transport characteristics of positive pressure beam tube system and its application to coal spontaneous combustion early warning

Coal spontaneous combustion (CSC) beam tube monitoring and early warning is one of the most important ways of mine fire prevention and achieve cleaning production. Aiming at solving the problem of distortion of gas leakage data collected by the CSC beam tube monitoring and warning system, a one-dimensional steady flow rate model of compressible gas was constructed based on the theory of hydrodynamics; and the calculation formula of gas transport flow is deduced. Besides, through laboratory research, the law of gas transmission caused by pipeline diameter, output pressure and pipeline length is revealed, and the gas transmission lag time is calculated. The theoretically calculated time is compared with the experimental data to verify the accuracy of the model. Research shows that when the pipeline diameter and pipeline length are constant, the gas steady flow rate is proportional to the output pressure; when the output pressure and pipeline length are constant, the gas steady flow rate is proportional to the fourth power of the pipeline diameter. The theoretically calculated gas transmission time is in good agreement with the experimental transmission time. Furthermore, the developed positive pressure beam tube monitoring system was used for early warning. CO, C2H4 and C2H6 are determined as the index gases for predicting CSC, the CH4, O2, CO and C2H4 concentrations under manual sampling and beam tube sampling were compared. The system would not lead to distorted composition and concentration of downhole gas after running for a long time and can timely and accurately reflect the real downhole situation. Finally, the concentrations of CO, O2, CH4 and C2H4 after fire extinguishing during the sealing period and test error is analyzed. The research presented in this paper has important practical value for promoting the accuracy of on-site CSC beam tube monitoring.

Hybrid Microporous Polymeric Materials with Outstanding Permeability and Increased Gas Transport Stability: PTMSP Aging Prevention by Sorption of the Polymerization Catalyst on HCPS.

The influence of hyper-crosslinked polystyrene (HCPS) MacronetTM MN200 on the gas transport properties and aging of the highly permeable glassy polymer poly(1-trimethylsilyl-1-propyne) (PTMSP) was studied and analyzed in detail. The gas transport characteristics of dense PTMSP membranes containing 0–10.0 wt % HCPS were studied. It was shown that the introduction of a small amount of HCPS into the PTMSP matrix led to a 50–60% increase of the permeability coefficients of the material for light gases (N2, O2, CO2) and slowed down the deterioration of polymer transport properties over time. The lowest reduction in gas permeability coefficients (50–57%) was found for PTMSP containing HCPS 5.0 wt % after annealing at 100 °C for 300 h. It was found that HCPS sorbed residues of tantalum-based polymerization catalyst from PTMSP. In order to investigate the influence of catalysts on transport and physical properties of PTMSP, we purified the latter from the polymerization catalyst by addition of 5 wt % HCPS into polymer/chloroform solution. It was shown that sorption on HCPS allowed for almost complete removal of tantalum compounds from PTMSP. The membrane made of PTMSP purified by HCPS demonstrated more stable transport characteristics compared to the membrane made of the initial polymer. HCPS has a complex effect on the aging process of PTMSP. The introduction of HCPS into the polymer matrix not only slowed down the physical aging of PTMSP, but also reduced chemical aging due to removal of active reagents.

Influence of Considering the Sorption Effect in the Betti-Maxwell Reciprocal Theorem on Gas Transport Capacity in Unconventional Reservoirs

Pore bulk modulus in continuum apparent permeability models was generally treated as an independent parameter or deduced from the Betti-Maxwell reciprocal theorem neglecting the sorption effect, which is not appropriate for the unconventional reservoirs with strong gas sorption. In this work, pore bulk modulus was firstly derived from the Betti-Maxwell theorem with the consideration of the gas sorption strain and was incorporated into the solid–fluid coupling apparent permeability model. Numerical simulation was then conducted under the uniaxial strain condition and the stress confined condition to investigate the gas transport characteristics. The results showed that when taking into account for the sorption effect in the Betti-Maxwell theorem, the pore bulk modulus dramatically decreased with the falling pore pressure during the gas transport process under two loading conditions. Moreover, gas transport ability under the stress confined condition was greatly degraded while it was hardly changed under the uniaxial strain condition. Both the pore bulk modulus and the stress distribution in the reservoirs were found responsible for the variation of gas flow capacity. Finally, the effects of the parameters associated with the pore bulk modulus were examined to further discuss the impact of involving the sorption effect in the Betti-Maxwell theorem.

Experimental investigation on shale gas transport characteristics in nanopores under high temperature and high pressure

Experimental investigation on shale gas transport characteristics in nanopores under high temperature and high pressure