Fundamental Investigation of the Effects of Hydrophobic Fumed Silica on the Formation of Carbon Dioxide Gas Hydrates

Abstract

Promoting the kinetics of CO2 hydrate formation using additives is of great importance in industrial applications of gas hydrates such as capture and storage of carbon dioxide. However, the mechanism of hydrate formation in the presence of solid particles is not well understood. This paper aims to gain a better understanding of the fundamental aspects of CO2 hydrates formation in the presence of hydrophobic silica nanoparticles. A novel mechanism for gas hydrate formation in the presence of hydrophobic particles was established from a series of well-designed experiments. Two types of particle stabilized systems, made by mixing hydrophobic silica nanoparticles and water, were tested: air-in-water foam and water-in-air powder (dry water). Gas consumption, CO2 conversion, induction time, and hydrate growth rate have been examined to establish the influence of particle hydrophobicity and concentration on hydrate formation kinetics. The results show that the promoting effect depends on the particle hydrophobicity and concentration. The most hydrophobic silica (dry water) enhances the kinetics of CO2 hydrate formation effectively. Cryogenic scanning electron microscopy, combined with energy-dispersive spectroscopy, was used to examine the morphology, microstructure, and pore characteristics of CO2 gas hydrates, as well as elemental composition of the samples. To provide further insight into the adsorption of gas molecules at the water/solid interface, surface potential of the hydrophobic fumed silica particles in aqueous system before and after exposing to N2 and CO2 gas was measured. These results, in combination with the result of our recent study on the structure of dry water, successfully provided further detailed information on how hydrophobic fumed silica promotes the formation of gas hydrates.