Guest Molecules In Hydrates Research Articles

Crystallization Mechanisms and Rates of Cyclopentane Hydrates Formation in Brine

AbstractClathrate hydrates most often grow at the interface between liquid water and another fluid phase (hydrocarbon) acting as a provider for the hydrate guest molecules, and some transfer through this shell is required for the hydrate growth to proceed, thus self‐limiting the reaction rate. An optical microscope and a horizontal reaction cell are utilized to capture the shell growth phenomenology and to estimate the hydrate layer growth rates from sequential pictures. Cyclopentane (CP) is chosen as the hydrate‐forming molecule to obtain hydrates at low pressure. Experimental hydrate layer growth rates are provided for the CP+brine system, using various combinations of salts and degrees of subcooling.

Analysis of Parameter Values in the van der Waals and Platteeuw Theory for Methane Hydrates Using Monte Carlo Molecular Simulations

The van der Waals and Platteuw (vdVVP) theory has been successfully used to model the thermodynamics of gas hydrates. However, earlier studies have shown that this could be due to the presence of a large number of adjustable parameters whose values are obtained through regression with experimental data. To test this assertion, we carry out a systematic and rigorous study of the performance of various models of vdWP theory that have been proposed over the years. The hydrate phase equilibrium data used for this study is obtained from Monte Carlo molecular simulations of methane hydrates. The parameters of the vdWP theory are regressed from this equilibrium data and compared with their true values obtained directly from simulations. This comparison reveals that (i) methane-water interactions beyond the first cage and methane-methane interactions make a significant contribution to the partition function and thus cannot be neglected, (ii) the rigorous Monte Carlo integration should be used to evaluate the Langmuir constant instead of the spherical smoothed cell approximation, (iii) the parameter values describing the methane-water interactions cannot be correctly regressed from the equilibrium data using the vdVVP theory in its present form, (iv) the regressed empty hydrate property values closely match their true values irrespective of the level of rigor in the theory, and (v) the flexibility of the water lattice forming the hydrate phase needs to be incorporated in the vdWP theory. Since methane is among the simplest of hydrate forming molecules, the conclusions from this study should also hold true for more complicated hydrate guest molecules.

Raman Spectroscopic Studies of Surfactant Effect on the Water Structure around Hydrate Guest Molecules

The addition of a small amount of surfactants enhances hydrocarbon enclathration rates without any mechanical agitation, but the role of surfactants has yet to be clarified. This work presents the effect of sodium dodecyl sulfate (SDS) on the water structure around cyclopentane (CP), a sII hydrate former, at the hydrate−water interface. The arrangement of local water molecules was obtained using surface-enhanced Raman spectroscopy (SERS). The Raman spectra reveal that the water structure around CP at the water−hydrate interface is identical to the hexakaidecahedron (51264) cavity of sII hydrates at a SDS concentration higher than 0.087 mM. However, at the CP−water interface, with and without SDS, the water arrangement is clathrate-like. Additionally, we found that the microenvironment of CP in bulk aqueous solutions is close to that in hydrates at a SDS concentration higher than 0.35 mM.

Measurement of Clathrate Hydrates via Raman Spectroscopy

Raman spectra of clathrate hydrate guest molecules are presented for three known structures (I (sI), II (sII), and H (sH)) in the following systems: CH4 (sI), CO2 (sI), C3H8 (sII), CH4 + CO2 (sI), CD4 + C3H8 (sII), CH4 + N2 (sI), CH4 + THF-d8 (sII), and CH4 + C7D14 (sH). Relative occupancy of CH4 in the large and small cavities of sI were determined by deconvoluting the ν1 symmetric bands, resulting in hydration numbers of 6.04 ± 0.03. The frequency of the ν1 bands for CH4 in structures I, II, and H differ statistically, so that Raman spectroscopy is a potential tool to identify hydrate crystal structure. Hydrate guest compositions were also measured for two vapor compositions of the CH4 + CO2 system, and they compared favorably with predictions. The large cavities were measured to be almost fully occupied by CH4 and CO2, whereas only a small fraction of the small cavities are occupied by CH4. No CO2 was found in the small cavities. Hydration numbers from 7.27 to 7.45 were calculated for the mixed hydrate.

Polar guest-molecules in natural gas hydrates. Effects of polarity and guest-guest-interactions on the Langmuir-constants

The effect of partial charges on guest-molecules in hydrates of structure I and II have been examined for H2S, CO2 and SO2 using Monte Carlo-simulations. The simulations indicate that Langmuir-constants for molecules with an average positive charge exposed towards the cavity-walls increases when compared to the corresponding models without charges. The opposite result is observed for molecules with a average negative charge exposed to the cavity-walls. Guest-guestinteractions increases the Langmuir-constants for both types but the effect is relatively larger for H2S than CO2 and SO2