Meitan xuebao (Mar 2024)
Research on the adsorption-diffusion mechanism of hydrogen sulfide based on Monte Carlo simulation
Abstract
In order to clarify the microscopic dynamics mechanism of hydrogen sulfide (H2S) adsorption and diffusion in coal, and to reveal the influence mechanism of different temperatures and pressures on the molecular adsorption and diffusion characteristics of coal adsorbed H2S, based on the Giant Canonical Monte Carlo (GCMC), Molecular Dynamics (MD), and Density Functional Theory (DFT) methods, the adsorption-diffusion characteristics of H2S in the gas-fertilized coal macromolecule model at temperatures ranging from 273.15 K to 313.15 K and pressures ranging from 1 to 1 000 kPa were investigated using Material Studio software. The results showed that the saturated adsorption of H2S decreased from 38.34 mL/g to 31.85 mL/g at an increase in temperature from 273.15 K to 313.15 K, which is a 16.93% decrease. The effect of temperature on adsorption is most sensitive when the pressure is 1 kPa. The most significant interaction energy increased from −39.391 kJ/mol to −34.301 kJ/mol when the pressure was increased from 1 kPa to 1 000 kPa at a temperature of 293.15 K. With the pressure increased, the most significant interaction energy increased first rapidly and then slowly. During the adsorption of H2S, the isocratic heat of adsorption of H2S was in the range of 36.63−41.43 kJ/mol, which is a physical adsorption. The isocratic heat of adsorption showed a negative exponential change with increasing adsorption volume. The Gibbs free energy ΔG of H2S was from −3.57 to −24.57 kJ/mol, and the entropy of adsorption ΔS was from −0.126 to −0.194 8 kJ/(mol·K). The absolute values of ΔG and ΔS linearly decreased with increasing adsorption amount, and the adsorption spontaneity of H2S and the chaos of the system decreased. The interaction energy of H2S with gas-fertilized coal was ranged from −492.47 to −3 390.95 kJ/mol, which was dominated by van der Waals’ energy accounting for 58.67% of the total energy, and supplemented by electrostatic energy accounting for 41.33%. As the adsorption capacity increased, the absolute value of interaction energy increased, and the changes in adsorption capacity and interaction energy were consistent. H2S interacted most strongly with the carboxyl group, followed by the hydroxyl group. Double layer adsorption of H2S occurred around —OH, —COOH, —C=O. The temperature was increased from 273.15 K to 313.15 K. The diffusion coefficient of H2S molecules was increased from 1.066×10−10 m2/s to 2.025×10−10 m2/s, and the activation energy of diffusion was 11.206 kJ/mol. An increase in temperature can lead to the opening of previously closed pores and channels, increasing the connectivity of cracks. As the temperature rises, it increases the average free path of H2S molecules, enhancing their diffusion ability. The limiting heat of adsorption of H2S was 42.898 kJ/mol. The H2S concentration distribution showed a multi-peak structure, and H2S was distributed in a laminar structure in the gas-fertilized coal macromolecule model. H2S had hydrogen bonding with —OH, —COOH, and —C=O reactive groups on the coal body, and there was a weak chemisorption of H2S in the early stage of adsorption.
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