Gas Adsorption Model Research Articles

Multimechanism of Gas Transportation in Micro–Nanoscale Pores of Coal Seam: An Advanced Model of Gas Adsorption and Diffusion

Multimechanism of Gas Transportation in Micro–Nanoscale Pores of Coal Seam: An Advanced Model of Gas Adsorption and Diffusion

Thermodynamically consistent modeling of gas flow and adsorption in porous media

In modeling of gas flow through porous media width adsorption, the thermodynamic properties of the adsorbed phase are usually approximated by those of the bulk liquid. Using non-isothermal, gaseous transport of moist air through a porous insulation material as example, we show that this leads to violation of the second law of thermodynamics and a negative entropy production. To resolve this violation, we use information about the adsorption and thermodynamic properties of bulk fluids to derive consistent thermodynamic properties of the adsorbed phase, such as the chemical potential, enthalpy and entropy. The resulting chemical potential of the adsorbed phase is a starting point for rate-based models for adsorption based on non-equilibrium thermodynamics. Incorporating the consistent thermodynamic description into the energy, entropy and momentum balances restores agreement with the second law of thermodynamics. We show that the temperature evolution in the porous medium from the consistent description differs from the standard formulation only if the adsorption depends explicitly on temperature. This highlights the importance of characterizing the temperature dependence of the adsorption with experiments or molecular simulations for accurate non-isothermal modeling of porous media.

3D nanostructure prediction of porous carbons via gas adsorption

Structural characterization of porous carbon materials is critical for the evaluation of their synthesis procedures and performance. Throughout the last decades, many methods have been employed to determine porosity properties from gas adsorption such as surface area, pore size distribution (PSD) and real density. However, gas adsorption models use 1D structures of carbon nanopores, although adsorption and separation properties of nanoporous carbons are governed by 3D pore parameters. Estimating the 3D nanostructure of nanoporous carbons using gas adsorption would accelerate progress in research and implementation of nanoporous carbons. We report here a promising 3D pore nanostructural characterization from gas adsorption. Using atomistic simulations, we have generated a database of realistic 3D porous carbon structures spanning a wide range of pore sizes and geometries. After calculating their gas adsorption isotherms, we employed a numerical procedure to find the relative contribution for each of the structures to the adsorption isotherm of a nanoporous carbon sample. These contributions allowed us to estimate the surface area and pore size distribution of carbon materials; moreover (and perhaps more importantly!), we will show that the plausible 3D pore structures correlate very well with the local carbon structure as experimentally determined by high-resolution TEM observations and can successfully predict adsorption of different molecules. This is a powerful procedure that can be extended to other materials, and with enough computer power, to larger pore sizes.

Development and application of gas adsorption model for coal based on particle flow code

This study investigated the deformation characteristics and engineering properties of gas-bearing coal. Based on the particle flow discrete element method, the gas was regarded as a flexible solid particle, and the relationship between the radius, elastic modulus and gas pressure of the flexible gas particle was derived through the ideal gas equation. Then, a gas adsorption model for coal based on the Langmuir equation was developed by coupling gas particles with rigid coal particles and applied to the coal and gas outburst evolution process. The results revealed that the flexible gas particles can effectively simulate the flow and deformation of gas, and the gas adsorption model exhibits adsorption deformation characteristics consistent with those of real coal specimens. Moreover, the adsorption model can indicate the adsorption saturation of coal particles at any moment, and the distribution characteristics of gas pressure within the microscopic pores, which is helpful to study the microscopic mechanism of gas adsorption process. Finally, the gas adsorption model for coal was applied to the coal and gas outburst and good simulation results were obtained, which further validated the reasonableness of the gas adsorption model. These studies promote the application of the particle method in the direction of gas-solid coupling research, and also provide a theoretical and numerical basis for the prevention and control of gas disasters.

Minireview on Lattice Boltzmann Modeling of Gas Flow and Adsorption in Shale Porous Media: Progress and Future Direction

As an important unconventional resource, shale gas reservoirs have unique characteristics different from conventional oil and gas reservoirs. The ultrasmall pore sizes in shale induce the nanopore confinement effect on shale gas flow. In addition, shale rocks are rich in organic matter, which has strong interactions with gas molecules and results in gas adsorption. The lattice Boltzmann method (LBM) for micro- and nanoscale gas flow, which is originally designed for micro-electro-mechanical systems (MEMS), has been modified to simulate gas flow and adsorption in shale rocks. This work reviews four types of lattice Boltzmann models developed recently for shale gas flow/adsorption: (1) the slip-velocity-based LBM, (2) high-order LBM, (3) diffusion-based LBM, and (4) REV-scale LBM. Among these models, the slip-velocity-based LBM is widely used for shale gas modeling, which incorporates the slip boundary condition and Knudsen number (Kn)-determined relaxation time to simulate the nanopore confinement effect. To model the gas adsorption, the fluid–solid interaction force is introduced into the model, and the magnitude of this interaction force is usually obtained from the molecular level simulations. LBMs have been regarded as an efficient numerical tool to estimate the shale gas apparent permeability, to describe the pore-scale flow behaviors, and to address the influence of gas adsorption on shale gas storage and transport. Nevertheless, some challenges exist in current applications of LBMs for shale gas flow and adsorption simulations that are discussed in this minireview as well.

The influencing factors of gas adsorption behaviors in shale gas reservoirs

The adsorption state is one of the main states for shale gas occurrence, and the gas adsorption behavior in shale directly affects shale gas content under reservoir conditions. This paper provides a comprehensive literature review on shale gas adsorption behavior and its affecting factors that have been developed in recent years. Influence factors of gas adsorption behavior are examined, including total organic carbon content (TOC), organic matter type, organic matter maturity, minerals and clay minerals, moisture content, pore characteristics and other characteristics of the shale itself. The characteristics of gas adsorption behavior under high temperature and pressure conditions showed that adsorption behaviors were difficult to describe by the Langmuir equation. This review indicates that shale contains higher organic matter content and organic matter maturity and has a higher adsorption capacity. The adsorption capacity with type III kerogen is higher than that for type II or type I. Clay minerals can provide free space for gas adsorption and promote adsorption. Normally, as the moisture content increased, adsorption capacity decreased. Micro pores provided a larger specific surface area for gas adsorption. As the temperature increased, the adsorption capacity decreased. As the pressure increased, shale adsorption characteristics showed two different behaviors as follows: one obeyed the Langmuir equation, and the other presented an inverted, U-shaped, single-peak distribution. However, there are some controversies surrounding adsorption, especially regarding the aspects of clay minerals, water content, pore characteristics, etc. The key is that the mechanism of adsorption in shale is unclear. There will be many new challenges in the field of shale gas adsorption research. Such challenges include studying the organic matter chemical structure, understanding the interaction between organic matter and clay minerals and how they affect adsorption, clarifying gas adsorption behavior changes, predicting favorable areas of adsorbed gas with the coupling of reservoir temperature and pressure, and building a better theory and model of shale gas adsorption.

Study on the Detection Characteristics and Response Mechanism of SnS2-Based Sensors for SO2 and SOF2

Based on the wide application of sulfur hexafluoride (SF6) gas in the power industry, the analysis and detection of its decomposition components have become important technical means for state detection and fault evaluation of gas-insulated equipment. The gas-sensitive characteristics and adsorption mechanism of the SnS2 sensor for SO2 and SOF2 gases were investigated using SO2 and SOF2, the main SF6 decomposition components, as the target detection gases. SnS2 gas-sensitive materials and components were prepared, and the temperature response, concentration response, response recovery and stability of the SnS2 sensor for the two SF6 decomposition components were tested based on the gas-sensitive test platform. The results demonstrate that the sensor had the best working performance at 200 °C, with obvious response and ideal recovery for both target gases and good stability in a certain time. Based on the first principle, the SnS2 surface structure model and the target gas adsorption model were established, and the adsorption mechanism was analyzed in terms of frontier molecular orbital theory to verify the correctness of the gas-sensitive test results. The gas-sensitive test analysis and simulation calculation can provide data basis and theoretical support for the study of SF6 decomposition components detected by gas sensors.

Noise spectroscopy based numerical modelling of chemisorption on SnO2 surface for CO gas sensing applications

The gas sensor response of the tin oxide (SnO2) semiconductor toward carbon monoxide (CO) gas has been simulated using the noise spectroscopy technique. The Wolkenstein gas adsorption model is used to calculate the surface coverage of environmental oxygen ions and target gas molecules. The interaction of CO gas molecules with the negatively charged oxygen ions has been incorporated in the numerical model. The equilibrium charge neutrality condition of the semiconductor material has been solved as a function of gas adsorption parameters like surface coverage, temperature, pressure and surface potential. Eventually, the power density spectrum (PDS) of the electrical conductance of the sensing layer was calculated before and after the gas adsorption conditions. It has been observed that the semiconductor band bending increases sharply at a certain concentration of environmental oxygen gas concentration (10−4 to 10−3 atm). At a temperature of 480 K, a steep decrease in surface potential was observed. This temperature range of maximum sensor response was optimized for calculating noise spectrum of electrical conductivity fluctuations at equilibrium condition of adsorption-desorption. The cutoff frequency (109 Hz) was calculated using normalized PDS of electrical conductance. The gas sensor response was calculated as a ratio of film conductance before and after the gas adsorption as a function of gas pressure and surface temperature at the calculated cutoff frequency. The increase in sensor response was observed with the increase in CO partial pressure as a function of temperature for a specific cut off frequency. The comparison of sensor response of the presented model has been demonstrated with another published numerical model. The presented model shows improved sensor response for higher (>10−8 atm) CO partial pressure.

Comparative DFT dual gas adsorption model of ZnO and Ag/ZnO with experimental applications as gas detection at ppb level

By experimental and density functional theory calculations, the toxic gases (O3 and NO2) sensing capability and mechanism of ZnO NRs and Ag/ZnO NRs have been comparatively studied in this work. Ag NPs arrays were employed for the growth of ZnO NRs. The experimental results show that when ZnO NRs are grown on Ag NPs, the response and adsorption rate towards the gases change significantly. The TDOS plot shows that the HOMO-LUMO gap changes after interaction with different oxidizing gases, and the peak intensity also decreases confirming the electron are transferred from ZnO to NO2 and O3. The response to gases decreases and the adsorption reaction rate increases in Ag/ZnO NRs, as calculated by the Eyring-Polanyi equation, which is very similar to our experimental data. We also find that the absorption coefficient is different for O3 and NO2. Finally, experimental response and theoretical results were compared and found to be in good agreement.

Microporous structure and gas adsorption model of fusain in lignite

Fusain is widely distributed in coal seams and studying its porosity and adsorption capabilities helps clarify the physical properties of coal reservoirs and gas adsorption mechanisms. The microporous structure characteristics and gas adsorption mechanism of fusain in lignite were studied based on methane and CO2 adsorption experiments, combined with nuclear magnetic resonance (13NMR) and high-resolution transmission electron microscopy (HRTEM) measurements. Numerous micropores were found in the sample, and the pore volume and specific surface area were relatively large, which is similar to the results of the micropore test for meta-anthracite. The adsorption capacity of fusain was higher under dry base conditions but significantly lower under air-dry base and equilibrium water conditions. The chemical structure analysis results indicate that the micropores of fusain are mainly the interlamellar pores of the aromatic layers and the pore channels formed by them. Under dry conditions, the water in the microporous structure was removed, which provides an effective space for gas adsorption, resulting in more micropores for CO2 adsorption and a higher methane adsorption capacity. Under wet conditions, the water occupies the adsorption site of methane, which hinders the gas entering the microporous channel of the fusain, resulting in a low adsorption capacity. On comparing the pore structure characteristics of fusain and meta-anthracite, it was found that despite possessing similar chemical structure characteristics of high polycondensation and similar micropore structure, there were clear differences in pore size, and their adsorption properties for methane were different.

Adsorption of CO2, N2 and CH4 on a Fe-based metal organic framework, MIL-101(Fe)-NH2

MIL-101(Fe)-NH2 was synthesized using solvothermal method to assess the selective adsorption of CO2 over N2 and CH4. The BET surface area of adsorbent sample was 915 m2.g-1. The adsorption capacities of CO2, N2 and CH4 on MIL-101(Fe)-NH2 at 4 MP and 298 K were achieved equal to 13, 3.8 and 5 mmol.g-1 respectively. Khan adsorption equilibrium model was applied to correlate the adsorption isotherms of pure gases. In addition to Khan model, the adsorption isotherm of CO2, which showed type II behavior, was also obtained by BET model as well as a newly presented hybrid model, which resulted in excellent agreement with the experimental data. Moreover, the selectivity of CO2 over N2 and CH4 was estimated by Ideal Adsorbed Solution Theory (IAST). Comparison of the achieved results from IAST method with experimental data verified the accuracy of this method in the lower pressure. The selectivity coefficient was about 2.2 for CO2 over N2 and CH4 at 16 bar. The achieved results from a classical micropore diffusion model showed that the diffusion coefficient of the pure gases was approximately constant with pressure. Moreover, the estimated values for diffusion time constants (DC/rC2 × 103) for CO2, N2 and CH4 were obtained equal to 1.178, 1.118, and 0.802 respectively. The adsorption kinetic data were also correlated with empirical models. Ultimately, the results showed that pseudo-nth order empirical model achieved the best agreement with the experimental data, and it can be successfully applied for kinetic modeling of gas adsorption by MIL-101(Fe)-NH2 MOF.

Statistical Mechanical Model of Gas Adsorption in a Metal-Organic Framework Harboring a Rotaxane Molecular Shuttle.

Metal-organic frameworks (MOFs) are modular and tunable nanoporous materials with applications in gas storage, separations, and sensing. Integrating flexible/dynamic, gas-responsive components into MOFs can give them unique or enhanced adsorption properties. Here, we explore the adsorption properties that could be imparted to a MOF by a rotaxane molecular shuttle (RMS) in its pores. In the unit cell of an RMS-MOF, a macrocyclic wheel is mechanically interlocked with a strut of the MOF scaffold. The wheel shuttles between stations on the strut that are also gas adsorption sites. At a level of abstraction similar to the seminal Langmuir adsorption model, we pose and analyze a simple statistical mechanical model of gas adsorption in an RMS-MOF that accounts for (i) wheel/gas competition for sites on the strut and (ii) gas-induced changes in the configurational entropy of the shuttling wheel. We determine how the amount of gas adsorbed, the position of the wheel, and the differential energy of adsorption depend on temperature, pressure, and the interactions of the gas and wheel with the stations on the strut. Our model reveals that, compared to a rigid, Langmuir material, the chemistry of the RMS-MOF can be tuned to render gas adsorption more or less temperature sensitive and to release more or less heat upon adsorption. The model also uncovers that, if gas-wheel competition for a station is fierce, temperature influences the position of the wheel differently depending on the amount of gas adsorbed.

Gas Adsorption and Controlling Factors of Shale: Review, Application, Comparison and Challenges

The selection of an optimized gas adsorption model is a quantitatively challenging task when characterizing shale gas reservoirs because every model has its own assumptions and mechanisms. In addition, excess when converting different models to absolute gas adsorption is reported in the literature, which differs due to variations in adsorbed phase density or volume. The roles of controlling factors change under various conditions and consequences, and it is hard to comprehend shale storage and production sources. Quantifying the contribution of adsorbed and absorbed gases to total gas storage is also challenging despite existing advanced techniques. This paper reviews gas adsorption in the light of applications, comparisons and challenges along with shale controlling factors, and it provides a better opportunity to comprehend shale storage and platforms to investigate further mechanisms of shale gas storage. It is revealed in this review that several crucial factors are found in shale that control and affect the gas adsorption capacity. Kerogen swelling and pore shrinkage, kerogen maturity, gas type and moisture content greatly influence gas adsorption capacity. The measurement of true adsorbed and bulk amount, selection of an optimized gas adsorption model, proper investigation of shale controlling factors, and accurate measurement of adsorbed and absorbed gases may enhance the accuracy of resource assessment and production performance predictions.

High-Performance Supercomputer Technologies of Simulation of Nanoporous Feedback Systems for Adsorption Gas Purification

High-performance methods and computer technologies for modeling of non-isothermal gas adsorption in nanoporous cyber-feedback systems based on the Heaviside operating method and Landau’s linearization approach to Langmuir’s nonlinear adsorption equilibrium using decomposition in a convergent series at the temperature transition point with subsequent efficient parallelization of the model over a small parameter are developed. The results of numerical experiments based on high-speed parallel computations on multicore computers are presented.

Ammonia sensing properties of metal–organic frameworks-derived zinc oxide/reduced graphene oxide nanocomposite

This work reported a high-performance room temperature ammonia (NH3) gas sensor, which was prepared by metal–organic frameworks (MOFs)-derived zinc oxide (ZnO)/reduced graphene oxide (rGO) nanocomposite. The sensor was prepared by depositing MOFs-derived ZnO/rGO film on an epoxy substrate with interdigital electrodes (IDEs). The microstructure and elementary composition of ZnO/rGO nanocomposite were measured and analyzed by using XRD, SEM, XPS and TEM measurements. The sensing performance of the prepared sensor was measured over a range of 0.5–30 ppm of ammonia gas. According to test results, the MOFs-derived ZnO/rGO sensor has a better sensing performance toward ammonia gas detection than that of ZIF8-ZnO and rGO sensors. The sensor exhibits high response (6.46@30 ppm), quick response/recovery time (50 s/25 s), low detection limit (0.5 ppm), excellent stability and well selectivity when the sensor was exposed to ammonia. The response–concentration curve shows good linear relationship (Y = 0.910 + 0.186X) within a certain range (0.5–30 ppm). Finally, the excellent performance of the sensor is explained by analyzing surface structure of MOFs ZnO/rGO and ammonia gas adsorption model.

Molecular Modeling of Ammonia Gas Adsorption onto the Kaolinite Surface with DFT Study

With high porosity and being one of the most abundant clay minerals, dried kaolinite may be an excellent adsorbent to remove ammonia gas (NH3). Here, the plane wave pseudopotential method based on density functional theory (DFT) was used to explore the mechanism of ammonia gas adsorption on the dried kaolinite, the Mulliken electric charge, and the partial density of states of atoms of the NH3/kaolinite (001) system. NH3 adsorption on kaolinite can happen in three different type adsorption positions: “top”, “bridge” and “hollow”. The “hollow” position is enclosed by two "upright" hydroxyl groups perpendicular to the (001) surface of kaolinite and a "lying" hydroxyl group parallel to the surface. At this position, the adsorption is the most stable and has the highest adsorption energy. The nitrogen atom of the NH3 molecule bonds with the hydrogen atom in the "upright" hydroxyl group on the (001) surface and its hydrogen atom forms HN…O hydrogen bond with oxygen atom in the "lying" hydroxyl group, which leads to the NH3 stably adsorbed on kaolinite (001) surface. A small part of electrons transfer between NH3 molecules and kaolinite creates weakly electrostatic adsorption between them.

Effects of surface roughness and derivation of scaling laws on gas transport in coal using a fractal-based lattice Boltzmann method

Unconventional energy such as coal seam gas is trapped in a low porosity/permeability environment and poses challenges for production. Of particular interest is the mode of microscale gas transport through so-called coal cleats which form a network of micro-channels/fractures and allow for gas transfer through impermeable coal seams. The problem has attracted interest from many fields such as microstructural characterization, permeability-, porosity-, fluid flow-, diffusion-, adsorption/desorption- and adsorption-induced deformation studies. The main mode of gas flow is supported by the cleat network pattern for which the surface roughness is a very significant property of gas transport. Roughness plays an important role because adsorption is very sensitive to the change in the area. Direct studies on the effects of surface roughness for methane migration in coal by experimental methods are difficult to perform because the micro-scale heterogeneous pore structure and methane sorption properties are difficult to observe and therefore robust numerical simulations provide the most comprehensive method of investigation. In this paper, we develop a numerical model of coal seam gas diffusion and adsorption considering surface roughness based on fractal statistics and the lattice Boltzmann method. The rough surface is characterized by a 3D-Laser profiler and the fractal dimension of the cleat surface is quantified. Generic variants of the measured rough profiles are generated by the Weierstrass-Mandelbrot function through altering the fractal dimension D and length-scale parameter G. The fractal-based lattice Boltzmann method is introduced to solve the governing equations for the gas flow and diffusion processes and scaling laws may be a standardized analysis method and are applied for the non-dimensionalization of gas adsorption. By considering D ranging from 1.5 to 1.8 and G ranging from 1 to 20 it is found that the fractal dimension D has a small effect on the flow but a significant effect on the gas diffusion. In addition, the length-scale parameter G significantly affects gas transport through varying characteristic cleat aperture, and gas diffusion and adsorption by changing the contact area. The data rescaled by scaling laws provide a better analysis of gas adsorption through simplifying the assessment of the effects of multiple variables. Results of this work allow direct quantification of the effect of surface roughness on gas diffusion and adsorption in coal cleats and can provide an improved assessment of the effect of microscopic mechanisms of gas transfer/adsorption in coal on the overall large recovery of gas in the reservoir.

Adsorption of active trace gases by ensemble of ultrafine porous particles with impermeable cores

In the present study, we analyze the combined influence of several factors on the rate of atmospheric trace gas adsorption by an ensemble of porous aerosol particles. The factors include the presence of a non-porous non-adsorbing inner core, kinetic effects, non-uniform distribution of concentration of adsorbed gas inside particles, inert admixtures in the atmosphere, radioactive decay, and particle number density. The particles size has the same order of magnitude as a free path length of molecules in air. The evolution of concentration of adsorbed active trace gas in solid porous particles is described by an integral equation, while the evolution of active trace radioactive gas in a gaseous phase is determined by an integro-differential equation. Numerical calculations are performed to investigate the adsorption of different isotopes of Radon and Iodine by porous particles. The comparison of rates of gas adsorption for radioactive isotopes of Iodine with a stable isotope I-127 shows that radioactive decay alters the scenario of the gas adsorption process by porous particles. The accuracy of the developed model of radioactive gas adsorption by porous particles is validated by available experimental data.

Hierarchical Nanoheterostructure of Tungsten Disulfide Nanoflowers Doped with Zinc Oxide Hollow Spheres: Benzene Gas Sensing Properties and First-Principles Study.

This paper reports an original fabrication of a benzene gas sensor based on tungsten disulfide nanoflowers (WS2 NFs)/zinc oxide hollow spheres (ZnO HMDs) hierarchical nanoheterostructure. The ZnO/WS2 hierarchical composite was characterized for the inspection of its nanostructure, elementary composition, and surface morphology. The benzene-sensing properties of the ZnO/WS2 nanofilm sensor were exactly investigated. The results illustrate that the ZnO/WS2 sensor exhibits a remarkable sensing performance toward benzene gas, including good sensitivity, rapid detection, outstanding repeatability, and stability. This is attributed to the fact that the ZnO/WS2 nanoheterostructure can dramatically enhance the benzene sensing performance. Furthermore, density functional theory was employed to construct the benzene gas adsorption model for the ZnO/WS2 heterostructure, from which the determined parameters in geometry, energy, and charge provided a powerful support for the mechanism explanation. This work suggests that the ZnO/WS2 nanoheterostructure is competent to detect trace benzene gas at room temperature.

Application of temperature-dependent adsorption models in material balance calculations for unconventional gas reservoirs

Langmuir isotherm is the most common adsorption model used in the prediction of gas adsorption in most shale and coal bed methane reservoirs. However, due to the underlying assumption of single temperature, it fails to model gas adsorption where temperature differential exists in the reservoir. To address this shortcoming, temperature-dependent gas adsorption models have been incorporated into material balance calculations for accurate prediction of original gas in place as well as determining both average reservoir pressure and future performance in coal/shale gas reservoirs. The material balance equation has been expressed as a straight line with both Bi-Langmuir and Exponential models used in prediction of gas adsorption rather than the Langmuir isotherm. With this methodology, several adsorption capacities can be obtained at multiple temperatures which will allow for better estimation of original gas in place and future gas production. The results from this works show that temperature-dependent gas adsorption models can be used in place of Langmuir isotherm to account for the effect of temperature variations and more accurate representation of the adsorption of gas in coal/shale gas reservoirs.