Raman Spectroscopic Study on a CO2-CH4-N2 Mixed-Gas Hydrate System

Abstract

Accurate determination of the characteristics of coal mine gas separation products is the key for gas separation applications based on hydrate technology. Gas hydrates are synthesized from gases with two types of compositions (CO2-CH4-N2). The separation products were analyzed by in situ Raman spectroscopy. The crystal structure of the mixed-gas hydrate was determined, and the cage occupancy and hydration index were calculated based on the various vibrational modes of the molecules according to the “loose cage-tight cage” model and the Raman band area ratio combined with the van der Waals-Platteeuw model. The results show that the two mixed-gas hydrate samples both have a type I structure. Large cages of mixed-gas hydrate are mostly occupied by guest molecules, with large cage occupancies of 98.57 and 98.52%; however, small cages are not easy to occupy, with small cage occupancies of 29.93 and 33.87%. The average cage occupancies of these two hydrates are 81.41 and 82.36%, and the stability of the crystal structure of the mixed-gas hydrate in the presence of 75% CO2 is better than that of the mixed-gas hydrate in the presence of 70% CO2. The hydration indices of the two hydrate gas samples are 7.14 and 6.98, which are greater than the theoretical value of structure l.

Instrumental Methods for Cage Occupancy Estimation of Gas Hydrate

“…They calculated the cage occupancy of the gas hydrate synthesized at different initial gas compositions and discussed its application on landfill gas treating. Similarly, Liu, et al [72] estimated the cage occupancy of mixed CH 4 -CO 2 -N 2 gas hydrate via in situ Raman spectroscopy. Chazallon, et al [58] analyzed the natural gas hydrate sample from the Western Black Sea and obtained the spatial variability of the methane cage occupancy via Raman imaging.…”

Comprehensive Review and Perspectives of CO2 Hydrate Storage in Subseafloor Saline Sediments: Formation, Stability, and Optimization Strategies

“…Raman spectroscopy is exceptionally beneficial due to its sensitivity to vibrational modes, which can reveal alterations in the hydrate phases and the influence of nanoconfinement on structural integrity. 144 It allows for the detailed identification of different cages within the CO 2 hydrate structures by examining shifts and intensities in the characteristic Raman peaks as a result of the CO 2 molecules interacting with the surrounding water lattice in hydrate form 145 In the latter, the CO 2 peaks within the small cage are located around ∼1274 and 1380 cm −1 . This contrasts with the large cage of sI CO 2 hydrate, where the lower frequency peak is situated at ∼1277 cm −1 ; a difference of ∼3 cm −1 for the lower frequency peak and ∼1 cm −1 for the higher frequency peak is noted.…”

“…Raman spectroscopy is exceptionally beneficial due to its sensitivity to vibrational modes, which can reveal alterations in the hydrate phases and the influence of nanoconfinement on structural integrity . It allows for the detailed identification of different cages within the CO 2 hydrate structures by examining shifts and intensities in the characteristic Raman peaks as a result of the CO 2 molecules interacting with the surrounding water lattice in hydrate form Figure shows an example of Raman spectra of CO 2 in various states and hydrate structures, indicating spectral changes relevant for nanostructured encapsulation characterization. The sensitivity to structural changes makes Raman spectroscopy indispensable for studying CO 2 hydrates encapsulated within the nanostructures.…”

Raman Spectroscopy Study on the Competition between CH4 and N2 to Form Mixed Gas Hydrate

“…Raman spectroscopy is a suitable tool for such work. For example, it was found through Raman spectroscopy that CBM hydrate is of sI type 32 and that an increased driving force can enhance the CBM purification efficiency. 33 The structure of gas hydrates formed in the CO 2 /CH 4 /N 2 /TBAB system was investigated using Raman spectroscopy, and the mechanism of CH 4 separation from the CO 2 /CH 4 /N 2 gas mixture was reported.…”