Investigation of dynamical properties of methane in slit-like quartz pores using molecular simulation

Abstract

The dynamical properties of adsorption media confined in micropores play an important role in the adsorptive separation of fluids. However, a problem is that it is difficult to directly use approaches based on experimental measurements. Molecular simulation has been an effective tool for investigating the diffusion of fluids on the microscale in recent years. In this work, the diffusion properties of methane in quartz were mainly investigated from a microscale viewpoint using MD (molecular dynamics) methods, and this paper primarily discusses the influence of parameters such as pressure, temperature, pore size and water content on the diffusion and thermodynamic parameters of methane in slit-like quartz pores. The results demonstrate that the transport ability of quartz pores decreases with an increase in pressure in pores of a fixed size at a certain temperature and increases with an increase in pore size or temperature at a fixed pressure, which is related to changes in the interaction between methane molecules and quartz. In the pressure range used in the simulation, the average isosteric heat of adsorption of methane increases with an increase in pressure and is in the range of 6.52-10.794 kJ mol^-1. Therefore, the gas adsorption behavior is classed as physical adsorption because the heat of adsorption is significantly lower than the minimum heat of gas adsorption for chemisorption. The increase in the total adsorption entropy is caused by an increase in temperature due to an increase in internal energy, which brings about a reduction in the interactions between gas molecules and walls of quartz. However, with an increase in pore size the total adsorption entropy increases, for which an explanation may be that in pores of a larger size methane molecules are adsorbed at higher-energy sites and generate a higher isosteric heat, which causes a reduction in interactions between the adsorbate and adsorbent. Regarding the influence of different water contents on the diffusion of methane, it was demonstrated that with an increase in moisture the mobility of methane molecules initially increases and then decreases, which is related to the distance between gas molecules.

Fig. 1. Fluid models and structure of slit-like quartz pores.

Fig. 2. Relationship between the excess adsorbed amount of CH₄ and the pressure for various sizes of slit-like pores.

Fig. 3. Isotherms for the excess adsorbed amount of CH₄ for different pore sizes.

Fig. 4. Comparison of the maximum excess adsorbed amount in this work with literature results from previous papers.

Fig. 5. Change in E_total of the adsorption system at various pressures (E_total represents the total energy in the system).

Fig. 6. Relationship between diffusion coefficient of methane and pressure in pores with a size of 1 nm at 60 °C.

Fig. 7. Correlation between heat of adsorption and pressure.

Fig. 8. Relationship between self-diffusion coefficient of methane and temperature in pores with a size of 3 nm at 21 MPa.

Fig. 9. Density distribution of methane in pores with a size of 3 nm at 21 MPa at different temperatures.

Fig. 10. Relationship between the adsorption entropy and temperature.

Fig. 11. Self-diffusion coefficient of methane at 5 MPa and 21 MPa for various pore sizes.

Fig. 12. Snapshots of methane molecules obtained for a pore size of (a) 10 Å, (b) 20 Å, and (c) 30 Å.

Fig. 13. Density distributions of methane at 21 MPa for various pore sizes.

Fig. 14. Relationship between the adsorption entropy and different pore sizes.

Fig. 15. Distribution of methane molecules with various water saturations in a 4 nm pore.

Fig. 16. Relationship between total gas adsorption and water saturation in a 4 nm pore.

Fig. 17. Methane density distributions at different water contents.

Fig. 18. Self-diffusion coefficient of methane for different water contents at 21 MPa.

Fig. 19. Correlation between heat of adsorption and water content.