Enhanced methane storage in clathrate hydrates induced by antifreezes

Abstract

• Antifreezes such as methanol and ammonia can enhance methane uptake ability of binary clathrate hydrates. • Methanol can be encaged in the distorted 5 12 cages of TBAB clathrate hydrate. • Methanol can accelerate the structural transformation of TBAB clathrate hydrate. • Methanol and ammonia can be key materials for hydrate-based gas storage systems. Antifreezes are widely used in preventing hydrate formation in oil and gas flowlines. Recent studies have revealed that methanol and ammonia can be incorporated into clathrate hydrate phases along with a more hydrophobic guest such as THF or propane and that these antifreezes act as catalysts for methane hydrate formation from ice. In this work, we demonstrated that these antifreezes can enhance the methane storage content of binary clathrate hydrates, namely those of THF and TBAB. THF + methane and TBAB + methane binary hydrates with/without methanol or ammonia were synthesized and analyzed with 13 C NMR spectroscopy and X-ray diffraction (XRD) methods. In the THF hydrate system, 84% and 81% of the 5 12 small cages were occupied by methane in the presence of methanol and ammonia respectively, while only 44% of the small cages were occupied in the absence of these antifreezes. In the TBAB-H 2 O system, the powder XRD (PXRD) patterns of 1TBAB:38H 2 O samples without antifreeze both before and after methane introduction showed mostly tetragonal structures. On the other hand, it was confirmed that methanol can easily induce TBAB hydrates to form the orthorhombic structure which is more suitable for methane storage than the tetragonal structures of TBAB hydrates. The single crystal XRD analysis of a crystal grown from the 1TBAB:1CH 3 OH:38H 2 O solution at 277 K showed that methanol was present in the 5 12 cage of the orthorhombic TBAB hydrate phase. The 13 C NMR spectra of TBAB + methane hydrates also showed an enhanced methane content in the presence of methanol. The present findings on the enhancement of methane storage induced by antifreezes suggest that methanol can be a key material for hydrate-based methane storage systems.