Molecular Dynamics Simulations about Adsorption and Displacement

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Molecular Dynamics Simulations about Adsorption and Displacement of Methane in Carbon Nanochannels Heng-An Wu, Jie Chen, and He Liu J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 27 May 2015 Downloaded from http://pubs.acs.org on May 28, 2015

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The Journal of Physical Chemistry

Molecular Dynamics Simulations about Adsorption and Displacement of Methane in Carbon Nanochannels Hengan Wua,*, Jie Chena and He Liub a

CAS Key Laboratory of Mechanical Behavior and Design of Materials,

Department of Modern Mechanics, University of Science and Technology of China, Hefei, Anhui, 230027, China b

PetroChina Research Institute of Petroleum Exploration & Development, Beijing, 100083, China

*

Corresponding author, Tel/Fax: 0086-551-63601245, Email: [email protected]

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Molecular Dynamics Simulations about Adsorption and Displacement of Methane in Carbon Nanochannels Abstract: Adsorption and displacement are two important issues in the exploitation of shale gas. In this study, molecular dynamics (MD) simulations are employed to study the mechanisms about adsorption and displacement of methane in carbon nanochannels. Here, the nanochannel is modeled as the slit pore. Due to the attractive potentials of the walls, more methane molecules can be stored in the slit pore compared to the bulk phase and part of them are in the adsorption state. As the width of slit pore increases, the structure of adsorbed methane transforms from single adsorption layer to four adsorption layers. Moreover, it is found that the small slit pore fills up quicker and can store more methane than the larger one under relatively low pressure due to its deeper potential well. To displace the adsorbed methane and enhance the gas recovery, injection gases such as carbon dioxide and nitrogen are simulated and investigated. The displacement mechanisms of the two gases are found to be different: carbon dioxide can replace the adsorbed methane directly while nitrogen works by decreasing the partial pressure of methane. The simulation results show that injection of carbon dioxide gives slow breakthrough time, sharp front while injection of nitrogen gives fast breakthrough time, wide front. Our work can be of great significance for revealing the mechanisms of adsorption and displacement and guiding the exploitation of shale gas. Key Words: Shale gas, Molecular dynamics simulation, Adsorption, Displacement 2 / 23


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1. Introduction Recently, the exploration and development of alternative energy sources have received extensive attention due to the growing demand for resources and climate problems.1-5 Shale gas, which is primarily composed of methane, is an attractive candidate because of its numerous advantages such as low pollution, wide distribution and abundant resource.6-7 It is found that large amounts of methane are stored as adsorption state in the pores especially in the nanochannels of organic-rich shale.8-9 The adsorption of methane not only impacts the estimating of shale gas reserves but also reduces the extraction efficiency. In engineering, only a small part of the adsorbed methane can be exploited due to the ultra-low porosity and permeability. To improve the recovery efficiency, new method such as injection gases to displace the adsorbed methane is proposed.10 In addition, adsorption in nanoporous materials such as slit pore,11-13 carbon nanotube14-18 is an important method for storing the shale gas. Hence, investigations about the adsorption and displacement of methane in nanochannels are of great significance for estimating and exploiting the shale gas. In terms of the adsorption of methane, there have been a series of experiments and computational studies. Rexer et al.19 studied methane adsorption on shale samples through experiments. At specific temperature and pressure, the methane excess uptake and isosteric enthalpy were measured and described with mathematical equations. Lithoxoos et al.20 investigated the adsorption capacity of 3 / 23



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single-wall carbon nanotubes (SWCNTs) experimentally and computationally. Both the experimental and simulation adsorption data were presented and compared, and the difference between them was discussed. Cao et al.21 studied the adsorption of methane on triangular arrays of SWCNT with different tube sizes and van der Waals (vdW) gaps. They found that the (15, 15) SWCNT arrays with a vdW gap of 0.8 nm was the optimal adsorbent. Zhu and Zhao22 investigated the methane adsorption in carbon nanopores (CNTs) through molecular dynamics simulations. Based on the simulation results and theoretical analyses, the equation of adsorbed methane was established. Furthermore, an optimal diameter was obtained considering both the curvature effect and size effect. However, these works mainly focused on the gases storage in SWCNT or CNTs. There is little correlative report about the mechanisms of adsorption phenomena and adsorption structures in the slit pore. In terms of the displacement of methane, most works at present were carried out through the experiment. Common choices of injection gases are carbon dioxide or nitrogen. Nitrogen (N2) is nontoxic, non-corrosive and abundant. Carbon dioxide (CO2) tends to adsorb on the pore surfaces more strongly than methane.23-24 Kowalczyk et al.25 studied the displacement of methane by coadsorbed carbon dioxide on various carbonaceous materials. In their work, two-stage mechanism of methane displacement was proposed. Bhowmik and Dutta26 discussed the methane displacement behavior by pure carbon dioxide injection. They found that carbon dioxide was preferentially adsorbed and methane was preferentially desorbed. Jessen 4 / 23


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et al.27 conducted experiments with carbon dioxide, nitrogen, and their mixtures to study the displacement process. To explain the experimental trends and results, a one dimensional model including gas and solid phase was employed. To the best of our knowledge, there is hardly any microscopic study about the mechanisms of methane displaced by carbon dioxide or nitrogen. According to the inadequacy as mentioned above, the mechanisms about the adsorption and displacement of methane in carbon slit pores are investigated through MD simulations in this paper. In section 3.1, the attractive potentials of pore walls are found to be the cause for adsorption. Due to the adsorption, more methane molecules can be stored in the slit pore and part of them are in adsorption state. The adsorption structures in different slit pores are investigated, together with the adsorption isotherms. Moreover, compared to the large pore, small pore can store more methane under relatively lower pressure because of its deeper potential well. In section 3.2, two gases, pure carbon dioxide and pure nitrogen, are injected to displace the adsorbed methane. Firstly, the adsorption capacity of these gases are investigated. Secondly, the displacement mechanisms of the injection gases are analyzed. Finally, the displacement processes of them are compared and discussed.

2. Simulation models and methods Considering that the adsorption and displacement of methane in carbon nanochannels occur at nanoscale, MD simulations implemented in LAMMPS28 are 5 / 23



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carried out for the investigations in this work. Our simulation models are shown in Fig. 1, where the gray part, blue part, green part and black part denote the slit pore, methane molecules, injection gases and the gas reservoir in bulk phase respectively. In this paper, complex structures of nanochannels in shale are modeled ideally as slit pores composed by two disconnected and independent graphite slabs. The distance between two slabs H can be adjusted to model various widths. Four H 7, 10, 15 and 20 Å are chosen to explore the influence of the slit pore’s width on the adsorption. The slabs are fixed during the whole process. In our simulation, the intermolecular interactions between the gases and carbon atoms are mainly caused by van der Waals forces. Thus, 12-6 Lennard-Jones (LJ) potential is adopted, which can be defined as: 12 6 φij = 4ε ij (σ ij rij ) − (σ ij rij )  ,





(1)

where ε ij , σ ij and rij denote the depth of the potential well, the finite distance at which the interparticle potential is zero and the distance between i and j molecules respectively. The LJ parameters used in the simulation are shown in Table 1.29-30 The interaction between different types of atoms can be calculated by the Lorentz −Berthelot (LB) rule directly. Particularly, ε CH 4 −C , ε CO2 −C and ε N2 −C determine the adsorption capacity of three gases and can be adjusted to simulate different adsorption capacity. In this paper, they are set as 0.15, 0.192 and 0.123 Kcal/mole respectively. The cutoff of LJ potential model is chosen to be 10 Å, which means the interaction beyond this range is negligible. 6 / 23


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Fig. 1 Schematic images of MD simulation models: (a) adsorption model, (b) displacement model.

Table 1. Parameters of LJ particles Molecules

σ (Å)

ε (Kcal/mole)

C-C

3.4

0.056

CH4-CH4

3.751

0.294

CO2-CO2

3.615

0.481

N2-N2

3.59

0.197

To simulate the adsorption of methane, the slit pore is connected with the bulk phase of methane, which is shown in Fig. 1(a). Under the pressure provided by the bulk phase of methane, the molecules enter into the slit pore and tend to adsorb on the walls due to the attractive potentials. All the simulations are conducted in the NVT ensemble with a time step of 1 fs. The temperature is controlled by a Nose/Hoover thermostat. The amount of methane N is randomly set and changed to obtain different pressures. The pressure is determined by the density of methane in 7 / 23



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bulk phase and can be computed directly in the simulation. For each simulation, the former 1 ns is used for system equilibrium and the latter 3 ns for data statistics. To simulate the process of adsorbed methane displaced by gas injections, the slit pore is connected with the injection gases, which is shown in Fig. 1(b). Initially, the methane is adsorbed in the slit pore and the injection gases (carbon dioxide or nitrogen) reach equilibrium in the left box. In the interest of simplicity, both carbon dioxide and nitrogen are modeled as a one-center LJ sphere.29-30 In this paper, a case that the pressure of adsorbed methane equals to 2.0MPa and the pressure of injection gases (carbon dioxide or nitrogen) equals to 2.3MPa is simulated. Accordingly, the number of CH4, CO2 and N2 are 263, 6260 and 5500 respectively. Similarly, the simulations are conducted in the NVT ensemble with a time step of 1 fs and the temperature is controlled by a Nose/Hoover thermostat. Due to the pressure difference, the injection gases enter into the slit pore and displace the adsorbed methane. The physical quantities of the output gases in the right box can be computed to study the displacement process.

3. Results and discussion 3.1 adsorption of methane in slit pores According to the theoretical analysis, the adsorption of methane is caused by the attractive potentials between the methane and pore walls.31 As Fig. 2 shows, when there is no interaction potential, the methane molecules in slit pore are in free 8 / 23


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phase and the potential of them equals nearly to zero. However, when the interaction potential takes effect, more methane molecules enter into the slit pore and adsorb on the walls. Meanwhile, the potential energy of adsorbed methane decreases.

Fig. 2 Snapshots of (a) the bulk phase, (b) the adsorbed phase. The colors represent different values of molecular potential energy. (c) The adsorption isotherms for the total, excess and bulk adsorption of methane.

Total gas content and excess adsorption are employed as two physical quantities to characterize the adsorption. They are defined as the total amount of gas present in the pore and the additional amount of gas in excess of the amount that present in bulk phase respectively. Fig. 2(c) shows the isotherms for the total, excess and bulk adsorption of methane in 1 nm pore at 298 K. Due to the adsorption, the total density is obviously higher than bulk density under the same pressure. With the 9 / 23



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pressure increases, the total density increases and tends to saturate at high pressure. The excess density is the difference between the total density and the bulk density. With the pressure increases, the excess density increases up and then decreases. This trend implies that there exits an optimum pressure for maximum adsorbed quantity. The bulk density is a reference state for methane with no adsorption impacts. The bulk density obtained by our simulation is consistent with that collected by the National Institute of Standards and Technology (NIST),32 which can verify our model and simulation methods. To explore the influence of pore size on adsorption and study the adsorption structures, the density distributions of methane in different slit pores are plotted in Fig. 3. The corresponding simulation results of adsorption structures are shown in the insets, which are consistent with the density distributions. Four pore sizes of 7, 10, 15, 20 Å and four pressure 4, 8, 12, 16 MPa are investigated. All the density peaks increase with the increase in pressure. The interval between the first adsorption layer and the wall is about 3.575Å, which is consisted with the LJ parameters.

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Fig. 3 Density distributions and adsorption structures of methane in different pores. (a) H= 7 Å, a single-layer chain; (b) H= 10 Å, two symmetrical adsorption layers; (c) H= 15 Å, two primary adsorption layers and a central single-layer chain; (d) H= 20 Å, two primary adsorption layers and two secondary adsorption layers.

For H=7 Å in Fig. 3(a), the methane molecules form single adsorption layer in the center of the pore. Since the attractive potentials of the two walls are superimposed on each other, the peak of density distribution is the highest. As pore size increases, the attractive potentials from the two walls begin to separate and two symmetrical peaks appear. For H=10 Å in Fig. 3(b), the methane molecules form two primary adsorption layers. Accordingly, the peaks of the density distribution 11 / 23



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decrease. When H increases to 15Å, a central single layer forms in the center of two primary adsorption layers, which is shown in Fig. 3(c). The peak of the central single layer is lower than that of the adsorption layers. For H=20 Å in Fig. 3(d), four adsorption layers, two primary peaks and two secondary peaks, form as indicated by the density distribution. The peaks of medium to large pores gradually tend to the same, which is due to the fact that the attractive potentials of these pores have been effectively divided into two single potential systems near each pore wall. Adsorption isotherms are another key aspect of understanding adsorption process. In this paper, adsorption isotherms plotted as adsorbed amount versus pressure for the four pores at 298K are presented in Fig. 4(a). The adsorbed amount is normalized as mass of adsorbed methane divided by mass of carbon.

Fig. 4 (a) Adsorption isotherms for four slit pores at 298K. (b) Snapshots and potential energy distributions of the four slit pores with pressure equaling to 1MPa. The colors represent different values of molecular potential energy.

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As can be seen by the figure, the adsorbed amount of all the pores increase with the pressure increases. The small pores reach saturation quickly while the large pores are not yet saturated even at the relatively high pressure. The methane molecules can continue to fill up the bulk region. In the low pressure region 0 - 2.5 MPa, the small pores have a larger initial rate of adsorption than the large pores. Pores of 15 Å or larger have the same number of adsorbed methane. This trend also indicates that the attractive potentials of small pores are superimposed and the potential energy is the lowest. For large pores, the attractive potentials from each pore wall are completely disassociated. Hence, these pores would have the same number of adsorbed methane because they have the same potential energy, which are shown in Fig. 4(b). Additionally, the amount of methane molecules that can be taken into bulk region increases with pore size increases. This can be seen clearly by the fact that the adsorption isotherms of different pores separate in the high pressure region.33

3.2 Displacement of the adsorbed methane To displace the adsorbed methane, injection gases are an alternative method. Carbon dioxide (CO2) and nitrogen (N2) are two common choices of injection gases due to their advantages. The adsorption capacity of the three pure gases is firstly investigated. The attractive potentials between gases and carbon atoms, which are defined as equation (1), are plotted in Fig. 5. As the figure shows, the potential 13 / 23



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energy decreases in the order CO2, CH4 and N2. Accordingly, the adsorption capacity of the three gases decreases in the same order. With stronger adsorption capacity, more gas molecules adsorb into the pore, which is shown in the insets of the figure.

Fig. 5 Lennard-Jones plot of CH4-C, CO2-C and N2-C. The insets show the adsorption of the three gases under the same temperature and pressure.

Comparing to methane, the adsorption capacity of CO2 is stronger while that of N2 is weaker. Therefore the displacement mechanisms of two gases should be different, which are simulated and analyzed in Fig. 6. Initially, pure methane is adsorbed in the slit pore under a certain temperature and pressure. When CO2 is injected, the CO2 molecules firstly adsorb on the vacancies. And then, some CO2 molecules replace the adsorbed methane due to the fact that the adsorption capacity of CO2 is stronger than that of methane. The adsorbed methane molecules are dismissed from the adsorption sites and return to the free phase. When N2 is injected, the N2 molecules can only adsorb on the vacancies since the adsorption capacity of 14 / 23


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N2 is weaker than that of methane. However, N2 can decrease the partial pressure of methane in the constant pressure condition. With its partial pressure decreases, methane molecules are desorbed and displaced. As shown in Fig. 6, the adsorbed methane decreases rapidly when both CO2 and N2 are injected.34

Fig. 6 Snapshots of (a) initial pure methane adsorption, (b) methane displaced by CO2, (c) methane displaced by N2.

Furthermore, the process of pure CO2 or N2 displacing the adsorbed methane is studied and compared. The pressure of adsorbed methane is about 2MPa and that of injection gas is about 2.3MPa for both CO2 and N2. The injection gas is sufficient enough to ensure that the pressure of injection gas keeps constant during the whole displacement process. Due to the pressure difference, the injection gas tends to enter into the pore and displace the adsorbed methane. The gases are produced into the right box and the concentrations of all output gases can be calculated. Fig. 7 shows the concentration curves of gases at the outlet for pure CO2 and N2

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injection. The data points are computed from the simulation and the solid lines are employed to fit the trends. For pure CO2 injection, the production gas shows a relatively slow breakthrough time and sharp front, which is shown in Fig. 7(a). Since CO2 gets adsorbed on the pore and CO2 in free phase reduces, it travels slowly through the pore. The sharp front is caused by an initial shock between the injection gases and the adsorbed methane. By the time CO2 breaks through, the concentration of methane decreases to zero quickly, which also indicates that the CO2 front moves very sharply inside the slit pore. In contrast, Fig. 7(b) shows a fast breakthrough time and wide front when injecting pure N2. Since N2 is less strongly adsorbed than methane, it travels quickly through the pore and causes methane to desorb early. Even after N2 breaks through, methane continues to produce. With enough N2 to inject, the concentration of methane in the output gases gradually tends to zero. It is found that both CO2 and N2 can displace the adsorbed methane efficiently. These simulation results are consistent with the experimental results in the literatures.23,27,35

Fig.7 (a) Composition profiles of pure CO2 displacing CH4, (b) Composition profiles of pure N2 displacing CH4. 16 / 23


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4. Conclusions In this study, the mechanisms about adsorption and displacement of methane in slit pore are studied and uncovered through molecular dynamics simulations. Due to the attractive potentials caused by the van der Waals forces, the slit pore can store more methane than the bulk phase. Within the slit pore, part of the methane molecules adsorb onto the walls and the potential energy of them decreases. With the increase of the slit pore’s width, the adsorption structure of methane transfers from single adsorption layer to four adsorption layers. In addition, by comparing the adsorption isotherms of different slit pores, it is found that the number of adsorbed methane is greater for smaller slit pore under relatively low pressure. The main reason is that, for smaller slit pore, the potentials of the walls are superimposed and the attractive interaction is stronger. To displace the adsorbed methane, carbon dioxide and nitrogen are injected and investigated. Under the same condition, the adsorption capacity of the three gases decreases in the order CO2, CH4, and N2, which indicates the different displacement mechanisms. Since the adsorption capacity of CO2 is stronger than that of methane, CO2 replaces the adsorbed methane directly. Though N2 cannot replace the adsorbed methane directly, it works by decreasing the partial pressure of methane. Both of the two gases can displace the adsorbed methane efficiently. Furthermore, the displacement processes of pure CO2 and N2 are compared. Injection of CO2 shows slow breakthrough time, sharp front while injection of N2 shows fast breakthrough 17 / 23



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time, wide front. These results and findings are of great significance for the efficient exploitation of shale gas.

Acknowledgement This work was supported by National Science Foundation of China (11472263).

References (1) Arora V.; Cai Y. Y. U.S. natural gas exports and their global impacts. Applied Energy 2014, 120, 95-103. (2) Weijermars R. Economic appraisal of shale gas plays in Continental Europe. Applied Energy 2013, 106, 100-115. (3) Li Y. X.; Nie H. K.; Long P. Y. Development characteristics of organic-rich shale and strategic selection of shale gas exploration area in China. Natural Gas Industry 2009, 29, 115-18. (4) Rivard C.; Lavoie D.; Lefebvre R.; Sejourne S.; Lamontagne C.; Duchesne M. An overview of Canadian shale gas production and environmental concerns. International Journal of Coal Geology 2014, 126, 64-76. (5) Bowker K. A. Barnett Shale gas production, Fort Worth Basin: Issues and discussion. AAPG Bulletin 2007, 91, 523-533. (6) Caulton D. R.; Shepson P. B.; Santoro R. L.; Sparks J. P.; Howarth R. W.; Ingraffea A. R.; Cambaliza M. O. L.; Sweeney, C.; Karion, A.; Davis, K. J.; Stirm, B. 18 / 23


Page 19 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


H.; Montzka, S. A.; Miller, B. R. Toward a better understanding and quantification of methane emissions from shale gas development. Proceedings of the National Academy of Sciences of the United States of America 2014, 111, 6237-6242. (7) Howarth R. W.; Santoro R.; Ingraffea A. Methane and the greenhouse-gas footprint of natural gas from shale formations. Climatic Change 2011, 106, 679-690. (8) Loucks R. G.; Reed R. M.; Ruppel S. C.; Jarvie D. M. Morphology, genesis, and distribution of nanometer-scale pores in siliceous mudstones of the Mississippian Barnett shale. Journal of Sedimentary Research 2009, 79, 848-861. (9) Sharma A.; Namsani S.; Singh J. K. Molecular simulation of shale gas adsorption and diffusion in inorganic nanopores. Molecular Simulation 2015, 41, 414-422. (10) Wang L. G.; Cheng Y. P.; Wang Y. K. Laboratory study of the displacement coalbed CH4 process and efficiency of CO2 and N2 injection. The Scientific World Journal 2014, 2014, 242947. (11) Tan Z. M.; Gubbins K. E. Adsorption in carbon micropores at supercritical temperatures. Journal of Physical Chemistry 1990, 94, 6061-6069. (12) Kowalczyk P.; Tanaka H.; Kaneko K.; Terzyk A. P.; Do D. D. Grand canonical Monte Carlo simulation study of methane adsorption at an open graphite surface and in slitlike carbon pores at 273 K. Langmuir 2005, 21, 5639-5646. (13) Mosher K.; He J. J.; Liu Y. Y.; Rupp E.; Wilcox J. Molecular simulation of methane adsorption in micro- and mesoporous carbons with applications to coal and 19 / 23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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gas shale systems. International Journal of Coal Geology 2013, 109, 36-44 (14) Bhatia S. K.; Nicholson D. Adsorption and diffusion of methane in silica nanopores: a comparison of single-site and five-site models. Journal of Physical Chemistry C 2012, 116, 2344-2355. (15) Bartus K.; Brodka A. Temperature study of structure and dynamics of methane in carbon nanotubes. Journal of Physical Chemistry C 2014, 118, 12010-12016. (16) Ohba T.; Kaneko K.; Yudasaka M.; Lijima S.; Takase A.; Kanoh H. Cooperative adsorption of supercritical CH4 in single-walled carbon nanohorns for compensation of nanopore potential. Journal of Physical Chemistry C 2012, 116, 21870-21873. (17) Vela S.; Huarte-Larranaga F. A molecular dynamics simulation of methane adsorption in single walled carbon nanotube bundles. Carbon 2011, 49, 4544-4553. (18) Cao D. P.; Wu J. Z. Self-diffusion of methane in single-walled carbon nanotubes at sub- and supercritical conditions. Langmuir 2004, 20, 3759-3765. (19) Rexer T. F. T.; Benham M. J.; Aplin A. C.; Thomas K. M. Methane adsorption on shale under simulated geological temperature and pressure conditions. Energy & Fuels 2013, 27, 3099-3109. (20) Lithoxoos G. P.; Labropoulos A.; Peristeras L. D.; Kanellopoulos N.; Samios J.; Economou I. G. Adsorption of N2, CH4, CO and CO2 gases in single walled carbon nanotubes: A combined experimental and Monte Carlo molecular simulation study. Journal of Supercritical fluids 2010, 55, 510-523. (21) Cao D. P.; Zhang X. R.; Chen J. F.; Wang W. C.; Yun J. Optimization of 20 / 23


Page 21 of 23

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single-walled carbon nanotube arrays for methane storage at room temperature. Journal of Physical Chemistry. Journal of Physical Chemistry B 2003, 107, 13286-13292. (22) Zhu X. Y.; Zhao Y. P. Atomic mechanisms and equation of state of methane adsorption in carbon nanopores. Journal of Physical Chemistry C 2014, 118, 17737-17744. (23) Zhu J. C.; Jessen K.; Kovscek A. R.; Orr F. M. Analytical theory of coalbed methane recovery by gas injection. SPE Journal 2003, 8, 371-379. (24) Yu H. G.; Yuan J.; Guo W. J.; Cheng J. L.; Hu Q. T. A preliminary laboratory experiment on coalbed methane displacement with carbon dioxide injection. International Journal of Coal Geology 2008, 73, 156-166. (25) Kowalczyk P.; Gauden P. A.; Terzyk A. P.; Furmaniak S.; Harris P. J. F. Displacement of methane by coadsorbed carbon dioxide is facilitated in narrow carbon nanopores. Journal of Physical Chemistry C 2012, 116, 13640-13649. (26) Bhowmik S.; Dutta P. Investigation into the methane displacement behavior by cyclic, pure carbon dioxide injection in dry, powdered, bituminous Indian coals. Energy & Fuels 2011, 25, 2730-2740. (27) Jessen K.; Tang G. Q.; Kovscek A. R. Laboratory and simulation investigation of enhanced coalbed methane recovery by gas injection. Transport in Porous Media 2008, 73, 141-159. (28) Plimpton S. Fast parallel algorithms for short-range molecular-dynamics. 21 / 23



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Journal of Computational Physics 1995, 117, 1-19. (29) Mosher K. The impact of pore size on methane and CO2 adsorption in carbon. M.S. Thesis, Stanford University, 2011. (30) JinHyeok C.; Chiashi S.; Shiomi J.; Maruyama S. Generalized model of thermal boundary conductance between SWNT and surrounding supercritical Lennard-Jones fluid-derivation from molecular dynamics simulations. International Journal of Heat and Mass Transfer 2012, 55, 2008-2013. (31) Delavar M.; Ghoreyshi A. A.; Jahanshahi M.; Khalili S.; Nabian N. Equilibria and kinetics of natural gas adsorption on multi-walled carbon nanotube material. RSC Advances 2012, 2, 4490-4497. (32) Rhoderick G. C.; Carney J.; Guenther F. R. NIST gravimetrically prepared atmospheric level methane in dry air standards suite. Analytical Chemistry 2012, 84, 3802-3810. (33) Ortiz L. Computational studies of methane adsorption in nanoporous carbon. M.S. Thesis, the University of Missouri-Columbia, 2012. (34) Zhang D. F.; Li S. G.; Cui Y. J.; Song W. L.; Lin W. G. Displacement behavior of methane adsorbed on coal by CO2 injection. Industrial & Engineering Chemistry Research 2011, 50, 8742-8749. (35) Turta A. T.; Sim S. S. K.; Singhal A. K.; Hawkins, B. F. Basic investigations on enhanced gas recovery by gas-gas displacement. Journal of Canadian Petroleum Technology 2008, 47, 39-44. 22 / 23


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Molecular Dynamics Simulations about Adsorption and Displacement

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