Concurrent Dynamics of Hydrate Crystal Growth and Vapor Bubble Formation at the Liquid–Liquid Interface

Abstract This study focuses on the dynamics of hydrate crystal growth at the interface between liquid water and a liquid guest compound with the vaporization of the liquid. For the development of hydrate-based technologies, exploration of methods for efficient hydrate formation is crucial. Utilizing the vaporization of a liquid guest compound is effective for the enhancement of exothermic hydrate formation. However, the sequence of hydrate crystal growth dynamics is not clearly known. To understand the hydrate crystal growth dynamics, we employed a hydrate-forming system with liquid HFC-134a, proven effective in hydrate-based tritiated water separation. The crystal growth dynamics at the liquid–liquid interface were observed under four different conditions with system subcooling temperatures of 2.8 K, 3.1 K, 5.0 K, and 6.3 K under vapor pressures of HFC-134a at 0.38 MPa to 0.43 MPa. At ΔTsub = 6.3 K, “chimney-like” hydrate films grew to trace the volumetric expansion and buoyant motion of the vapor bubbles formed at the hydrate-covered liquid–liquid interface. Subsequently, the bulk layer of the liquid water phase became slushy through the formation of multiple chimneys that ruptured, extended, and expanded. At the lower system subcooling temperatures of 2.8 K, 3.1 K, and 5.0 K, hydrate film growth with vapor bubble formation was not observed. However, as a rare phenomenon, at ΔTsub = 3.1 K, liquid–liquid interface “necking” was observed as the hydrate film composed of polygonal plate crystals grew from the cell-wall side toward the center of the liquid–liquid interface. Understanding the concurrent dynamics of the chimney-like hydrate film growth and vapor bubble formation demonstrates the utility of hydrate formation processes that use liquefied guest compounds.