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

Abstract

CO2 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 CO2 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 additives i.e. salt and hydrophobic particles were examined. Initially sodium halides were tested for their effect on the kinetics of the formation of CO2 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 CO2 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 CO2 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 CO2 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 CO2 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. 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, CO2 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 CO2 hydrate formation were enhanced effectively by dry water i.e. the most hydrophobic silica particles. To identify the role of dry water on promoting the formation of gas hydrates, the specific structure of dry water was studied using high-resolution X-ray micro computed tomography (HRXMT) technique. Using this method the inner structure of dry water was characterised quantitatively. Useful data on the statistical characteristics (number, surface area and volume distributions) of dry water were successfully obtained. This technique was also successful in characterising the structure of dry water and coalescence of droplets after exposure to low temperature, high pressure and stirring. To provide further insight into the adsorption of gas molecules at the water/solid interface, the surface (zeta) potential of the hydrophobic fumed silica particles in aqueous system before and after exposing to N2 and CO2 gas was measured. The measured Zeta-potential indicated that gas molecules accumulate at the water hydrophobic solid interface when it is saturated with gas. From the collective results obtained a new mechanism was inferred for the promotion of gas hydrates by hydrophobic fumed silica. The overall conclusions produced during this body of research are: sodium halide solutions are CO2 hydrate kinetic promoters at low concentrations of approximately 50 mM. CO2 hydrate formation was found to be enhanced in the presence of hydrophobic nano-particles. The influence of hydrophobic particles on water structure and the formation of a gas layer around hydrophobic particles, are the main reasons for this effectiveness. However, the special structure of dry water also plays an important role and boosts the formation of hydrates. All in all, any additive (e.g. salts and hydrophobic particles) that influences the structure of water can affect the formation of gas hydrates. Depending on the level of perturbation on water structure, additives can promote or inhibit the formation of gas hydrate. The concept of this research could assist to design a novel process for the capture and permanent storage of CO2 in the form of gas hydrate.