Roles of amino acid hydrophobicity on methane-THF hydrates in the context of storage and stability

• A well-defined amino acid hydrophobicity ranking for CH4-THF hydrate formation. • A proposed mechanism for roles of amino acid hydrophobicity on the hydrate formation. • A morphology insight on effects of different amino acids on the hydrate formation. • Extremely stable hydrates for a long-term storage and transportation. This report reveals the success of the formation of mixed methane-THF hydrates at room temperature (298.2 K) and a pressure of 9.2 MPa equipped with a hybrid stirring in the presence of amino acids as kinetic promoters. Five different amino acids in increasing order of hydrophobicity: glutamic acid, valine, leucine, methionine, and tryptophan, were used to investigate their effects on methane uptake, formation, and dissociation kinetics. The addition of hydrophobic amino acids (tryptophan, methionine, and leucine) promotes the hydrate formation to completion, which doubly increases the methane uptake compared to that without amino acids and with hydrophilic amino acids (glutamic acid and valine) at the same condition. The results indicate that the hydrophobicity has a positive contribution to the methane uptake by creating a hydrophobic area, enhancing hydrate growth to completion. Moreover, the time taken to complete hydrate by 90 percent has a negative correlation to the molecular weight of the amino acids. The morphological studies demonstrate that the hydrates with hydrophobic amino acids were more consolidated than the addition of hydrophilic amino acids. Lastly, the hydrates with hydrophobic amino acids show excellent stability for 30 days even if they are stored at a temperature higher than their equilibrium temperature at atmospheric pressure. The small amount of gas lost was due to intrinsic loss only. This opens the possibility for cost-reduction for methane hydrate in commercialization.