Chemically Influenced Self-Preservation Kinetics of CH4 Hydrates below the Sub-Zero Temperature

Abstract

The self-preservation property of CH4 hydrates is beneficial for the transportation and storage of natural gas in the form of gas hydrates. Few studies have been conducted on the effects of chemicals (kinetic and thermodynamic promoters) on the self-preservation properties of CH4 hydrates, and most of the available literature is limited to pure water. The novelty of this work is that we have studied and compared the kinetics of CH4 hydrate formation in the presence of amino acids (hydrophobic and hydrophilic) when the temperature dropped below 0 °C. Furthermore, we also investigated the self-preservation of CH4 hydrate in the presence of amino acids. The main results are: (1) At T < 0 ℃, the formation kinetics and the total gas uptake improved in the presence of histidine (hydrophilic) at concentrations greater than 3000 ppm, but no significant change was observed for methionine (hydrophobic), confirming the improvement in the formation kinetics (for hydrophilic amino acids) due to increased subcooling; (2) At T = −2 °C, the presence of amino acids improved the metastability of CH4 hydrate. Increasing the concentration from 3000 to 20,000 ppm enhanced the metastability of CH4 hydrate; (3) Metastability was stronger in the presence of methionine compared to histidine; (4) This study provides experimental evidence for the use of amino acids as CH4 hydrate stabilizers for the storage and transportation of natural gas due to faster formation kinetics, no foam during dissociation, and stronger self-preservation.

Screening of Low-Dosage Methanol as a Hydrate Promoter

“…Although SDS B (100 ppm) and MeOH B have a comparable gas uptake, the different dissociation behavior at T = −4 °C clearly shows the weaker self-preservation due to the presence of SDS. The weaker self-preservation effect due to the presence of SDS has been confirmed in other similar studies for CO2 hydrates and CH4 hydrates [45,53]. As our system is identified as a low-hydrate saturation based system, our conclusion is based on the assumption that for the low-hydrate saturation case, the effect of a different hydrate saturation, hydrate morphology, and hydrate surface area is at a minimum across all samples [62].…”

“…This indicates that the presence of chemicals delays the nucleation, as compared to water. Delay in nucleation on account of the chemicals has been observed in our previous publications too [21,45,64]. In general, CO 2 /N 2 hydrate in the presence of amino acids (hydrophobic and hydrophilic) had a higher onset nucleation temperature (To~3.9 to 5.4 • C for amino acids) and for SDS (To~3.8 to 5.8 • C) during the formation [21].…”

“…CSMGem software is based on Gibbs energy minimization concept, and more details can be referred to in this article [42]. T op is the operating temperature mostly referred to as the temperature during the isothermal test [21,[43][44][45]. Subcooling measurement explains the effect of concentration on driving force and growth profile.…”

“…The rocking cell system is a closed system volume, and dissociation experiments are performed on a constant volume system so that dissociation behavior might be observed via increasing the rate of system pressure. Such a technique to observe dissociation has been tested by various groups previously [45,48]. The dissociation behavior of gas hydrates below 0 • C is of great value for understanding hydrate stability under self-preservation tendency with and without additives.…”

“…Compared to the additive-based solutions, the pure water solution showed the lowest pressure decrease and increase and the slowest dissociation profile curve due to the low-hydrate saturation and high-ice saturation. Detailed discussion about self-preservation and the effect of different factors are also available in our previous two manuscripts on similar topics [45,49].…”