Molecular Dynamics Study on the Equilibrium and Kinetic Properties of Tetrahydrofuran Clathrate Hydrates (original) (raw)
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Molecular dynamics study of structure H clathrate hydrates of methane and large guest molecules
The Journal of Chemical Physics, 2008
Methane storage in structure H ͑sH͒ clathrate hydrates is attractive due to the relatively higher stability of sH as compared to structure I methane hydrate. The additional stability is gained without losing a significant amount of gas storage density as happens in the case of structure II ͑sII͒ methane clathrate. Our previous work has showed that the selection of a specific large molecule guest substance ͑LMGS͒ as the sH hydrate former is critical in obtaining the optimum conditions for crystallization kinetics, hydrate stability, and methane content. In this work, molecular dynamics simulations are employed to provide further insight regarding the dependence of methane occupancy on the type of the LMGS and pressure. Moreover, the preference of methane molecules to occupy the small ͑5 12 ͒ or medium ͑4 3 5 6 6 3 ͒ cages and the minimum cage occupancy required to maintain sH clathrate mechanical stability are examined. We found that thermodynamically, methane occupancy depends on pressure but not on the nature of the LMGS. The experimentally observed differences in methane occupancy for different LMGS may be attributed to the differences in crystallization kinetics and/or the nonequilibrium conditions during the formation. It is also predicted that full methane occupancies in both small and medium clathrate cages are preferred at higher pressures but these cages are not fully occupied at lower pressures. It was found that both small and medium cages are equally favored for occupancy by methane guests and at the same methane content, the system suffers a free energy penalty if only one type of cage is occupied. The simulations confirm the instability of the hydrate when the small and medium cages are empty. Hydrate decomposition was observed when less than 40% of the small and medium cages are occupied.
Journal of Molecular Graphics & Modelling, 2022
In this paper we report a successful molecular simulation study exploring the heterogeneous crystal growth of sI methane hydrate along its [001] crystallographic face. The molecular modeling of the crystal growth of methane hydrate has proven in the past to be very challenging, and a reasonable framework to overcome the difficulties related to the simulation of such systems is presented. Both the microscopic mechanisms of heterogeneous crystal growth as well as interfacial properties of methane hydrate are probed. In the presence of the appropriate crystal template, a strong tendency for water molecules to organize into cages around methane at the growing interface is observed; the interface also demonstrates a strong affinity for methane molecules. The maximum growth rate measured for a hydrate crystal is about 4 times higher than the value previously determined for ice I in a similar framework (
The kinetic modeling of methane hydrate growth by using molecular dynamic simulations
International Journal of Heat and Mass Transfer, 2019
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 À1.05 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.
The Journal of Physical Chemistry C
A sound knowledge of thermodynamic properties of sII hydrates is of great importance to understand the stability of sII gas hydrates in petroleum pipelines and in natural settings. Here, we report direct molecular dynamics (MD) simulations of the thermal expansion coefficient, the compressibility and the specific heat capacity of C3H8, or tetrahydrofuran (THF), in mixtures of CH4 or CO2, in sII hydrates under a wide, relevant range of pressure-and temperature conditions. The simulations were started with guest molecules positioned at the cage center of the hydrate. Annealing simulations were additionally performed for hydrates with THF. For the isobaric thermal expansion coefficient, an effective correction method was used to modify the lattice parameters, and the corrected lattice parameters were subsequently used to obtain thermal expansion coefficients in good agreement with experimental measurements. The simulations indicated that the isothermal expansion coefficient and the specific heat capacity of C3H8-pure hydrates were comparable, but slightly larger than those of THF-pure hydrates, which could form Bjerrum defects. The considerable variation in the compressibility between the two, appeared to be due to crystallographic defects. However, when a second guest molecule occupied the small cages of the THF hydrate, the deviation was smaller, because the subtle guest-guest interactions can offset an unfavorable configuration of unstable THF hydrates, caused by local defects in free energy. Unlike the methane molecule, the carbon dioxide molecule, when filling the small cage, can increase the expansion coefficient and compressibility as well as decrease the heat capacity of the binary hydrate, similar to the case of sI hydrates. The calculated bulk modulus for C3H8 pure and binary hydrates with CH4 or CO2 molecule varied between 8.7 and 10.6 GPa at 287.15K between 10 and 100MPa. The results for the specific heat capacities varied from 3155 to 3750.0 J kg-1 K-1 for C3H8 pure and binary hydrates with CH4 or CO2 at 287.15K. These results are the first of this kind reported so far. The simulations show that the thermodynamic properties of hydrates largely depend on the enclathrated compounds. This provides a much-needed atomistic characterization of the sII hydrate properties, and gives an essential input for large-scale discoveries of hydrates and processing as a potential energy source.
Molecular dynamics simulations of methane clathrate hydrate and methane/water mixtures
The melting of structure I methane clathrate hydrate has been investigated using NVT molecular dynamics simulations, for a number of potential energy models for water and methane. The equilibrated hydrate crystal has been heated carefully from 270 K, in steps of 5 K, until a well de® ned phase instability appears. At a density of 0± 92 g cm Õ $ , an upper bound for the mechanical stability of the methane hydrate lattice over a timescale of 11 nanoseconds is 330 K. Finite size eOE ects have been investigated by simulating systems of 1 and 8 units cells of methane hydrate. The properties of the melted system upon cooling are examined.
Phase Stability of Hydrates of Methane in Tetrahydrofuran Aqueous Solution and the Effect of Salt
Journal of Chemical & Engineering Data, 2014
Phase equilibrium data are generated for clathrate hydrates of methane in THF aqueous solution for (0.040, 0.016, 0.010, and 0.005) mass fraction and with NaCl ((0.03, 0.05, and 0.10) mass fraction) in combination with THF ((0.010 and 0.005) mass fraction) for the methane hydrate system to study the effect of salt. The pressure−temperature curves for equilibrium points have been generated by employing an isochoric pressure-search method. The phase stability conditions were reported for a wide range of pressure (2.17 MPa to 6.43 MPa) and temperature (276.15 K to 297.70 K). Contending effects of THF and NaCl at various concentrations on the phase stability of the clathrate hydrate of methane have been studied. The inhibition effect of NaCl is limited by the promotion effect of THF for the clathrate hydrate of methane, even though there is a shift in the hydrate equilibrium curve toward the inhibition zone. The inhibition effect shown by salt is more enunciated at higher pressures compared to lower pressures. The promotion effect is found to decrease as the concentration of NaCl is increased. Moreover, the promotion effect of THF at a lower concentration is commendable on NaCl with higher concentration for methane hydrate formation. The values of heat of dissociation of hydrates for the (THF + CH 4) system and the (THF + NaCl + CH 4) system at different experimental pressure and temperature conditions are calculated using the Clausius−Clayperon equation for the obtained phase equilibrium data and reported. This study shows that the clathrate hydrates of methane in THF and in (THF + NaCl) aqueous system are anticipated to be more stable as compared to the hydrates of pure methane, thus promising their use for formation, storage, and transportation of the hydrates of natural gas in a real environment.
Journal of Natural Gas Science and Engineering, 2017
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.
International Journal of Hydrogen Energy, 2014
Hydrogen hydrate formation and decomposition kinetics using tetrahydrofuran (THF), tetra-n-butylammonium bromide (TBAB) and cyclopentane (CP) as promoters under similar experimental conditions was studied. First set of experiments on hydrate formation were conducted at same promoter concentrations, experimental pressure and experimental temperature and the second set of experiments were conducted at same experimental pressure, same driving force and varying promoter concentrations. Hydrogen storage capacity of THF/Hydrogen hydrate was the highest amongst the three promoters under the experimental conditions studied. Hydrogen uptake of 0.0173 mole of gas/mole of water was obtained for the 3.5 mol% THF solution and it was about 2 times higher than hydrogen uptake obtained for 3.5 mol% TBAB solution under similar experimental conditions. With the same amount of heat supplied, TBAB/Hydrogen mixed semi-clathrates took longer time to dissociate compared to THF/Hydrogen hydrates under similar decomposition conditions. It was very difficult to form CP/Hydrogen hydrates even at a very high driving force.
Industrial & Engineering Chemistry Research, 2009
In this work, experimental dissociation data for the clathrate hydrates of tetrahydrofuran + hydrogen sulfide and tetrahydrofuran + methane are reported. The experimental data were generated using an isochoric pressuresearch method. The dissociation data for the tetrahydrofuran + methane clathrate hydrates are compared with the corresponding experimental data reported in the literature, and the acceptable agreement demonstrates the reliability of the experimental method used in our work. Moreover, we extend the literature data for the latter system to a low concentration of tetrahydrofuran in its aqueous solution. As addition of high concentrations of tetrahydrofuran in aqueous solution diminishes its pressure-reducing effect, we therefore measured and report the experimental dissociation data for the tetrahydrofuran + hydrogen sulfide clathrate hydrates at low concentrations of tetrahydrofuran in its aqueous solution.