Kinetics of the formation of CO2 hydrates in the presence of sodium halides and hydrophobic fumed silica nanoparticles

Abstract

CO 2 capture by trapping the greenhouse gas in clathrates hydrates is proving to be one of the promising options for long term carbon storage. This technology is desirable as it can be deposited in the deep ocean where the suitable pressure and temperature is ready without any extra cost. To date, this method has mostly been of academic interest because the formation of gas hydrates is very slow and requires lots of energy and resources. It is important therefore to develop an efficient and economical process to be able to apply this method at an industrial scale. Scientists and researchers have been looking for suitable additives to accelerate the formation of CO 2 hydrate. To meet these objectives numerous additives have previously been tested and several were found to be suitable hydrate promoters. The mechanism for the formation of gas hydrates in the presence of additives is not clear yet. Understanding this mechanism would help us to select and synthesise the most suitable and cost effective additive.In this thesis, two groups of chemicals i.e. salt and hydrophobic particles were added to the hydrate formation reactor and examined. Initially sodium halides were tested for their effect on the kinetics of the formation of CO 2 hydrates in an isochoric system. The effect of different anion types and concentrations was investigated by directly measuring pressure and temperature changes in the reactor and evaluating maximum CO 2 uptake, conversion, storage capacity, induction time, and hydrate growth rate. The results indicated that sodium halides at a concentration of 50 mM (mmol/L) increase CO 2 consumption and conversion to hydrates. In addition, sodium iodide and sodium bromide in a range of concentrations between 50 and 250 mM significantly increased the hydrate formation kinetics. Measurements of the surface potential of CO 2 hydrates formed in the presence of sodium halides showed negative charge on hydrates with the highest zeta potential observed at the concentration of 50 mM. The findings of this part of the research led us to propose a mechanism for the formation of CO 2 hydrates in the presence of sodium halides.The second group of additives extensively investigated in the current research were hydrophobic nano-particles. Two types of hydrophobic fumed silica were studied.iii They produced two types of particle stabilised systems when mixed with water (i.e. silica foam and dry water). The influence of particle hydrophobicity and concentration on hydrate formation kinetics was established by monitoring gas consumption, CO 2 conversion and induction time. The morphology, microstructure, and pore characteristics were elucidated by cryogenic scanning electron microscopy (cryo-SEM), and the elemental distribution and composition were determined by X-ray energy dispersive spectroscopy (EDS). The results indicated that the promoting effect of hydrophobic fumed silica was dependent on the particle hydrophobicity and on the weight ratio of silica to water. The kinetics of CO 2 hydrate formation were ...

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