Quantitative analysis of gas hydrates using Raman spectroscopy

Abstract

A calibration protocol to quantify the compositional information of gas hydrates using Raman spectroscopy is proposed. Structure I pure CH 4 -, CO 2 -and C 2 H 6 -hydrates in their deuterated and hydrogenated forms with known cage occupancies were investigated by Raman spectroscopy. Raman scattering cross sections of CH 4 in the large and small cages were found to be very similar, but not identical. Some C 2 H 6 bands of C 2 H 6 -hydrate were tentatively reassigned or newly reported and assigned. Our results show that the relative cross sections of guest vibrational modes in the deuterated hydrate are in agreement with those in the hydrogenated hydrate, whereas they are considerably different from those in fluid phase. Using our Raman quantification factors, the relative cage occupancies can now be determined more reliably in CH 4 -hydrates. Moreover, with additional assumptions, the absolute cage occupancies, the bulk guest composition and hydration number of pure or mixed gas hydrates become accessible by Raman spectroscopy. m h and m f indicate Raman frequencies of ethane in hydrate lattices and fluid phase, respectively. R is percentage of peak area. a,b and c represent the values given by Helvoort et al. 67 , Fernandez and Montero 71 , and Domingo and Montero 70 , respectively. Dm is the difference between m h and m f .Figure 6. Raman spectra of the deuterated sI CH 4 -CO 2 -C 2 H 6 -hydrate measured at ambient pressure and 113 K.

DFT calculation of the potential energy landscape topology and Raman spectra of type I CH4and CO2hydrates

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Self-preservation and structural transition of gas hydrates during dissociation below the ice point: an in situ study using Raman spectroscopy

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