Gas Diffusion Coefficient Research Articles

Multiscale measurements of gas diffusion coefficient of coal using counter-diffusion and image-based methods

Coal seam gas (CSG), also known as Coalbed Methane (CBM), is an important natural gas resource owing to its abundance and high energy density. During production, the gas desorbs from the coal matrix owing to pressure depletion. It then diffuses primarily within the micropores of the coal matrix toward the coal fractures and then migrates to the wellbore. Therefore, diffusivity plays a significant role in the gas production process, and this process needs to be explicitly characterized. However, experimental gas diffusion measurements are commonly based on coal particles rather than bulk coal samples, and thus, the impacts of coal fractures are typically neglected. This study aims to test the diffusion coefficients of fractured and tight bulk coal samples to provide insights into gas diffusion within the different pores and fractured structures. Diffusion coefficients of bulk coal samples were measured at different diffusion times, confining pressures, and gas pressures using counter-diffusion experiments. The results show that the bulk diffusion coefficient varies with confining pressure and gas pressure, which demonstrates the impacts of internal pore/fracture structure of coal on the diffusivity. The time-dependent diffusion coefficient results show that the diffusion coefficients became constant after 24 h, ranging from 1 × 10−10 m2/s to 2 × 10−10 m2/s. Local diffusion coefficients were obtained using X-ray micro-computed tomography images of krypton gas diffusing through a coal sample. These were higher than the diffusion coefficients of bulk samples, which were in the order of ∼10−9 m2/s. The variations in local diffusion coefficients indicated the heterogeneity of coal, thereby validating the importance of both local and bulk diffusion measurements in coal characterization.

Pore network modeling as a new tool for determining gas diffusivity in peat

Abstract. Peatlands are globally significant carbon stocks and may become major sources of the greenhouse gases (GHGs) carbon dioxide and methane in a changing climate and under anthropogenic management pressure. Diffusion is the dominant gas transport mechanism in peat; therefore, a proper knowledge of the soil gas diffusion coefficient is important for the estimation of GHG emissions from peatlands. Pore network modeling (PNM) is a potential tool for the determination of gas diffusivity in peat, as it explicitly connects the peat microstructure and the characteristics of the peat pore network to macroscopic gas transport properties. In the present work, we extracted macropore networks from three-dimensional X-ray micro-computed tomography (µCT) images of peat samples and simulated gas diffusion in these networks using PNM. These results were compared to the soil gas diffusion coefficients determined from the same samples in the laboratory using the diffusion chamber method. The measurements and simulations were conducted for peat samples from three depths. The soil gas diffusion coefficients were determined under varying water contents adjusted in a pressure plate apparatus. We also assessed the applicability of commonly used gas diffusivity models to peat. The laboratory measurements showed a decrease in gas diffusivity with depth due to a decrease in air-filled porosity and pore space connectivity. However, gas diffusivity was not extremely low close to saturation, which may indicate that the structure of the macropore network is such that it enables the presence of connected diffusion pathways through the peat matrix, even in wet conditions. The traditional gas diffusivity models were not very successful in predicting the soil gas diffusion coefficient. This may indicate that the microstructure of peat differs considerably from the structure of mineral soils and other kinds of porous materials for which these models have been constructed and calibrated. By contrast, the pore network simulations reproduced the laboratory-determined soil gas diffusion coefficients rather well. Thus, the combination of the µCT and PNM methods may offer a promising alternative to the traditional estimation of soil gas diffusivity through laboratory measurements.

Gas desorption law of granular coal in negative-pressure environment and calculation of gas loss during negative-pressure sampling

• The gas desorption law at different AEPs and NP conditions was revealed. • The variation law of the diffusion coefficient was obtained by inversion. • The method of gas loss in the process of negative pressure sampling is analyzed. To explore the gas desorption law of granular coal during negative-pressure (NP) slag discharge sampling, atmospheric-pressure (AP) desorption, BP desorption and negative-to-atmospheric pressure (NP-AP) desorption experiments were performed. Based on the experimental results, the variation laws of the gas diffusion coefficient in the NP and NP-AP environments were analyzed through numerical simulation, and the variation laws of gas desorption in the processes of NP desorption and AP desorption were comparatively analyzed. The results show that the gas diffusion coefficient under the NP condition conforms to a power function. At the same adsorption equilibrium pressure (AEP), the initial diffusion coefficient monotonically increases with the rise of NP, that is, the NP environment facilitates gas desorption. At the same desorption environment pressure, the diffusion coefficient is smaller and varies more stably under a higher AEP. Within the same NP sampling time, a higher AEP corresponds to a larger volume of cumulative gas desorption, and a higher NP corresponds to a greater gas loss in the sampling process. This proves that shortening the sampling time can reduce the gas loss at the NP stage and weaken the influence on the desorption at AP. During NP sampling, the logarithmic method is more accurate than the t method in terms of gas loss calculation, its relative error ranging from −56.25% to 16.45%. The relative error is pretty small in the high NP environment and decreases with the passage of the sampling time. Based on the linear relationship between the environmental NP value and the calculation error, the gas loss calculation formulas corrected by error analysis were obtained. The research results can serve as a guideline for further mastering the NP sampling technology.

Investigating the kinetics of CO2 absorption in aqueous sodium serinate/piperazine solutions: An experimental and mathematical modeling

Aqueous solutions of amino acids are some of the most important alternatives to conventional alkanolamines for gas absorption applications. This study investigates the kinetics of CO2 absorption in sodium serinate solutions and their blend with piperazine. To that end, the kinetic analysis for sodium serinate was performed at 0.5, 1, 2, and 4 kmol/m3 concentration levels. The blended solution was prepared with 1 kmol/m3 of sodium serinate solution as the main component and piperazine in 0.5, 1, and 2 kmol/m3 concentration levels as the additive. The temperature ranged from 293 to 323 K. The density and viscosity of the solutions, gas solubility in the liquid, and physical mass transfer coefficient of the liquid phase were measured directly. The gas diffusion coefficient in the solution was calculated using the modified Stokes-Einstein relation. At constant concentration, the CO2 absorption rate increased by increasing temperature, while at a constant temperature, the rate directly correlated with concentration up to an optimum level. Beyond the optimum concentration, an increase in concentration decreased the rate. Here, a rate equation for the absorption of CO2 in sodium serinate/piperazine solutions was developed. It can be concluded that aqueous solution of sodium serinate can be used as a CO2 absorbent. Moreover, adding piperazine to the aqueous solution of sodium serinate can improve the CO2 absorption efficiency.

Effect of sodium dodecyl sulfate on CO2 and H2S absorption enhancement of functionalized multiwall carbon nanotubes in water: Experimental study and empirical model

A single bubble absorption column was used to examine the effect of hydrodynamic on carbon dioxide (CO2) and hydrogen sulfide (H2S) absorption in pure water and water-based nanofluids dispersed with neat, and OH and NH2 functionalized multiwall carbon nanotubes (MWCNTs). Sodium dodecyl sulfate (SDS) was used as a surfactant and stabilizer. The maximum absorption of CO2 and H2S were found to be 0.0038 mmol/m2·s and 0.056 mmol/m2·s using NH2-MWCNTs /nanofluid with 0.5 wt% content, respectively. The diffusion coefficients of gases into the nanofluids were computed by using an equation attained based on Dankwert’s theory. A last, an empirical correlation was proposed to determine the Sherwood number for the absorption of the aforementioned gases into the nanofluids.

Quantification of the asynchronous gas diffusivity in macro-/micropores using a Nelder-Mead simplex algorithm and its application on predicting desorption-based indexes

Accurate and intelligent calculation of diffusion coefficient not only helps reduce greenhouse gas emissions, but also conduces to CO2 geological storage and CH4 displacement in coal seams. In this study, a kind of object-oriented software for conveniently and quickly obtaining bidisperse (i.e., macro-pore and micro-pore) diffusion coefficients and grasping main controlling factors of macroscopic desorption curves was designed based on the Nelder-Mead simplex algorithm. Furthermore, the gas desorption characteristics under different pressures were analyzed by performing conventional desorption experiments. The results show that the apparent diffusion coefficient obtained by the bulk technique displays three different trends as the pressure rises, namely a rising, falling and constant diffusion coefficient respectively. The three variation trends mainly result from the difference in deformation caused by adsorption swelling and mechanical compression. As desorption continues, the gas diffusion coefficient values in the macro-pores and micro-pores fluctuate for a short time, but increase in the long term. This is because the early stage corresponds to intense mechanical expansion and a flow boundary effect, while the later stage corresponds to an increasingly intense effect of desorption-induced shrinkage. Moreover, the time-dependent contribution law of macro-pore diffusion and micro-pore diffusion affects the reliability of outburst risk index, making △h2 more stable than K1. The conclusions provide theoretical guidance for the selection of rational gas drainage methods for diffusion-controlled coal seams.

Mobility of Dissolved Gases in Smectites under Saturated Conditions: Effects of Pore Size, Gas Types, Temperature, and Surface Interaction

In a nuclear waste repository, the corrosion of metals and the degradation of the organic material in the waste matrix can generate significant amounts of gases. These gases should be able to migrate through the multibarrier system to prevent a potential pressure build-up that could lead to a loss of barrier integrity. Smectite mineral particles form a tortuous pore network consisting of larger interparticle pores and narrow interlayer pores between the platelets of the smectite minerals. These pores are normally saturated with water, so one of the most important mechanisms for the transport of gases is diffusion. The diffusion of gases through the interparticle porosity depends on the distribution of gas molecules in the water-rich phase, their self-diffusion coefficients, and the tortuosity of the pore space. Classical molecular dynamics simulations were applied to study the mobility of gases (CO2, H2, CH4, He, and Ar) in Na-montmorillonite (Na-MMT) under saturated conditions. The simulations were used to estimate the gas diffusion coefficient (D) in saturated Na-MMT as a function of nanopore size and temperature. The temperature dependence of the diffusion coefficient was expressed by the Arrhenius equation for the activation energy (Ea). The predicted D values of gases were found to be sensitive to the pore size as the D values gradually increase with increasing pore size and asymptotically converge to the gas diffusion coefficient in bulk water. This behavior is also observed in the self-diffusion coefficients of water in Na-MMT. In general, H2 and He exhibit higher D values than Ar, CO2, and CH4. The predicted Ea values indicate that the confinement affects the activation energy. This effect is due to the structuring of the water molecules near the clay surface, which is more pronounced in the first two layers of water near the surface and decreases thereafter. Atomic density profiles and radial distribution functions obtained from the simulations show that the interaction of the gas with the liquid and the clay surface influences mobility. The obtained diffusion coefficient for different gases and slit pore size were parameterized with a single empirical relationship, which can be applied to macroscopic simulations of gas transport.

The effect of CO2-philic thickeners on gravity drainage mechanism in gas invaded zone

The rate of mass transfer between the fractures and matrix in gas invaded zone can significantly influence on the oil recovery during the forcedgravitydrainageprocess.However, in this study, a new approach was suggested to improve the gravity drainage process in gas invaded zone. Poly(fluoroacrylate) (PFA), as a CO2-philic thickener, was injected into the gas invaded zone to illustrate the impact of interfacial mechanisms such as gas diffusion coefficient and interfacial tension (IFT) on oil recovery. Also, the cloud point pressures were measured to ensure that the PFA did not come out of the solution due to a phase change during IFT, gas diffusion coefficient, and gravity drainage experiments.Results showed that the CO2-PFA thickener (20000 ppm) could decrease the IFT from 56 to 24 dyne/cm compared to the pure CO2 scenario, improving the gravity drainage mechanism in the gas invaded zone. In addition, the CO2 diffusion coefficients were increased approximately more than two times during CO2-PFA injection in comparison with pure CO2 injection in both porous media and bulk oil phase scenarios at reservoir conditions. Also, an incremental oil recovery of 16 percent was achieved during PFA/CO2 compared to pure CO2 injection in the gas invaded zone. Therefore, gas gravity drainage is the most important mechanism once gas thickener or CO2 enters the fractures in the gas invaded zone.

Mechanism and influence factor of hydrocarbon gas diffusion in porous media with shale oil

Due to the compactness of shale reservoir matrix and the high conductivity of fractures, the hydrocarbon gas injection huff and puff method or displacement is the most realistic technology to improve shale oil recovery. The diffusion mechanism plays an important role in shale oil development; therefore, it is crucial to figure out the factors influencing diffusion, which could enhance shale oil recovery. In this paper, a physical simulation experiment is designed to evaluate the diffusion ability of hydrocarbon gas. Diffusion experiments are conducted to simulate diffusion in the bulk fluid and in the porous media, to learn about how the pressure, permeability and fracture affect the diffusion behavior. The diffusion coefficients between the bulk diffusion and core sample diffusion are compared. The experimental results show that the diffusion coefficient and mass transfer capacity are positively correlated with permeability and pressure: increasing these parameters can promote the diffusion process. The diffusion coefficient of hydrocarbon gas in a saturated oil core is significantly less than that in crude oil, which indicates that the porous media seriously affects the process of gas diffusion in crude oil. Fractures have little impact on the diffusion behavior. Combined with numerical simulation, the influencing factor of diffusion on the development effect of hydrocarbon gas injection is clarified. The recovery enhances and then decreases with the increasing diffusion coefficient. Cited as: Wanyan, Z., Liu, Y., Li, Z., Zhang, C., Liu, Y., Xue, T. Mechanism and influence factor of hydrocarbon gas diffusion in porous media with shale oil. Advances in Geo-Energy Research, 2023, 7(1): 39-48. https://doi.org/10.46690/ager.2023.01.05

Evaluation of Performance of Polyacrylamide-Modified Compacted Clay as a Gas Barrier: Water Retention and Gas Permeability and Diffusion Characteristics

In this paper, the performance of a gas barrier that consisted of polyacrylamide (PAM)-modified compacted clayey soil was experimentally explored. The moisture content and water loss characteristics of the tested soils were adopted as indicative indices of water retention capacity (WRC). The gas permeability (Kp) and gas diffusion coefficient (Dp) of the modified compacted clays were evaluated via gas permeability and gas diffusion tests. The test results showed that the moisture content of the modified compacted clay samples subjected to drying tests increased with increasing polyacrylamide content. Kp and Dp decreased with increasing PAM content. Compared with 0.2% PAM content, the Kp of the sample with 1.0% PAM was reduced by ten times, and the Dp was reduced to ~35%. Compared to the unmodified clay, the liquid limit of the PAM-modified clay increased by 45~55%. Comparison of the liquid limit tests between this study and previous studies revealed that the liquid limit ratio of the zwitterionic polyacrylamide (ZP)-modified soil was much higher than the other material-modified soils. The results of this study are useful to facilitate the application of modified compacted clays as gas barrier materials at industrial contaminated sites.

Gas-Diffusion Coefficient in Organic Matter Affects Estimation of Shale-Gas Reservoirs

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 201653, “On the Inference of Gas Diffusion Coefficient in Organic Matter of Shale Gas Reservoirs,” by Esmail Eltahan, SPE, Mehran Mehrabi, and Kamy Sepehrnoori, SPE, The University of Texas at Austin, et al. The paper has not been peer reviewed. US shale reservoirs are tight and inherently heterogeneous, with an abundant presence of kerogenic material. Modeling fluid flow in shale reservoirs is complex because of flow physics such as pressure flow and diffusion. Many field-performance forecasts underestimate production from these reservoirs constantly because most current models ignore important governing physics. This study provides new insights on diffusion in organic matter to correct a main source of underestimation of gas production in shale-gas models. In the complete paper, the authors implement a multiscale diffusion model to estimate gas-diffusion coefficient in organic matter. Background Pores in shale-gas resources, which are on the order of a few to tens of nanometers in diameter, are much smaller than pores in conventional resources. At such small pore size, the ratio of surface area to pore volume becomes large, giving importance to transport mechanisms that depend on the surface area. According to other researchers, in addition to the free gas, considerable amounts of gas molecules will exist on the surface of organic material (adsorbed gas) and inside the solid matrix of kerogen (dissolved gas). Although typically neglected in conventional gas resources, the adsorbed and dissolved gas should receive attention in the case of shale-gas resources because of an abundance of organic material and substantial pore surface area. Gas production from shale is largely facilitated by maximizing the contact a well creates with the formation. In practice, the contact is established by numerous hydraulic fractures along a wellbore that extends for a lengthy horizontal distance in the formation. The induced-fracture networks grant direct access to the free gas in organic and inorganic pores as well as the adsorbed and dissolved gas through organic pores. Gas is depleted in the same manner as it is stored (in laboratory experiments, but in reverse order). Free gas is expanded first, creating local pressure sinks that force adsorbed gas to detach from the pore surface. As the desorption process continues, the number of unoccupied spots on the pore walls increases. Local concentration disruptions occur at those spots, which cause the dissolved gas to diffuse back into the pores. The contribution of dissolved gas in field cases has been reported in several studies, which provides motivation to study the physics of dissolved-gas diffusion on the pore scale and to develop consistent predictive models on the mesoscale. In a previous work by the authors, they presented a mathematical model for the evolution of dissolved gas that considers detailed pore geometry. However, the authors write that they have shown that pore geometry has minor influence, in contrast to the involved diffusion-length scales. Thus, the key components of previously obtained insights are used to build a simple model. The proposed model is considerably simpler in implementation, yet descriptive of the key components that govern dissolved-gas evolution. The mathematical model and its associated equations are detailed in the complete paper.

Separation of propylene/propane using IL/Silver ion facilitated transport: Insights from computational molecular approach

• Facilitated transport ionogel membranes are analyzed to reveal separation mechanism. • The interactions between gas, solid scaffold, and IL electrolyte are discussed. • MD simulation combined with DFT calculation are employed to investigate interactions. • The sorption and transport properties are analyzed by GCMC and MD simulations. • The membranes with or without silver salt show stronger interaction toward propylene. Membrane technology is being used increasingly for olefin/paraffin separation, and the facilitated transport of ionogel membranes provides unprecedented possibilities to circumvent the permeability/selectivity trade-off. An in-depth understanding of the essence of the concept of gas sorption and diffusion in silver-assisted facilitated transport mechanisms remains relatively unfathomed, which motivated the present work. Herein, the computational molecular approach consisting of molecular dynamics (MD) simulation, grand canonical Monte Carlo (GCMC), and density functional theory (DFT) shed light on the possible interactions between the components of membrane and gas species. The solvation free energy as a function of silver concentration was calculated to determine the contribution of coulombic and van der Waals forces quantitatively. The solvation-free energy of propylene in the IL electrolyte is highly favorable and follows a decreasing trend when compared to propane. The DFT calculations were performed to establish a clear connection between molecular structure and solubility results. From radial distribution functions (RDFs), it was found that the π-π interaction of propylene/cation of IL and propylene/Ag + is the key point in its higher solubility. Solubility of propylene in IL electrolyte increases from 0.0016 to 0.013 cm 3 (STP)/cm 3 .cmHg upon the addition of silver salt up to 1 M. Relative concentration distribution proves the layering of the components adjacent to the pore wall which could be the reason for larger gas diffusion coefficient (50–100%) in the membrane compared to bulk IL. This simulation work is expected to be noteworthy toward a detailed understanding of facilitated mechanisms and provides useful guidance for designing new concept membranes for olefin/paraffin separation.

Support vector machine regression to predict gas diffusion coefficient of biochar-amended soil

Measurement of gas diffusion coefficient (Dp) of biochar-amended soil (BAS) under varying conditions is essential for assessing the adsorption capacity and water/gas diffusion in compacted BAS. However, there is no established equation of Dp available on this topic. Also, the factors influencing gas diffusion in BAS have not been properly studied and remain unclear. Various machine learning models were employed in this paper to learn and predict the Dp of BAS based on experimental data. Six factors (i.e., degree of compaction (DOC), biochar content (BC), soil air content (SAC), gravimetric water content (GWC), degree of saturation (DS), and porosity) are considered for testing the prediction models. The epsilon radial basis function support vector regression model showed better accuracy and predictive performance (R=0.9925) than other models and was further improved by applying the feature selection technique using the multiple linear regression and tree-based models (R=0.9937). The results reveal that SAC, DS, and porosity are the main predictor variables. The SAC proved to be the most influential predictor variable based on the estimated p-value. Furthermore, the optimal Dp was established for the various DOC and BC, which could be useful in designing engineered landfill covers. The accurate model prediction and relative importance of the predictor variables could significantly minimize the experimental work volume required to determine Dp, thereby saving time and cost.

Screening Additives for Amending Compacted Clay Covers to Enhance Diffusion Barrier Properties and Moisture Retention Performance

The cover systems in contaminated sites have some problems, including desiccation cracks, which would lead to degradation of the barrier performance. This study presented a systemic laboratory experimental investigation on the liquid–plastic limit, moisture retention, hydraulic conductivity (k), and gas diffusion barrier properties of amended compacted clay by attapulgite and diatomite for controlling desiccation cracks and migration of water and volatile organic compounds (VOCs). The results showed that the attapulgite could enhance the moisture retention and liquid limit of amended compacted clay. Diatomite could reduce the gas diffusion coefficient (Dθ) significantly. The compacted clay amended by the dual-additives component of attapulgite and diatomite could enhance the liquid limit, moisture retention percent, gas barrier property, and hydraulic performance compared with the unamended clay. Based on the experimental data obtained, the dosage of additives was targeted to be 5%. The moisture retention percent of dual-additives (attapulgite 4% and diatomite 1%) amended clay increased by 82%, the k decreased by 25%, and the Dθ decreased by 42% compared with unamended clay. Scanning electron microscopy (SEM), BET-specific surface area test method (BET), Mercury Intrusion Porosimetry (MIP), and thermogravimetric analysis (TGA) indicated the enhancement mechanism of additives-amended compacted clay.

Comparison of analytical film theory and a numerical model for predicting concentration polarisation in membrane gas separation

Accurate prediction of the concentration polarisation (CP) effect is very important in the design of an efficient membrane-based gas separation process. This study analyses the reliability of analytical film theory (FT) for evaluating the performance of gas separation membranes in terms of CP and flux. The analytical model is compared against a more rigorous numerical model developed by using Computational Fluid Dynamics (CFD) for various operating variables. The results show that the FT prediction is less accurate at high CP conditions when gas permeation through the membrane increases, due to higher permeance selectivity and pressure ratio. Hence, the results suggest that FT is not recommended for membranes with high permeance or high-pressure conditions. Given that the typical range of feed composition and temperature has little impact on fluid properties (i.e., gas diffusion coefficient, densities, and viscosities), the resulting CP does not vary much and hence both FT and CFD models predict a similar CP. The analysis also suggests that the FT model is more accurate in predicting CP in the region closer to the membrane entrance. Overall, the analytical film theory serves as a reliable approximation in membrane gas applications under low CP at high crossflow and low flux conditions.

Integrated Thermo-Electric Measurement of Electrospun Nanofiber for Polymer Electrolyte Fuel Cell

Polymer electrolyte fuel cell (PEFC), as an efficient hydrogen energy conversion device, has been developed for decades. Catalyst layer (CL), where the oxygen reduction reaction occurs, predominantly determines the performance of PEFC. Since 1889, the mass activity of catalyst layer has been improved for more than three orders of magnitudes, thanks to the invention of PFSA ionomer and highly dispersed catalyst on nano-sized carbon support. In recent years, introducing order into CL is bringing the CL performance to a higher level. The CLs with state-of-the-art performance feature nanofiber structures. One type of is ionomer nanofibers deposited with Pt/C catalyst on the surface, and another type is composite nanofiber containing ionomer, Pt/C catalyst, and binding polymer. Their superior performance may come from the ordered mass transport network and the size effect of nanofiber conductivity. To further improve the CL performance, the structure-property-performance relationship of ionomer nanofibers or composite nanofibers needs to be clarified.In this work, we designed a setup to measure the proton, electron and thermal conductivity as well as the performance of a single ionomer or composite nanofiber. We developed a self-bonding method to fix the nanofiber on the micro-electrode. For proton and electron conductivity measurement, four-probe method was adopted to eliminate the electrical contact resistance and interfacial effect. For thermal conductivity measurement, Raman assisted steady state method was used to directly measure the temperature profile along the nanofiber. So, the measurement error resulting from the thermal contact resistance between the sample and the temperature sensor/heat sink can be eliminated. For performance measurement, three-electrode system was adopted using the Ag/AgCl electrode as the reference electrode and the Zn/ZnCl2 electrode as the counter electrode. We prepared nanofibers with different size, polymer content, and cross-sectional component distribution by controlling the ink recipe and electrospinning parameters. The integrated thermo-electric measurement found that the proton and thermal conductivity of nanofiber are one order of magnitude higher than bulk properties and increases with decreasing fiber radius. This is attributed to the oriented ionic morphology along the nanofiber. Furthermore, a nanofiber model was developed to extract the catalytic activity and gas diffusion coefficient of nanofiber. These characterization and modelling work can provide guidance to the design and optimization of catalyst layer with electrospun ionomer or composite nanofibers.

Mixed gas diffusion and permeation of ternary and quaternary [formula omitted] gas mixtures in Matrimid®, polyetherimide and poly(lactic acid) membranes for [formula omitted] separation

The CO2/CO mixed-gas separation performance of a polyimide (Matrimid®), polyetherimide (PEI) and polylactic acid (PLA) membrane, were characterized in the presence of CO2- rich ternary (CO2/ CO/ O2) and quaternary (CO2/ CO/ O2/ N2) feed gas mixtures mimicking the products of CO2 reforming conversion reactions. The membrane-based separation of this mixture is poorly characterized and original data were obtained in a novel mass spectrometric apparatus that permits to monitor the instantaneous permeate composition, thus allowing to evaluate both mixed gas diffusion and permeability coefficients of all gases. CO2, CO, O2andN2 permeability and diffusivity in single gas tests were measured between 298 and 353 K up to 1 atm feed pressure and relevant activation energies were evaluated. At 298 K Matrimid® exhibits CO permeability of 0.50 ± 0.03 Barrer and an ideal CO2/ CO selectivity of 16 ± 1. PEI and PLA exhibit similar ideal selectivity values but lower CO transport rates. In all examined polymer films the CO2/ CO selectivity has absorption-selective character that favours the permeation of CO2. The ideal CO2/ CO selectivity of all membrane samples decreases with temperature, reaching values of 10 ± 1 at 335 K in Matrimid®. The CO2/ CO selective performances of all examined membrane do not show markable variations exposing the membrane samples to CO2- rich gas mixtures as feed gas. The upper bound correlation among selectivity and permeability for the CO2/ CO gas couple is here for the first time proposed.

Tuning the Morphology and Gas Separation Properties of Polysulfone Membranes

The present work deals with the modification of casting solutions for polysulfone gas separation membranes fabricated by wet-phase inversion. The aim was to fabricate membranes with thin gas separation layers below one micrometer of thickness and a sponge-like support structure. With decreasing thicknesses of the separation layers, increasing permselectivities were observed. For the first time, we could show that permeabilities and diffusion coefficients of certain gases are orders of magnitude lower in separation layers of membranes below 500 Å of thickness compared to separation layers with a thickness above 1 micrometer. These results indicate that the selection of the solvent system has a huge impact on the membrane properties and that the permeability and diffusion coefficient are not material-related properties. Thus, they cannot be applied as specific indicators for gas-separating polymers. In this publication, scanning electron microscopy and gas permeation measurements were carried out to prove the gas separation properties and morphologies of polysulfone membranes.

Effect of Lifting Gas Diffusion on the Station-Keeping Performance of a Near-Space Aerostat

During the long-endurance flight of a near-space aerostat, the characteristics of lifting gas diffusion have a great influence on the flight altitude adjustment and station-keeping performance. Thus, in this study, a lifting gas diffusion model and a dynamic model that consider thermal effects, which had not been studied in similar models before, were developed. The dynamic model and thermal model were validated by historic flight data, and the calculated lifting gas diffusion results were compared with the experimental data of other researchers. The variations in the flight endurance, flight altitude, lifting gas diffusion rate, and diffusion coefficient of a near-space aerostat were analyzed. The effects of the ratio of porosity to tortuosity and envelope radiation properties on the mass of the lifting gas and flight altitude were considered in detail. To analyze the effect mechanism of the ratio of porosity to tortuosity and the envelope radiation properties, the envelope and gas temperature, as well as the gas pressure, were studied. The results show that the lifting gas diffusion rate and diffusion coefficient are very sensitive to the change in the ratio of porosity to tortuosity and envelope temperature. The results obtained from the analysis of the lifting gas diffusion can lay a solid foundation for improving the flight performance of near-space aerostats and for providing improved design considerations for aerostats.

Experimental and numerical analysis of CO2 and CH4 hydrate formation kinetics in microparticles: A comparative study based on shrinking core model

The promotion of gas hydrate formation kinetics through mass transfer enhancement has been an important research topic, for which microparticles have been considered an effective method. In this work, carbon dioxide (CO2) and methane (CH4) hydrate formation kinetics in microparticles are investigated using “dry water” particles made of 3, 5 and 8-wt% silica. A modified shrinking-core model is established to study the CO2 and CH4 hydrates formation kinetics. It is the first model that integrates the effects of dissolved gas, the capillary effect of porous hydrate shell, and the volume change from water to hydrate. The experimental results reveal that “dry water” particles with 8-wt% silica has the highest normalized gas uptake due to their small particle size. The simulation results show an initial effective diffusion coefficient of 6.41–6.50×10–14 m2s−1 for CO2 and a slightly higher 6.83×10–14 m2s−1 for CH4 hydrate formation. The average effective diffusion coefficient of gas is higher in smaller particles. The water consumed through capillaries is more prominent in smaller particles, but it only accounts for less than 10% of water consumed at the hydrate–water interface. Furthermore, a decoupled heat transfer model was developed to quantify the effect of heat transfer in gas hydrate formation. The instantaneous temperature gradient in the hydrate shell is of a small magnitude of 10–2 K m−1, indicating that the impact of the heat transfer on hydrate formation kinetics is negligible. This work provides comprehensive insights into gas hydrate formation in microparticles and contributes as a theoretical basis for the improvement of gas hydrate kinetics through mass transfer enhancement.