Effects of Low Concentrations of NaCl and EG on Hydrate Formation Kinetics and Morphology in the Presence of SDS

Abstract

The research on gas hydrates can be divided into two subfields: risk prevention and control of pipeline blockage by hydrates and industrial applications of solidified natural gas (SNG) in gas storage and transportation, seawater desalination, and gas recovery. The two opposing properties of hydrates have stimulated research into promoting and inhibiting methods. A considerable number of studies have reported that the same type of additive can play a promoting or inhibiting effect at different concentrations. This study evaluated the kinetic effects of low concentrations of ethylene glycol (EG) and sodium chloride (NaCl) on methane hydrate production in sodium dodecyl sulfate (SDS) solution. The results showed that both NaCl and EG had kinetic inhibitory effects on the hydrate reaction in SDS solution, and the inhibitory effects gradually increased with the increase of the concentration. Among them, the inhibitory effect of NaCl was stronger than that of EG at the same content. 5 wt % NaCl can reduce the reaction rate of hydrate in SDS solution by 81%, but 0.1 wt % NaCl helped SDS to act on hydrate growth, and the rate of hydrate growth stage was increased by 100%. When 0.1 wt % NaCl or 5 wt % EG existed in the SDS solution, the hydrate mainly grew in the bulk solution and on the inner wall surface of the container, forming a peak-like structure below the interface, and 5 wt % EG also formed a ridge-like structure above the interface. Exploring the macroscopic formation characteristics of hydrate is helpful to industrial process optimization and predicting the additive concentration in the pipeline. This work emphasizes that the selection of substances can be broadened and the rational use of resources can be achieved as much as possible when selecting promoters that are conducive to hydrate formation kinetics or inhibitors that control hydrate growth.

Experimental Study on the Effect of Mixed Thermodynamic Inhibitors with Different Concentrations on Natural Gas Hydrate Synthesis

“…At low concentrations of inhibitors, the amount of NaCl used to achieve the same inhibitory effect is significantly lower than that of alcohol. In the experiments of Bai et al [28], NaCl was inhibited more effectively than EG at low concentrations of inhibitors, so NaCl can be preferred as an inhibitor at low concentrations of inhibitors. However, when the amount of inhibitor is increased, NaCl exerts a less potent inhibitory effect than increasing the alcohol concentration, and when the alcohol inhibitor is increased to a certain level, it can completely inhibit the growth of the hydrate.…”

“…The presence of NaCl and EG in the aqueous phase appeared to minimize the interactions between hydrate particles by encircling the hydrate particles, a phenomenon that demonstrated that NaCl and EG acted as synergistic inhibitors under insufficiently inhibitory conditions, further limiting the hydrate synthesis. Bai et al [28] evaluated the kinetic effects of low concentrations of EG (0.1 wt%, 0.5 wt%, 1 wt%, 5 wt%) and NaCl (0.1 wt%, 0.5 wt%, 1 wt%, 5 wt%) on natural gas hydrate synthesis in SDS solutions, and the results showed that both NaCl and EG had kinetic inhibitory effects on hydrate synthesis in SDS solutions, and the inhibitory effect of NaCl was greater than that of EG at the same content. Sui et al [29] experimentally investigated the synergistic inhibition of natural gas hydrate synthesis by EG and the kinetic inhibitor PEO-co-VCap-1 in the presence of fine sand, and the experimental results showed that the use of PEO-co-VCap-1 in combination with EG not only retarded hydrate nucleation but also effectively reduced the catastrophic growth of hydrates.…”