Experimental investigation of hydrate formation kinetics and microscopic properties by a synthesized ternary gas mixture with combination additives

Given the recycling, environmental friendliness and other advantages, hydrate-based gas separation (HBGS) technology has recently gained considerable attention. This work examined synthesized ternary gas mixture (CH4/CO2/N2) hydrate formation kinetics with both cyclopentane (CP) and sodium dodecyl sulfate (SDS) as additives under 7.0 MPa at different temperatures (275.1, 280.1, 295.1 and 290.1 K). Two volume ratios of CP and SDS aqueous solution were used. Hydrate samples were analyzed by powder X-ray diffraction (PXRD), Raman spectroscopy, and cryo-scanning electron microscopy (Cryo-SEM) to reveal the microscopic influence mechanism of additives on hydrates. It was found that the use of mixed additives clearly promotes the hydrate formation kinetics, while enhancing gas separation under some special conditions. Ternary mixed gas with CP and SDS can form a hydrate, in which structure I and II coexisted. CO2 molecules can be encaged in both small and large cages in hydrate. However, CH4 molecules occupied only the small cage of structure I hydrate except with 0.01% SDS with volume ratio of CP and solution at 0.308 and 0.03% SDS with the volume ratio of CP and solution at 0.154. The increasing SDS concentration can smoothen the hydrate crystal surface, thereby inducing more complete crystallization.