Gas Diffusion Process Research Articles

Uncertainty Quantification of Multiple Gas Transport and Sorption in Porous Polymers

A high-fidelity physics-based model of mixed-gas transport coupled with kinetic and equilibrium adsorption is derived, and experiments were performed in order to calibrate and exercise the model. In the literature, a continuum-scale model that couples Fickian diffusion with Henry’s law absorption, and kinetic Langmuir adsorption was previously developed to describe the diffusion and sorption of moisture in porous materials. Here, we expand the model to gases, rather than moisture, derive, and implement a competitive adsorption mechanism into the model to enable mixed-gas sorption. This model facilitates a mechanistic-based understanding of the sorption and diffusion processes of mixed gases in polymeric materials. Diffusion and sorption experiments were conducted for a range of partial pressures; model validation and calibration were carried out by comparing modeled mass uptake and experimental data considering the uncertainties of conceptualized (or assumed) physical processes and system parameters. Uncertainty quantification and sensitivity analysis methods are described and exercised here to demonstrate the capability of this predictive model.

Multiple diffusion mechanisms of shale gas in nanoporous organic matter predicted by the local diffusivity lattice Boltzmann model

Shale gas diffusion properties in organic matter (OM) are crucial for estimating gas yield. The gas diffusion process involves multiple diffusion mechanisms namely molecular diffusion, Knudsen diffusion and transition diffusion due to the nanoporous features of OM. Although a single diffusion mechanism in OM has been well-studied, the coexistence of multiple diffusion mechanisms remains unclear. In this work, a local diffusivity lattice Boltzmann model is proposed to consider multiple diffusion mechanisms at the pore-scale. Algorithms of pore network and random-placing sphere are integrated to reconstruct the nanoporous structures of OM. A local Knudsen number is involved to automatically identify local diffusion mechanisms, thus determining the corresponding local diffusivity. The diffusion properties of shale gas in OM are investigated at various shale reservoir pressures and temperatures and OM structural parameters. OM diffusivity is overestimated in the typical pressure range if multiple diffusion mechanisms are ignored. Furthermore, the OM diffusion mechanism gradually transfers from Knudsen diffusion to molecular diffusion with continuously increased pressure. To conveniently predict the diffusion capability of OM, a general empirical formula, which is mainly expressed as a function of porosity and average Knudsen number, is established on the basis of the present simulation results.

Simulation of pressure tank leakage based on PHAST

Chemical plants usually store hazardous chemicals in high-pressure or low-pressure storage tanks in the form of pressurization, liquefaction, and low temperature. The poisonous gas in the tank is easily diffused. If a leakage accident occurs, it will cause personal injury and environmental pollution. In this paper, based on the propane acrylonitrile project in an industrial park, Major hazard identifications were identified for the tank area (risk level is Level 1), then PHAST software was used to simulate the leakage and diffusion of acrylonitrile storage tanks. It was concluded that the greater the wind speed, the leakage pore and the more stable atmosphere, the greater the leakage and diffusion of hazardous areas. And timely and intuitive prediction of the diffusion process and scope of toxic gases provide a scientific reference for taking corresponding countermeasures and on-site control after the accident.

Three-dimensional simulation of a PEM fuel cell with experimentally measured through-plane gas effective diffusivity considering Knudsen diffusion and the liquid water effect in porous electrodes

A fundamental understanding of the gas diffusion properties in porous electrodes, such as catalyst layers (CLs), micro porous layers (MPLs) and gas diffusion layers (GDLs) is necessary for the optimal design of polymer electrolyte membrane (PEM) fuel cell. The conventional Bruggeman correlation has been proven to overestimate through-plane gas effective diffusivity (TP-GED) in GDLs; however, few studies have shed light on the influence of gas diffusion in MPLs and CLs in simulations of PEM fuel cells. In this study, to accurately predict the gas diffusion process and cell performance, a three-dimensional, two-phase and non-isothermal PEM fuel cell model was developed with experimentally measured TP-GED considering Knudsen diffusion and the liquid water effect in porous electrodes. The numerical results indicate that the conventional Bruggeman correlation greatly overpredicts TP-GED in porous electrodes and the oxygen reduction reaction (ORR) within the cathode CL and thus cannot accurately reflect the mass transport loss at high current density. The gas diffusion properties in the cathode porous electrode play a dominant role in determining cell performance. Furthermore, MPLs and CLs exhibit lower TP-GED than GDLs due to Knudsen diffusion; therefore, the distinction among the gas diffusion properties of GDLs, MPLs and CLs must be considered in quantitative modeling and optimization of PEM fuel cells.

Evaluation of electrochemical performance of solid-oxide fuel cell anode with pillar-based electrolyte structures

Among the gas, ion, and electron diffusion processes in solid-oxide fuel cell (SOFC) electrodes, it is generally known that ionic conduction has the most impact on their electrochemical performance. Therefore, enhancement of the effective ionic conductivity of electrodes is a useful approach to reduce the overpotential. Yttria-stabilized zirconia (YSZ) pillars can be effective solutions to enhance the effective ionic conductivity of SOFC anodes. In this study, the influence of YSZ pillar structures on the electrochemical performance of SOFC anodes was evaluated by numerical simulation and experiments. First, to reveal the electrochemical reaction kinetics of anodes with pillar structures, a three-dimensional electrochemical simulation was conducted by the lattice Boltzmann method. The microstructure without pillars obtained by a focused ion beam scanning electron microscopy (FIB-SEM) measurement was used as the reference structure. Then, the original structure was replaced with YSZ phase to obtain virtual microstructures with YSZ pillars. With YSZ pillars, predicted area specific resistance became smaller than that of the reference structure, in spite of decrease in percolated TPB density. The electrochemical potential distribution of oxide ion and charge-transfer currents clearly show increase in the effective ionic conductivity. Relationships between overpotential and pillar geometries were parametrically discussed. Then, electrochemical performance of Ni-YSZ anode with the YSZ pillar structure formed by modifying the YSZ electrolyte surface was evaluated. By sputtering Ni-YSZ on pillar structures, stable electrochemical performance was obtained.

Model-based gas source localization strategy for a cooperative multi-robot system—A probabilistic approach and experimental validation incorporating physical knowledge and model uncertainties

Sampling gas distributions by robotic platforms in order to find gas sources is an appealing approach to alleviate threats for a human operator. Different sampling strategies for robotic gas exploration exist. In this paper we investigate the benefit that could be obtained by incorporating physical knowledge about the gas dispersion. By exploring a gas diffusion process using a multi-robot system. The physical behavior of the diffusion process is modeled using a Partial Differential Equation (PDE) which is integrated into the exploration strategy. It is assumed that the diffusion process is driven by only a few spatial sources at unknown locations with unknown intensity. The objective of the exploration strategy is to guide the robots to informative measurement locations and by means of concentration measurements estimate the source parameters, in particular, their number, locations and magnitudes. To this end we propose a probabilistic approach towards PDE identification under sparsity constraints using factor graphs and a message passing algorithm. Moreover, message passing schemes permit efficient distributed implementation of the algorithm, which makes it suitable for a multi-robot system. We designed an experimental setup that allows us to evaluate the performance of the exploration strategy in hardware-in-the-loop experiments as well as in experiments with real ethanol gas under laboratory conditions. The results indicate that the proposed exploration approach accelerates the identification of the source parameters and outperforms systematic sampling.

Shale gas transport in rock matrix: Diffusion in the presence of surface adsorption and capillary condensation

One characteristic of unconventional shale rock is the abundance of nanoscale pores and pore throats. For many shale reservoirs, these are the dominant factors controlling the total reserve, production rate, and overall production. Accordingly, many new phenomenon related to nanopore physics need to be considered for shale reservoir. A key mechanism controlling hydrocarbon gas transport from pores to fractures in shale reservoirs is molecular diffusion. As a result, the diffusion coefficient of light hydrocarbon becomes an important parameter in reservoir simulation to estimate production rate, especially for the long-term or late time production. The diffusion coefficient is also a key parameter in nuclear magnetic resonance (NMR) logging for fluid typing in determining hydrocarbon-in-place.Diffusion in nanopore systems is complicated by two important factors. First, the gas molecules collide more with pore surfaces than with each other; therefore, simple diffusion theory based on gas molecule collision is not applicable. Second, the occurrence of capillary condensation in small pores or pore throats and the adsorption to the pore surface makes the hydrocarbon transport a mixed process of liquid diffusion and gas diffusion in the reservoir.We derived a theoretical model for mixed phase diffusion under fast exchange for shale gas reservoirs. We obtained that the effective diffusion coefficient of mixed fluid is a fractional average of the liquid and gas diffusivity. We verified this model using NMR to measure n-hexane diffusion in a well-characterized hierarchical carbon with both nanometer pores and micrometer pores so that both gas and liquid can be prepared to exist in the sample simultaneously. We found that the diffusion coefficient of hexane in these hydrophobic nanopores is approximately one magnitude of order larger than the bulk liquid value. The effective diffusion coefficient in this study can be used in reservoir simulation and interpretation of NMR logs for shale gas reservoirs.

Mesoscopic exploration on mass transfer in porous thermochemical heat storage materials

Mass transfer is the key characteristic that affects the charging and discharging performance of the porous materials used in thermochemical heat storage systems. Therefore, an algorithm is required to simulate the transport process within heat storage materials and to accurately evaluate the effective diffusion coefficient. This study is about porous calcium oxide, which is widely used in CaO-Ca(OH)2 or CaO-CaCO3 heat storage systems. In this paper, the fractal porous media that resemble real heat storage materials are reconstructed by a random walk method. Furthermore, the gas diffusion process through complicated porous microstructures within fractal porous media is simulated by an efficient lattice Boltzmann method. Results indicate that thermochemical heat storage materials have great fractal characteristic and materials with larger fractal dimension have more diffusion resistance. The effective gas diffusion coefficients of CaO and Ca(OH)2 are obtained from the ratios of effective gas diffusion coefficient and bulk gas diffusion coefficient that are 0.228 and 0.188 for CaO and Ca(OH)2, respectively. Additionally, it is found that calcium oxide samples prepared at higher temperatures have the poorer exothermic performance because of the poorer gas diffusion. Moreover, the correlation between the effective diffusion coefficients and the porosity of porous materials is numerically predicted and evaluated. The fractal model-LBM method developed in this paper may serve as a great tool for easy estimation of effective diffusion coefficients and mass transfer.

Design and control of gas diffusion process in a nanoporous soft crystal.

Design of the gas-diffusion process in a porous material is challenging because a contracted pore aperture is a prerequisite, whereas the channel traffic of guest molecules is regulated by the flexible and dynamic motions of nanochannels. Here, we present the rational design of a diffusion-regulatory system in a porous coordination polymer (PCP) in which flip-flop molecular motions within the framework structure provide kinetic gate functions that enable efficient gas separation and storage. The PCP shows substantial temperature-responsive adsorption in which the adsorbate molecules are differentiated by each gate-admission temperature, facilitating kinetics-based gas separations of oxygen/argon and ethylene/ethane with high selectivities of ~350 and ~75, respectively. Additionally, we demonstrate the long-lasting physical encapsulation of ethylene at ambient conditions, owing to strongly impeded diffusion in distinctive nanochannels.

Estimation of Gas Absorption and Diffusion Coefficients for Dissolved Gases in Liquids

The multifaceted process of gas absorption and diffusion of dissolved gases has several useful applications in engineering, medicine, pharmaceutics, and science. A new systematic study for modeling of this process using an inverse problem for concentration dependent diffusion with free boundary at the gas–liquid interface is presented in this work. The study includes the model derivation based on the Fick’s second law of diffusion, the Henry’s absorption law, the Stefan’s principles of free boundary problems, and the development of new techniques for estimation of concentration dependent diffusion coefficient and the gas absorption coefficient governing propagation of gas–liquid interface due to absorption. The developed estimation technique utilized the dissolved gas concentration profiles measured and different time moments and is applied to real X-ray CT measurement data for a binary gas liquid mixture containing dimethyl ether and bitumen.

Temporal scale analysis of shale gas dynamic coupling flow

Shale gas is one of the world’s strategic energy reserves. The microscopic pores and the fracture structure of the shale rocks play an important role in shale gas flow. An effective reservoir simulation must account for the heterogeneity and anisotropy of the microscopic pore structure as well as the dynamics between the gas flow and the gas diffusion processes. In this study, the time lag effect, the dynamic coupling of gas flow and diffusion as well as the heterogeneity and anisotropy of the pores are taken into account in order to properly model shale gas production on a large scale. The effect of these variables on the temporal scale analysis of the wellbore gas pressure is presented, as the reliable estimate of wellbore pressure would allow better prediction of the duration of well shut-in periods. Gas production under the intermittent operation scheme at different temporal scales is discussed. The results showed that the well startup is composed of five stages, while the well shut-in is composed of only three stages. The crossflow stage is prolonged by the dynamic coupling. The heterogeneity, anisotropy and dynamic coupling phenomena are accurately presented on the temporal scale diagram. Moreover, the impact of time lag on the gas pressure is better captured by the dynamic coupling between the gas flow and the gas diffusion. The proposed model better simulates actual shale reservoirs and, hence, would be helpful in selecting suitable production rates.

Effects of porous support microstructure enabled by the carbon microsphere pore former on the performance of proton-conducting reversible solid oxide cells

Proton-conducting reversible solid oxide cells (PC-RSOCs) have attracted extensive attention due to their high efficiencies as energy conversion devices. Generally, the performance of the cell is affected to a certain extent by the microstructure of the electrodes, which is closely related to the gas diffusion and surface reaction processes. Herein, different contents of the carbon microspheres (CMSs) are used as the pore formers to control the microstructure of the hydrogen electrode. Experimental results reveal that the porosity, line shrinkage, and thermal expansion coefficient of the hydrogen electrode support simultaneously increase with the CMS content. The support with 30 wt% CMS presents high porosity (39.27 vol%) with uniform-size pores. Subsequently, the corresponding single cells were fabricated successfully, particularly, the cell with 30 wt% CMS exhibiting the best electrochemical performance in both fuel cell (0.46 W cm−2 at 700 °C) and electrolysis cell (1.41 A cm−2 at 1.3 V and 700 °C) operational modes. Further results demonstrated the highest performance was attributed primarily to the maximal three-phase boundary length, which mainly originates from the high porosity and unique microstructure of the hydrogen electrode.

Cathode Optimization for an Inert-Substrate-Supported Tubular Solid Oxide Fuel Cell

Inert-substrate-supported tubular solid oxide fuel cells with multi-functional layers were fabricated in this work. The tubular single cells consisted of a porous yttira-stabilized zirconia inert-substrate supporting layer, a Ni anode current collecting layer, a Ni-Ce0.8Sm0.2O1.9 anode electrochemical layer, a yttira-stabilized zirconia/Ce0.8Sm0.2O1.9 bi-layer electrolyte, and a La0.6Sr0.4Co0.2Fe0.8O3-δ cathode. Thickness of the La0.6Sr0.4Co0.2Fe0.8O3-δ cathode layer could be varied from 2.5 to 25.0 μm by controlling the number of dip-coatings in the single cell fabrication process. Electrochemical performance of the tubular single cells was investigated as a function of cathode thickness. Electrode resistance and maximum power density of the single cell were significantly affected by the thickness of the cathode. Increasing the cathode thickness to 15 μm was effective in reducing the sheet resistance of the layer and the electrode resistance of the single cell. Further increasing the cathode thickness induced a higher electrode polarization loss, which originated from insufficient gas diffusion and transport processes. Therefore, the optimum thickness of the La0.6Sr0.4Co0.2Fe0.8O3-δ cathode layer was determined to be 15 μm. At 800 °C, the tubular single cell with the optimum cathode thickness displayed the highest observed maximum power density of 559 mWcm-2 under the hydrogen/air operation mode. Additionally, the tubular single cell exhibited good thermal cycling stability between 800 and 25 °C for five cycles. These results illustrate the advantages of this system for future applications of the inert-substrate-supported tubular single cells in repeated startup and shut down conditions.

Controlling the adsorption behavior of hydrogen at the interface of polycrystalline CVD graphene

Polycrystalline graphene films were synthesized from renewable biomaterials in ambient air using a facile and rapid thermal chemical vapour deposition technique. Characterization of the graphene reveals a large surface area, the presence of nanoscale domains and open edges, atomic-level stacking, and high electrical conductivity, which are favorable features for electrochemical hydrogen evolution reactions (HERs). The numerous boundaries and open edges accelerate the gas diffusion process and enlarge the effective reactive surface area for gas evolution, which is responsible for a significant improvement of HER performance and stability compared to a commercial graphene film. The hydrogen adhesion behavior in investigated for both bare Ni foil/foam and graphene grown on Ni foil/foam samples. The hydrogen gas bubbles adhere to the polycrystalline graphene for a long period of time before detaching, in contrast to their behavior on the pristine Ni foil surface. Post treatment of the graphene film using plasma treatment increases the desorption rate of hydrogen bubbles from the surface. The results indicate a wide range of possibilities for use of graphene-based catalysts in electrocatalytic gas evolution reactions.

Experimental and modeling study on controlling factor of methane hydrate formation in silica gels

In order to study the mechanism of methane hydrate formation in porous media, the formation experiments of methane hydrate in porous media at the constant pressure were performed in the temperature range of 274.15–276.15 K and the pressure range of 5–8 MPa. The silica gels with the average pore diameters of 129.5, 179.6, and 332 Å were used as the porous media for the experiments. The experimental results indicate that the final gas consumption increases with the increase of the formation pressure and the decrease of the formation temperature. Based on the shrinking core model, the reaction-controlled kinetic model and the diffusion-controlled kinetic model for hydrate formation in silica gels were built, respectively. The reaction-controlled kinetic model well fits the kinetic data in 129.5 Å and 179.6 Å silica gels, and the diffusion-controlled model well fits the kinetic data in 332 Å silica gels with a relatively high regression coefficient (R2 > 0.99). The formation rate of the methane hydrate is controlled by the gas diffusion process in 129.5 Å and 179.6 Å silica gels, and is controlled by the reaction process in 332 Å silica gels.

Research regarding the vacuuming of liquid steel on steel degassing

When the liquid steel comes in contact with the atmosphere of the elaboration aggregates, a process of gas diffusion into the metal bath takes place on the one hand, and on the other hand a process that allows them to pass from the metal bath into the atmosphere. The meaning of these processes is determined by a number of factors as follows: the quality of raw and auxiliary materials (moisture content, oils, etc.), the boiling intensity, the evacuation duration, the properties of used slags, the values of the casting ladle processing parameters (bubbling, vacuuming, etc.). The research was carried out at an electrical steelwork, equipped with an electric arc furnace type EBT (Electric Bottom Tapping) capacity 100t, LF (Ladle-Furnace) and VD (Vacuum Degassing) facilities, establishing some correlations between the vacuuming parameters from the V.D.facility and the amounts of hydrogen and nitrogen removed from the metal bath, as well as their removal efficiency, were taken into consideration. The obtained data was processed in MATLAB calculation program, the established correlations form was presented both in analytical and graphical form. The validity of these correlations was verified in practice, being particularly useful in research.

Optical perturbation of atoms in weak localization

We determine the microscopic transport parameters that are necessary to describe the diffusion process of the atomic gas in optical speckle. We use the self-consistent theory to calculate the self-energy of the atomic gas. We compute the spectral function numerically by an average over disorder realizations in terms of the Greens function. We focus mainly on the behaviour of the energy distribution of the atoms to estimate a correction to the mobility edge. Our results show that the energy distribution of the atoms locates the mobility edge position under the disorder amplitude. This behaviour changes for each disorder parameter. We conclude that the disorder amplitude potential induced modification of the energy distribution of the atoms that plays a major role for the prediction of the mobility edge.

Formation Behavior and Controlling Factor of Methane Hydrate in Porous Media

In order to study the formation behavior and controlling factor of methane hydrate in porous media, the formation experiments of methane hydrate in porous media at the constant pressure were carried out in the temperature range of 274.15 - 276.15 K and the pressure range of 6 - 8 MPa. The silica gels with the mean pore diameter of 12.95 nm, 17.96 nm and 33.20 nm were used as the porous media for the experiments. The experimental results indicated that the hydrate formation rate and the final gas consumption in porous media increases with the increase of the formation pressure, the decrease of the formation temperature. Based on the shrinking core model, the kinetic models for hydrate formation in silica gels under the reaction control and gas diffusion control were built, respectively. The methane hydrate formation is controlled by the gas diffusion process in silica gels with the mean pore diameter of 12.95 nm and 17.96 nm, and is controlled by the reaction process in silica gels with the mean pore diameter of 33.20 nm.

Large scale synthesis of α-Si3N4 nanowires through a kinetically favored chemical vapour deposition process

Understanding the kinetic barrier and driving force for crystal nucleation and growth is decisive for the synthesis of nanowires with controllable yield and morphology. In this research, we developed an effective reaction system to synthesize very large scale α-Si3N4 nanowires (hundreds of milligrams) and carried out a comparative study to characterize the kinetic influence of gas precursor supersaturation and liquid metal catalyst. The phase composition, morphology, microstructure and photoluminescence properties of the as-synthesized products were characterized by X-ray diffraction, fourier-transform infrared spectroscopy, field emission scanning electron microscopy, transmission electron microscopy and room temperature photoluminescence measurement. The yield of the products not only relates to the reaction temperature (thermodynamic condition) but also to the distribution of gas precursors (kinetic condition). As revealed in this research, by controlling the gas diffusion process, the yield of the nanowire products could be greatly improved. The experimental results indicate that the supersaturation is the dominant factor in the as-designed system rather than the catalyst. With excellent non-flammability and high thermal stability, the large scale α-Si3N4 products would have potential applications to the improvement of strength of high temperature ceramic composites. The photoluminescence spectrum of the α-Si3N4 shows a blue shift which could be valued for future applications in blue-green emitting devices. There is no doubt that the large scale products are the base of these applications.

Constitutive model for methane desorption and diffusion based on pore structure differences between soft and hard coal

This paper aims to improve the accuracy and applicability of gas diffusion mathematical models from coal particles. Firstly, a new constitutive model for gas diffusion from coal particles with tri-disperse pore structure is constructed by considering the difference in characteristics between soft coal and hard coal. The analytical solution is then derived, that is, the quantitative relationship between gas diffusion rate (Qt/Q∞) and diffusion time (t). The pore structure parameters of soft coal and hard coal from Juji coal mine are determined. Gas diffusion rules are numerically calculated and investigated by physical simulation methods. Lastly, the applicability of this model is verified. The results show that the homogeneous model only applies to the gas diffusion process of hard coal during the initial 10min. The calculation results from this model and the physical experimental results of soft coal and hard coal are nearly identical during the initial 30min.