Structure and Composition Analysis of Natural Gas Hydrates: ¹³C NMR Spectroscopic and Gas Uptake Measurements of Mixed Gas Hydrates

Abstract

Gas hydrates are becoming an attractive way of storing and transporting large quantities of natural gas, although there has been little effort to understand the preferential occupation of heavy hydrocarbon molecules in hydrate cages. In this work, we present the formation kinetics of mixed hydrate based on a gas uptake measurement during hydrate formation, and how the compositions of the hydrate phase are varied under corresponding formation conditions. We also examine the effect of silica gel pores on the physical properties of mixed hydrate, including thermodynamic equilibrium, formation kinetics, and hydrate compositions. It is expected that the enclathration of ethane and propane is faster than that of methane early stage hydrate formation, and later methane becomes the dominant component to be enclathrated due to depletion of heavy hydrocarbons in the vapor phase. The composition of the hydrate phase seems to be affected by the consumed amount of natural gas, which results in a variation of heating value of retrieved gas from mixed hydrates as a function of formation temperature. 13C NMR experiments were used to measure the distribution of hydrocarbon molecules over the cages of hydrate structure when it forms either from bulk water or water in silica gel pores. We confirm that 70% of large cages of mixed hydrate are occupied by methane molecules when it forms from bulk water; however, only 19% of large cages of mixed hydrate are occupied by methane molecules when it forms from water in silica gel pores. This result indicates that the fractionation of the hydrate phase with heavy hydrocarbon molecules is enhanced in silica gel pores. In addition when heavy hydrocarbon molecules are depleted in the vapor phase during the formation of mixed hydrate, structure I methane hydrate forms instead of structure II mixed hydrate and both structures coexist together, which is also confirmed by 13C NMR spectroscopic analysis.

Instrumental Methods for Cage Occupancy Estimation of Gas Hydrate

“…The calculation method will be introduced in detail in subsequent sections. 13 C-NMR is widely used in hydrate structure determination and composition analysis [12,18,[111][112][113][114][115][116]. Ripmeester et al [18] confirmed that NMR can be used to distinguish sI and sII hydrates due to the chemical shift pattern of methane encaged in large and small cages.…”

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

“…The value of ⌬ w o ͑h o ͒ was reported as 883.8 J/mol for structure II. [18][19][20][21] The 13 C NMR spectra in Fig. 5 provide the cage occupancy ratio ( L,C 3 H 8 / S,CH 4 , L,C 2 H 6 / S,CH 4 , and L,CH 4 / S,CH 4 ) from the ratio of the integrated peak intensities, I s /I L , in Table 2, which then can be used to calculate the cage occupancies of each molecule.…”

“…The occupancy of hydrocarbon molecules in hydrate cages can be estimated from 13 C NMR spectroscopy, which is important information to calculate the amount of gas stored in the hydrate phase. [15][16][17][18][19][20][21][22] This information can be incorporated with the data from gas consumption profiles to determine the capability of gas hydrates to capture natural gas.…”

Calculations of NMR properties for sI and sII clathrate hydrates of methane, ethane and propane

“…However, 13 C NMR spectra recorded to date did not show signals attributable to ethane residing in the small cavity, which can be explained in terms of sensitivity of this method. Propane on its own forms a sII hydrate structure, in which bigger 5 12 6 4 cages are occupied and smaller 5 12 cages are left empty [ 6 , 35 , 37 , 49 , 50 ].…”