Raman Spectroscopic Analysis on the Hydrate Formed in the Hydrate-Based Flue Gas Separation Process in Presence of Sulfur Dioxide and Tetra-n-butyl Ammonium Bromide

In this work, Raman spectroscopic analysis was applied to determine the structures and cage occupancies of the hydrates that formed from the system of flue gas (simulated by carbon dioxide–nitrogen–sulfur dioxide)–sulfur dioxide aqueous solution, and from the system of flue gas–sulfur dioxide containing tetra-n-butyl ammonium bromide (TBAB) aqueous solutions (sulfur dioxide mass concentration 0, 1.0, and 7.0 wt%). Comprehensive TBAB (solid, aqueous, and hydrate) Raman spectra were also obtained. The results reveal that when TBAB is used as promoter, both sulfur dioxide and carbon dioxide are encaged in the hydrate from systems of flue gas-TBAB solution with low sulfur dioxide concentration (0, 1.0 wt%), whereas in the hydrate from the system of flue gas-sulfur dioxide highly concentrated (7.0 wt%) TBAB solution, sulfur dioxide will be the sole gas guest encaged in the semi-clathrate hydrate. This suggests the sulfur dioxide concentration significantly influences the hydrate cage occupancies and separation selectivity of the hydrate-based technology. A two-stage hydrate-based flue gas purification process is proposed: one aims at desulfurization when sulfur dioxide concentrates to a relatively high level with the solutions recycling and in the other we can remove the sulfur dioxide and carbon dioxide simultaneously.