Investigation of kinetics of tetrabutylammonium chloride (TBAC) + CH4 semiclathrate hydrate formation

Abstract

For potential application in advanced gas storage at moderate temperatures, a systematic study on tetrabutylammonium chloride (TBAC) + CH₄ semiclathrate hydrate formation kinetics was conducted using isobaric kinetics measurements to evaluate the effects of pressure (6.0, 3.0 MPa), temperature (278 K, subcooling degree of 6 K), and salt concentration (0.10, 0.20, 0.30, 0.34, 0.45 mass fraction). The results revealed that the systems showed shorter induction time, higher normalized gas consumption and higher rapid growth rate under a higher supersaturation environment, represented by higher pressure or lower temperature. Besides, the effect of salt concentration was complicated. With the increase of salt concentration, the total gas consumption was almost the same while the normalized gas consumption decreased greatly, indicating that the amount of CH₄ trapped in the hydrate unit greatly decreased and the system with low salt concentration was a good choice for advanced gas storage. In addition, a Raman device was employed to reveal the structural properties. The spectra showed that the (TBAC + CH₄) semiclathrate hydrates were formed with hexagonal structure or tetragonal structure under different salt concentrations, which were different from the structures of pure TBAC hydrate. It was assumed that at low salt concentrations the addition of CH₄ induced the formation of hexagonal structure since it had three 5¹² cages per TBAC which was higher than that of the tetragonal structure.