Investigation on the Effect of Ionic Liquids and Quaternary Ammonium Salts on the Kinetics of Methane Hydrate

This study investigates the kinetics of gas hydrate (methane gas) in the presence of Ionic Liquids (ILs) and organic Quaternary Ammonium Salts (QASs). Aqueous solutions of eight ILs (five aromatic ILs and three aliphatic ILs) and three QASs have been studied for the kinetics of methane hydrate formation. In this study, five aromatic-based ILs with cations, viz., 1-hexyl-3-methylimidazolium, 1-butyl-3-methylimidazolium, and 1-octyl-3-methylimida-zolium, and anions such as [Br]-, [Cl]-, and [HSO4]- were selected. Three aliphatic ILs with cations, viz., diethylammonium, tripropylammonium, and tributylammonium, attached with the [HSO4]- anion were also selected. In the case of QASs, varying carbon chain lengths such as tetra-n-methyl-, tetra-n-ethyl-, and tetra-n-butylammonium bromide (TMAB, TEAB, TBAB) have been considered for the study. The kinetics of methane hydrate formation were investigated at a pressure of 7.5 MPa and a temperature of 276.15 K in the presence of aqueous solutions of QASs at 0.05 mf (mass fraction) and 0.1 mf concentrations and ILs at 0.01 mf. The results show that 0.05 mf of TBAB assists the methane hydrate to nucleate very rapidly and enhances the rate of growth. Hence, TBAB at 0.05 mf is observed to be most promising for gas storage and gas separation/processing among all investigated QASs. TMAB and TEAB have shown slow kinetics; therefore, their presence in a gas hydrate system may not aid in hydrate gas storage/gas separation applications; nevertheless, they could be used as an inhibitor for the prevention of hydrates in systems where other inhibitors may become unsuitable. The aqueous solution of aromatic-based ILs generally shows hydrate promotion, except for shorter chain lengths. The shorter chain length of ionic liquid with suitable anion (Cl-) behaved as a hydrate as the inhibitor and with Br- as the promoter. In general, a bigger cation acts as a good nucleating agent. The anion [HSO4]- is common in both the categories of ILs (aromatic and aliphatic), and it looks like the hydrate inhibition (no hydrate formation) may have occurred due to its presence. ILs offer more scope to understand the tunability of cations and anions to derive a better solution for gas hydrate inhibition and promotion, which have more applications in the ares of methane storage and transportation.