OS5A-2:RECOVERING CH4 FROM CLATHRATE HYDATES WITH CO2 + N2 MIXTURE: QUANTITATIVE RAMAN STUDY

Junfeng QIN and Werner F. KUHS
GZG Abt. Kristallographie, Universität Göttingen, Germany

    Exploiting methane from hydrate deposits has proven to be a considerable challenge where aside from engineering issues, economic and safety concerns are raised. Storing CO2 as solid hydrate and concomitantly recovering methane from gas hydrate reservoirs is considered one of the promising methods. Laboratory studies have reported that CO2 + N2 gas mixture, are more efficient than pure CO2 and a recent field-scale trial on Alaska North Slope shows this combining method applicable. Here, we report laboratory experiments on this gas exchange reaction of methane hydrates with CO2 + N2 mixture; the characteristic molecular signatures make Raman spectroscopy a valuable tool to study gas exchange reactions. Raman scattering was previously used in a qualitative and semi-quantitative manner. In this work, Raman spectroscopy is established as a quantitative tool to determine the cage fillings in mixed gas hydrates involving N2. Raman quantification factors of guest to guest and guest to host framework in hydrate phase are established using pure gas hydrates for which the absolute cage occupancies were established by crystallography [1]. Using empirical Raman quantification factors, the cage occupancies of guests, the resulting bulk guest composition and hydration number of pure and mixed gas hydrates containing CH4, C2H6, CO2, N2 molecules are determined. In this way, we gain insight into details of the CH4 exchange reaction with CO2 + N2 or CO2 and the preference of certain gas species for certain types of hydrate cages. This is important in order to optimize the conditions of the intended gas exchange, largely occurring in the solid state. 

[1] Junfeng Qin and Werner F. Kuhs (2013), "Quantitative Analysis of Gas Hydrates Using Raman Spectroscopy", AIChE Journal, 59, 2155-2167.