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In the present work, the thermodynamic, structural and dynamical properties of methane hydrate system were predicted by molecular dynamic simulations. Having knowledge of how methane/water system undergoes thermodynamic, structural and dynamical changes is practical, when methane hydrate is formed. So the aim of this work is to investigate the differences in the properties between two systems; methane/water and methane/water/hydrate systems. The results showed the thermodynamic properties of methane/water/hydrate system are lower than that of another one implying the hydrate structure is more stable and decreases the energy surface of system. The potential energy, density and MSD profiles were determined to distinguish the transition position and width of the interface of hydrate clathrate. Also, the diffusivity reduction proves that the molecular structure varied from liquid-like to the solid-like. A procedure was used to calculate the water/hydrate surface tension that is consistent with the previously reported. Finally, the comparison of oxygen-oxygen, carbon-carbon and carbon-oxygen radial distribution functions indicated that the heights of peaks increase and become narrow in methane hydrate system confirming the regular arrangement of methane and water molecules in the hydrate phase. [ABSTRACT FROM AUTHOR]

In this work, cationic (Mg, Ca, Sr, and Ba) substitution in X and Y faujasite zeolite structures and their effects on capacity of zeolites for methane adsorption were studied by applying multiple linear regression and expert adaptive neuro-fuzzy inference system (ANFIS) . Temperature, pressure, and molecular weight of cations were used as the input parameters. The results obtained from application of the proposed ANFIS model showed that at high pressures, the zeolite with smaller cation in their structure have higher methane adsorption capacity. The root-mean-square error, square correlation coefficient ( R ), mean absolute error, and percentage of mean absolute relative error for X and Y faujasite zeolites were evaluated, which indicated that ANFIS model can accurately predict the adsorption of methane gas on X and Y zeolites in the presence of the substituted cations. [ABSTRACT FROM AUTHOR]

Molecular dynamic simulations have been served to investigate various properties of methane/water system at two extreme conditions: away from hydrate formation and threshold it. The results that compare the two conditions have been discussed in the terms of surface tension, mean square displacement (MSD), diffusion coefficient, radial distribution function and thermodynamic functions. Investigation over a timescale of 500 ps demonstrated that the thermodynamic functions at 275 K and high pressures (corresponding to the hydrate formation threshold) are smaller than that of at 298 K, implying that the hydrate formation threshold is more stable. An increase in the system pressure and temperature leads to a decrease in the surface tension because of decreasing the intermolecular attractive forces. Besides, the comparison of oxygen–oxygen, carbon–carbon and carbon-oxygen radial distribution functions at temperatures of 275 and 298 K has been indicated that the height of peaks decreases due to increasing the temperature, demonstrating that the system become more stable at the threshold of hydrate formation. [ABSTRACT FROM AUTHOR]

Highlights: [•] The adsorption and separation properties of new siliceous zeolites were investigated. [•] The force fields of COMPASS, PCFF, CVFF, DREIDING and Universal were used in the GCMC. [•] The PCFF is the best force field for simulation of adsorption in zeolites. [•] RWY zeolite is the best zeolite types for the separation of binary mixture. [Copyright &y& Elsevier]

• MD simulation was applied to study the kinetics of methane hydrate growth. • A kinetics model based on the non-equilibrium thermodynamics was considered. • Two-parametric model had a constant parameter in all the operational conditions.. • The hydrate formation is a process proceeding on a natural path. In the present work, the molecular dynamic simulation was applied for studying the kinetics of methane hydrate growth. Several parameters such as the changes of potential energy, the MSD of molecules, the number of methane molecules near the solid/liquid interface and the position of liquid/solid interfaces with time were considered. The results showed that the potential energy and MSD of molecules in the layers near the interfaces significantly decreases implying that the growth proceeds in these layers. The maximum rate of growth or migration of methane molecules to the interfaces is observed around 2 ns. Moreover, a kinetics model was considered to predict hydrate growth kinetics. It is based on the irreversible and non-equilibrium thermodynamics and the concept of the thermodynamic natural path. The proposed model is a two-parametric model that one parameter was estimated to be a nearly constant value in the range from −1.05 to 1.46, but another one is a kinetic parameter dependent on the operational conditions. The model can well predict the entire process of hydrate formation, since the affinity, as a driving force of the process, shows that the hydrate formation is a process proceeding on a natural path. [ABSTRACT FROM AUTHOR]