Morphology Study of Hydrate Shell Crystal Growth on a Microbubble Interface in the Presence of Additives

Abstract

Bubble dispersions in water can be observed in both wellbore or subsea piping as well as subsea gas seepages. The hydrate risk in subsea pipelines or methane leaks from the cold spring into seawater can be strongly influenced by the hydrate crystal growth on bubbles. In addition, the bubbling method has been proven to be an effective method to enhance the gas–liquid mixing pathway and promote hydrate’s rapid formation in the application of hydrate-based technologies, yet the microscopic mechanism of hydrate film growth on the bubbles still needs to be further clarified. In this study, the hydrate crystal growth morphology evolution at the microbubble interface was investigated by a high-resolution microscope combined with the temperature-controlled stage. The results showed that the hydrate crystals nucleated rapidly at the bubble interface to form a hydrate shell-coated structure in a pure water system. The defect growth mechanism dominated the hydrate crystals growth at the bubble interface in additive solutions. Hydrate did not nucleate directly on the surface of bubbles but by trapping guest molecules around the shrinking bubbles to form aggregated hydrate agglomeration in the presence of surfactants. In addition, the bubbles tend to provide a steady stream of gas source for hydrate formation. The growth of hydrate shells contributed to a normal distribution of bubble shrinking rates, with an average rate of 9 μm/s in 0.5 wt % NaCl solution. The existence of a high concentration of ions or organic molecules at the microbubbles’ interface and the exclusive effect played a dominant role in the process of hydrate formation. In addition, it was found that the repulsive effect of ions at the bubble interface on hydrate formation was weak under a high subcooling degree. However, other organic molecule promoters still accelerated the hydrate’s formation rate because they increased the dissolution and diffusion rate of gas in solution. The results are expected to strengthen further the applications relating to hydrate-based phase transition promotion technology in the industry in energy storage, gas separation, or inhibition techniques in flow assurance of the offshore industry.