Diffusion Of Gas Molecules Research Articles

Normal and anomalous diffusion of non-interacting particles in linear nanopores

The diffusion of gas molecules in pores is determined by the collisions between the molecules as well as by the collisions of the molecules with the pore walls. In many applications the so-called Knudsen regime is of particular interest. In this regime the collisions of the molecules with the pore walls play the crucial role, while the inter-molecular collisions can be neglected. Here we study the influence of surface roughness on the coefficients of self (or tracer) diffusion and transport diffusion. Considering the first four iterations of a generalised fractal Koch surface, we construct pore models of different roughness. For these model pores we have performed detailed simulations of both diffusion coefficients using a cube-based algorithm. The molecular trajectories can be mapped onto L´evy walks to determine the diffusion properties. In linear two-dimensional (2d) channels we observe anomalous diffusion, which can also be induced in smooth and rough three-dimensional (3d) pores by anomalous reflection laws. Normal diffusion is found in convoluted 2d pores and in all 3d pores when a diffuse reflection law is applied.

The nature of permeability anisotropy and catalytic activity

The phenomena of permeability anisotropy and an increase in the rates of catalytic reactions in porous membranes modified with highly dispersed catalytic systems were analyzed. A model of stochastic gas motions was proposed; this model is based on the hypothesis of the specific interaction of molecules with the inner surface of pores resulting in a nonisotropic distribution of molecules over traveling directions. The effects of asymmetric gas transfer in porous and gradient-porous membranes were considered to explain differences in the rates of heterogeneous catalytic reactions in a nanoporous membrane reactor under changes in the direction of supplying a reaction mixture. From the model proposed, it follows that the transversal diffusion of gas molecules is most probable in the porous medium of a ceramic membrane with a pore-size distribution gradient from large to small pores along the flow direction. This diffusion results in an increase in the frequency of molecular collisions with the wall of a microchannel and, correspondingly, in an increase in the contact time. The model proposed explains the intensification of a number of heterogeneous catalytic reactions performed in the porous media of catalytic porous membranes.

On the Knudsen transport of gases in nanochannels

We investigate the diffusion of gas molecules in nanochannels under the combinational effect of the vibration of the channel, gas-wall binding energy, and channel size through molecular dynamics simulations. It is found that the molecular vibration of the channel plays a critical role in gas transport process when the gas-wall binding energy is strong. For small binding energies, the influence of the flexibility of the wall can be neglected. In rigid channels, the gas self-diffusion coefficient increases with increasing gas-wall binding energy, while it decreases in nonrigid channels. The effect of the channel size on the self-diffusion coefficient is not significant except that a local maximum in the gas self-diffusion coefficient is found in 2 nm channels due to the strong repulsive force caused by the surface curvature of the channels.

Temperature-Gradient Assisted Gas-Dissolved Liquid-Phase Synthesis of Clathrate Hydrates

A novel laboratory method to synthesize clathrate hydrates in sealed glass tubes is described in this manuscript. It was known that the major obstacle in the synthesis of a gas hydrate is from the slow mass transport through the hydrate layer. To circumvent this problem in a sealed glass tube, we developed a temperature-gradient assisted gas-dissolved liquid-phase reaction. This method allows a clathrate hydrate start to form at the bottom of a glass tube and then grow through the diffusion of gas molecules from the gas phase passing through the liquid water phase to the water−gas hydrate interface. This method takes advantage of the fact that gases diffuse much faster in a liquid phase than in a gas-hydrate solid phase. Three examples to synthesize D2O/Xe, D2O/THF/Xe, and D2O/propane gas hydrates are given. The formations of the first two gas hydrates were verified with 129Xe and 129Xe T1 NMR experiments, and that of the last one was verified with 13C and 2H−1H QEDOR NMR experiments.

Gas transport through nano and micro composites of natural rubber (NR) and their blends with carboxylated styrene butadiene rubber (XSBR) latex membranes

The gas permeability coefficient of nano and micro composites of natural rubber, carboxylated styrene butadiene rubber and 70:30 natural rubber/carboxylated styrene butadiene rubber blend membranes has been investigated with special reference to type of filler, gases, filler loading and pressure. The layered silicates such as sodium bentonite and sodium fluorohectorite were the nanofillers used and the conventional micro fillers were clay and silica. Latex nanocomposites were characterized by X-ray diffraction technique. The dispersion of layered silicates in the polymer matrix was analysed using transmission electron microscopy. The fluorohectorite silicate showed excellent dispersion in natural rubber matrix. The effect of free volume on the gas barrier properties was investigated by positron annihilation lifetime spectroscopy. It was observed that due to the platelet like morphology and high aspect ratio of layered silicates, the gas barrier properties of nano filled latex membranes were very high. The crosslink density values and extent of reinforcement were estimated in order to correlate with the gas barrier properties. The oxygen/nitrogen selectivity of these membranes were investigated. The diffusion of gas molecules through the polymer was determined by time-lag method and diffusion selectivity of the membranes was computed.

Gas permeation through a polymer network

We study the diffusion of gas molecules through a two-dimensional network of polymerswith the help of Monte Carlo simulations. The polymers are modelled as non-interactingrandom walks on the bonds of a two-dimensional square lattice, while the gas particlesoccupy the lattice cells. When a particle attempts to jump to a nearest-neighbour emptycell, it has to overcome an energy barrier which is determined by the number of polymersegments on the bond separating the two cells. We investigate the gas currentJ as a function of themean segment density ρ, the polymer length and the probability qm forhopping across m segments.Whereas J decreasesmonotonically with ρ for fixed , its behaviour for fixed ρ and increasing depends strongly on q. For small, non-zero q, J appears to increase slowly with . In contrast, for q = 0, it is dominated by the underlying percolation problem and can be non-monotonic. Weprovide heuristic arguments to put these interesting phenomena into context.

Sulfur Sorption Rate and Equilibrium Content of Reduced Iron in H2-H2S Mixtures

Contents and rates of the sulfur sorption on the reduced iron from hematite and limonite were measured in the atmosphere of H 2 -H 2 S mixture of sulfur activity less than unity by combining the thermogravimetric method with the reaction interruption method. The sulfur content by the former was a half the content by the latter presumably owing to the sorbed sulfur exchanged with sorbed oxygen. In this study, the saturated content of sulfur, which is attained after enough long time, was estimated by the latter and the sorption rate was measured by the former. The equilibrium content was properly expressed by the Langmuir's isotherm. The simulation of the sorption rate by the parallel model, in which the diffusion of gas molecules in the pore and the diffusion of adsorbed species on the wall of the pore were taken into account, was tried. It was successful with the sulfur sorption of the reduced iron from limonite and the diffusion was mainly surface diffusion at 873 K, but it gradually shifted to the diffusion in pore with increasing temperature. But any trial was unsuccessful with that from the hematite. The reason may be attributed to, in the reduced iron from the hematite, the contribution of sulfur segregated on the grain boundary of reduced iron is significant relative to the total sorption content to the content of sulfur adsorbed on the surface pore.

Segregation of platinum from mordenite channels on calcination and reduction pretreatments

Samples of Pt/NaMOR (Pt/NaM) and Pt/HMOR (Pt/HM) were prepared through ion exchange of MOR with [Pt(NH 3) 4](NO 3) 2 and subsequently treated with calcination at 723 K in air and reduction at 673 K in H 2. The segregation of Pt in the prepared samples during the treatment was monitored with different characterization techniques. Transmission electron microscopy results showed that platinum in freshly exchanged and in calcined samples stayed dominantly in channels of MOR. However, a significant portion of platinum in channels segregated to and sintered at the external surface during the reduction. A high N H/ N Pt ratio of 0.80 in 0.5Pt/NaM compared to that of 0.55 in 0.5Pt/HM in hydrogen chemisorption suggested that the segregation of platinum was partially retarded by Na + ions in NaM. Xenon tended to physisorb on reduced Pt particles in the channels of Pt/NaM and caused a downfield shift to 129Xe NMR peak. The shift revealed that a fraction of segregated platinum (or a fraction of Pt retained in the channels) remained constant for Pt/NaM samples with different Pt loadings. Pt particles retained in channels of highly loaded 5Pt/NaM blocked pores ( d∼0.70 nm) and thereby substantially blocked the diffusion of gas molecules. Consequently, a low uptake of both hydrogen and xenon in 5Pt/NaM was observed in isotherm measurements.

Ultrasonic characterization performed during chemical foaming of cross-linked polyolefins

Injection molding of cross-linked low density foams made from poly(ethylene-co-octene) resins results from simultaneous reactions occurring during the process. The ultrasonic quasi-static characterization technique (no flow) can mimic adequately the conditions prevailing during the molding process (pressure, temperature, time) and monitor the blowing process through sound velocity, attenuation and specific volume measurements. In this work, compounds prepared from resins with different melt flow indices are investigated, showing the influence of the degree of cross-linking on the chemical blowing agent (CBA) decomposition, as well as the effect of viscosity on the degassing conditions. Experiments demonstrate the complexity of CBA decomposition and of gas molecule diffusion in the polymer matrix.

Development of carbon dioxide separation process using continuous hollow-fiber membrane contactor and water-splitting electrodialysis

Studies on the development of carbon dioxide (CO2) separation process using continuous hollow-fiber membrane contactor (HFMC) and water-splitting electrodialysis (WSED) as an alternative method for the conventional processes are reported. Several experiments for CO2 recovery and absorbent regeneration using HFMC–WSED hybrid system were performed. In this study, four widely used absorbents (KOH, NaOH, K2CO3, and N-methyldiethanolamine) were examined and compared in terms of the process efficiency. The results showed that KOH was the best absorbent for this hybrid system due to its high ionic conductivity. In addition, several operational problems including the proton leakage and gas molecule diffusion through the membranes were considered. The results of continuous experiments showed that the continuous recovery of CO2-rich absorbents was reliably performed during the tests without any problem.

Gas Permeation through Ultrathin Liquid Films

We report measurements of gas permeation through ultrathin liquid layers. The gas flux penetrating nanometer thick free-standing smectic films is determined experimentally as a function of membrane thickness and temperature. We observe an unusual film thickness dependence of the permeation in very thin films. For its interpretation, we introduce a molecular model that considers sorption and diffusion of gas molecules in the liquid film as well as the dynamic exchange between the film and the external gas phase. From the fit of experimental data, we obtain the ratio of the solubility of gas in the smectic phase and the diffusion coefficient for gas molecules in the liquid and an energy barrier for gas molecules to enter the liquid film. The measured permeation is strongly temperature dependent; at the transition to low-temperature smectic modifications with in-plane molecular positional order, the films become almost impermeable.

Diffusion at anodically bonded interfaces

The diffusion of gas molecules into cavities closed by anodicbonding is quantified by the annealing of specially designed teststructures. Annealing is performed at temperatures in the range150-430 °C for several days. An increased concentration ofmolecules within the closed cavities after heat treatments is verifiedboth electrically and optically. The diffusion of gas into the cavities isfound to be substantial at temperatures above 300 °C. A diffusionparameter, the diffusion coefficient times the height of the bondedinterface, is found from curve fitting of experimental data with ananalytic expression for diffusion.

Diffusion of gas molecules in the polystyrene matrix

Diffusion of gas molecules in the polystyrene matrix

Diffusion of gas molecules in the polystyrene matrix

We have studied the diffusion of gas molecules mside an amorphous polystyrene matrix. The diffusion constant of several gases at T = 450 K in polystyrene was computed. Particular attention was given to CO 2 for terroeratures between 300 and 800 K. The temperature dependence of the diffusion constant and the relationship between the diffusion constant and the diameter of the gas molecule were analysed. We further examined the motion of the gas molecules on the short time scale not readily accessible to experimental observation. Here we used the cage overlap function which gives information on the typical cage sizes and distribution times. On the short time scale the gas molecules show a hopping behaviour The distribution of the time period between hopping events, the distance between the cages and the size of the cages in the polystyrene matrix in the presence of guest molecules were calculated. Through the analyzing, we gel a clearer picture from the behaviour of gas molecules on short time scale in the polymer matrix.

Cavitation Nuclei and Bubble Formation—A Dynamic Liquid-Solid Interface Problem

A model of the formation of cavitation nuclei is developed assuming local detachment of the liquid at locations of concave solid surface topography. The detachment is attributed to diffusion of gas molecules into the interfacial liquid, where the liquid-solid bonds are strained due to interfacial tension in the liquid. Calculations indicate that attached interfacial voids may grow into stabilized cavitation nuclei as a consequence of broad-band resonance, excited by external sources of noise or vibration in the whole range of frequencies up to the MHz regime. The gas content in the liquid and the amplitude and frequency of the sound field determine a balance between rectified diffusion of gas into the void and diffusion out of it due to the excess pressure in the void. In strong acoustic fields and in supersaturated liquids the voids may grow into bubbles that detach and form free gas bubbles. [S0098-2202(00)01503-0]

Gas-generating porous electrodes at low overvoltages: Allowance for the outside-electrode limitations

Characteristics of gas-generating porous electrodes (GPE) are calculated and analyzed at low over-voltages, when all the electrode pores are still filled with electrolyte. The calculations assume the existence of limitations outside the electrode, specifically, the diffusion of gas molecules dissolved in electrolyte and their conglomeration into bubbles. Separate solutions are found and then sewn for GPE and the electrolyte chamber outside it, where the generated gas is collected. An important parameter is revealed, namely, the ratio of a characteristic gas-generation current inside GPE to a characteristic gas-removal current inside the outside-electrode region. The parameter determines both the net current density in GPE and the depth of the electrochemical process penetration into the electrode’s porous space. The two limiting cases studied are the hydrogen-generating water electrolysis on a porous platinum electrode and the chlorine generation on dimensionally stable anodes (DSA). A way to estimate all quantities that characterize the gas removal into the outside-electrode region is shown. It is established that only a narrow (no greater than a micrometer) region adjacent to the front surface of GPE takes part in the chlorine generation process on DSA of standard thickness (5 μm).

Alteration of methane pressure in the closed pores of fossil coals

The kinetics of gas molecule diffusion is considered in a solid with pores. The results obtained describe the effects of porosity and gas solubility on the gas pressure alteration in surrounding closed volume. Estimations have been made for the case of methane absorption by coal substance.

Correlation between fractional free volume and diffusivity of gas molecules in glassy polymers

Correlation between fractional free volume and diffusivity of gas molecules in glassy polymers

Correlation between fractional free volume and diffusivity of gas molecules in glassy polymers

Despite its oversimplifications, the free-volume approach has proven to provide very useful correlative and even semipredictive capabilities. This article is concerned with the correlation between the diffusivity, D, of gas molecules in glassy polymers and the fractional free volume, FFV, determined by the Bondi method. The diffusivities were taken from a database, generated by the authors in connection with work on the new Landolt Börnstein series “Diffusion in Non-Metallic Solids”, which encompasses a very large variety of glassy polymers. For a given diffusant log D is a linear function of 1/FFV as predicted by the free volume theory. However, the deviations from this relationship are significant and strongly correlated between O2, N2, CO2, and CH4. The systematic deviations are opposite to the predicted effect of the polymer jumping unit size in the free-volume concept of Vrentas and Duda and are interpreted in terms of an increase in the activation energy with increasing chain stiffness. Correlation analysis further suggests that the diffusion mechanisms of H2 and particularly of He differ markedly from that of larger gas molecules. Furthermore the influence of the cohesive energy and the glass-transition temperature of the polymers is investigated. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 3344–3358, 1999

Arterial oxygen partial pressures during nitrous oxide and xenon elimination

A fall in arterial oxygen partial pressure (PaO2) during nitrous oxide (N2O) elimination is a well known phenomenon denominated as diffusive hypoxia that is caused by the relatively fast N2O-elimination compared with the slower simultaneous N2-uptake. On the one hand, the occurence of diffusive hypoxia during Xenon (Xe) elimination could be supposed because of the rapid recovery from an inhalational (Xe)-anesthesia that can be explained by the low blood/gas partition coefficient (λXe = 0.l4, λN20 = 0.47). On the other hand, since diffusion of gas molecules is highly dependent on molecular weight (MWN2 = 28, MWN2O = 44, MWXe = 133), the velocity of Xe-elimination should not exceed N2-uptake very much. Therefore we compared PaO2 during the course of Xe- and N2O elimination.