Promoting Effect of Ultra-Fine Bubbles on CO2 Hydrate Formation

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.

Nucleation Behavior of Single-Gas Hydrates in the Batch-type Stirred Reactor and Their Promotion Effect with Ultrafine Bubbles: A Mini Review and Perspectives

“…They also used Raman spectroscopy to estimate their inner pressure, finding it to be as high as that expected by the Laplace equation . Then, they confirmed that the UFB-contained water prepared by a UFB generator that experienced no hydrate structures exhibited the “memory effect” equivalent to that of dissociated water. − These results indicate that bulk UFBs are a key material for promoting gas hydrate formation. Table summarizes research on UFBs relevant to gas hydrates.…”

“…These values were roughly the same for the three hydrocarbon types, 52,53 but the CO 2 case had a larger average diameter and a smaller number density. 54 Although the stability of UFBs in water is still under debate, the results of TEM observations show that UFBs can remain stable long enough to have a role in the memory effect.…”

“…We previously developed a larger reaction vessel to study the nucleation process under stirred conditions. − Here, we apply the apparatus to hydrates of CH 4 , C 2 H 6 , C 3 H 8 , and CO 2 . In this apparatus, the volume of the high-pressure vessel HV (Figure ) is 232.2 cm 3 , and the amount of water is fixed at about 50 cm 3 .…”

Molecular Insights into the Effect of Nitrogen Bubbles on the Formation of Tetrahydrofuran Hydrates

“…Currently, the formation behavior of THF hydrates has been investigated in both experimental [ 17 , 18 , 19 , 20 ] and simulation studies [ 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 ]. There are many factors that may affect hydrate formation, and one of the most important is micro- or nano-bubbles in liquid phase [ 29 , 30 , 31 , 32 , 33 , 34 ]. In nature, around gas chimneys and cold springs, it is a common phenomenon that clathrate hydrate will preferentially crystallize surrounding bubbles [ 35 , 36 ].…”

“…In nature, around gas chimneys and cold springs, it is a common phenomenon that clathrate hydrate will preferentially crystallize surrounding bubbles [ 35 , 36 ]. Many previous studies reveal that the presence of micro- and nano-bubbles can shorten the induction time for hydrate nucleation, and they attributed this phenomenon to the provision of gas sources and the high Laplace pressure of micro- and nano-bubbles [ 29 , 30 , 31 , 32 , 33 , 34 ]. In our prior experiment, in which we observed the crystallization of THF hydrates in the existence of micro- and nano-bubbles based on atomic force microscopy (AFM), it was found that even air bubbles can also promote the crystallization of hydrates [ 19 ].…”