Visual observations of CH 4 + CO 2 hydrate crystal growth formed at the gas/liquid interface and in liquid water presaturated with a mixed gas have been made. The compositions of the CH 4 + CO 2 gaseous mixture were 40 : 60 and 30 : 70 for the gas/liquid interface observations, 30 : 70 and 70 : 30 for water saturated with the guest gas. The feed gas compositions of the CH 4 and CO 2 gaseous mixture were 40 : 60 and 30 : 70 for the gas/liquid interface observations, or 30 : 70 and 70 : 30 for liquid water. The crystal morphology of the CH 4 + CO 2 hydrate observed in both feed gas compositions was similar. This may be ascribed to the fact that the molar ratios of CO 2 to CH 4 in the liquid phase ranged from 90 : 10 to 97 : 3 due to the greater solubility of CO 2 in water. These results suggest that the crystal morphology of the CH 4 + CO 2 hydrate may be controlled by the guest composition in the liquid phase, not by the feed gas composition. As the system subcooling increased, the shape of the hydrate crystals changed from polygons to sword-like or dendrites. The implications for the process design of the hydrate-based technologies are discussed based on the observations.

Formation and growth of ionic semiclathrate hydrate crystals on the surface of a liquid droplet of tetra-nbutylammonium bromide (TBAB) aqueous solution exposed to CO 2 gas have been visually observed.Experiments were conducted at temperature range between 280 K and 290 K under the pressure of 2.3 MPa at w TBAB = 0.10 and w TBAB = 0.40, where w TBAB is defined as the mass fraction of TBAB in the aqueous solution.It was found that the hydrate crystals initially grew in the liquid phase, instead of growing at the gas/liquid interface. Then the hydrate grew to form a polycrystalline film covering the droplet only at w TBAB = 0.40. The individual crystals that constitute the polycrystalline hydrate film were observed and the morphology was classified according to the system subcooling ∆T sub (∆T sub ≡ T eq -T ex, where T eq is the equilibrium temperature and T ex is the experimental temperature). In all ∆T sub at w TBAB = 0.40, hydrate crystals with stepshaped and thin polygonal-shaped morphologies were observed. The difference in the size of the individual hydrate crystals due to the difference in ∆T sub was not observed. ABSTRACTFormation and growth of ionic semiclathrate hydrate crystals on the surface of a liquid droplet of tetra-n-butylammonium bromide (TBAB) aqueous solution exposed to CO 2 gas have been visually observed. Experiments were conducted at temperature range between 280 K and 290 K under the pressure of 2.3 MPa at w TBAB = 0.10 and w TBAB = 0.40, where w TBAB is defined as the mass fraction of TBAB in the aqueous solution. It was found that the hydrate crystals initially grew in the liquid phase, instead of growing at the gas/liquid interface. Then the hydrate grew to form a polycrystalline film covering the droplet only at w TBAB = 0.40. The individual crystals that constitute the polycrystalline hydrate film were observed and the morphology was classified according to the system subcooling ∆T sub (∆T sub ≡ T eq -T ex, where T eq is the equilibrium temperature and T ex is the experimental temperature). In all ∆T sub at w TBAB = 0.40, hydrate crystals with step-shaped and thin polygonal-shaped morphologies were observed. The

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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.