Phase behavior and structure identification of the mixed chlorinated hydrocarbon clathrate hydrates

The primary objective of this research is to verify thermodynamic feasibility for recovering chlorinated hydrocarbons (CH2Cl2, CCl4, and CH3CCl3) from aqueous solutions using clathrate hydrate formation with guest gases of CO2 and CH4. First, the four-phase (H-L-W-L-CHC-V) clathrate hydrate equilibria of the ternary CO2 (CH4) + water + chlorinated hydrocarbon (CHC) systems were measured at various temperature and pressure conditions and particularly up to the upper quadruple point for the CO2-containing solutions. The inclusion of CHCs in the clathrate hydrate lattice greatly reduced the clathrate hydrate-forming pressure at a given temperature, which confirmed the mixed CHC clathrate hydrates to be more stabilized than the pure CO2 (CH4) clathrate hydrate. The structure of the mixed CHC clathrate hydrates was newly identified as sII through NMR and Raman spectroscopies, and the quantitative analysis results were directly used for determining both small and large cage occupancies. From spectroscopic results of the mixed CHC clathrate hydrates, it was found that the large guest molecules of chlorinated hydrocarbons exclusively occupied the large sII 5(12)6(4) cages and thus restricted CH4 molecules predominantly to the small sII 5(12) cages. The highlight feature of this study is that the basic information drawn from both phase behavior and structure-related properties of cage occupancies might play a key role in understanding the phenomenological characteristics of the mixed CHC clathrate hydrates composed of multiguest molecules.