Experimental investigation on structure H hydrates formation kinetics: Effects of surfactants on interfacial tension

Among the known gas hydrate structures, structure H (sH) is the most fitting crystal for storage and transportation of natural gas due to its moderate formation pressure and high storage capacity. In the present work, the kinetics of methane (as help gas) hydrate formation is investigated in a high pressure batch stirred reactor with and without additives. Methylcyclohexane (MCH), methylcyclopentane (MCP) and tertbutylmethylether (TBME) were used as large molecule guest substance (LMGS). The effects of five surfactants on sH hydrate promotion are experimentally investigated. These surfactants belong to the family of Nonyl Phenol Ethoxylates (NPE), Lauryl Alcohol Ethoxylates (LAE), Ethylene Oxide/Propylene Oxide copolymer (EO/PO), polyoxyethylene sorbitan mono palmitate (Tween), Butyl Phenol ethoxylates (TritonX100) and Sodium alkyl sulfate. The results show that while the system with MCH and MCP exhibit high induction times and low formation rates, addition of surfactants can highly decrease the induction times and increase the formation rates. Investigation of the Liquid/Liquid (L/L) interfacial tension (IFT) of these systems in their pure state and in present of surfactants indicates that IFT plays an important role in the kinetic promotion of these systems. Studied surfactants can reduce L/L IFTs dramatically. Furthermore our results indicate that, in general, the systems with lower interfacial tension tend to have higher hydrate formation rates and lower induction times. Among the studied surfactants, TritonX100 and NPE6EO are the best kinetic promoters of structure H methane hydrate. The rate of hydrate formation in MCH system is increased more than 400% and the induction time is lowered to 0.05 its original value. Also effects of non-ionic surfactants on MCP and TBME follow the same trend.