Crystal Growth of Ionic Semiclathrate Hydrate Formed at the Interface between CO₂ Gas and Tetra-n-butylammonium Bromide Aqueous Solution

Abstract

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

Crystal growth of ionic semiclathrate hydrate formed at interface between CO2+N2 gas mixture and tetrabutylammonium bromide aqueous solution

“…In this study, CO 2 partial pressure was 0.46 MPa at the total pressure of 2.3 MPa, and 0.94 MPa at the total pressure of 4.7 MPa. The surface tension in this study would be higher than that in the previous study (CO 2 +TBAB system at 2.3 MPa) [25] as the CO 2 partial pressure is lower. However, the addition of TBAB resulted in the hydrate crystals to be more wettable with liquid compared to the general water+gas systems.…”

“…On the other hand, small crystals had the following sizes; the shortest side was less than 0.02 mm, and the longest side was 0.07 mm. As mentioned in the introduction, the step-shaped crystals and polygonal-shaped crystals were previously observed, which had sizes of 0.14 mm×0.38 mm and 0.05 mm×0.50 mm, respectively in CO 2 +TBAB system [25]. Fig.…”

“…The morphology of CO 2 +TBAB hydrate was replaced from the wedge crystals to polygonal columnar, needle and sword-like crystals. Akiba et al [25] reported the semiclathrate hydrates formed of CO 2 +TBAB aqueous droplet system to understand the hydrate growth at the gas/liquid interface at w TBAB =0.10 and w TBAB =0.40, at 2.3 MPa. The observation of hydrates crystals growing inside a droplet of TBAB aqueous solution is unique as most hydrates grow at liquid/gas interface.…”

“…One was step-shaped crystals, the other was polygonalshaped crystals. Because in both previous studies [24,25] the nucleation occurred at gas/liquid interface, it is important to observe the crystal growth at gas/ liquid interface.…”

Reaction rate constant of CO2–Tetra‐n‐butylammounium bromide semi‐clathrate formation

“…[29,33] Lipkowski et al examined 1:24, 1:26, 1:32, and 1:36 with results indicating that 1:26 is more stable than other structures in the range of roughly 0.33-0.43 g/g (33 to 43 wt%) TBAB. [35,36] Given the number of possible crystal structures, liquid phase composition of TBAB is measured throughout growth to verify the semi-clathrate structure present. [34] Due to its stability at its stoichiometric concentration of 0.4 g/g (40 wt%) in water, 1:26 has been chosen as the focus of this study.…”