In situ Raman and ¹³C NMR spectroscopic analysis of gas hydrates formed in confined water: application to natural gas capture

Abstract

This study investigates the formation characteristics of gas hydrate from bulk water as well as dispersed water in silica gel and dry water particles when they are exposed to natural gas. The inclusion process of methane, ethane, and propane molecules in hydrate cages were observed with in situ Raman spectroscopy, and the resulting cage occupancies were estimated from 13 C NMR spectra. A high-pressure autoclave was used to monitor the formation process to determine hydrate onset time, initial growth rate, and conversion ratio. The obtained data from Raman spectra and gas consumption profiles suggested that hydrate formed within less than 20 min when the temperature is sufficiently lower than the hydrate equilibrium condition at a given pressure. Methane molecules started to occupy the small cages of structure II, but about 6 min later ethane and propane were also included in hydrate cages. 13 C NMR spectroscopy confirms that only 23% of large cages of structure II are occupied by methane molecules when hydrate formed from dispersed water in silica gel, which was much less than 68% from dry water. These results suggest that the dispersion of water in silica gel and dry water would enhance the formation process by increasing gas-to-water ratio, although the composition of hydrate phase may vary depending on the formation condition. However the formation of hydrate in silica gel and dry water still provide an effective option to capture the natural gas without using complex rotating machineries.Résumé : La présente étude a pour but d'étudier les caractéristiques de formation d'hydrates de gaz dans l'eau liquide et à partir d'eau dispersée dans du gel de silice ou de particules sèches contenant de l'eau lorsque ces corps sont exposés au gaz naturel. Nous avons observé le processus d'inclusion des molécules de méthane, d'éthane et de propane dans les cages d'hydrates au moyen de la spectroscopie Raman in situ et nous avons estimé les taux d'occupation des cages à partir des spectres RMN 13 C. Nous avons suivi le processus de formation grâce à un autoclave à haute pression afin de déterminer le délai de formation des hydrates, la vitesse de croissance initiale et le taux de conversion. Les données obtenues à partir des spectres Raman et des profils de consommation de gaz laissent supposer que les hydrates se sont formés en moins de 20 minutes lorsque la température est suffisamment en deçà des conditions d'équilibre des hydrates à une pression donnée. Les molécules de méthane ont d'abord occupé les petites cages de structure II, toutefois, environ 6 minutes plus tard, les molécules d'éthane et de propane étaient aussi incluses dans les cages d'hydrates. La spectroscopie RMN 13 C confirme que seulement 23 % des grandes cages de structure II sont occupées par des molécules de méthane lorsque les hydrates sont formés à partir d'eau dispersée dans du gel de silice, ce qui représente beaucoup moins que les 68 % qui sont occupées dans le cas des particules sèches contenant de l'eau. Ces résultats laissent supposer que la dis...

Instrumental Methods for Cage Occupancy Estimation of Gas Hydrate

“…Raman spectroscopy is a very good instrument method for hydrate qualitative study (including composition [26], structure type identification [57,76]) and quantitative study [19,[80][81][82] (including cage occupancy and hydration number). This information will be applied to the hydrate kinetics process study (including formation [46,51,60,65,[83][84][85], dissociation [33,36,64,86], gas replacement [50,60,87,88], and inhibitors [47,89]), structure changes [8,11,31,34,35,41,45,49,90], hydrate distribution [26,38,43], and structural characterization [91][92][93].…”

Phase Behavior and Raman Spectroscopic Analysis for CH4 and CH4/C3H8 Hydrates Formed from NaCl Brine and Monoethylene Glycol Mixtures

“…This can be considered a direct result of the dissolution of NaCl in water, which causes a decrease in the number of hydrogen bonds in the intermolecular structure. In the C–H stretching region, two peaks from dissolved MEG could be observed at 2887 and 2945 cm –1 , and the intensities decrease significantly after hydrate formation. , …”

“…In the C−H stretching region, two peaks from dissolved MEG could be observed at 2887 and 2945 cm −1 , and the intensities decrease significantly after hydrate formation. 13,14 Figure 5b shows the Raman spectra of C−H stretching region during hydrate formation, which provides evidence of the hydrate structure. The Raman spectra of CH 4 hydrates with pure water show two peaks at 2905 and 2915 cm −1 , corresponding to CH 4 gas molecules occupying the small and large cages, respectively, in sI hydrates.…”

Investigation on the Structure, Formation Mechanism, and Surface Morphology of CP–CH4 Binary Hydrate

“…To determine the absolute occupancy of the guest molecules in the small and large cages, the statistical thermodynamic model of van der Waals–Platteeuw is used. The chemical potential difference between sII hydrate and the empty hydrate lattice can be calculated by the following equation μ normalw ( h ) − μ normalw ( h 0 ) = R T 17 [ ln false( 1 − θ normalL , normalC normalP − θ normalL , normalC normalH 4 false) + 2 ⁢ ln false( 1 − θ normalS , normalC normalH 4 false) ] where μ w (h) is the chemical potential of the host water molecules in the hydrate lattice and μ normalw ( h 0 ) is the chemical potential of the water molecules in the empty lattice.…”

“…A portion of the CP−CH 4 hydrate sample was stored in liquid nitrogen and prepared for taking cryo-SEM images. The remaining CP−CH 4 hydrate sample was grinded into powder and stored in liquid nitrogen for further 13 C NMR analysis.…”