Diameter Of Gas Molecules Research Articles

Molecular dynamics simulation of strained controlled gas diffusion in polyimide membranes

The study of the diffusion and separation performance of small molecules within polyimide membranes under uniaxial stress is crucial for their applications under high-temperature and high-pressure conditions. In this study, we systematically investigated the diffusion behavior and diffusion selectivity of N2, CF4, C2F6, SF6, C2H4, C2H6, CH4, and SO2 gases, as well as their gas mixtures, within polyimide membranes under different tensile temperatures (300 K, 400 K, 500 K, 600 K) and tensile rates (1011 s−1, 1010 s−1, 109 s−1, 108 s−1) using molecular dynamics simulations. Our findings indicate that the diffusion coefficients of gas molecules within the polymer membrane increase with increasing strain or temperature. Excessive or overly rapid tensile rates are not favorable for the most efficient diffusion of molecules within the membrane. The strain has a relatively small impact on the diffusion coefficients of gas molecules that strongly adsorb to the polymer membrane. In contrast, the differences in the kinetic diameters of gas molecules affect diffusion selectivity under varying strain. These results suggest that a combination of molecular kinetic diameter, gas-polymer interactions, internal free volume, and pore distribution influences the effective gas diffusion within polyimide membranes. By controlling strain appropriately, the diffusion separation performance of this membrane material can be effectively improved.

Encapsulation of small molecules in two-dimensional clathrate hydrates

We performed first-principles computations to investigate four new structures of two-dimensional hydrates (denoted as TDH-I, TDH-II, TDH-III, and TDH-IV), and to evaluate their potential applications in gas separation for CH4, CO2, N2 and NH3. The lattice parameters, the cage occupancy, and the energetic stability of the hydrates are determined and compared, and it is feasible to use the studied hydrates for gas separation. On the other hand, the dynamic analysis shows that these hydrates can exist stably in a highly confined environment, following the order of N2 > CO2 > NH3 > CH4, and that the kinetic diameter of gas molecules play a primary role in stabilizing the hydrate structure. As the temperate increases, the dynamic stability of the hydrates decreases, but they can still be stabilized at a temperature of 270 K.

ZIF-8-based dual layer hollow fiber mixed matrix membrane for natural gas purification

To prevent pipe damage and lower operating costs, natural gas must be cleaned of pollutants like carbon dioxide (CO2) and hydrogen sulphide (H2S). High selectivity of the functional material in the mixed matrix membranes (MMMs) allows for a more efficient gas separation compared to traditional polymeric membranes. However, poor dispersion and compatibility between the functional material and the polymer matrix become one of the major problems of MMMs. The use of zeolite imidazole (ZIF-8) in MMMs has been explored for its ability to separate CO2/CH4 due to its versatile gas molecule diameter range and effective CO2 adsorption. In this study, the effects of ZIF-8 at various loadings were examined on the physical and chemical properties of dual layer hollow fiber MMMs and gas separation efficiency. Following the successful production of 86.25 nm ZIF-8 nanoparticles, ZIF-8-based DLHF MMMs were created. The results showed that 0.5 wt% ZIF-8 loading provided the best results, with excellent interaction between the polymer PSf and the inorganic filler ZIF-8, strong heat resistance, and chemical stability. Increased CO2 permeability and CO2/CH4 ideal selectivity by 72.5 % and 1196.2 % respectively, compared to the unmodified membrane. It was inferred through this study that minimal loading of 0.5 wt% ZIF-8 in the DLHF MMMs improved their ability to separate gases, making them a good option for capturing CO2.

Dense gas flow simulations in ultra-tight confinement

Modeling dense gas flows inside channels with sections comparable to the diameter of gas molecules is essential in porous medium applications, such as in non-conventional shale reservoir management and nanofluidic separation membranes. In this paper, we perform the first verification study of the Enskog equation by using particle simulation methods based on the same hard-sphere collisions dynamics. Our in-house Event-Driven Molecular Dynamics (EDMD) code and a pseudo-hard-sphere Molecular Dynamics (PHS-MD) solver are used to study force-driven Poiseuille flows in the limit of high gas densities and high confinements. Our results showed (a) very good agreement between EDMD, PHS-MD, and Enskog solutions across density, velocity, and temperature profiles for all the simulation conditions and (b) numerical evidence that deviations exist in the normalized mass flow rate vs Knudsen number curve compared to the standard curve without confinement. While we observe slight deviations in the Enskog density and velocity profiles from the MD when the reduced density is greater than 0.2, this limit is well above practical engineering applications, such as in shale gas. The key advantages of promoting the Enskog equation for upscaling flows in porous media lie in its ability to capture the non-equilibrium physics of tightly confined fluids while being computationally more efficient than fundamental simulation approaches, such as molecular dynamics and derivative solvers.

An adjustable permeation membrane up to the separation for multicomponent gas mixture

The mixture separation is of fundamental importance in the modern industry. The membrane-based separation technology has attracted considerable attention due to the high efficiency, low energy consumption, etc. However, the tradeoff between the permeability and selectivity is a crucial challenge, which is also difficult to adjust during the separation process. Based on the salt water-filled carbon nanotubes, a separation membrane with the adjustable molecular channels by the electric field is proposed in this work. The separation mechanism is clarified on the basis of the characteristic size of the molecular channel and the overall effective diameter of gas molecules. The molecular dynamics simulation is performed to examine the feasibility and validity of the designed separation membrane. The simulations on the binary gas mixture (H2 and N2) reveal the flow control and high-purity separation as the electric field intensity varies. As for the mixed gas with the three components (H2, N2 and Xe), the successive separations and the switch between the high-efficiency and high-purity separation could be achieved only through adjusting the electric field intensity. This work incorporates the control into the membrane-based separation technology, which provides a novel solution for the complex industrial separation requirement.

Gas diffusion characteristics as criteria of nonequilibrium state of amorphous glassy polymers

Three amorphous glassy polymers (polyetherimide Ultem-1000, polyhexafluoropropylene and poly(3,4-bis(trimethylsilyl)tricyclonon-7-ene) were subjected to different physical treatments (biaxial stretching, removal of residual solvent (drying), annealing, aging, etc.). This work summarizes the observations made earlier in the papers of the same group of authors but also presents some new results. The treatments were shown to produce various changes of gas permeability and diffusivity. In all the cases with good accuracy the diffusion coefficients followed a well-known equation logD=a−b∙d2, here d is the kinetic diameter of gas molecules. The parameters a and b were shown to be sensitive to the treatments of the polymers and, in addition, linearly correlate with each other. By combining equations of Eyring and Meares, it was demonstrated that the parameters a and b can characterize the deviation from equilibrium state of polymers after particular treatment.

Some parameters of coal methane system that cause very slow release of methane from virgin coal beds (CBM)

In some worldwide hard coal basins recovery of methane from virgin coal beds is difficult. In general, mentioned difficulties are related to geo-mechanical, petrographical and physical-chemical properties of coals in question, occurring for example in the Bowen Basin (Australia) or the Upper Silesian Coal Basin (Poland). Among numerous properties and parameters, the following are very essential: susceptibility of coal beds to deformation connected with coal stress state change and contemporary shrinkage of the coal matrix during methane desorption. Those adverse geo-mechanical and physical-chemical effects are accompanied by essential change of the porous coal structure, which under these disadvantageous conditions is very complex. This study aims to show difficulties, which occur in phase of recognition of the methane-reach coal deposit. Volume absorbed methane (not surface adsorbed) in sub-micropores having minimal size comparable with gas molecule diameter must possess energy allowing separation of the nodes and methane release to micropores.

Using Polysulfone Hollow-Fiber Member to Separate H<sub>2</sub> from Petrochemical Process Tail Gas

The rate of gas diffusion is inversely proportional to the gas molecule diameter; hydrogen gas molecular has smaller diameter and thus it has a greater diffusion coefficient than methane. Results of laboratory studies shown that when a gas mixture consisting of H2/CH4 = 30:70 is treated in a Hollow-fiber member under 5 kg/cm2, the hydrogen concentration is raised from 30 mol% to 71 mol%. If the Hollow-fiber member is re-arranged in series connection without controlling the operating pressure (free permeation), the recovered hydrogen concentration can be as high as 88 mol%. Additionally, using H2/CH4 = 50:50 as the influent gas mixture and controlling the influent pressure at 5 kg/cm2, the resulting hydrogen concentration can be raised from 50 mol% to 92 mol%, if the Hollow-fiber member is re-arranged in series connection without controlling the permeating pressure (free permeation), the recovered hydrogen concentration can reach 94 mol%. Therefore, Hollow-fiber member can be implemented to recover hydrogen gas from the hydrogen-rich petrochemical process tail gas at low pressure; the recovered hydrogen can be further utilized with higher add-on value. This hydrogen-recovery method has the advantage of low capital cost, simple to operate and small energy consumption.

Microporous Metal–Organic Framework Based on Supermolecular Building Blocks (SBBs): Structure Analysis and Selective Gas Adsorption Properties

A new supermolecular building blocks (SBBs) based microporous metal–organic framework (MOF) was prepared with both 1,3,5-benzenetricarboxylic acid (H3btc) and 2,4,6-tris(4-pyridyl)-1,3,5-triazine (tpt) as ligands. The obtained CoII MOF reveals an uncommon ftw-a-like structure from the assembly of cubohemioctahedra SBBs under the direction of tpt and exhibits selective gas adsorption properties depending on gas molecule diameter and temperature which might have application in gas separation purposes.

Microstructural parameters controlling gas permeability and permselectivity in polyimide membranes

The present paper deals with the relationships between the gas permeation properties and the microstructural parameters of amorphous polyimides. The correlation is deduced from a data compilation of about 100 polyimides, taken from the literature or synthesized and characterized in our laboratory. The microstructure of polyimides is described on different scales through density measurement, wide-angle X-ray scattering (WAXS), UV–visible spectrophotometry, dynamic mechanical thermal analysis (DMTA) and positronium annihilation lifetime spectroscopy (PALS). From these experiments, there are only three extracted probes which display strong relationships with gas transport properties. These probes are the temperature of the gamma relaxation (Tγ), the color index ( λ 50) and the radius ( R) of the spherical free volume hole. The empirical models are based on the gas molecular diameter and on four constants for each microstructural probe. The prediction is achieved for the gas permeability coefficients of seven gas molecules and also for diffusion and sorption coefficients in some cases. The proposed fits are also consistent with the dependence of the permselectivity on the microstructural probe and gas molecule diameter.

Relation between Electrical Mobility, Mass, and Size for Nanodrops 1–6.5 nm in Diameter in Air

A large number of data on mobility and mass have been newly obtained or reanalyzed for clusters of a diversity of materials, with the aim of determining the relation between electrical mobility (Z) and mass diameter d m = (6m/ π ρ ) 1/3 (m is the particle mass and ρ the bulk density of the material forming the cluster) for nanoparticles with d m ranging from 1 nm to 6.5 nm. The clusters were generated by electrospraying solutions of ionic liquids, tetra-alkyl ammonium salts, cyclodextrin, bradykinin, etc., in acetonitrile, ethanol, water, or formamide. Their electrical mobilities Z in air were measured directly by a differential mobility analyzer (DMA) of high resolution. Their masses m were determined either directly via mass spectrometry, or assigned indirectly by first distinguishing singly (z = 1) and doubly (z = 2) charged clusters, and then identifying monomers, dimers, … n-mers, etc., from their ordering in the mobility spectrum. Provided that d m > 1.3 nm, data of the form d m vs. [z(1+m g /m) 1/2 /Z)] 1/2 fall in a single curve for nanodrops of ionic liquids (ILs) for which ρ is known (m g is the mass of the molecules of suspending gas). Using an effective particle diameter d p = d m + d g and a gas molecule diameter d g = 0.300 nm, this curve is also in excellent agreement with the Stokes-Millikan law for spheres. Particles of solid materials fit similarly well the same Stokes-Millikan law when their (unknown) bulk density is assigned appropriately.

Nanoscale Effect on Ultrathin Gas Film Lubrication in Hard Disk Drive

Since the current thickness of the gas film between the slider and the disk in Hard Disk Drive is already only one order of magnitude larger than the diameter of gas molecules, the nanoscale effect cannot be neglected any longer. In this paper a nanoscale effect function, Np is proposed by investigating the unidirectional flow of the rarefied gas between two parallel plates, and four kinds of formerly and currently employed lubrication models are modified. The calculated results using the modified Reynolds equations indicate that the nanoscale effect weaken the rarefaction effect to some extent for ultra-thin gas film lubrication.

Gas permeability, diffusivity, and solubility of poly(4-vinylpyridine) film

Although poly(4-vinylpyridine) is believed to have good gas permselectivity, the intrinsic gas permeation property is rarely reported in the literature. The objective of this work is to study the the intrinsic gas permeation property of poly(4-vinylpyridine) using a free-standing film. Because of its brittleness and strong adhesion with most solid surfaces, a free-standing poly(4-vinylpyridine) film was therefore prepared from casting on a liquid mercury surface. The permeation behavior of He, H2, O2, N2, CH4, and CO2 through the film was tested over a pressure range of 252 to 800 cm Hg at 35°C. The permeability and solubility decrease slightly with an increase in pressure, whereas the diffusivity increases as pressure increases. The pressure-dependent phenomenon can be explained using the partial immobilization model and the dual sorption model. An effective gas molecule diameter, which is defined as the square root of the product of gas collision and kinetic diameters, was used to correlate the diffusivity and gas molecule size, and an empirical equation was derived. Solubility is also a strong function of gas physical properties such as critical temperature and Lennard–Jones force constant, which are the measures of gas condensability and molecule interaction, respectively. In general, higher solubility in a polymer is obtained for gases with greater condensability and stronger interaction. Typical gas permeabilities of poly(4-vinylpyridine) measured at 619 cm Hg and 35°C are: 12.36 (He), 12.64 (H2), 3.31 (CO2), 0.84 (O2), 0.14 (CH4), and 0.13 (N2) barrers. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2851–2861, 1999

A Simple Method for Prediction of Gas Permeability of Polymers from Their Molecular Structure

A method for the prediction of gas permeabilities (P) through polymers from their chemical structure has been developed on the basis of the ratio of molar free volume to molar cohesive energy, V(f)/E(coh). The permeation of small gas molecules through polymer membranes is dependent on the chain packing density measured by V(f) and segmental motion of polymer chains measured by E(coh). But no simple relationship between P and V(f) or E(coh) alone was found. The permeability data of more than 60 polymers covering 7 orders of magnitude for six gases have been treated with linear regression analysis. All plots of log P vs. V(f)/E(coh) gave good straight lines. It is also found that a linear relationship holds when plotting both the intercepts and slopes of log P vs. V(f)/E(coh) lines against square of the diameters of gas molecules. Therefore, the permeabilities of all the non-swelling gases through a great variety of polymers can be estimated using two correlations above. Moreover, this method is more accurate than others in the literature and may found useful for the selection of gas separation or barrier membrane materials.

Selective permeation of hydrocarbon gases in poly(tetrafluoroethylene) and poly(fluoroethylene–propylene) copolymer

AbstractThe permeabilities and diffusivities of methane, ethane, propane, n‐butane, and isobutane in commercially available poly(tetrafluoroethylene) (TFE) and poly(fluoroethylene–propylene) (FEP) Teflon have been measured in a Pasternak‐type permeation cell. Experiments were carried out at upstream hydrocarbon partial pressures up to 50 torr (1000–60,000 ppm gas phase concentration) and temperatures from 40 to 195°C with films of 0.0508 and 0.127 mm thickness using nitrogen as carrier gas on the upstream and downstream sides of the membrane. The transient and steady‐state permeation data are described well by a combination of Henry's law and Fick's law with a concentration‐independent diffusion coefficient. Linear Arrhenius plots of both permeabilities and diffusivities were obtained. Linear correlations were found both between the activation energy for diffusion and the square of the gas molecule diameter, and between the logarithm of solubility at 90°C and the penetrant boiling point. Separation factors for binary mixtures of hydrocarbons were measured for TFE at 140°C and found to be similar to those predicted by individual permeabilities in most cases. Measurements with mixed gases were not made for FEP Teflon, but selectivities of FEP are expected to be similarly well described by the ratios of the pure gas permeabilities at the low partial pressures studied. The effect of annealing FEP Teflon for 24 hr at 200°C was found to produce an average of 20–30% reduction in solubility as well as a 9% increase in the activation energy for diffusion compared to as‐received films. These effects are believed to be due to increased crystallinity in the sample upon annealing.