Figure 6 - uploaded by Aleksander Bródka
Content may be subject to copyright.
Diffusion coefficient D as a function of linear density of methane, , for (a) rigid and (b) flexible nanotube.
Source publication
The behaviour of methane molecules inside carbon nanotubes at room temperature is studied using classical molecular dynamics simulations. A methane molecule is represented either by a shapeless super-atom or by a rigid set of five interaction centres localized on atoms. Different loadings of methane molecules ranging from the dense gas density to t...
Contexts in source publication
Context 1
... diffusion coefficient values for CH 4 in the rigid CNT and flexible one for both methane models are presented in Figure 6. In all cases under consideration the diffusion coefficients obtained from Equations (6) and (7) are very similar, and hence we show only results obtained from velocity autocorrelation func- tion, and the coefficient errors were estimated using results obtained from the consecutive MD simulation runs. ...
Context 2
... other words, the realistic model of the methane molecule becomes important at low densities. Moreover, comparing results presented in Figure 6(a) and (b) one may conclude that the diffusivity of methane molecules in a flexible nanotube differ very little from that for a rigid one, suggesting that the vibration of the nanotube carbon atoms which interact through the REBO potential has a little influence on translational motion of methane molecules. ...
Similar publications
We have used Molecular Dynamics simulations to investigate the structure and dynamics of TIP4P/2005 water confined inside nanotubes. The nanotubes have distinct sizes and were built with hydrophilic or hydrophobic sites, and we compare the water behavior inside each nanotube. Our results shows that the structure and dynamics are strongly influenced...
Citations
... Employing the Steele potential [21] with LB mixing rules is a common approach in describing fluid/graphene-like interfaces [9,[22][23][24][25][26][27][28][29][30]. Other studies [18,20,31] employed the same mixing rules but considered different CNT potentials, such as the AMBER96 [8,19] . ...
Carbon nanotubes and graphene are promising nanomaterials to improve the performance of current gas separation membrane technologies. From the molecular modeling perspective, an accurate description of the interfacial interactions is mandatory to understand the gas selectivity in these materials. Most of the molecular dynamics simulations studies considered available force fields with the standard Lorentz-Berthelot (LB) mixing rules to describe the interaction among carbon dioxide (CO$_2$), methane (CH$_4$) and carbon structures. We performed a systematic study in which we showed the LB underestimates the fluid/solid interaction energies compared to the density functional theory (DFT) calculation results. To improve the classical description, we propose a new parametrization for the cross-terms of the Lenard-Jones (LJ) potential by fitting DFT forces and energies. The obtained model enhanced fluid/carbon interface description showed excellent transferability between single-walled carbon nanotubes (SWCNTs) and graphene. To investigate the effect of the new parametrization on the gas structuring within the SWCNTs with varying diameters, we performed Grand Canonical Monte Carlo (GCMC) simulations. We observed considerable differences in the CO$_2$ and CH$_4$ density within SWCNTs compared to those obtained with the standard approach. Our study highlights the importance of going beyond the traditional Lorentz-Berthelot mixing rules in the studies involving solid/fluid interfaces of confined systems.
... 14 It was found that when the external pressure reaches 5 MPa, the adsorption isotherms for bulk methane is about 1.5 mmol cm À3 , whereas it is about 10 mmol cm À3 in nanopores. Bartus et al. 16 studied the behavior of methane molecules inside rigid and exible carbon nanotubes (CNTs) at room temperature through classical MD simulations and obtained that the diffusion coefficient in rigid and exible CNTs are similar. Mahdizadeh et al. 17 used grand canonical Monte Carlo (GCMC) simulations to investigate the methane adsorption in single-walled carbon nanotubes (SWCNTs) and their results indicate that SWCNTs can be stylized for methane adsorption. ...
In this work, we investigate the release of methane in quartz nanochannels through the method of displacement using carbon dioxide. Molecular dynamics (MD) simulations and theoretical analysis are performed to obtain the release percentage of methane for nanochannels of various diameters. It is found that both the pressure of CO2 and the channel size affect the release percentage of methane, which increases with increasing pressure of CO2 and channel diameter. Without CO2, the majority of methane molecules are adsorbed by the channel surface. When CO2 is injected into the channel, CO2 molecules replace many methane molecules due to the relatively strong molecular interactions between CO2 and the channel, which leads to the desorption of methane, reduces the energy barrier for the transport of methane, and consequently increases the release rate. Theoretical predictions using the kinetic energy of methane and the energy barrier inside the channel are also conducted, which are in good agreement with the MD simulations. This journal is
... 53 Because of hydrogen atoms in methane molecules, the time step is 4 times shorter than that used previously for rigid molecules. 27,28 Moreover, such a reduction of time step decreases frequency of the velocity scaling described above, which is illustrated in Figure S1 of the Supporting Information. The MD run lengths presented in Table 1 give approximately zero value of the velocity and angular velocity correlation functions after half of the simulation run length. ...
We report structural and dynamic properties of methane inside the (15,15) carbon nanotube (CNT) obtained from molecular dynamics simulations of flexible methane molecules, and intramolecular interactions are introduced by the reactive empirical bond order potential. The calculations are performed for wide range of temperature and loading, that corresponds to states from the dense gas phase to the liquid state. The properties of flexible and rigid models of methane molecules are compared. The diffusivity of molecular translations along the CNT and rotations increase with temperature and they decrease with pressure. Temperature dependences of the diffusion coefficients for flexible molecules are predicted by the Arrhenius equation. Internal motions of the CH4 atoms diminish the activation energies of the translational diffusion, and increase the energies of the rotational diffusion, especially for higher pressures. The results mean that possibility of changes of molecular bond lengths and valence angles in methane molecules causes a reduction of hindrances of their translations and at the same time it leads to the increase of rotational motion interruptions.
... As the insoluble organics in shale, MD method to choose carbon nanotubes in place of the shale organics. The adsorption law of the mixtures was predicted by the IAS adsorption model, and a new equation was proposed to describe the adsorption phase densities of the mixtures [35,36]. Billemont et al. studied the adsorption law of CO 2 , CH 4 and their mixtures by combining the experimental and molecular simulation methods and taking into account the adsorption law of gas under water preload conditions [37]. ...
As kerogen is the main organic component in shale, the adsorption capacity, diffusion and permeability of the gas in kerogen plays an important role in shale gas production. Based on the molecular model of type II kerogen, an organic nanoporous structure was established. The Grand Canonical Monte Carlo (GCMC) and Molecular Dynamics (MD) methods were used to study the adsorption and diffusion capacity of mixed gas systems with different mole ratios of CO 2 and CH 4 in the foregoing nanoporous structure, and gas adsorption, isosteric heats of adsorption and self-diffusion coefficient were obtained. The selective permeation of gas components in the organic pores was further studied. The results show that CO 2 and CH 4 present physical adsorption in the organic nanopores. The adsorption capacity of CO 2 is larger than that of CH 4 in organic pores, but the self-diffusion coefficient of CH 4 in mixed gas is larger than that of CO 2 . Moreover, the self-diffusion coefficient in the horizontal direction is larger than that in the vertical direction. The mixed gas pressure and mole ratio have limited effects on the isosteric heat and the self-diffusion of CH 4 and CO 2 adsorption. Regarding the analysis of mixed gas selective permeation, it is concluded that the adsorption selectivity of CO 2 is larger than that of CH 4 in the organic nanopores. The larger the CO 2 /CH 4 mole ratio, the greater the adsorption and permeation selectivity of mixed gas in shale. The permeation process is mainly controlled by adsorption rather than diffusion. These results are expected to reveal the adsorption and diffusion mechanism of gas in shale organics, which has a great significance for further research.
... In the experimental study, the researchers mainly used the physical and chemical method to separate the sample from the shale, and used the method of volume or weight to test the adsorption capacity of methane (Hu, 2014;Zhang et al., 2012). However, in the aspect of theoretical research, the adsorption of methane in carbon nanopores, such as kerogen, carbon nanotubes and graphene, were studied by Monte Carlo method (Collell et al., 2014;Bartuś and Bródka, 2011;Billemont et al., 2010;Billemont et al., 2013). Previous research was mainly on the description and analysis for adsorption isotherm which could reflect the adsorption capacity of kerogens for methane, and has already obtained many achievements. ...
Organic matter plays a vital role in the adsorption of shale gas. The current study about adsorption capacity and adsorption law of organic matter have made abundant achievements, but the effect of functional groups on the adsorption of shale gas is still not clear. In this paper, the adsorption of CH4 on the surface of kerogen fragments with different functional groups (OH, CH2OH, COOH, CH3, NH2, O) was studied based on first principles density functional theory (DFT). The structures, electronic properties were calculated by the generalized gradient approximation (GGA). The result shows that the existence of oxygen and nitrogen in functional groups makes kerogen has greater methane adsorption capacity. The fragment of kerogen with NH2- has strongest methane adsorption capacity and the adsorption energy is -5.959eV, while the adsorption energy of the fragments of kerogen with OH, CH2OH, COOH, O is between -0.274eV~-0.395eV, which is close to each other, but far less than the former; the adsorption effect is very weak for methane on the surface of kerogen fragments RH、RCH3; Difference value of the adsorption energy is less than 0.03eV on the surface of same fragment of kerogen, indicating that adsorption site has little influence on methane adsorption and methane is easy to spread on the same surface.
... Examples include: metal organic frameworks (MOFs), 28,29 zeolites, 27,30,31 activated carbons including the amorphous carbons, 32,33 and well-shaped carbon nanotubes. 7,9,34 However, these simulations all apply periodic boundary conditions in the direction of flow, in effect expanding the seamlessly connected unit cells to infinite length, and therefore cannot estimate the individual resistances separately of the boundary and internal regions. ...
We investigate the transport diffusion of methane at 300 K in a series of short (10, 10) carbon nanotubes with length of up to 100 nm, using a novel equilibrium molecular dynamics simulation (EMD) method. The calculated transport diffusivities for methane in the short CNTs were validated by gravity-driven non-equilibrium molecular dynamics (NEMD) simulations. Due to the dominant interfacial resistance, that collectively accounts for the entrance and exit interfacial barriers, the transport diffusivities of methane in the finite CNTs are generally 2-3 orders of magnitude lower than those in an infinitely long CNT, and depend significantly on the tube length. The EMD simulations show that interfacial resistance is the major source of resistance to transport, and that the ratio of the interfacial resistance to the overall resistance decreases with increase in the length of the CNTs. Significant correlation between the motion of methane near the interfaces and that deep inside the CNT was found, showing that the interfacial region is not limited to a narrow range at the entrance and exit, but extends to more than 50 nm inside the CNT. We find that high net flux in the NEMD simulations increases the heat release/supply at the entrance/exit region, in accord with the exothermic/endothermic adsorption/desorption processes occurring at the interface. As a result, a temperature gradient is generated inside the CNT, which in turn enhances the diffusion of methane. Nevertheless, good agreement between NEMD and EMD results is obtained. The generalized EMD method proposed in this work for determining the interfacial resistance is readily applied to any nanoporous material.
... [22] In regards to CO 2 and CH 4 , CO 2 molecules demonstrate a bulk Fickian diffusion mode in CNTs whose diameter is larger than 1.36 nm [23]. For CH 4 molecules, Fickian diffusion is also the dominating mode in large CNTs of diameter larger than 2.0 nm [24,25]. When it comes to CO 2 /CH 4 mixtures, we expect that CNTs with proper diameters will accommodate single-file and Fickian diffusion modes respectively to species with different molecular sizes: larger particles diffuse in the single-file mode while the smaller ones in the Fickian mode [20]. ...
... It is obvious that such a large number of molecules impose a significant challenge to the computational resources. On the other hand, the use of the Langevin thermostat can greatly reduce the number of molecules required while maintaining the same accuracy, as discussed in literature [20,25]. For example, by using a stochastic Langevin thermostat, a single-file mode was reported for about 100 L-J particles confined in CNTs [20]. ...
The diffusion of a CO2/CH4 mixture in carbon nanotube (CNT) bundles was studied using molecular simulations. The effect of diameter and temperature on the diffusion of the mixture was investigated. Our results show that the single-file diffusion occurs when CO2 and CH4 are confined in CNTs of diameter less than 1.0 nm. In CNTs of diameter larger than 1.0 nm, both molecules diffuse in the Fickian style. The transition from single-file to Fickian diffusion was demonstrated for both CO2 and CH4 molecules. A dual diffusion mechanism was observed in the studied (20, 0) CNT bundle, single-file diffusion of CO2 in the interstitial sites of (20, 0) CNT bundle and Fickian diffusion of CO2 and CH4 in the pores. For CO2, the interaction energies (CO2–CO2 and CO2–CNT) are larger than that of CH4 in all cases. But only a very small difference in the diffusion coefficient was observed between CO2 and CH4. Temperature has a negligible effect on the difference between diffusion coefficients of CO2 and CH4 in the studied CNT bundles. The adsorption, diffusion and permeation selectivities are discussed and compared, and the adsorption is demonstrated to be the rate limiting step for the separation of CO2/CH4 in CNT bundle membranes.
For the effective exploration and development of shale gas, it is important to understand the transport and adsorption mechanisms of methane, the main composition of shale gas. The wide variety of mineral components in shale matrices makes the pore surface complex, causing difficulty to understand the mass transport and adsorption behaviors within the shale pores. Molecular dynamics (MD) have high fidelity to simulate the domains with complex geometries at nanoscale. However, most of the previous MD studies focused on the mechanisms of methane in slit nanopores, which could not reflect the real adsorption and transport behavior of shale gas in shale pores with complex structures. This work studied the fluid properties in shale nanopores with high relative roughness and complex boundary geometries. The graphene sheets with different geometrical structures are used to characterize the complex boundary of shale pores. In addition, the effects of temperature, pressure, driving force, relative roughness, and absolute roughness on the adsorption and transport behavior are investigated. In this study, shale gas diffusion is developed by the Einstein method to calculate the spatial diffusion coefficient D, planar diffusion coefficient Dxy, and vertical diffusion coefficient Dz of gas molecules to quantify and analyze the effect of temperature and pressure on shale gas diffusion. The results indicate that the main diffusion mechanism of methane is planar diffusion. The diffusion coefficient is sensitive to temperature and pressure changes in rarefied gases. Then, Grand Canonical Monte Carlo (GCMC) method is carried out to investigate the adsorption behavior. The simulation results show that the adsorption density increases with the increase of pressure and the decrease of temperature. The pressure-driven flow behavior of methane in nanopores is investigated by using non-equilibrium molecular dynamics (NEMD). The results indicate that the flow velocity of methane increases with the driving force. Moreover, the rough elements near wall restrict the transport of methane molecules.
The fracture network in fractured reservoirs is the main channel for fracturing fluid intrusion and gas flow, and the presence of liquid has a great influence on the gas flow pattern, and the gas flow shows more diversity in fluids with different properties such as fracturing fluid or formation water. In order to study the characteristics of gas-liquid two-phase flow in the microfracture network of the fractured gas reservoir, the topology of the microfracture network is simplified to T-junction and Y-junction. Based on the volume of fluid model, the gas-liquid two-phase flow interface is tracked. The model is solved by computational fluid dynamics software OpenFOAM, and the correctness of the model is verified by comparing it with laboratory microscopic experiment results. The flow characteristics of the gas phase in different fluids at the microscale and the influence of fluid parameters on flow pattern are analyzed. The results show that the properties of the liquid phase have a strong influence on the gas flow pattern, and the two-phase flow velocity and the properties of fluid parameters affect the gas flow to reach the stable state. The findings of this study can be used for reference to analyze the flow characteristics of gas in fracturing fluid and the flowback technology of fracturing gas well.
