Experimental Investigation on Interfacial and Surface Phenomena of Clathrate and Semi-clathrate Hydrates
Creators: Wei, Yu
Publisher: name: University of Alberta Library
Year: 2023

<p>Clathrate hydrates are compounds where guest molecules are trapped in cages formed between water molecules via hydrogen bonding. They have many promising applications, such as gas storage, energy storage, gas mixture separation and seawater desalination. They may also form inside the oil &amp; gas pipelines at a fast rate and pose a threat to flow assurance. Despite extensive research on physicochemical properties of clathrate hydrates, many knowledge gaps still remain to be filled.</p>

<p>This study presented a systematic investigation on interfacial and surface phenomena of clathrate and semi-clathrate hydrates, including the determination of specific surface free energy and tangential adhesive strength, the quantification of CO2 hydrate nucleation kinetics, the elucidation of the memory effect and the evaluation of the effect of dispersion on nucleation kinetics and gas uptake capacity of CO2 hydrate.</p>

<p>The specific surface free energy (γsv) value of THF hydrate was deduced using an indirect method in the range from 60.4 to 124.2 mJ/m2, with an average of 92.3 mJ/m2, which was somewhat lower than that of ice. The determination of this fundamental parameter is essential to the understanding of surface phenomena of clathrate hydrates.</p>

<p>Tangential adhesive strength (τ) of THF hydrate and TBAB semi-clathrate hydrate on substrates with different hydrophobicity was measured. Results showed: 1) diminished values of τ as heating which was speculated to be caused by pre-melting; 2) increased values of τ upon the addition of PVP (a kinetic hydrate inhibitor) on all substrates and 3) higher values of τ on the hydrophobic substrates than on the hydrophilic ones. Our findings may be valuable to hydrate-related flow assurance problems in oil &amp; gas pipelines during hydrodynamic transport. </p>

<p>Nucleation rate of CO2 hydrate (sI) was determined using a linear cooling ramp method in the presence and absence of a solid wall. It was found: 1) the convergence of nucleation curves with an increasing number of data points; 2) the promoting effect of a stainless-steel wall on hydrate nucleation kinetics; 3) the limitation of Classical Nucleation Theory in explaining hydrate nucleation and 4) the major effect of the guest type on hydrate nucleation kinetics. Our findings offered new insights into the quantification of CO2 hydrate nucleation kinetics and impact factors of it. </p>

<p>The mechanism of the memory effect was investigated by comparing nucleation curves of CO2 hydrate formation in fresh and dissociated water. Despite different “amount”, the memory effect was detected when CO2 hydrate formed in dissociated water both in the presence and absence of a solid wall. A new mechanism was proposed——the memory effect was caused by two components, interfacial gaseous states and bulk guest supersaturation (with or without nanobubbles), with the former one being more effective in size. Our hypothesis supplemented and improved existing hypotheses and might solve this mystery. </p>

<p>The effect of dispersion on CO2 hydrate formation kinetics was researched in dry water. Results showed that dry water remained stable after being doped with a small amount of nucleation promoters. The presence of SDS and nucleation promoters had no promoting effect on the nucleation kinetics of CO2 hydrate but significantly promoted crystal growth kinetics of the CO2 hydrate formation in dry water. We concluded that dispersion and additives promoted CO2 hydrate nucleation and crystal growth, respectively. Our findings can be utilized to store more CO2 in a hydrate form at a faster rate.</p>

<p>Overall, we deduced the essential parameter that characterizes the surface properties of hydrates, specific surface free energy. This offered a pathway for the estimation of the work of adhesion and nucleation work. We investigated the influencing factors on hydrate-substrate shear adhesive strengths, which is meaningful for flow assurance. We derived nucleation rates of CO2 hydrate and elucidated the influencing factors of it, providing a baseline for the industry. We proposed a mechanism explaining the memory effect and it shed light on this long-standing mystery. We investigated the effect of dispersion and additives on CO2 hydrate formation kinetics. The main findings are significant for the application of gas hydrates in carbon capture and sequestration and relief of the greenhouse effect. This thesis not only advanced the basic understanding and the fundamental knowledge of the surface and interfacial properties of gas hydrates but also provided insightful guidance for the hydrate-related industry.</p>