Size Of Gas Molecules Research Articles

A coordination polymer based on ionic-type ligand for efficient separation of C2H2/C2H4 and C3H4/C3H6 mixtures

C2–C3 alkyne/alkene adsorption and separation is a critical process in industrial production, but it remains challenging due to the similar size of gas molecules. Herein, based on an ionic-type ligand, a porous coordination polymer with inherent positive charged sites has been successfully constructed by 1D chains through non-conventional CH···O hydrogen bonds. The flexible framework shows adsorption ability for C2H2 while negligible adsorption of C2H4; and a much earlier steep increase for C3H4 isotherm compared to C3H6 isotherm in the low-pressure region. Ideal adsorbed solution theory further illustrated that the framework has the ability to separate C2H2/C2H4 and C3H4/C3H6 mixtures.

The p-diethanolaminomethylcalix[4]arene-incorporated polyacrylonitrile-based facilitated-transport-nanofiber mat for O2/N2 separation.

Separation of gases from air mixture is one of the most challenging and laborious separations due to the remarkably uniform molecular size of gas molecules. Therefore, the present study aimed to synthesize polyacrylonitrile-based nanofibers mat(NM) impregnated with p-diethanolaminomethylcalix[4]arene (PAN/p-DEAC4 NM) for the separation of two crucial gases O2 and N2. The affinity of the prepared PAN/p-DEAC4 NM for O2 was examined by optimizing the loading concentration of p-DEAC4 in the range from 5% to 20% (w/v). The results showed remarkable performance of the PAN/p-DEAC4 NM for O2/N2 separation with a superior O2/N2 selectivity of 12.75 and excellent permeance of 10.2 GPU for O2 and 0.8 GPU for N2 at 2 bar. The PAN/p-DEAC4 NM followed a facilitated transport mechanism for the separation of gases and it was revealed that the p-DEAC4 platform in the PAN NM is facilitating the transport of O2 due to its greater affinity towards O2. BET analysis revealed that the prepared NM possesses non-porous morphology with a surface area of 12.69 m2 g-1. SEM micrographs also confirmed the formation of defect-free NM. Thus, this study presents a unique perspective and direction for fabricating highly permeable nanofiber mats for O2/N2 separation.

Fabrication of ZIF-8 membranes by direct assembly of nanosheets from bottom-up synthesis growth solution

Metal-organic frameworks (MOFs) host intrinsically porous structure and rich structural and chemical features. Several MOFs with pore aperture comparable to the size of gas molecules have attracted interest to form the selective layer of membranes. Synthesis of MOFs in nanosheet morphology is highly attractive for this because one can use highly-scalable filter coating to make MOF membranes. However, conventional nanosheet based processing route require several processing steps including centrifugation to prepare a coating dispersion. Herein, we report facile preparation of zeolitic imidazolate frameworks (ZIF) membranes by a straightforward assembly of nanosheets. ZIF-8 nanosheets were obtained by a direct bottom-up synthesis route where crystallization was optimized to obtain large, well-faceted 40-nm-thick nanosheets. Membranes, preferentially oriented along the c-out-of-plane direction, were fabricated by directly filtering the growth solution over a porous polymeric support without any further purification of nanosheets. This also ensures that only a small amount of precursor solution is used minimizing waste. The obtained membranes were compact and free of pinhole defects and yielded H2/C3H8 ideal selectivity over 3000 at 25 °C. We anticipate that this approach can be applied to several MOFs which can be synthesized in nanosheet morphology, advancing the scalability prospects of MOF membranes.

Study on the effect of physical structure and chemical modification of molecular sieve on the adsorption properties of C4F7N and its decomposition products

The environmentally friendly insulation medium C4F7N mixed gas decomposes after long-term overheating and partial discharge (PD). Some of the decomposition products not only affect the stable operation of gas insulated switchgear but also cause personal safety hazards. Therefore, the adsorption materials need to be equipped for adsorbing decomposition products in the equipment. Molecular sieves are considered to be the most promising adsorbent materials with wide variety and ionic modification space. In order to better apply the molecular sieve to adsorb the decomposition products of C4F7N, the effects of its physical structure and chemical modification on the adsorption of C3F8, C3F6, C2F6, and CF4 are investigated based on Grand Canonical Monte Carlo (GCMC) and Density functional theory (DFT) in this paper. Firstly, the adsorption amounts and sites of various pure silica-type molecular sieves (ATS, BEA, BOG, CAN, CHA, LAU, LTA, MTW, STT, TON, and ZSM) are investigated by GCMC. Then the adsorption energy, charge transfer, and density of states of the molecular sieve modified by cations (H+, Na+, Mn2+, and Fe2+) for C4F7N and its decomposition products are obtained based on DFT. The results reveal that the molecular sieve topology, size of gas molecules and molecular sieve windows are important factors affecting the adsorption process. After modification with Mn2+ and Fe2+, the adsorption energy and electron transfer of C4F7N decomposition products by ZSM molecular sieve increased significantly and the adsorption distance became closer. The main action site of C4F7N is CN bond, the main action sites of C2F6 are CC double bond and F atom, and C3F8, C2F6 and CF4 is F atom.

Quadrature-based lattice Boltzmann model for non-equilibrium dense gas flows

The Boltzmann equation becomes invalid as the size of gas molecules is comparable with the average intermolecular distance. A better description is provided by the Enskog collision operator, which takes into account the finite size of gas molecules. This extension implies nonlocal collisions as well as an increase in collision frequency, making it computationally expensive to solve. An approximation of the Enskog collision operator, denoted the simplified Enskog collision operator, is used in this work to develop a quadrature-based lattice Boltzmann model for non-ideal monatomic dense gases. The Shakhov collision term is implemented in order to fine-tune the Prandtl number. This kinetic model is shown to be able to tackle non-equilibrium flow problems of dense gases, namely, the sound wave and the shock wave propagation. The results are compared systematically with the results of the more accurate but computationally intensive particle method of solving the Enskog equation. The model introduced in this paper is shown to have good accuracy for small to moderate denseness of the fluid (defined as the ratio of the molecular diameter to the mean free path), and due to the efficiency in terms of computational time, it is suitable for practical applications.

Separation of H2/CH4 gas mixture through graphenylene membrane with functionalized nanopore: A computational study

Using molecular dynamics (MD) simulations, we investigated the performance of graphenylene membrane with functionalized nanopore in the H2/CH4 separation. In the present work, we studied the impact of functionalized nanopore, system temperature (298, 323, and 348 K), applied difference pressure (up to 2 MPa), and feed composition on the H2/CH4 separation performance. The passage of gas molecules across the nanopore was monitored within the simulations, and the permeance was determined under applied conditions. The results revealed that the size of gas molecules and its interaction with the membrane nanopore are two important factors in the separation performance of H2/CH4 gas mixture. It is also found that H2 molecules can easily pass through the studied membranes, whereas no CH4 molecule was seen in the permeate side, which confirms the ultrahigh selectivity of H2 over CH4. Furthermore, the maximum H2 permeance of 1.95 × 105 GPU through the pore 1 was obtained at 1.5 MPa, which was higher than that of 1.93 × 105 GPU through pore 2. The results also demonstrated that the system temperature doesn't have any effect on the membrane performance. To this end, the permeance of H2 molecules through the studied membranes obviously increased with raising the ration of H2 molecules in the feed composition. Due to high selectivity and permeance, the graphenylene membrane with functionalized nanopore is expected to have promising applications in hydrogen separation from H2/CH4 mixed gas.

Effect of Small Cage Guests on Dissociation Properties of Tetrahydrofuran Hydrates.

It is well understood that tetrahydrofuran (THF) molecules are able to stabilize the large cages (51264) of structure II to form the THF hydrate with empty small cages even at atmospheric pressure. This leaves the small cages to store gas molecules at relatively lower pressures and higher temperatures. The dissociation enthalpy and temperature strongly depend on the size of gas molecules enclathrated in the small cages of structure II THF hydrate. A high-pressure microdifferential scanning calorimeter was applied to measure the dissociation enthalpies and temperatures of THF hydrates pressurized by helium and methane under a constant pressure ranging from 0.10 to 35.00 MPa and a wide THF concentration ranging from 0.25 to 8.11 mol %. The dissociation temperature of binary He + THF and methane + THF hydrates increases along with an increase in the THF concentration in the liquid phase at a fixed pressure (e.g., 30 MPa), reaching a maximum value of 280.8 and 312.8 K, respectively, at stoichiometric concentration (5.56 mol % THF), and then remains nearly constant for even higher THF concentrations (>5.56 mol %). The effect of gas occupancy in the small cages on the dissociation enthalpy of He + THF and methane + THF mixed hydrates was further examined by using molecular dynamics (MD) simulations. The dissociation enthalpy of the He-THF mixed hydrates is independent of pressure with an average of 5.68 kJ/mol H2O over the pressure ranging from 0.10 to 30.0 MPa, consistent with the MD results of the He-THF mixed hydrates with low single occupancy (<23%) of helium molecules in the small cages. Consequently, the heat of adsorption of helium molecules in the small cages of the He-THF mixed hydrates is rather too weak to be identified. On the other hand, the dissociation enthalpy of the methane-THF hydrates increases from 9.11 to 10.01 kJ/mol H2O along with an increase in methane pressure over the pressure ranging from 5.0 to 30.0 MPa, consistent with the MD results of the methane-THF mixed hydrates with full occupancy of methane molecules in the small cages. These findings provide important information for the design of a potential medium of gas storage and transportation.

Tunable sieving of small gas molecules using horizontal graphene oxide membrane

Graphene oxide membrane (GOM) has attracted extensive attraction as a molecular sieve for the separation of water, ions, and gases. However, Å-level control of the GOM interlayer spacing for separating the specific size of gas molecule is difficult and the vertical transport mechanism of molecules through the GOM is highly complex because of the existence of several possible pathways. Here we fabricated GOMs with different Å-scale interlayer spacing through heat treatment and made them allow horizontal entrance and transport of gas molecules. The GOMs with angstrom channels exhibited superior performance in permeance in horizontal mode and moderate selectivity for H2/CO2 and He/CO2 by tuning the sieving size of the channels. Molecular dynamics (MD) simulations reveal that the precisely controlled interlayer spacing plays a primary role as a molecular sieve indeed, but increased graphitic area, a result of heat treatment, also affects observed selectivity. Our results provide insight into the transport mechanism of gas molecules through GO nanochannel, which has not been accurately identified due to its complex behavior and suggest the possibility of studying nanochannels as effective vehicle for separating molecules precisely.

Effect of side branch on gas separation performance of triptycene based PIM membrane: A molecular simulation study

In this study ab initio calculations, molecular dynamics (MD) and Monte Carlo (MC) simulation techniques are used to investigate the structural properties of triptycene based polymers of intrinsic microporosity (PIMs), consisting of polyimide branched with the side groups: C4H9, C3H7, CH3 and CF3, to evaluate their performance as polymeric membrane for separation of gases, O2, N2, CO2, CH4 and H2S, which are the constituents of natural gas and their separation is of high industrial interest. In the course of MD simulation, initially, the branched polyimide membranes are built to obtain the PIMs' model. Then the low-density membrane models undergo a consecutive simulation procedure of compression and relaxation to achieve the experimental density of equilibrated membrane. The structure of the constructed membranes is analyzed by calculating: dihedral angles, radius of gyration, fractional free volume, accessible free volume, cavity size distribution, and surface area. The behavior of the membranes against penetration and permeation of the studied gases is determined by evaluating the diffusion and solubility coefficients of the gases and by employing MD and MC simulation techniques, respectively. Comparison of the structural properties of the membranes shows that the PIM membranes with larger side branch groups in their polymeric chain structure are more rigid and therefore, due to restriction in chain packing and cavity formation between polymer chains, the free volume in the membrane's structure increases which as a result would promote the diffusion and permeation of gases into the membrane, where, the obtained results indicate that the membrane with C4H9, as the largest side branch in its polymer chain, has the greatest diffusivity and permeation. Also, the highest selectivity for all studied binary gas mixtures is manifested by the PIM membrane with C4H9 at its side branch, however, for (CO2/CH4) and (H2S/CH4) binary mixtures CF3 as the side branch of PIM membrane represents an acceptable selectivity. The obtained results illustrate that in addition to the membrane free volume, other parameters are influential in gas separation by these polymeric membranes which require further consideration. These parameters include gas adsorption, specific surface area of the membrane for adsorption, the size of gas molecules and their interaction with the PIM membranes which need to be investigated and discussed in the light of the obtained results. To the best of knowledge, based on a thorough investigation of the literature, no similar work can be cited which includes detailed properties of PIM membranes at the atomic scale by using quantum mechanical and simulation techniques in order to elucidate the behavior of PIMs for gas separation.

The kinetic Shakhov–Enskog model for non-equilibrium flow of dense gases

When the average intermolecular distance is comparable to the size of gas molecules, the Boltzmann equation, based on the dilute gas assumption, becomes invalid. The Enskog equation was developed to account for this finite size effect that makes the collision non-local and increases the collision frequency. However, it is time-consuming to solve the Enskog equation due to its complicated structure of collision operator and high dimensionality. In this work, on the basis of the Shakhov model, a gas kinetic model is proposed to simplify the Enskog equation for non-ideal monatomic dense gases. The accuracy of the proposed Shakhov–Enskog model is assessed by comparing its solutions of the normal shock wave structures with the results of the Enskog equation obtained by the fast spectral method. It is shown that the Shakhov–Enskog model is able to describe non-equilibrium flow of dense gases, when the maximum local mean free path of gas molecules is still greater than the size of a molecular diameter. The accuracy and efficiency of the present model enable simulations of non-equilibrium flow of dense gases for practical applications.

Pore size dependent gas-sensing selectivity based on ZnO@ZIF nanorod arrays

Abstract Recently, a metal-organic framework (MOF) coating method has been reported for improving the poor gas selectivity of metal oxide semiconductor (MOS) gas sensor. However, the correlation between MOF’s pore size and the size of gas molecule is not fully discussed, as well as gas-sensing properties to gases with similar molecular size. In this paper, two zeolitic imidazolate frameworks (ZIFs) with different pore sizes (∼3.4 A for ZIF-8, ∼4.8 A for ZIF-71) are synthesized on the surface of ZnO nanorod arrays to form a ZnO@ZIF core-shell structure, where ZIFs act as gas molecule sieve membrane that shields ZnO from gas molecules larger than the pore size of ZIFs. Hydrogen, ammonia, ethanol, acetone and benzene (molecular size in the range of 2.89–5.85 A) are selected as testing gases. The ZnO@ZIF-8 sensor exhibited a clear response to smaller molecules of hydrogen and ammonia rather than larger molecules of ethanol, acetone and benzene. While the ZnO@ZIF-71 sensor only exhibited inhibited response to benzene, which has the largest molecular size. These results indicate that the selectivity of ZnO gas sensor can be regulated by surface MOF coating with specific pore size. But this pore size depending selectivity regulation method is insufficient effective for targeting gases with similar molecular sizes, which may be improved by further modification on MOF materials for selective adsorption in our future work.

Two-dimensional graphene oxide membrane for H2/CH4 separation: Insights from molecular dynamics simulations

As an ultrathin, high-flux, and energy-efficient membrane, graphene oxide (GO) has shown great potential for gas separation. Using molecular dynamics (MD) simulations, we studied the H2/CH4 separation performance of GO membrane. It is found that the size of gas molecule and its interaction with GO sheet are two key factors in determining the diffusivity in interlayer gallery of GO, which results in significant difference of selectivity and permeability for H2 and CH4. The influences of two interlayer configuration parameters, the interlayer spacing and oxidation degree, on the separation of H2/CH4 are firstly studied. Then the effects of gas thermal behaviors, simulation temperature and initial gas pressure are also examined. Our studies uncover the underlying mechanism of H2/CH4 separation in GO membrane, and these understandings could promote efficient two-dimensional layered gas separation membrane materials development by engineering assemblies.

Thermodynamics and Kinetics of Gas Storage in Porous Liquids.

The recent synthesis of organic molecular liquids with permanent porosity opens up exciting new avenues for gas capture, storage, and separation. Using molecular simulations, we study the thermodynamics and kinetics for the storage of CH4, CO2, and N2 molecules in porous liquids consisting of crown-ether-substituted cage molecules in a 15-crown-5 solvent. It is found that the intrinsic gas storage capacity per cage molecule follows the order CH4 > CO2 > N2, which does not correlate simply with the size of gas molecules. Different gas molecules are stored inside the cage differently; e.g., CO2 molecules prefer the cage's core whereas CH4 molecules favor both the core and the branch regions. All gas molecules considered can enter the cage essentially without energy barriers and leave the cage on a nanosecond time scale by overcoming a modest energy penalty. The molecular mechanisms of these observations are clarified.

Gas‐transport properties of novel composites based on addition poly(3‐trimethylsilyltricyclononene‐7) and substituted cyclodextrines

In this work we report the preparation and properties of composite materials for gas separation membranes based on highly permeable addition poly(3‐trimethylsilyltricyclononene‐7) (PTCNSi1) as a matrix and substituted cyclodextrines (CD) as fillers. CD studied had different structure and this allowed us to explore correlations between the structure of the fillers and gas permeation parameters of the resulting compositions. Namely α‐, β‐, and γ‐cyclodextrines with Me‐ or Me3Si‐substituents were synthesized and the composites containing them were prepared. Permeability coefficients of the obtained composites were determined for He, H2, O2, N2, CO2, CH4. It was found that the introduction of cyclodextrines with bulky Me3Si‐groups rather weakly influenced the permeability coefficients for He and H2, while for other gases a marked decrease in permeability was observed. On the other hand, in the case of the composites containing Me‐substituted CD decreases in permeability were observed for all the studied gases. The changes of gas permeability dramatically depend on the size of gas molecules and this results in an increase in selectivities for some gas pairs. Thus, introduction of α‐CD with Me‐groups resulted in an increase in selectivity α(H2/N2) from 5.2 for pure polymer to 9.1. The increase in CD content in a composite gave an extra growth of selectivities. BET, TEM, WAXD, positron annihilation data (free volume) and mechanical properties of the composites were also studied and discussed. POLYM. COMPOS., 36:1029–1038, 2015. © 2015 Society of Plastics Engineers

Modeling chromatographic separation of produced gas in shale wells

Field evidence exists showing temporal variation in produced gas composition in shale wells. Preferential gas flow and sorption of the species in shale formations cause compositional variations in the gas produced from shale. This process is similar to gas chromatographic (GC) separation, in which the size of gas molecules and their affinity for walls cause separation. As in gas chromatography, shale gas contains molecules of different gases (methane, ethane, propane, carbon dioxide, etc.). When reservoir pressure is greater than critical sorption pressure, sorption process is negligible and the separation process is mainly due to differences in gas molecule speeds in pores. The extremely small size of the pores in shale adds different flow physics, such as Knudsen diffusion and slip flow that intensifies separation of gas components. Understanding and modeling chromatographic separation (CS) in shale can improve our knowledge and help us produce more valuable gas from gas shale. We have developed a numerical model to study temporal variations of the composition of gas produced from shale gas wells. The model is a physical transport model of single-phase multicomponent gas flow in nanoporous media. The governing equations are implemented into a one-dimensional numerical model and solved using a fully implicit solution method. A sensitivity study of the effect of different parameters such as reservoir pressure, length of the system, tortuosity, and permeability on the CS process is performed. The model results confirm strong CS process in shale. In an early stage of production, the component with the highest Knudsen diffusivity and slip coefficient is produced with a higher mole fraction than its in-situ composition. At a later time, the same component comprises a smaller mole fraction than its in-situ composition in the gas production stream. Lower Darcy permeability, a longer system, and higher reservoir pressure intensify the CS process.

The compression of chemically inert gases in a closed volume and their equation of state

The totality of processes that occur when gases are compressed in a closed volume were studied. An equation describing the removal of molecules from the gas phase undergoing compression was suggested. The solution to this equation predicted the existence of two modes in the development of this process and the possibility of the transition from one mode to another at a point. The influence of the size of gas molecules on the form of the suggested equation and, accordingly, on the removal of molecules from the gas phase was analyzed. The case of the equisignificance of the effective volume of molecules in the gas and liquid phases and the situation which allowed us to ignore this parameter for particles in the gas phase were studied. The predicted differences in condensation for these models were substantial and could likely be checked experimentally. The results obtained were used to suggest an equation of state of real gases, which physically correctly described the regions of their compression and condensation and the compression of condensed liquids. The influence of liquid compressibility on the form of the equation of state and the corresponding isotherms was studied.

Transient coherent anti-Stokes Raman scattering spectroscopy as a tool for measuring the diffusion coefficient and size of gas molecules

Formulas are derived for evaluating the diffusion coefficient and size of gas molecules from transient coherent anti-Stokes Raman scattering measurements. Numerical estimates are presented for hydrogen.

Enhancement of Gas Permeability in HPC, CTA and PMMA under Microwave Irradiation

Gas permeability in hydroxypropyl cellulose (HPC), cellulose triacetate (CTA), and poly(methyl methacrylate) (PMMA) membranes under microwave irradiation was investigated in comparison with permeability obtained without microwave (conventional method). Permeability coefficients for several gases in HPC, CTA and PMMA under microwave were higher than those obtained by conventional methods. That is, the enhancement of gas permeability (microwave irradiation effect) by microwave was made clear, additionally it was suggested that accelerated molecular motion of –OH and –COO– moiety in side chains contribute to the enhancement under microwave irradiation. The degree of the enhancement of gas permeability depended on the combination of polymers and gases. Consequently, the size of gas molecule, polar groups and free volume spaces in polymer are assumed to affect the microwave irradiation effect.

Explanation of a Selectivity Maximum as a Function of the Material Structure for Organic Gas Separation Membranes

Many previous studies have examined structure-property relationships of both polymeric and carbon molecular sieve gas separation membranes, which show a widely known tradeoff between permeability and selectivity. This work presents an explanation of a lesser-known trend of a selectivity maximum as a function of the material structure for certain rigid polymers and a variety of carbon molecular sieving materials. Interestingly, this surprising trend of a maximum in selectivity over a range of materials is observed for certain gas pairs and not others over the same range of material structures. First-order modeling efforts, which treat molecular sieving membranes as effective heterogeneous composite materials comprising a distribution of selective entities, are used to explain and understand this trend of a maximum in selectivity. Several composite models are considered, and specific examples are demonstrated using both a parallel model and effective medium theory. The modeling results reflect trends in experimental data and suggest that subtle changes in the distribution of selective entities or the size of gas molecules can result in vastly different overall separation properties.

Temperature parameters of the gas permeability of polymers

The paper relates to statistical treatment of published data on diffusion coefficients, and on the solubility and permeability of inert (and some polyatomic) gases in various polymers at elevated temperatures. The found correlation dependences relate temperature parameters of the gas permeability of polymers to the size of gas molecules and to their force constants of the Lennard-Jones potentials. It is shown that activation energies of diffusion and permeability, and heats of dissolution of gases in polymers may be calculated.