Contribution of Ultra-Fine Bubbles to Promoting Effect on Propane Hydrate Formation

Abstract

To investigate experimentally how ultra-fine bubbles (UFBs) may promote hydrate formation, we examined the formation of propane (C 3 H 8 ) hydrate from UFB-infused water solution using two preparation methods. In one method, we used C 3 H 8 -hydrate dissociated water, and in the other, C 3 H 8 -UFB-included water prepared with a generator. In both solutions, the initial conditions had a UFB number density of up to 10 9 mL −1 . This number density decreased by only about a half when stored at room temperature for 2 days, indicating that enough amount of UFBs were stably present at least during the formation experiments. Compared to the case without UFBs, the nucleation probabilities within 50 h were ∼1.3 times higher with the UFBs, and the induction times, the time period required for the bulk hydrate formation, were significantly shortened. These results confirmed that UFB-containing water promotes C 3 H 8 -hydrate formation. Combined with the UFB-stability experiments, we conclude that a high number density of UFBs in water contributes to the hydrate promoting effect. Also, consistent with previous research, the present study on C 3 H 8 hydrates showed that the promoting effect would occur even in water that had not experienced any hydrate structures. Applying these findings to the debate over the promoting (or "memory") effect of gas hydrates, we argue that the gas dissolution hypothesis is the more likely explanation for the effect.

Promoting Effect of Ultra-Fine Bubbles on CO2 Hydrate Formation

“…Based on these properties, UFBs are considered to have a long lifespan in liquids [22][23][24]. Our previous studies [18][19][20] confirmed that a large amount of UFB is formed in water when gas hydrate is dissociated. This process has also been supported by molecular dynamic simulations [25][26][27].…”

“…We used three types of liquid samples to evaluate the effect of UFBs on the promotion of CO 2 hydrate formation, as used in our previous studies [18][19][20], including deionized pure water, hydrate-dissociated water prepared by dissolving CO 2 hydrate, and CO 2 -UFB-containing water. Pure water was prepared by ion exchange (Essential Elix, Merck KGaA, Darmstadt, Germany), with a resistivity of approximately 15 MΩ cm.…”

“…Co., Ltd., Tokyo, Japan, type SK-620PHII). We followed the FFT method in our previous studies [18][19][20]; therefore, we have only briefly described it here. A small amount (less than 10 µL) of the liquid sample was quenched in liquid nitrogen, and set in the replication system (JEOL Ltd., Tokyo, Japan, type JFD-9010).…”

“…This process is a major barrier to crystallization. Although this hypothesis was proposed in a simulation study [17], Uchida et al [18][19][20] demonstrated the existence of a large amount of ultra-fine bubbles (UFBs) in dissociated water, along with the memory effect expression of UFB-containing water without hydrate dissociation, which supported the "gas dissolution hypothesis".…”

“…In our previous studies, we explored the expression of the memory effect (or promoting the phenomenon of gas hydrate formation) using hydrocarbon gases, such as methane (CH 4 ) [18], ethane (C 2 H 6 ) [19], and propane (C 3 H 8 ) [20], and demonstrated the promotion of gas hydrate formation, not only with hydrate-dissociated water, but also with the UFBcontaining water prepared by an UFB generator without any hydrate dissociation processes. Based on these studies, UFBs are considered to play important roles in the hydrate memory effect.…”