Occupation and Release Behavior of Guest Molecules in CH4, CO2, N2, and Acetone Mixture Hydrates: An In Situ Study by Raman Spectroscopy

Abstract

For practical applications of gas hydration (formation of gas hydrates) in environmental and technological processes, considerable knowledge regarding the thermodynamic stability and structural features of these hydrates, as well as the occupation behavior of specific components of gas mixtures within them, is essential. Herein, the hydrate phase equilibria of a system comprising CH4/CO2/N2 (55/40/5) + aqueous acetone solutions (1, 3, and 5.56 mol %) were determined in the temperature range 273–285 K and under pressures up to 4.5 MPa. Gas compositions in the hydrate phase were also obtained by evaluating the following variables: (1) hydrate-formation temperature and pressure, (2) concentration of acetone, and (3) type of hydrate structure: (a) structure I or (b) structure II. The crystal structures of the gas hydrates formed from the acetone and CH4 + CO2 + N2 mixture gas were also evaluated by both X-ray diffraction and Raman spectroscopy. In addition, structural identification of the CH4 + CO2 + N2 + acetone hydrates formed by varying the concentration of acetone (0, 1, 3, and 5.56 mol %) was performed. Further evaluation of the temperature-dependent occupation behavior of CH4 and CO2 in structure II hydrate cages in the temperature range 150–290 K indicates that CH4 and CO2 gradually escaped from the hydrate frameworks with increasing temperature, up to 255 K, at which point the CH4 + CO2 + N2 + acetone hydrate completely decomposed.