Toward Economical Seawater-Based Methane Hydrate Formation at Ambient Temperature: A Combined Experimental and Computational Study

Clathrate hydrates are emerging as a novel storage medium for safe and compact methane storage. However, their industrial-scale applicability is hindered by sluggish formation kinetics and intense energy cooling requirements. The present study is the first report on binary methane-tetrahydrofuran (THF) formation using a combination of seawater and an unstirred reactor at ambient temperature (298.2 K) that would improve the process economics. Acidic zeolites with different Si/Al ratios (USY-40 and USY-10) as well as aliphatic (L-valine) and aromatic (L-tryptophane) amino acids are employed as environmentally benign kinetic hydrate promoters. The experimental study is combined with DFT calculations to shed light on the role of kinetic promoters in hydrate formation. The set of experimental data revealed that hydrophobic zeolites with a higher Si/Al ratio performed better than the more hydrophilic zeolites. Moreover, the aliphatic amino acid Lvaline showed slightly better kinetic promotion performance for hydrate formation in natural and artificial seawater than the aromatic amino acid L-tryptophan. The optimization of the experimental conditions allowed a controlled hydrate growth, boosting the gas uptake to 40 mmol of gas/mol of water, which is the highest reported under mild conditions using seawater. In addition, the induction time is reduced to less than 10 min, and a methane recovery of 97% is reached without any signs of foaming. Thus, this study demonstrates the possibility of controlling the stochastic nature of nucleation and hydrate growth by properly manipulating the reaction system. Our results provide a better understanding of hydrate nucleation enhancement under realistic conditions and open the door for a possible application of these environmentally benign kinetic hydrate promoters (KHPs) for synthetic natural gas (SGH) on a continuous process and industrial scale.