Adsorption Behavior of Methane on Kaolinite - Industrial

May 10, 2017 - In this work, the adsorption behaviors of CH4 in slit-like kaolinite pores were investigated using the grand canonical Monte Carlo meth...
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Adsorption Behavior of Methane on Kaolinite Jian Xiong,† Xiangjun Liu,*,† Lixi Liang,† and Qun Zeng‡ †

State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, Sichuan, China ‡ Institute of Chemical Materials, Engineering Physical Academy of China, Mianyang 621999, Sichuan, China S Supporting Information *

ABSTRACT: In this work, the adsorption behaviors of CH4 in slit-like kaolinite pores were investigated using the grand canonical Monte Carlo method. The research results show that the isosteric heat of adsorption of CH4 decreases with increasing pore size and that CH4 adsorption on kaolinite can be characterized as physical adsorption. The potential energy between CH4 and kaolinite was found to decrease with increasing pressure or decreasing pore size, indicating that the adsorption sites of CH4 changed from higher-energy adsorption sites to lower-energy adsorption sites. The CH4 adsorption capacity decreased with increasing pore size in mesopores. With increasing temperature, the isosteric heat of adsorption of CH4 decreased, and the adsorption sites of CH4 changed from higher-energy adsorption sites to lower-energy adsorption sites, resulting in a decrease of the CH4 adsorption capacity. As a result of van der Waals force interactions, Coulomb force interactions, and hydrogen-bonding interactions, the water molecules in the kaolinite pores occupies the pore walls in a directional manner, causing the water molecules in the kaolinite pores to accumulate. With increasing accumulation of water, the water molecules occupied adsorption spaces and adsorption sites of CH4, leading to a decrease of the CH4 adsorption capacity. The gas adsorption capacity on kaolinite was found to decrease in the following order: CO2 > CH4 > N2. With increasing mole fraction of N2 or CO2, the mole fraction of CH4 in the gas phase decreased, the adsorption sites of CH4 changed, and the adsorption space of CH4 decreased, resulting in a decrease of CH4 adsorption capacity.

1. INTRODUCTION In 2013, a report released by the U.S. Energy Information Administration (EIA) estimated that the global technical recoverable reserves of shale gas amounted 220.73 × 1012 m3, suggesting that the global shale gas resource was abundant and had a large potential for production.1 Shale gas is mainly in the free state, the adsorbed state, and the dissolved state.2 Free gas mainly exists in microfractures or larger pores in organic matter and among mineral grains in shale gas reservoirs, whereas adsorbed gas exists on mineral grains or organic matter in shale gas reservoirs. After studying the reservoir characteristics of shale gas reservoirs in the United States, Curtis3 summarized the conditions for changing the content of adsorbed gas in shale gas reservoirs, considering that the proportion of adsorbed gas ranged from 20% to 85% and that adsorbed gas made up a sizable share of the shale gas. Therefore, it is important to study the CH4 adsorption capacity on shales for the evaluation of shale gas resources. Some researchers4−7 have suggested that organic matter is the major factor controlling the CH 4 adsorption capacity. Other researchers,8−14 however, have © XXXX American Chemical Society

suggested that clay minerals could also contribute to the CH4 adsorption capacity. In addition, Yang et al.7 observed that the average contribution of clay minerals to the CH4 adsorption capacity was approximately 28.6%. Rexer et al.14 estimated that the contribution of clay minerals to CH4 adsorption capacity was approximately 45−60%. These works indicate that clay minerals play a significant role in the CH4 adsorption capacity on shales. One important type of clay mineral is kaolinite. To date, isothermal adsorption experiments have been conducted to investigate the CH4 adsorption capacity on different materials. Many researchers have carried out isothermal adsorption experiments targeted at determining the CH4 adsorption capacity on kaolinite. Cheng and Huang,15 Ross and Bustin,8 Ji and co-workers,9,10 Liu et al.,11 Fan et al.,12 and Liang et al.13 researched the CH4 adsorption capacity on Received: Revised: Accepted: Published: A

February 27, 2017 May 7, 2017 May 10, 2017 May 10, 2017 DOI: 10.1021/acs.iecr.7b00838 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure S1, the morphology of the shape of the pores in kaolinite can be simplified as slit-shaped. The simulation cell was established in a rectangular box with periodicity in the x and y directions according to the kaolinite unit cell structure, including an 8a × 4b supercell structure in the x × y directions, indicating that the size of the kaolinite supercell structure in the x × y directions was 4.123 nm × 3.577 nm. On the basis of this structure, we added a vacuum between the inner planes of the two supercell structures, that is, the z direction, to build the slitlike kaolinite pore structure. The length in the z direction is defined by the pore size of the kaolinite supercell structure and the vacuum. The pore size H was determined by the vacuum. In this way, slit-like kaolinite pores with different pore sizes could be obtained. A schematic representation of the slit-like kaolinite pores is shown in Figure 1, and the parameters of slit-like kaolinite pores with different pore sizes are listed in Table S1.

clay-rich rocks (kaolinite) at different temperatures and pressures using isothermal adsorption experiments and discussed the factors influencing the adsorption capacity, including temperature, pressure, and sample size. All of the research works mentioned above contrasted and compared the CH4 adsorption capacity on kaolinite using the Langmuir parameters under equilibrium conditions according to the results of their isothermal adsorption experiments. However, the adsorption amounts reported reflected only the specific surface area of kaolinite. Thus, the adsorption amounts per unit surface area, which can be regarded as macroscopic values, failed to deeply reflect the adsorption of CH4 on kaolinite and were not able to describe the microscopic adsorption mechanism of CH4 in the kaolinite nanopores. As a theoretical research approach for investigating the adsorption properties of adsorbents, computer molecular simulation technology that is able to investigate the microscopic adsorption mechanisms between porous materials and fluid molecules at the molecular level has gained increasing attention in the past few years. For example, Jin and Firoozabadi16,17 and Kadoura et al.18 studied the structural properties of CH4, CO2, and their mixture in montmorillonite pores using the grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) methods and discussed some influencing factors, including pore size, temperature, and water content. Chen et al.19 and Zhang et al.20 investigated the adsorption behaviors of CH4, C2H6, and CO2 in illite pores using the GCMC method. Xiong and co-workers21,22 studied the adsorption behaviors of CH4 in chlorite and quartz pores using the GCMC and MD methods and discussed the effects of pore size, temperature, and water content. In addition, Zhang and Wang,23 Tanaka et al.,24 Yang et al.,25 and Thierfelder et al.26 investigated the adsorption behaviors of CH4 on carbon or graphite using the density functional theory method. These studies indicate that molecular simulation methods have been widely applied to investigate the adsorption properties of adsorbents and that the GCMC method has proven to be an effective method for investigating the microscopic adsorption mechanisms of adsorbents. At the same time, these efforts also obtained some knowledge on the adsorption of methane on montmorillonite, illite, and chlorite. However, a detailed microscopic mechanism for the adsorption of CH4 in slit-like kaolinite nanopores has not been well studied. In this work, the GCMC method was used to investigate the microscopic adsorption mechanism of CH4 in slit-like kaolinite pores. We investigated the effects of pore size, temperature, and water content on the adsorption behaviors of CH4 on kaolinite, as well as the effects of N2 and CO2 on the competitive adsorption of N2 or CO2 and CH4 in binary gas mixtures in slitlike kaolinite pores. On this basis, the microscopic adsorption mechanism of CH4 in slit-like kaolinite pores was studied. Moreover, the effects of temperature, water content, and composition on the adsorption behaviors of CH4 on kaolinite are discussed, along with their interaction mechanisms.

Figure 1. Schematic representation of a slit-like kaolinite pore (H represents different pore sizes).

2.2. Force Field Parameters. The Lennard-Jones (L-J) potential parameters and charges of the sites in the unit cell of kaolinite are reported in Table S2. In Table S2, the L-J potential parameters of the sites in the unit cell of kaolinite are described in ref.were obtained from ref 28, and the charges of the sites in the unit cell of kaolinite were obtained from the literature.16,17 The potential model used for the methane molecule was from the TraPPE force field.29 The potential model used for the nitrogen molecule was also from the TraPPE force field.30 The potential model used for the water molecule was from the SPCE force field.31 The carbon dioxide molecule was simulated using the EPM2 model.32 The L-J potential parameters and the charges of each atom in the liquids are reported in Table S2 and were obtained from the above-cited references. During the simulations, the kaolinite was assumed to be a rigid body. In the simulations, the interactions between kaolinite and the liquids included van der Waals forces and Coulomb forces. The van der Waals forces were described by the L-J (12,6) potential model, and the combined effects of the van der Waals and Coulomb forces were described by the following potential model ⎡⎛ ⎞12 ⎛ ⎞6 ⎤ qiqj σij σij E = εij⎢⎢⎜⎜ ⎟⎟ − ⎜⎜ ⎟⎟ ⎥⎥ + r ⎝ rij ⎠ ⎦ 4πε0rij ⎣⎝ ij ⎠

2. MODEL AND METHOD 2.1. Model. The parameters of the kaolinite crystal cell are given in the literature.27 Kaolinite is a 1:1 clay mineral, And the 1:1 layer structure consists of the repetition of one tetrahedral sheet and one octahedral sheet. The unit cell parameters are as follows: a = 0.5154 nm, b = 0.8942 nm, c = 0.7391 nm, α = 91.92°, β = 105.05°, and γ = 89.90°. According to the results of N2 adsorption−desorption isotherm, which can be observed in

(1)

where qi and qj are the charges of the atoms in the system (in units of coulombs); rij is the distance between atoms i and j (in units of nanometers); ε0 is the dielectric constant (8.854 × 10−12F/m); and σij and εij are the L-J well depth and the L-J size, respectively. Unlike interactions were calculated by the standard Lorentz−Berthelot combining rules B

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Figure 2. (a) Total amount isotherms of the methane and (b) absolute adsorption isotherms of CH4 on kaolinite with different pore sizes.

σij = (σii + σjj)/2

εij =

εiiεjj

To investigate the effects of N2 on the competitive adsorption of N2 and CH4 in the kaolinite pores, five simulations considering five different mole fractions of N2 in CH4/N2 binary gas mixtures were carried out; the CH4/N2 mole ratios (yCH4/yN2) were 0.8:0.2, 0.4:0.6, 0.6:0.4, and 0.2:0.8 (yCH4 = 0.80 means that the mole fraction of CH4 was 80% whereas the mole fraction of N2 was 20%). In these simulations, the pore size of the kaolinite pores was 4 nm, and the temperature was 333 K. To investigate the effects of CO2 on the competitive adsorption of CO2 and CH4 in the kaolinite pores, five simulations considering five different mole fractions of CO2 in CH4/CO2 binary gas mixtures were carried out; the CH4/CO2 mole ratios (yCH4/yCO2) were 0.8:0.2, 0.4:0.6, 0.6:0.4, and 0.2:0.8 (yCH4 = 0.80 means that the mole fraction of CH4 was 80% whereas the mole fraction of CO2 was 20%). In these simulations, the pore size of the kaolinite pores was 4 nm, and the temperature was 333 K. 2.4. Absolute Adsorption Amount. In the simulations, we assumed that the total amount of gas in the adsorption system was N, so the excess adsorption amount can be expressed as

(2)

2.3. Grand Canonical Monte Carlo Simulations. The grand canonical Monte Carlo (GCMC) simulation method has been widely used to study the adsorption equilibria of adsorbates. During a GCMC simulation cycle, the moves constituting exchange, conformation, rotation, and translation were set as 40%, 20%, 20%, and 20%. In this work, we used GCMC simulations to investigate the adsorption behaviors of CH4 in the silt-like kaolinite nanopores. In the grand canonical ensemble, the chemical potential, volume, and temperature are independent variables. It is worth mentioning that the chemical potential is a function of the fugacity rather than the pressure. Therefore, the Soave−Redlich−Kwong (SRK) equation33 was employed to calculate the fugacity of CH4 in this study, which can be observed in Figure S2. The simulations were conducted using the sorption module in Materials Studio 6.0. The maximum simulated pressure was 40 MPa, and the simulations were under constant pressure point by point, divided into 15 points. The Dreiding force field was chosen as the force field type in the simulations, and the Ewald and group methods were employed to calculate the Coulomb force interactions. We used the atom-interaction-based method to calculate the van der Waals forces with an L-J potential cutoff distance of 1.55 nm. The maximum load step in each simulation was 3 × 106, in which the balance step was 1.5 × 106 and the process step was 1.5 × 106. Later, 1.5 × 106 types of configurations were applied for related statistics. To investigate the effects of pore size on the adsorption behaviors of CH4 on kaolinite, 10 simulations considering 10 different pore sizes were carried out using a temperature of 333 K in these simulations. To investigate the effects of temperature on the adsorption behaviors of CH4 on kaolinite, four simulations considering four different temperatures were carried out. The simulated temperatures ranged from 313 to 373 K, and the temperature interval was 20 K. The pore size of the kaolinite pores in these simulations was 4 nm. To investigate the effects of water on the adsorption behaviors of CH4 on kaolinite, four simulations were carried out considering four different water contents, namely, 0, 2, 4, and 8 wt % (where the weight percentage is given by the ratio of the mass of water molecules to the mass of kaolinite mass). The adsorption sites of the water molecules in the slit-like kaolinite pores were determined first using the annealing simulation method. In addition, the pore size of the kaolinite pores was 4 nm, and the temperature was 333 K in these simulations.

nex =

ρg (Va + Vg) ρg Vp N N − = − S MS S MS

(3)

where nex is the excess adsorption amount (mol/m2); N is the total amount of gas (mol/m2); Vg is the gas-phase volume (g/ cm3); Vp is the free volume (g/cm3); M is the molar mass of the gas (g/mol); S is the surface area of the unit cell (m2); and ρg is the bulk density of the free gas (g/cm3), which was calculated using the SRK equation of state.33 The bulk density of CH4 can be seen in Figure S3. The free volume in the pores could be determined using a helium probe.34 For each pore size and each temperature, the free volume of the kaolinite pores was calculated. The excess adsorption amount is equal to the absolute adsorption amount when the temperature is lower than the critical temperature of the gas, whereas the excess adsorption amount is not equal to the absolute adsorption amount and the difference between the two increases with increasing pressure when the temperature is higher than the critical temperature of the gas. Therefore, a realistic adsorption amount for the adsorption system is not reflected by the excess adsorption amount obtained under supercritical conditions and excess adsorption amount should be converted into absolute C

DOI: 10.1021/acs.iecr.7b00838 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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ρg Va MS

(4)

where nab is the absolute adsorption amount (mol/m2) and Va is the adsorbed-phase volume (cm3). The adsorbed-phase volume could be calculated from the region of the adsorbed phase, and detailed information on this method can be found in ref 22. Figure 3. Absolute adsorption isotherms of CH4 from both simulation results and experimental results.

3. RESULTS AND DISCUSSION 3.1. Effects of Pore Size on CH4 Adsorption on Kaolinite. According to the simulation results, we could obtain the total amount of CH4. The isotherms for the total amounts of CH4 in the kaolinite pores at different pore sizes are shown in Figure 2a. Based on the total amounts of CH4, the absolute adsorption amounts of CH4 were calculated using eq 3 and eq 4. The absolute adsorption isotherms for CH4 in the kaolinite pores are depicted in Figure 2b. From Figure 2a, one can see that the total amount of CH4 in the kaolinite pores increased with increasing pore size; moreover, the total amount of CH4 in the kaolinite pores first increased rapidly and then increased slowly with increasing pressure. These findings are in line with the results of previous work on chlorite,21 quartz,22 and graphite with different O/C ratios,35,37 which were investigated by molecular simulations. Furthermore, it can be seen from Figure 2b that the absolute adsorption capacity of CH4 increased with the pressure, first rapidly and then slowly. This phenomenon is in agreement with the experimental results of previous studies on pure minerals8−13 and organic-rich shales4−7 and is also in line with the results of previous works on quartz22 and graphite with different O/C ratios.35,37 In addition, the absolute adsorption capacity of CH4 in pores with a size of 1 nm was the lowest, which can be related to the pore volume. The difference in the absolute adsorption capacity of CH4 in 1.5- and 2-nm pores was smaller and was more than that in mesopores. In mesopores, however, the absolute adsorption capacity of CH4 in the pores decreased with increasing pore size. This might be because the potential superimposed effects of the pore walls can significantly affect CH4 adsorption in kaolinite pore in micropores, so that the CH4 adsorption capacity in the pores would be limited by the pore volume. However, the CH4 adsorption in kaolinite mesopores is mainly affected by the surface potential effects of two sides of the pore walls, so that the CH4 adsorption capacity in these pores decreases with increasing pore size. Along with the simulation results, we present the available experimental results for methane adsorption on kaolinite samples at 338.4 K reported by Ji et al.10 in Figure 3. As can be seen in Figure 3, there were differences between the simulation results and the experimental results. The pore size of the kaolinite used in the experiments exhibited a continuous distribution and had a variable pore size distribution, so the CH4 absolute adsorption capacity obtained from the experiments reflects the synthesis results of the continuous pores. However, the pore skeleton of the kaolinite modeled in the simulations had a single pore size, so the CH4 absolute adsorption capacity obtained from the simulations reflects the results of for a single pore size and changed with changing pore size. Furthermore, the simulation results for methane

adsorption in kaolinite pores and the experimental results for CH4 adsorption on kaolinite matched well. In other words, the CH4 adsorption capacity in pores of a specific pore size was approximately equal to that on kaolinite. At the same time, one can see from Figure 3 that the equivalent pore size of the kaolinite was approximately 20 nm. Therefore, in this respect, the simulated models and simulated parameters were reasonable. According to the simulation results, we could obtain the absolute adsorption capacity of CH4 and the total adsorption amount of CH4. Therefore, the amount of gas adsorbed and the amount of free gas in the kaolinite−CH4 system could be obtained, from which we could calculate the volume proportion of adsorbed gas in the total amount of gas. The relationships between the volume proportion of adsorbed gas in the total amount of gas and the pressure in the kaolinite pores are shown in Figure 4. It can be noted from Figure 4 that the volume

Figure 4. Relationships between the volume proportion of adsorbed gas in the total amount and the pressure in pores with different pore sizes.

proportion of adsorbed gas decreased with increasing pore size or pressure, indicating that the volume proportion in the lowerpressure or smaller pores was greater than that in the higherpressure or larger pores. Furthermore, the volume proportion of adsorbed gas in pores at the same pore size first decreased rapidly and then decreased slowly with increasing pressure. Moreover, the volume proportion of adsorbed gas in pores under the same pressure decreased with increasing pore size. In addition, when the pore size was greater than 6 nm, the volume proportion of adsorbed gas in kaolinite pores under a pressure of 20 MPa was 25.76%, and the volume proportion of adsorbed gas in kaolinite pores under a pressure of 40 MPa was reduced to 19.03%. These findings indicate that the volume proportion of adsorbed gas under higher pressure in the kaolinite pores D

DOI: 10.1021/acs.iecr.7b00838 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Furthermore, from Figure 5, one can see that the average isosteric heats of adsorption of CH4 in kaolinite pores of different pore sizes were less than 42 kJ/mol, indicating that the CH4 adsorption on kaolinite was physical adsorption. This conclusion is in line with previous works 8−13 using experimental investigations. According to the simulation results, we could obtain the potential energy between CH4 and kaolinite. The potential energy distribution curves of CH4 and kaolinite at different pressures (for a pore size of 4 nm) are presented in Figure 6a. As shown in Figure 6a, with increasing pressure, the potential energy distribution curves of CH4 and kaolinite gradually moved toward the left (toward larger negative values). In addition, the most probable potential energy between CH4 and kaolinite gradually decreased with increasing pressure, indicating that the adsorption sites of the CH4 molecules in the kaolinite pores gradually changed from higher-energy adsorption sites to lower-energy adsorption sites with increasing pressure. It can also be observed that the adsorption state of the CH4 molecules in the kaolinite pores at low pressures was not as stable as that at high pressures. The potential energy distribution curves of CH4 and kaolinite with different pore sizes under a pressure of 20 MPa are shown in Figure 6b. We note that the potential energy distribution curves of CH4 and kaolinite gradually moved toward the right (toward smaller negative values) with increasing pore size. In addition, the most probable potential energy between CH4 and kaolinite gradually decreased as the pore size increased, indicating that the adsorption sites of the CH4 molecules in the kaolinite pores gradually changed from lower-energy adsorption sites to higherenergy adsorption sites with increasing pore size, which also suggests that the adsorption capacity in the kaolinite micropores is stronger than that in the kaolinite macropores. 3.2. Effects of Temperature on CH4 Adsorption on Kaolinite. The absolute adsorption isotherms of CH4 in the kaolinite pores at different temperatures are shown in Figure 7. According to Figure 7, the absolute adsorption capacity of CH4 decreased with increasing temperature at the same pressure. This is because the adsorption of CH4 on kaolinite is a physical adsorption. With increasing temperature, the thermal motion of the CH4 molecules would increase, resulting in an increase of the mean kinetic energy of the CH4 molecules, which would make escaping from the kaolinite pore walls easier, thus resulting in a reduction of the CH4 adsorption capacity. This conclusion is in accord with the results of the isothermal adsorption experiments conducted by Ji and co-workers,9,10 which suggested that the CH4 adsorption capacity on kaolinite

was lower, which indicates that the CH4 in pores existed mainly as free gas. According to the results of low-pressure N2 adsorption experiments,9,10 we found that the pore size of the kaolinite in the experiments was mainly between 10 and 70 nm, that is, the CH4 in the kaolinite pores of the organic-rich shales could be mainly in free state. According to the simulation results, we could obtain the isosteric heat of adsorption of CH4. The average isosteric heats of adsorption of CH4 in kaolinite pores at different pore sizes are shown in Figure 5. From Figure 5, one can see that the

Figure 5. Average isosteric heats of adsorption of CH4 in kaolinite pores with different pore sizes.

average isosteric heat of adsorption of CH4 decreased gradually with increasing pore size. During the simulations, the average isosteric heat of adsorption of CH4 corresponding to a pore size of 1 nm was the maximum, with a value of 13.57 kJ/mol, and the average isosteric heat of adsorption of CH4 corresponding to a pore size of 20 nm was the minimum, with a value of 6.12 kJ/mol. On the basis of experimental data, Ji and co-workers9,10 obtained the average isosteric heat of adsorption of CH4 adsorption on kaolinite as 9.6 kJ/mol. There is a difference between our simulated result and their experimental result, but they also show similarities to a certain extent. This might be related to the differences in the research methods and research objects used in the previous and current works. The pore size of the kaolinite in the experiments was a continuous distribution and was mainly between 10 and 70 nm,10,11 so that the isosteric heat of adsorption of CH4 obtained from the experimental results reflects the synthesis results of the continuous pores. However, the pore skeletons of the kaolinite in the simulation had a single pore size, and the isosteric heats of adsorption of CH4 obtained from the simulation results reflect the results for the single pore size and changed with changing pore size.

Figure 6. Potential energy distribution curves of methane and kaolinite at (a) different pressures (pore size of 4 nm) and (b) different pore sizes (pressure of 20 MPa). E

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Figure 9. Potential energy distribution curves of CH4 and kaolinite at different temperatures.

Figure 7. Absolute adsorption isotherms of CH4 in kaolinite pores at different temperatures.

formation increase will result in accelerating the desorption of CH4 molecules to a certain extent. 3.3. Effects of Water on CH4 Adsorption on Kaolinite. The distributions of water molecules with different contents in kaolinite pores are presented in Figure 10. As shown in Figure

decreased as the temperature increased. The same conclusion was also reported in refs 18−22, 35, 38, and 39 for systems investigated by molecular simulations, indicating that the CH4 adsorption capacities on minerals, carbon, and simplified kerogen decrease with increasing temperature. The average isosteric heats of adsorption of CH4 in kaolinite pores at different temperatures are shown in Figure 8. From

Figure 10. Distributions of water molecules in kaolinite pores at different water contents. Figure 8. Average isosteric heats of adsorption of CH4 in kaolinite pores at different temperatures.

10, the water molecules occupied the walls of the kaolinite pores in a directional manner, so that the oxygen atoms of the water molecules were close to or pointed toward the surface of the kaolinite pore walls or the hydrogen atoms of the surrounding water molecules and the hydrogen atoms were pointing away from the surface of the kaolinite pore walls. This might be because the aluminum and silicon atoms of kaolinite have positive charges on the surface of the kaolinite pore walls and the oxygen atoms of the water molecules have negative charges, causing the pattern that the oxygen atoms of the water molecules are close to or point toward the surface of the kaolinite pore walls. The reason for this phenomenon is that there are not only the Coulomb force interactions but also the van der Waals force interactions between the water molecules and the kaolinite, leading to the accumulation of water molecules in the chlorite pores. In addition, the oxygen atoms of the water molecules could point toward the hydrogen atoms of surrounding water molecules through the hydrogenbonding interactions. In conclusion, water molecules can adsorb and accumulate on the surface of the kaolinite pore walls and occupy the adsorption space of CH4 molecules. The absolute adsorption isotherms of CH4 in the kaolinite pores at different water contents are presented in Figure 11. According to Figure 11, the absolute adsorption capacity of CH4 decreased with increasing water content at the same temperature and pressure, indicating that the water molecules would hinder CH4 adsorption on kaolinite under the same

Figure 8, one can see that the average isosteric heats of adsorption of CH4 decreased as the temperature increases, illustrating that the interactions between the CH4 molecules and the kaolinite decreased with increasing temperature, thereby resulting in a decrease of the CH4 adsorption capacity. Within the range of simulated temperatures, the values of the average isosteric heats of adsorption of CH4 in kaolinite pores with a pore size of 4 nm ranged from 7.594 to 8.213 kJ/mol, which are all less than 42 kJ/mol, indicating that the adsorption of CH4 on kaolinite is physical adsorption. The potential energy distribution curves of CH4 and kaolinite at different temperatures can be seen in Figure 9. According to Figure 9, the potential energy distribution curve of CH4 and kaolinite gradually moved toward the right when the temperature was increased. In addition, the most probable potential energy between CH4 and kaolinite gradually increased with increasing temperature, indicating that the adsorption sites of the CH4 molecules in the kaolinite pores gradually changed from lowerenergy adsorption sites to higher-energy adsorption sites with increasing temperature, thus resulting in a decrease of the CH4 adsorption capacity on kaolinite. Therefore, tectonic movements can result in changes of the temperature and pressure in shale formations, leading to changes in the CH4 adsorption capacity. In addition, increasing the temperature in a shale F

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conclusions suggest that, when water molecules enter the nanoscale pores in shale formation, they occupy the adsorption space of methane molecules, leading to an increased desorption velocity of the CH4 molecules to a certain extent. 3.4. Effects of N2 on CH4 Adsorption on Kaolinite. The absolute adsorption isotherms of CH4 in the kaolinite pores at different mole fractions of N2 are shown in Figure 13. From

Figure 11. Absolute adsorption isotherms of CH4 in kaolinite pores at different water contents.

temperature and pressure conditions. This conclusion is in line with those of previous works on montmorillonite16,17 and chlorite,21 suggesting that the water greatly decreased the CH4 adsorption capacity in the pores. This observation is also in agreement with those of previous studies on quartz22 and carbon.35,39 The potential energy distribution curves of CH4 and kaolinite at different water contents are shown in Figure 12. According

Figure 13. Absolute adsorption isotherms of CH4 in kaolinite pores at different mole fractions of N2.

Figure 13, it can be seen that the CH4 absolute adsorption capacity decreased with increasing mole fraction of N2 or with decreasing mole fraction of CH4 at the same experimental temperature, illustrating that the lower the mole fraction of CH4 in the CH4/N2 binary gas mixture, the lower the CH4 adsorption capacity on kaolinite. For the CH4/N2 binary gas mixture adsorption system, the potential energy distribution curves at different mole fractions of N2 are shown in Figure 14. As shown in Figure 14, the most

Figure 12. Potential energy distribution curves of CH4 and kaolinite at different water contents.

to Figure 12, the potential energy distribution curves of CH4 and kaolinite at different water contents exhibited two peaks. The main peak of the potential energy distribution curve was located in the area where the energy was higher, whereas the area of the secondary peak was at lower energy. The main peak of the potential energy distribution curve did not change significantly with increasing water content, indicating that the higher-energy adsorption sites of the CH4 molecules in the kaolinite pores did not change with the change of the water content. However, the secondary peak of the potential energy distribution curves gradually became gentler as the water content increased, indicating that the water molecules in the kaolinite pores occupied the lower-energy adsorption sites of the CH4 molecules. Therefore, the water molecules in the kaolinite pores mainly occupied the lower-energy adsorption sites of the CH4 molecules rather than the higher-energy adsorption sites, and the water molecules and the CH4 molecules in the kaolinite pores mainly competed for adsorption space and adsorption sites. As a result, the water molecules occupied the adsorption space and adsorption sites of the CH4 molecules, causing reductions of the adsorption space and adsorption sites for the CH4 molecules and then leading to a decrease of the CH4 adsorption capacity. These

Figure 14. Potential energy distributions of methane and kaolinite at different mole fractions of N2.

probable potential energy between CH4 and kaolinite ranged from −3.975 to −2.720 kJ/mol, and the most probable potential energy between N2 and kaolinite was between −1.464 and −2.720 kJ/mol. That is, the most probable potential energy between CH4 and kaolinite at different mole fractions of N2 was less than that of N2, indicating that the potential energy distribution between CH4 and kaolinite was different from that between N2 and kaolinite. Specifically, the adsorption sites of CH4 molecules on the kaolinite pore walls were located in the lower-energy adsorption sites, whereas the adsorption sites of N2 on the kaolinite pore walls were in the higher-energy adsorption sites. This conclusion indicates that the adsorption of N2 on kaolinite in the adsorption system of CH4/N2 binary gas mixtures is less stable than the adsorption of CH4. That is, G

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Industrial & Engineering Chemistry Research the CH4 adsorption capacity on kaolinite is greater than the N2 adsorption capacity on kaolinite. Furthermore, from Figure 14, one can see that interactions between the CH4 and N2 molecules could lead to changes in both potential energy distribution curves. At the same time, the potential energy distribution curves of CH4 and kaolinite gradually moved toward the right, and the most probable potential energy between CH4 and kaolinite gradually increased as the mole fraction of N2 increased. These findings indicate that the adsorption sites of CH4 molecules in the kaolinite pores gradually changed from lower-energy adsorption sites to higherenergy adsorption sites with increasing mole fraction of N2, thereby resulting in a decrease of the CH4 adsorption capacity on kaolinite. The results also illustrate that the adsorption sites of N2 molecules in the pores led to a change in the adsorption sites of the CH4 molecules and a reduction of the adsorption space of the CH4 molecules. Thus, the CH4 adsorption capacity on kaolinite was greater than the N2 adsorption capacity in the CH4/N2 binary gas mixture adsorption system. Therefore, according to the above analysis, the CH4 adsorption capacity on kaolinite decreased with increasing mole fraction of N2 in the CH4/CO2 binary gas mixture, which can be related to the decrease of the mole fraction of CH4 in the CH4/N2 binary gas mixture, the change in the adsorption sites of the CH4 molecules, and the reduction of the adsorption space of the CH4 molecules. These findings indicate that the N2 adsorption capacity in the pores was less than the CH4 adsorption capacity. However, when the N2 molecules entered into the nanoscale pores, they could change the adsorption sites of the CH4 molecules and reduce the adsorption space of the CH4 molecules, resulting in accelerating the desorption of the CH4 molecules to a certain extent. 3.5. Effects of CO2 on CH4 Adsorption on Kaolinite. The absolute adsorption isotherms of CH4 in the kaolinite pores at different mole fractions of CO2 are shown in Figure 15.

Figure 16. Potential energy distributions of methane and kaolinite at different mole fractions of CO2.

−10.669 and −6.485 kJ/mol. That is, the most probable potential energy of CH4 at different mole fractions of CO2 was greater than that of CO2, suggesting that the potential energy distribution between CH4 and kaolinite was different than that between CO2 and kaolinite. Specifically, the adsorption sites of CO2 molecules on the kaolinite pore walls were located in the lower-energy adsorption sites, whereas the adsorption sites of CH4 molecules adsorbed on the kaolinite pore walls were in the higher-energy adsorption sites. This conclusion indicates that the adsorbed states of the CH4 molecules on kaolinite were less stable than the adsorbed states of the CO2 molecules. That is, the CO2 adsorption capacity on kaolinite was greater than the CH4 adsorption capacity. Furthermore, from Figure 16, one can observe that the interactions between the CH4 molecules and the CO2 molecules could lead to changes in both potential energy distribution curves. At the same time, when the mole fraction of CO2 increased, the potential energy distribution curves of CH4 and kaolinite gradually moved toward the right, and the most probable potential energy between CH4 and kaolinite increased, indicating that the adsorption sites of CH4 molecules in the kaolinite pores gradually changed from lowerenergy adsorption sites to higher-energy adsorption sites with increasing mole fraction of CO2, thereby resulting in a decrease of the CH4 adsorption capacity on kaolinite. This result also illustrates that the adsorption of CO2 molecules in the pores led to a change in the adsorption sites of the CH4 molecules and a reduction of the adsorption space of the CH4 molecules. Thus, the CH4 adsorption capacity on kaolinite was less than the CO2 adsorption capacity in the CH4/CO2 binary gas mixture adsorption system. Therefore, according to the above analysis, the CH4 adsorption decreased with increasing mole fraction of CO2 in the CH4/CO2 binary gas mixture, which can be related to the decrease of the mole fraction of CH4 in the CH4/CO2 binary gas mixture, the change in the adsorption sites of the CH4 molecules, and the reduction of the adsorption space of the CH4 molecules. These findings indicate that the CO2 molecules could result in accelerating the desorption of the CH4 molecules, which could improve the methane recovery rate in shale gas reservoirs to a certain extent.

Figure 15. Absolute adsorption isotherms of CH4 in kaolinite pores at different mole fractions of CO2.

According to Figure 15, the CH4 absolute adsorption capacity decreased with increasing mole fraction of CO2 or decreasing mole fraction of CH4 at the same experimental temperature, illustrating that the lower the mole fraction of CH4 in the binary gas mixture, the lower the CH4 adsorption capacity on kaolinite. For the CH4/CO2 binary gas mixture adsorption system, the potential energy distribution curves at different mole fractions of CO2 are shown in Figure 16. As can be seen from Figure 16, the most probable potential energy between CH4 and kaolinite ranged from −7.322 to −4.393 kJ/mol, and the most probable potential energy between CO2 and kaolinite was between

4. CONCLUSIONS In this work, the adsorption behaviors of CH4 in slit-like kaolinite pore were investigated by using the GCMC method, and the effects of pore size, temperature, water content, and composition on the adsorption of CH4 in kaolinite pores were examined, and their interaction mechanisms were also discussed. The following conclusions can be made: H

DOI: 10.1021/acs.iecr.7b00838 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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(1) With increasing pressure or decreasing pore size, the adsorption of CH4 in the kaolinite pores gradually changed from higher-energy adsorption sites to lowerenergy adsorption sites, leading to an increase in the CH4 adsorption capacity. The CH4 adsorption capacity decreased with increasing pore size in mesopores. (2) The average isosteric heat of adsorption of CH4 decreased with increasing temperature, and the adsorption sites of CH4 in the kaolinite pores gradually changed from lower-energy adsorption sites to higher-energy adsorption sites, leading to a reduction of the CH4 adsorption capacity. Under the combined van der Waals force interactions, Coulomb force interactions, and hydrogen-bonding interactions, water molecules in the kaolinite pores occupied the pore walls in a directional manner and took up the adsorption sites and adsorption space of CH4 molecules, leading to a decrease of the CH4 adsorption capacity. (3) In gas adsorption systems, the potential energy between the gas and kaolinite was found to decrease in the order N2 > CH4 > CO2, indicating that the adsorption capacity decreased in the opposite order, namely, CO2 > CH4 > N2. When the mole fraction of N2 or CO2 in the gas phase increased, the mole fraction of CH4 in the gas phase decreased; the adsorption sites of CH4 molecules changed; and the adsorption space of CH4 molecules reduced, resulting in a decrease of the CH4 adsorption capacity.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b00838. Low-pressure N2 adsorption−desorption isotherms of a kaolinite sample sourced from Shanxi, China (Figure S1); fugacity coefficients of CH4 at different temperatures and pressures (Figure S2); densities of CH4 at different temperatures and pressures (Figure S3); parameters for slit-like kaolinite pores with different pore sizes (Table S1); and Lennard-Jones (L-J) potential parameters and charges of each atom in kaolinite and the investigated liquids (Table S2) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xiangjun Liu: 0000-0002-0633-0989 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (NSFC) (Nos. 41602155 and 51504202) and the Young Scholars Development Fund of SWPU (No. 201599010137).

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REFERENCES

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DOI: 10.1021/acs.iecr.7b00838 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX