Experimental Investigations of Nucleation Rates of Ice and Clathrate Hydrates
Creators: Zhang, Xin
Publisher: name: University of Alberta Library
Year: 2024

<p>Clathrate hydrate is a multi-component system in which the “guest” gas molecules are accommodated by the “host” water. It has many potential applications in CO2 capture and sequestration, gas separation and others. We wish to promote the formation of clathrate hydrate in these applications since the formation of clathrate hydrate is desired. However, in oil and gas industry, we wish to prevent the formation of hydrate due to the flow assurance problems. <br>
Nucleation is the first step of clathrate hydrate formation. It is a kinetic effect and occurs stochastically. For nucleation, the nucleation rate which is defined as the nucleation probability per unit size in unit time is the most important measure that characterizes the nucleation phenomenon of a given system.<br>
For now, there’s relatively few experimental studies focused on the nucleation of clathrate hydrate, especially quantitative studies that reliably quantify the nucleation rates of clathrate hydrate. In addition, it is challenging to understand the mechanism of clathrate hydrate nucleation due to the reasons such as the uncertainties in how to apply classical nucleation theory, experimental difficulties, and the inability of determining the nucleation rate reliably. Therefore, studying a structurally similar and less complex system, such as ice, may provide an indirect method to help understanding the mechanism of clathrate hydrate nucleation.<br>
Other than laying the foundation for the study of clathrate hydrate nucleation, ice nucleation itself has applications in global climate change, food engineering and other fields. Although ice nucleation has been studied over decades, many problems and mechanisms still remain unclear. <br>
In this study, in order to enhance the understanding of mechanisms of both clathrate hydrate nucleation and ice nucleation, we plan to take dual approaches, studying the nucleation of ice and clathrate hydrate and using findings in one of them to help understanding of the other. Firstly, a new experimental setup was established for the measurement of ice nucleation rate. The nucleation rates of ice were investigated in three microliter-sized water systems: 1) quasi-free water droplet supported by stable wetting films; 2) quasi-free water droplet suspended between two immiscible liquids; and 3) water directly in contact with a hydrophobic Teflon wall. The results showed that the nucleation rates measured in two quasi-free water droplet systems were broadly similar to each other, which suggested the quasi-free droplet systems could provide a reliable baseline for future studies in the presence of additives. The nucleation rates measured in water directly contact with a Teflon wall were somewhat higher than those of two quasi-free droplet systems, but the difference was not big.<br>
After the baseline was set up, the nucleation rates of ice in the presence of seven nucleation promoters were investigated in water directly in contact with a Teflon wall. The efficacy of these seven promoters could be ranked as: Snomax ≈ AgI ≥ kaolinite &gt; steroid &gt; cholesterol ≈ celluloses ≥ Teflon wall. <br>
Snomax was found to be the most effective promoter among the seven tested promoters and it was postulated that the efficacy of Snomax might be aided by its larger interfacial area for heterogeneous nucleation since it could be dispersed in water. Therefore, the three nucleation promoters that we previously found to be effective- AgI, kaolinite and cholesterol – were attempted to be dispersed in water to increase the interfacial area. It was found that the dispersion of these promoters into water was difficult and required the addition of TBAB to the aqueous phase before they could be dispersed. The nucleation rates measured in the dispersed nucleation promoter suspensions were investigated. And the results showed that dispersing AgI into 1 mM TBAB solution further promoted the ice nucleation while dispersing kaolinite or cholesterol in TBAB solutions did not.<br>
Other than promoters, the impact of monovalent electrolytes with concentrations lower than 100 mM was investigated in quasi-free droplets suspended between two immiscible liquids. It was found that some monovalent salts unexpectedly increased the nucleation rate of ice at low supercoolings.<br>
Finally, the formation of CO2 hydrate in quasi-free droplets of dilute electrolytes was investigated. The results showed that NaCl had no inhibition effect while KI had a weak promotion effect at low concentrations. </p>