New promoters based on amino acids modified with nitrilotriacetic acid for efficient storage of methane as gas hydrates without foaming

Abstract

Solidified natural gas (SNG) is a perspective method for transportation and storing hydrocarbon gases under mild conditions. The aim of this work is to synthesize new compounds based on several amino acids (leucine, methionine, phenylalanine, norleucine, aspartic acid, valine, glutamic acid, norvaline, proline, threonine, alanine, 6-aminohexanoic acid, cysteic acid) with nitrilotriacetic acid (NTA + AA) to improve the efficiency of methane transition from gas phase into hydrate form and compare them with to pure amino acids and sodium dodecyl sulfate (SDS) as well known gas hydrate promoters. According to the results of high-pressure autoclave experiments, 7 NTA + AA samples out of 13 increase methane uptake and water to hydrate conversion better than SDS, which is a well-known effective promoter of hydrate formation, and 12 NTA + AA samples out of 13 have shorter induction times than SDS. In addition, visual observations showed that all the novel promoters synthesized from amino acids and nitrilotriacetic acid do not cause foaming throughout the process of methane hydrate dissociation, which improves the performance of this type of promoter in comparison with SDS. The efficiency of kinetic hydrate promoters at 0.05 wt% concentration by maximum mole consumption of methane decreased in the range of samples: NTA + Phenylalanine > NTA + Glutamic acid > NTA + Leucine > NTA + Norleucine > NTA + Aspartic acid > NTA + Alanine > NTA + Valine > NTA + Norvaline > SDS > NTA + Proline > NTA + Methionine > NTA + Threonine > NTA + Cysteic acid > NTA + 6-aminohexanoic acid. NTA + Phe (nitrilotriacetic acid + phenylalanine) was able to achieve 98.2 % water to hydrate conversion (0.164 mole gas/mole water) at 0.05 wt% concentration, and its induction time was independent of concentration and approximately equal to 30 min, which was about half that of SDS and pure phenylalanine. Thus NTA + Phe can be considered as one of the best promoters of methane gas hydrate formation, which can be used at low concentrations. Modification of amino acids with nitrilotriacetic acid was shown to significantly improve the efficiency of the compounds as kinetic promoters of hydrate formation, both in terms of induction time and methane uptake, compared to pure amino acids, which makes it possible to create more effective eco-friendly non-foaming promoters of methane hydrate formation used at low concentrations.