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.

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This study reports a visual observation of the formation and growth of ionic semiclathrate hydrate on the surface of a Tetrabutylammonium bromide (TBAB) aqueous solution and CO 2 +N 2 gas mixture. The composition of CO 2 +N 2 gas mixture was 20 : 80. The experimental temperature range was from 280 K to 290 K, under the pressures of 2.3 MPa and 4.7 MPa, at w TBAB =0.10 and w TBAB =0.40, where w TBAB denotes the mass fraction of TBAB in the aqueous solution. At w TBAB =0.40, the hydrate crystals were initially observed to grow within the droplet, and followed by lateral growth at the droplet surface; but at w TBAB =0.10, the hydrate crystals grew exclusively in the liquid phase and did not cover the droplet surface. Two types of different crystals with different sizes were clearly observed.

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