Diffusion Coefficients Of CH4 Research Articles

Microstructure and transport properties of metakaolin-based geopolymers subjected to accelerated leaching

Towards the development of net-zero carbon concrete and the wider application of geopolymers, understanding the durability of geopolymers has become essential. However, knowledge on the evolution of microstructure and transport properties of the materials under aggressive conditions due to their interaction with external chemicals is limited. Therefore, this study investigated the leaching behaviors of metakaolin-based geopolymers with high water-to-binder (w/b) ratios under a 6 M NH4NO3 attack, focusing on their microstructure and transport properties. Microstructural insights were assessed through N2-adsorption, Mercury intrusion porosimetry, and water porosity analyses. After 28 days of leaching, total porosity slightly decreased, but pore size distribution (PSD) shifted towards larger pore sizes. The materials close to the exposed surface exhibited a relatively higher porosity and larger PSD than the deeper parts. The potential formation of Al(OH)3 and silica layers as a result of dealumination, confirmed by mass gain and SEM/EDS, contributed to the pore structural evolution. These alterations led to slight reductions in water permeability and CH4 diffusion coefficients but mostly increased the He diffusion coefficients of the geopolymers after 28-day leaching. These findings underscored the inherent microstructural characteristics of geopolymers (e.g., porosity, pore size distribution, and tortuosity) and the size of diffusing species as key factors shaping the transport properties. Notably, the permeability and diffusivity of both leached and intact geopolymers exhibited exponential correlations with the accessible porosity.

The use of small emitter flow rate in drip irrigation favored methane uptake in arid potato fields

Drip irrigation has been widely used in agriculture, while the mechanism underlying the effect of drip irrigation system design and management on soil methane (CH4) uptake have not been fully studied. A two-year experiment was conducted to explore the effects of different emitter flow rates of drip irrigation (1.3, 2.0, and 3.0 L/h) on CH4 uptake in potato fields in an arid region of Northwest China. Results showed that compared to emitter flow rates of 2.0 and 3.0 L/h, the 1.3 L/h flow rate significantly increased the cumulative CH4 uptake by 6.3–16.2%, mainly due to the increase in the abundance of the pmoA gene by 15.2–49.5%, and by the decrease of water-filled pore space (WFPS), increasing the effective CH4 diffusion coefficient (Dp) and soil oxygen concentration in the topsoil. A hump-shaped relationship was found between WFPS and CH4 uptake flux, and the optimum WFPS for CH4 uptake was about 44.8%. The treatment with an emitter flow rate of 1.3 L/h exhibited the highest frequency (28.6%) of favorable WFPS for CH4 uptake, compared to the 2.0 and 3.0 L/h treatments. In addition, significant negative quadratic relationships were observed between both Dp and soil oxygen with CH4 uptake, and optimal values were 0.02 cm2 s−1 and 18.7%, respectively. There was no significant difference in CH4 uptake between 2.0 and 3.0 L/h treatments in 2021 and 2022, due to high WFPS and low oxygen concentrations in both treatments. Multiple linear regressions indicated that soil oxygen concentration became a major limiting factor under emitter flow rates of 2.0 and 3.0 L/h. This study provides insights into mechanisms of soil CH4 uptake in drip-irrigated farmland, which gives guidance on developing irrigation strategies for sustainable production and CH4 mitigation in irrigated arid land.

Influence of competitive adsorption, diffusion, and dispersion of CH4 and CO2 gases during the CO2-ECBM process

The displacement mechanisms of adsorbed CH4 by injected CO2 at high pressures in CO2-enhanced coalbed methane (CO2-ECBM) processes involve competitive adsorption, diffusion, and dispersion. Inherent coal heterogeneity and the complexity of the CO2-CH4 displacement model make CO2-ECBM modelling and optimal process design challenging. The pure gas isotherms and Langmuir adsorptions constants for CO2 and CH4 were determined using the manometric method. Notably, CO2 shows higher adsorption capacity than methane, and Langmuir volume (VL) for CO2 is 2 to 2.3 times that of methane. High pressure adsorption isotherms and experimentally determined diffusion coefficients set the sorption kinetics. The diffusion coefficient of CH4 for all four samples was in the range between 1.35×10-12 and 7.42×10-12 m2/s, while for CO2, it varied from 10.9×10-12 to 15.48×10-12 m2/s. The new concept of caucus time was introduced in the sorption kinetics of CO2 and CH4. Each sample has distinct caucus time and was positively correlated with Langmuir volumes of CO2 and CH4 with R2 > 0.9. The critical desorption pressure above which CH4 exhibits retrograde adsorption behaviour is evaluated from co-adsorption isotherms adopting Ideal Adsorption Solution Theory (IAST). The critical desorption pressure for the four samples were 1220, 460, 575 and 740 psi respectively. IAST also predicts a critical mole fraction (∼30 %) in the gas phase above which CO2 becomes the dominant adsorbed phase on coal surfaces. The results of Advective-Dispersion modelling at a larger scale illustrate the value of the Dispersion coefficient in forecasting and screening. Since the time available for diffusion versus dispersion depends upon flowrate, high values of dispersion coefficient can negatively influence favourable competitive adsorption expectations through higher operational cost in CO2 separation. This work suggests the existence of both an optimal operational pressure and flowrate to maximize the potential of CO2-ECBM technology demonstrated in competitive adsorption modelling.

Modeling competitive adsorption and diffusion of CH4/CO2 mixtures confined in mature type-II kerogen: Insights from molecular dynamics simulations

In this study, the adsorption and competitive diffusion of methane (CH4) and carbon dioxide (CO2) mixtures through a molecular model of mature type-2 kerogen was investigated using molecular dynamics simulations. The Dual Control Volume-Grand Canonical Molecular Dynamics method was used to analyze the system and the results were focused on the selectivity of kerogen towards mixtures under a wide range of thermodynamic conditions. The results showed that the adsorption selectivity of CO2/CH4 was around 2–4 and decreased with temperature in the range 350–450 K. The Extended Langmuir (EL) model was found to underestimate the selectivity, and a correction was proposed in the form of the Corrected Extended Langmuir (CEL) model, which improved the estimations of the adsorption of CH4/CO2 mixtures in kerogen with only one adjustable parameter independent of thermodynamic conditions. Interestingly, we noticed that, out of equilibrium, the adsorption selectivity was not affected by the flow conditions when a pressure gradient was imposed, and the CEL model can thus be used to describe the contribution of adsorption to the selectivity under flow conditions. The results showed that the ratio of the collective diffusion coefficients of CH4 and CO2 was around 0.3−0.4, which was about two times lower than other simulations found in the literature. To describe the global selectivity, a simple approach combining the CEL model and the average ratio of diffusion coefficients was proposed, resulting in a selectivity of around 1.

Enhancing the CO2, CH4, and N2 adsorption and kinetic performance on FAU zeolites for CO2 capture from flue gas by metal incorporation technique

Various metals including alkali (Na+, K+), alkaline earth (Mg2+ and Ca2+), and transition metals (Cu2+ and Fe3+) were successfully incorporated into the structure of faujasite zeolites (X and Y) by novel post-synthesis modification method. The effect of metal-doping on the CO2, CH4, and N2 adsorption and kinetic performance in the context of flue gas carbon capture and natural gas purification was examined by four approaches; (i) the impact on structural properties was examined by XRD, FTIR, UV–visible, H2-TPR, FESEM, EDX Dot-Mapping, and BET analysis; (ii) the influence on the CO2, CH4, and N2 adsorption capacity was acquired by volumetric method up to 100 kPa at 25 °C (iii) the effect on binary CO2/CH4 and CO2/N2 selectivity was determined by the Henry constant and IAST selectivity; (iv) the micropore - macropore kinetic model was utilized to study the influence on diffusion coefficients. The dual-site Langmuir-Freundlich (DSLF) isotherm was found to be the best adaptability to the experimental isotherms data. The results showed an extraordinary increase in CO2 absorption capacity to 4.9 and 7 for KX and KY, respectively. Faujasite zeolite with a lower Si/Al ratio (zeolite X) outperforms CO2/CH4 and CO2/N2 separation performance than higher ones (zeolite Y). Among all samples, KX showed the highest CO2/CH4 and CO2/N2 IAST selectivity with values of 61 and 152 at 100 kPa, respectively, in addition to the highest diffusion coefficients for CH4 and N2. The incorporated potassium metal into the faujasite zeolites showed promising sorbents for the separation of CO2 from flue gas and natural gas streams.

Molecular interactions of CO2 and CH4 and their adsorption behaviour in kerogens with different grades of maturity

ABSTRACT CO2 sequestration (CS) into the shale formations can reduce not only carbon emissions but also enhance gas recovery (EGR). The adsorption and diffusion behaviour of CO2 and CH4 on kerogens with different maturity play a crucial role in CS-EGR as they determine the efficiency of CO2 storage and energy recovery. In this work, GCMC and MD simulations were performed to investigate the adsorption and diffusion behaviour of CO2 and CH4 on kerogen models at different grades of maturity. It indicated that, in the same maturity, the CO2 adsorption capacity was more significant than that of CH4. With increasing maturity, the adsorption capacity and diffusion rate increased. With the increase of water contents, the swelling ratio of kerogens increased, and the adsorption capacity and the diffusion coefficients of CH4 and CO2 decreased. However, the adsorption selectivity of CO2 over CH4 significantly increased. H2O and CO2 molecules both preferred to adsorb on functional groups of oxygen, nitrogen and sulfur. This study consolidated our hypothesis that an injected CO2 to shale gas and oil formations contributed positively to enhanced energy recovery owing to the difference in adsorption and diffusion behaviour of CH4 and CO2 in kerogens with different maturity in gas and oil reservoirs.

Adsorption Equilibrium and Diffusion of CH4, CO2, and N2 in Coal-Based Activated Carbon.

Coal-based activated carbon is an ideal adsorbent for concentrating CH4 from coalbed methane and recovering CO2 from industrial waste gas. In order to upgrade the environmentally protective preparation technology of coal-based activated carbons and clarify the adsorption equilibrium and diffusion rules of CH4, CO2, and N2 in these materials, we prepared granular activated carbon (GAC) via air oxidation, carbonization, and physical activation using anthracite as the raw material. Also, we measured the adsorption isotherms and adsorption kinetic data of GAC by the gravimetric method and characterized its surface chemical properties. According to the results, GAC had abundant micropore structures with a pore size mainly in the range of 5.0-10.0 Å, and its surface was covered with plentiful oxygen-containing functional groups. The specific pore structure and surface chemical properties could effectively improve the separation and purification effects of GAC on CH4 and CO2. In the temperature range of 278-318 K, the equilibrium separation of CH4/N2 by GAC with a coefficient between 3 and 4 could be achieved. Also, the CO2/CH4 separation coefficient decreased with the increase in temperature but remained around 3. The bivariate Langmuir equation could describe the adsorption behaviors of GAC on CH4/N2, CO2/N2, and CH4/CO2. With the increase in the concentrations of CH4 and CO2 in the gas phase, the difference between the adsorption capacity of CH4 or CO2 and that of N2 became greater. The change of the gas ratio did not affect the characteristics of preferential adsorption of CH4 and CO2. At different temperatures (278, 298, and 318 K), the diffusion coefficients of CH4, N2, and CO2 at various pressure points showed predominately a small variation without an obvious trend. These results demonstrated that the separation of CH4/N2, CO2/N2, and CH4/CO2 by the activated carbon could only rely on the equilibrium separation effect rather than the kinetic effect.

Influence of Supercritical CO2 Fluid on CH4 and CO2 Diffusion in Vitrinite-Rich Coals and Inertinite-Rich Coals

Coal maceral composition has a great effect on gas adsorption and diffusion. The interaction between maceral composition and supercritical CO2 (SCCO2) fluid will affect gas diffusion behavior in coals. Thus, the diffusivity derived from adsorption kinetics of CH4 and CO2 in vitrinite- and inertinite-rich coals with low-violate bituminous rank collected from the Hancheng mine of the Weibei coalfield pre- and post-SCCO2 fluid exposure (SFE) were tested at the conditions of 45 °C and 0.9 MPa. In combination with pore distribution and functional group content, the possible mechanism of the alterations in gas diffusion characteristics in coals with various maceral compositions was addressed. The results show that for vitrinite-rich coals, SFE increases the macropore apparent diffusion coefficient of CH4, while this treatment decreases the micropore apparent diffusion coefficient of CH4. However, the reverse trend is found for CO2 diffusion–adsorption rate. For inertinite-rich coals post-SFE, CH4 diffusion–adsorption rate increases, while an increase and a decrease in diffusivity CO2 occur for macropore and micropore, respectively. Generally, SFE shows a stronger impact on CO2 adsorption rate than CH4 in coals. The results suggest that the diffusion of CH4 and CO2 in coals with different maceral compositions show selectivity to SCCO2 fluid. The possible reason can be attributed to the changes in pore structure and surface functional group content. SFE causes an increase in macro/mesopore volume of all samples. However, SFE induces a reduction in oxygen-containing species content and micropore volume of inertinite-rich coals, while the opposite trend occurs in vitrinite-rich coals. Thus, the changes in pore volume and surface functional group account for the difference in gas diffusivity of coals with different maceral compositions. With regard to the micropore diffusion–adsorption behavior of CH4 and CO2, the impact of oxygen-containing species is superior to pore volume. The oxygen-containing species favor CO2 diffusion–adsorption but go against CH4 transport. This effect accounts for the reduction in the micropore diffusion–adsorption rate of CH4 and the increase in micropore diffusivity of CO2 in vitrinite-rich coals, respectively. However, the aforementioned effect is the opposite for inertinite-rich coals. Overall, the changes in gas diffusion in coals with different maceral composition during the CO2-ECBM process requires further attention.

Molecular Simulation of Methane Adsorption Properties of Coal Samples with Different Coal Rank Superposition States.

Obvious differences exist in the structures of different coal rank coals, resulting in many differences and similarities in the amount of methane (CH4) gas adsorbed by coal and the control mechanism. In this study, we conducted adsorption simulations of three different coal rank coals in the superposition state using the Materials Studio software with simulated temperatures of 293.15, 313.15, and 333.15 K and adsorption pressures ranging from 0 to 10 MPa. We used the grand canonical ensemble Monte Carlo calculation method to calculate and study the adsorption amount, adsorption process, isosteric heat, and diffusion coefficient of CH4 in detail. We found that the adsorption of CH4 by coal samples of three coal rank coals (i.e., anthracite, bituminous coal, and lignite), which were mixed and stacked separately, was concentrated in the effective pores of coal molecules. Their effect on the amount of CH4 adsorption was dominated by the coal samples with more effective pores in the coal molecules. The isosteric heat of the coal samples showed an obvious exponential relationship with the adsorption pressure, and the isosteric heat of coal samples as a nonhomogeneous adsorbent gradually decreased with an increase in adsorption pressure. During the transformation of free-state CH4 into an adsorbed state, we obtained the best adsorption effect for bituminous-lignite superposition-state coal samples, and the diffusion coefficient of CH4 was the lowest.

Molecular simulation of alkyl glycoside surfactants with different concentrations inhibiting methane diffusion in coal

The injection of surfactant solutions into coal seams is one of the most critical technical means of preventing coal mine disasters. It can effectively prevent coal and gas outburst disasters by reducing the gas concentration at the coal mining face. The Wiser model of coal was utilized to establish the initial CH4-containing coal. Different concentrations (0%, 0.2%, 0.4%, 0.8%, 1.2%, and 1.6%) of alkyl poly glucoside surfactant (APG) were injected into CH4-containing coal to investigate the microkinetic mechanism of inhibiting gas desorption. The diffusion parameters of CH4 (i.e., diffusion coefficient, relative concentration, interaction radius), the gyration radius of APG, and the interaction energy between surfactant solution and CH4-containing coal were analyzed on the basis of the kinetic results. Results showed that with the increase in APG concentration, the diffusion coefficient of CH4 decreased gradually. The distribution of CH4 in coal molecules and the change rate of diffusion coefficient increased remarkably. When the concentration of APG was 0%, the diffusion coefficient was 12.00 × 10−7 m2 s−1, and that of 1.6% was 5.65 × 10−7 m2 s−1, a reduction of 52.91% compared with the concentration of 0%. With the increasing concentration of APG, the gyration radius increased, and the interaction energy between APG and CH4-containing coal molecules decreased. The CH4 diffusion coefficient was negatively correlated with the APG gyration radius, and the number of hydrogen bonds was positively correlated with the interaction energy. With the increase in the gyration radius of APG molecule, more hydrogen bonds were formed between APG surfactant and coal molecules, resulting in a decrease in the interaction energy. The system gradually stabilized to effectively inhibit the diffusion of CH4 in coal, which can provide a basis to prevent coal and gas outburst disasters.

Experimental study on the methane desorption-diffusion behavior of Longmaxi shale exposure to supercritical CO2

Desorption-diffusion behaviors of CH4 in shale after supercritical CO2 (ScCO2) injection are closely related to CO2 enhanced shale gas recovery (CO2-ESGR). A self-developed gas desorption test system was used to measure the desorption kinetics of CH4 in shale collected from Sichuan Basin at different ScCO2 exposure pressures (8, 12, 16 MPa) and temperatures (40, 60, 80 °C), and the low-pressure N2 adsorption was performed to evaluate the variations of pore structure of shale. Results indicate that the desorption kinetics curves of CH4 in shale exhibit typical Langmuir characteristics under different desorption pressures, and the kinetics data were fitted by unipore and bidisperse diffusion models, manifesting that the diffusion of CH4 in the shale samples are dominated by macroporous channel (>95%). With increasing desorption pressure, the desorption capacity of CH4 in shale (nde) increased, while the diffusion coefficient of CH4 in shale (D) decreased. Additionally, the nde and D increased after ScCO2 exposure, and exhibited a slight exposure pressure and temperature dependence, which is mainly caused by the decrease of specific surface area and the widening of microstructure in shale, implying that the ScCO2-shale interaction can promote the desorption-diffusion of CH4 in shale. This study provides a theoretical reference for CO2-ESGR.

Permeation Characteristics of CH4 in PVDF with Crude Oil-Containing.

The liner of reinforced thermoplastic composite pipes (RTPs) used for oil and gas gathering and transportation experienced blister failure due to gas permeation. Few reports have appeared on the problem of gas permeation in thermoplastics with absorbed crude oil. Accordingly, the permeability of CH4 in polyvinylidene fluoride (PVDF) containing crude oil was studied at the normal service conditions by molecular simulations. The results showed that the solubility coefficients of CH4 in PVDF containing crude oil were much lower than those in pure PVDF. It can be concluded that the crude oil molecules absorbed into PVDF occupied certain adsorption sites, resulting in a decrease in the adsorption capacity of CH4 molecules in PVDF. The diffusion coefficients of CH4 in oil-containing PVDF were significantly greater than in PVDF. This is because the absorption of oil molecules leads to the volume swelling of PVDF and then increases the free volume for diffusion. The permeation process showed that CH4 molecules were selective-aggregate adsorbed in the region with low potential energy in oil-containing PVDF firstly, and then they vibrated within the holes of PVDF containing oil in most cases and jumped into the neighboring holes at high temperatures and pressures.

Experimental study on kinetic characteristics of gas diffusion in coal under nitrogen injection

Gas extraction after the permeability of the coal seam is enhanced is an important direction of gas control to improve the pre-drainage rate of coal seam gas and shorten the pre-drainage time. Gas injection displacement is an effective method to enhance gas extraction. The interaction mechanism between gas molecules and gas containing coal in the process of gas injection to promote desorption and the kinetic effect of gas desorption diffusion need to be further improved. In this paper, the PCTPro high-pressure adsorption instrument and Trace 1300 gas chromatograph were used to conduct gas desorption experiments on coal-containing gas at different temperatures, and the kinetic characteristics and laws of gas desorption diffusion in the process of N2 injection were analyzed. The CH4 concentration gradually decreased with the decrease of the equilibrium pressure, and the N2 concentration showed a negative correlation with the equilibrium pressure. The variation of N2 and CH4 desorption capacity at different temperatures conforms to the power function curve, and N2 and CH4 desorption capacity increases with the decrease of equilibrium pressure. The CH4 diffusion coefficient ranges from 6.555 × 10−10 m2/s to 8.388 × 10−10 m2/s. The CH4 diffusion coefficient increases with increasing temperature. The timeliness of gas diffusion during gas injection was analyzed. The dynamic correction model of coal gas diffusion under the condition of gas injection is established and verified by experimental data with high stability and accuracy. The research results are of certain significance for clarifying the mechanism of N2 injection on gas containing coal and promoting the mechanism of gas desorption, providing a theoretical basis for the field application of N2 injection in coal seam to improve CH4 extraction rate, and realizing efficient and safe gas extraction in a low permeability coal seam.

Diffusion of guest molecules in coal: Insights from simulation

The ever-rising consumption of fossil fuels has led to dramatically increased CO2 emissions and notably global warming. Investigations of diffusion behaviors of small guest molecules such as CH4, CO2, N2, H2O, and O2 were critical for the relief the global warming, effective implementation of the injection of CO2 and N2 to enhance the CBM (coalbed methane) recovery (CO2-ECBM, N2-ECBM), as well as the prediction and prevention of coal spontaneous combustion. Herein, using the self-created coal vitrinite macromolecular representation, the self-diffusion coefficients and transport diffusion coefficients of CH4, CO2, N2, H2O, and O2 were clarified via MM (molecular mechanics), GCMC (grand canonical Monte Carlo), and MD (molecular dynamics) to clarify the impacts from different ensembles, as well as the pressure and temperature dependence. The self-diffusion mechanism was also discussed with the aid from the trajectory analysis. For the identified gas species and temperature, the self-diffusion coefficients (Ds) and transport diffusion coefficients (Dt) were higher for NPH (constant parameter: particle number, system pressure, thermodynamic enthalpy) and NPT (constant parameter: particle number, system pressure, temperature) ensemble than NVE (constant parameter: particle number, system volume, system energy) and NVT (constant parameter: particle number, system volume, temperature). For all ensembles, the DsH2O has always jumped up with the increasing temperature independent of the ensembles. The DtH2O were higher than CH4 and CO2 for NPH, NVE, and NVT ensembles. DtCH4 has steep increase points for NPT and NPH ensemble at high temperatures, resulting in the higher DtCH4 than DtCO2. However, DtCH4 was overall lower than DtCO2 for NVE and NVT ensemble at 298 ∼ 358 K. The diffusion activation energy increases with the increasing pressure, indicating that the diffusion barrier rises as the pressure increases. Also, the higher swelling deformation of H2O suggested that the water injection during the drainage and depressurization process should be reduced to achieve the successful ECBM engineering. The results in this paper verify the feasibility of ECBM and provides the innovative theory and technology for “carbon neutralization and carbon peak” target.

Impact of land use change on soil methane fluxes and diffusivity in Pampean plains, Argentina

Upland soils are the main methane (CH4) biological sink, and may be affected by land-use change. Changes in land uses and soil management affect soil properties that control diffusion of gases, which in combination with microbial activity, determine CH4 flux (fCH4) through the soil. Net CH4 fluxes and diffusivity -estimated by the CH4 diffusion coefficient- were measured in three common land uses typical from Pampean plains, South America (natural grassland NG; Eucalyptus globulus Labill. afforestation E; and agricultural land AL: oat, soybean and red clover in successive cultivation) during two years (March 2017–March 2019). Methane fluxes in the soil-atmosphere interface were measured using the static chamber technique, and a diffusion model was applied to estimate soil CH4 diffusivity from soil porosity. We aimed to quantify the effect of land use change (both E and AL vs. NG, the reference system) on fCH4 and gas diffusivity due to changes in the soil parameters. Soils were net sinks in the three land uses, with mean CH4 flux higher in the afforestation, intermediate in the natural grassland and lower in the agricultural land (− 10.99 ± 5.85, − 8.9 ± 5.32 and − 4.58 ± 4.19 ng CH4 m−2 s−1, respectively). CH4 fluxes varied significantly through seasons and space coinciding with variations in water-filled pore space and air-filled pore space variables (ρ > 0.7 and <−0.7 respectively; p < 0.05). Land-use change metric for methane flux ΔfCH4 was − 2.1 ± 3.7 and 4.4 ± 2.5 for NG-E and NG-AL, respectively, indicating a significant increment in net CH4 uptake when the natural grassland is afforested and a decrease when it was converted to agricultural use. This change was mainly explained by changes in soil physical properties (bulk density, soil water content, WFPS and air filled porosity). In relation to this, soil CH4 diffusion coefficient followed the same pattern as fCH4 (0.024 ± 0.011; 0.015 ± 0.007 and 0.008 ± 0.007 cm2 s−1 for E, NG and AL respectively); and allowed us to recalculate mean CH4 fluxes. Theoretical and in situ measured CH4 fluxes were similar and followed the same patterns across land uses, suggesting the possibility to determine CH4 fluxes by means of simple measures of soil properties (bulk density and soil water content) and soil CH4 gradient concentration.

Residual solvent induced physical morphology and gas permeation in polyamide-imide membrane: Experimental investigation and molecular simulations

The present study focuses on understanding various residual solvent effects on the physical morphology and separation characteristics of polyamide-imide (Torlon) membranes. The dense membranes were indigenously synthesized by phase inversion technique using various solvents including, N,N-dimethyl formamide (DMF), N,N-dimethyl acetamide (DMAc) and N-methyl pyrrolidone (NMP) respectively. The effect of various solvents on the physical morphology and separation characteristics were studied using a combination of both experimental techniques and atomistic simulations. The Hansen solubility and Floury-Huggins parameters were used to understand the polymer–solvent interactions and further extended to correlated to separation characteristics. Scanning electron microscope (SEM) and atomic force microscopy (AFM) methods were used to investigate the resulted physical morphology of the membranes. Thermogravimetric analysis (TGA) and 1H NMR spectra revealed residual solvent, hydrogen-bonding interactions with the Torlon polymer membrane. The experimental gas permeation studies demonstrated that the CO2 permeability in Torlon membranes follows the order T-NMP > T-DMAc > T-DMF. However, the membrane with the T-DMAc solvent exhibited higher ideal selectivity of 34.65 at 12.5 bar for CO2/CH4 separation due to the lower diffusion coefficient of CH4 in the presence of residual DMAc. The predicted solubility and diffusion coefficients of CO2 and CH4 were shown to be in good agreement with the experimental results. Additionally, structural analysis of simulated models provided further insights into residual solvent effects on the microscopic properties and gas permeation. Overall, the Torlon membrane with the residual DMAc solvent showed positive effects on the transport properties and in turn enhance the diffusive selectivity.

Molecular Simulation of the Adsorption Characteristics of Methane in Pores of Coal with Different Metamorphic Degrees.

In order to study differences in the methane adsorption characteristics of coal pores of different metamorphic degrees, 4 nm pore structure models based on three typical coal structure models with different metamorphic degrees were constructed. Based on the molecular mechanics and dynamics theory, the adsorption characteristics of methane in different coal rank pores were simulated by the grand canonical Monte Carlo (GCMC) and molecular dynamics methods. The isothermal adsorption curve, Van der Waals energy, concentration distribution, and diffusion coefficient of methane under different conditions were analyzed and calculated. The results showed that at the same pore size, the adsorption capacity of CH4 is positively correlated with pressure and metamorphic degree of coal, and the adsorption capacity of CH4 in high metamorphic coal is more affected by temperature. The relative concentration of CH4 in high-order coal pores is low, and the relative concentration at higher temperature and pressure conditions is high. The CH4 diffusion coefficient in high-rank coal is low, corresponding to the strong Van der Waals interaction between CH4 and coal. The research results are of great significance for further exploration of the interaction mechanism between CH4 and coal with different metamorphic degrees and can provide theoretical support for the selection of gas extraction parameters.

Molecular simulation of N2 and CO2 injection into a coal model containing adsorbed methane at different temperatures

To research the dynamic mechanism of nitrogen and carbon dioxide displacement of methane, we used the grand canonical Monte Carlo (GCMC) simulation method to determine the lowest energy coal model containing adsorbed methane. The desorption behavior of CH4 after the injection of N2 and CO2 at different temperatures was studied. Results show that CO2 and N2 were mainly used to drive off methane gas by occupying adsorption sites. The total energy of the CH4-CO2 model was lower than that of the CH4-N2 model. With the increased of temperature, the average relative concentration and motion velocity of CH4 in the vacuum layer increased. The relationship of the average relative concentration and average velocity distribution of the three gases in the vacuum layer was CH4>CO2>N2. Under the same time conditions, the relationship between the mean square displacement and diffusion coefficient of CH4, CO2, and N2 in different models was CH4>CO2>N2, and they all increased with temperature. The diffusion activation energy of CH4 in the model injected with CO2 was reduced by 20.53%, and the effect of injecting CO2 to promote the desorption of methane was better than that of N2.

Transport mechanisms and emission of landfill gas through various cover soil configurations in an MSW landfill using a static flux chamber technique

This study evaluated the transport mechanisms and emission rates of landfill gas (LFG) from 200- (vegetated with short grass), 300- (vegetated with short grass), and 450-mm-thick (non-vegetated) interim cover soils within a municipal solid waste landfill. LFG emission and diffusion mechanisms were evaluated using static flux chambers and laboratory-scale diffusion columns. Overall, the greatest CH4 and CO2 emissions were consistently observed from the 200-mm-thick cover soil with an average flux rate of 39.2 mg m−2 h−1 and 3.07 × 103 mg m−2 h−1, respectively. In addition to CH4 and CO2, H2S migration through a 450-mm interim cover soil was also evaluated. The H2S emission rate was relatively more uniform at an average of 2.47 × 10−5 mg m−2 h−1. Long-term LFG emission was predicted using an emission model based on a first-order decomposition rate equation and compared with the static flux chamber method. The field-measured CO2, CH4 and H2S emissions were less than the estimated emissions from the emission model, by 22%, 85%, and 91%, respectively. Further, the diffusion coefficients of CH4, CO2, and H2S for the interim cover soils were determined using a laboratory-scale diffusion column test and compared with a three-parameter diffusion model. The measured and estimated diffusion coefficients for the three landfill gases were within the 10% variation limits. Based on these findings, the LFG emission rate varied depending on the physical-chemical properties of the cover soil (e.g., cover thickness, moisture content, compaction ratio, uneven distribution of soil), organic material content and age of buried refuse, and seasonal environmental conditions (such as temperature). Test results showed that fugitive CH4 emissions can be reduced one fourth by utilizing an appropriate cover soil (300-mm to 450-mm, CL) compared to cases with a thinner cover soil.

Gas supersaturation and diffusion joint controlled CH4 nanobubble evolution during hydrate dissociation

Nanobubbles have attracted wide scientific attention due to their large number of potential applications in nanotechnology, and they are ubiquitous during natural gas hydrates (NGHs) mining which is one of the strategic energy resources. Molecular dynamic simulations were performed to investigate CH4 nanobubble characteristics during NGHs dissociation. The nucleation mechanism and evolution characteristics of nanobubbles vary with hydrate dissociation temperature. The higher the temperature, the earlier the bubble nucleation, the more small methane clusters are produced, and the more complex the evolution path of nanobubbles, the initial position of bubble nucleation is closer to the decomposition interface. The gas reservoir can provide another possibility for methane molecules to overcome solubility barriers to avoid the formation of nanobubbles at lower dissociation temperature. CH4 nanobubble evolution was found to be governed by the gas supersaturation and the diffusion of CH4 molecules. The self-diffusion coefficient of CH4 molecules near the decomposition interface area is small due to the adsorption effect of the incomplete cages, nanobubbles are more likely to form outside the adsorption zone where the CH4 diffusion coefficient is larger at a lower temperature. A direct proof of speeding up of hydrate dissociation by nanobubble growth was given. The acceleration of dissociation is later than nanobubble growth and the accumulation of nanobubbles on dissociation interface may have a negative influence. The new findings on nanobubble evolution may help design and optimize nanobubble formation techniques in nanotechnology and efficient gas hydrate production protocols.