Surfactant Effects on Crystal Growth Dynamics and Crystal Morphology of Methane Hydrate Formed at Gas/Liquid Interface

Abstract

This paper presents visual observations aiming to clarify the underlying physics of surfactant effects on clathrate hydrate crystal growth at the interface between methane gas and water with surfactant. Sodium dodecyl sulfate (SDS), which is commonly used in industrial processes, was used in this study. Various SDS mass fractions from 0 to w SDS = 100 ppm with a step of size 10 ppm were examined, where w SDS represents the mass fraction of SDS aqueous solution. The crystal growth behavior and the crystal morphology of the methane hydrate at the interface varied depending on surfactant concentration and ΔT sub. In systems with w SDS ≤ 20 ppm, the nucleation occurred on the droplet surface (gas/liquid interface), then grew laterally, and finally covered the whole droplet surface. On the contrary, at w SDS ≥ 30 ppm, the droplet shape was not maintained and enhanced hydrate crystal growth was observed compared to those systems with w SDS ≤ 20 ppm. Individual hydrate crystals at w SDS = 20 ppm were observed to be smaller than those in a pure water system at a given ΔT sub, which is ascribed to the enhanced hydrate nucleation by the addition of SDS.

Investigation on Hydrate Growth at the Oil–Water Interface: In the Presence of Wax and Kinetic Hydrate Inhibitor

“…Figure 9 and a video in the Supporting Information illustrate the typical process of the vertical growth. To the best of our knowledge, experimental phenomena similar to vertical growth have only been reported by Dong et al 46 and Hayama et al 47 when forming hydrates using water droplets in CP−water−AA systems and methane−water−surfactant systems, respectively. Different from the two abovementioned studies, in our experiments, the vertical growth of hydrate shell was closely related to the wax content and the growth rate of hydrates.…”

“…Under these circumstances, wettability of the plate surface would increase in response to the decreasing interfacial tension. 47 Consequently, it becomes much easier for the unconverted water inside the droplet to permeate out from the bottom of the droplet after the initial formation of hydrates, as shown in Figure 10b. Afterward, once the unconverted water that permeates out contacts with CP at the bottom of the droplet, hydrate formation and growth would happen, as can be seen in Figure 10c.…”

“…According to the Gibbs adsorption isotherm given in eq , where γ is the interfacial tension, C is the interfacial concentration, Γ is the amount of the “absorbed” wax, R is the gas constant, and T is the temperature, the interfacial tension of the oil–water interface would decrease due to the “absorption” of wax, which could soften the oil–water interface. Under these circumstances, wettability of the plate surface would increase in response to the decreasing interfacial tension . Consequently, it becomes much easier for the unconverted water inside the droplet to permeate out from the bottom of the droplet after the initial formation of hydrates, as shown in Figure b.…”

Sodium Dodecyl Sulfate Preferentially Promotes Enclathration of Methane in Mixed Methane-Tetrahydrofuran Hydrates

“…A similar peak for sI hydrate growth is observed in the presence of 100 ppm SDS, albeit at a later time, and is of smaller magnitude. This observation suggests that compared with 100 ppm SDS, 1,000 ppm SDS shows better kinetics, resulting in faster enclathration of gaseous species in the solid hydrate (Hayama et al., 2016, Kumar et al., 2015). The SDS-concentration-dependent promotion of hydrate growth is well known in the literature (Kumar et al., 2015).…”

Methane hydrate production using a novel spiral‐agitated reactor: Promotion of hydrate formation kinetics

“…Hydrate formation is governed by the mass transfer of gas phases, so individual hydrate crystals first form at gas–liquid interface, and the lateral growth of hydrate crystals gradually give rise to a hydrate film, 14,15 which restricts further hydrate growth on liquid side, causing slow hydrate formation kinetics 16 . In order to overcome the limitation, various methods have been proposed to enhance mass transfer, committing to fast hydrate nucleation and growth kinetics.…”