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 À1 . 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.

Simulation of Adsorption–Desorption Behavior in Coal Seam Gas Reservoirs at the Molecular Level: A Comprehensive Review

“…The adsorption isotherm is a criterion used to examine the adsorption capacity of a gas under diverse temperatures and pressures. ,,,,− ,,,,,,,, …”

“…The single absolute adsorption pattern of CO 2 and CH 4 matches the type I Langmuir adsorption curve well, ,,,,,,,, which increases at first and then stabilizes with increasing pressure at constant temperature, whereas with increasing temperature, the Langmuir adsorption curve reduces. ,,,,,,,, This performance is a consequence of the physisorption process being exothermic, that is, the adsorbate exhibiting a negative response with increasing temperature, thereby displaying a reduced loading pattern. ,,,,,,,,, The aggregation of gas molecules on the adsorbent surface typically depends on two factors, where one is the collision number between gas molecules and the adsorbent and the other is the duration of gas molecules being attached to the adsorbent. , As the temperature is relevant to kinetic energy, a more reinforced thermal motion is achieved with increasing temperature. Thus, the increased kinetic energy allows the gas molecules to escape from the energy barrier easily, consequently decreasing the adsorption loading. ,,,,,, Additionally, the duration of gas molecules being attached to the adsorbent surface becomes shorter with increasing temperature due to the thermal motion, , which results in lower gas adsorption. Moreover, the increased temperature is more beneficial to the desorption process; ,, therefore, a higher injection temperature should be considered to enhance CH 4 production.…”

“…The isosteric heat of adsorption is an important thermodynamic parameter for evaluating energy liberation, and displaying the strength of an interaction. ,, The following formula is used to investigate the adsorption characteristics, ,,,,,, where Q st is the isosteric heat of adsorption in the absolute number of n (kJ/mol), R g is the gas content (kJ/(mol·K)), T is the temperature (K), U ad is the total adsorption energy for the adsorbed phase (kJ), and N ad is the total molar number of adsorbates of n (mol).…”

“…The COMPASS (condensed-phase optimized molecular potential for atomistic simulation studies) force field involves a covalent-bond model, ion model, quasi-ion model, and metal model . The covalent-bond model can be employed for both organic and inorganic systems. − …”

“…The NVT ensemble is usually used to examine the transport properties of gas molecules. ,,, Additionally, it is also applied to calculate the electrostatic interaction and van der Waals potential to obtain the dynamic properties and density distribution of gas molecules …”