To develop the crucial concepts of clathrate hydrates toward the hydrate-based technological implications, it is indispensable to comprehend their formation and growth mechanisms and stability in pure and saline water environments. In this view, we provide fundamental insights into the gas hydrate dynamics by carefully conducting a molecular dynamics simulation in an extensive range of carbon dioxide (CO 2 ) gas (CO 2 /H 2 O from 1:5 to 1:18) and sodium chloride (NaCl) salt (from 0.0 to 18.0 wt %) concentrations. Using the F 4φ order parameter and the radial distribution function, we assess the important information about the time evolution of the visualization states and the formation and growth of small (5 12 ) and large (5 12 6 2 and 5 12 6 4 ) cages of hydrates along with their crystalline nature. We found that (1) CO 2 forms the pure S−I type of hydrate structure irrespective of guest gas and salt concentrations; (2) lower CO 2 gas concentration (CO 2 /H 2 O from 1:8 to 1:18) leads to fast but incomplete conversion of water to hydrate, while the higher CO 2 gas concentration (1:6) causes the phase separation and consequent sluggish hydrate growth; (3) 1:7 is an optimum CO 2 /H 2 O ratio for the rapid, complete, and properly ordered hydrate growth; (4) at the optimum amount of CO 2 and H 2 O, the lower range of salt concentrations (0.0−5.0 wt %) has a slight inhibition effect on the hydrate growth, while there is a notable inhibition effect for the higher salt concentrations (7.0−18.0 wt %); (5) the number of oxygen atoms of water present in the first coordination sphere remains constant for the lower salt concentrations (0.0−5.0 wt %), and they get reduced with the higher salt concentrations (7.0−18.0 wt %); (6) the inhibition effect is due to the reduction of CO 2 solubility in the aqueous phase in the presence of salt ions. These novel findings provide useful assistance for choosing an appropriate combination of the imperative elements of gas hydrate systems toward carbon dioxide separation, sequestration, storage, and transportation.

The growth dynamics of natural gas hydrates in saline water has been studied using copious experiments and spectroscopic observations; however, the microscopic evidences to the structural and molecular transformations that they have provided are poorly understood. In this view, we perform extensive molecular dynamics simulations to gain physical insights into the formation and growth mechanism of naturally occurring gas hydrates with a wide variation in the amount of methane (1:5 to 1:18 methane/water ratio) in pure and salt (0–5 wt %) water environments at 50 MPa and 260 K. A couple of new findings analyzed from the number of cages and F 4φ order parameter are as follows: (a) 1:6 (methane/water ratio) is an optimum ratio for the rapid growth of a properly ordered hydrate in pure water at which the hydrate growth retards with increasing salt concentration, (b) there is an inconsequential difference between methane hydrate dynamics in pure water and 0.8 and 1.5 wt % salt water at a ratio of 1:12 (methane/water), and (c) lower methane (1:18) and salt (0.8 wt %) concentrations promote hydrate growth. Besides, this study observes the structural coexistence of S-I and S-II methane hydrates as the large 51264 cages appear along with the small 512 and large 51262 cages, in which the low methane concentration favors the S-II structure.

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This work aims at formulating the noncovalent interactions of the mixed guest hydrate lattices of sI and sII structures. The reduced density plots obtained at the B3LYP level with 6‐31G(d) split valence set unveil the crucial contributions of hydrogen bonding, dispersion, van der Waals interaction, and steric effects toward lattice stability. These contributions are rather unreported in the hydrate phase equilibrium modeling. With this research gap, we attempt to formulate the natural regularity in the nonstoichiometric hydrates by the spherical harmonics shape descriptor. The van der Waals and hydrogen‐bond interactions are described with Kihara spherical cell potential and ab initio based calculations, respectively. Further, the ionization potential and polarizability of the guest molecule describe the dispersion interactions. Combining these contributions, we introduce a theoretical framework for the phase equilibrium modeling of mixed guest hydrates. This novel formulation offers various appealing advantages as: significantly reduced parametric structure, standardization of the parameter value for host–host pair, and generalized model framework. Despite of making the proposed theory so simple, it consistently outperforms the existing latest models, which employ various state equations, ab initio based calculations and CSMGem, for a wide variety of systems (total 26 systems tested) with reference to the experimental data.

High latent heat storage capacity with naturally assisted salt rejection makes the clathrate compounds appropriate for applications towards load management and desalination processes. Adding to these energy savings are the ease of operations provided by water and the mild conditions at which the refrigerant hydrates are occurred. A direct comparison between these hydrates becomes unfeasible due to the scattered experimental data. Though thermodynamics can streamline this dispersed data, they are currently limited to being a proof of concept most accurately representing the experimental observations. We address this critical deficit of phase assessment and identify, from among R13, R14, R22, R23, R125, R134a and R152a, the most suitable hydrate former for the concerned application. An approach based on van der Waals and Platteeuw model is undertaken and the estimates are quantified in terms of percent average absolute relative deviations (% AARD). An average AARD of 1.75% and 2.68% is observed in pure and aqueous electrolytic phase of NaCl, KCl, CaCl2 and MgCl2, respectively. The model predictions are then estimated at temperature/salinity of 281 K/0 wt% and 284 K/3.5 wt%. Together with the qualitative assessment of the hydrate phase, viz, vapor pressure, compressibility and dissociation enthalpy, R152a refrigerant is observed to be the appropriate former for applications to both load management and desalination.