Abstract
When gas hydrates dissociate into gas and liquid water, many gas bubbles form in the water. The large bubbles disappear after several minutes due to their buoyancy, while a large number of small bubbles (particularly sub-micron-order bubbles known as ultra-fine bubbles (UFBs)) remain in the water for a long time. In our previous studies, we demonstrated that the existence of UFBs is a major factor promoting gas hydrate formation. We then extended our research on this issue to carbon dioxide (CO2) as it forms structure-I hydrates, similar to methane and ethane hydrates explored in previous studies; however, CO2 saturated solutions present severe conditions for the survival of UFBs. The distribution measurements of CO2 UFBs revealed that their average size was larger and number density was smaller than those of other hydrocarbon UFBs. Despite these conditions, the CO2 hydrate formation tests confirmed that CO2 UFBs played important roles in the expression of the promoting effect. The analysis showed that different UFB preparation processes resulted in different promoting effects. These findings can aid in better understanding the mechanism of the promoting (or memory) effect of gas hydrate formation.
Highlights
Methane hydrates existing below the deep seafloor are attracting significant attention as an unconventional natural gas resource, and many studies and developments have been conducted on this topic [1,2,3,4]
We explored the expression of the memory effect using hydrocarbon gases, such as methane (CH4) [18], ethane (C2H6) [19], and propane (C3H8) [20], and demonstrated the promotion of gas hydrate formation, with hydrate-dissociated water, and with the UFBcontaining water prepared by an ultra-fine bubbles (UFBs) generator without any hydrate dissociation processes
Most of the UFBs were spherical or oval, and relatively larger than those observed in other gas UFBs [18,19,20]
Summary
Methane hydrates existing below the deep seafloor are attracting significant attention as an unconventional natural gas resource, and many studies and developments have been conducted on this topic [1,2,3,4]. Gas hydrates, or more generally clathrate hydrates, have attracted attention as refrigerants owing to their melting points that are higher than that of ice [8,9,10,11], as they have a large latent heat similar to ice. Research on gas hydrate utilization has increased in recent years, and such research has focused on efficient gas hydrates formation. Gas hydrate crystal formation requires relatively high supersaturation (or supercooling) conditions, similar to ice formation. The most promising approach to promoting the formation process is applying the “memory effect” phenomenon
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