Growth rate and morphology study of tetrahydrofuran hydrate single crystals and the effect of salt

Abstract

For an advanced gas hydrate management strategy in flow assurance as well as the application of a gas hydrate technology (e.g., desalination), it is important to understand the kinetics and morphologies of gas hydrate crystals during the growth period in saline water. Single crystal growth provides a unique method of assessing the hydrate growth kinetics and morphology in a controlled and precise manner; yet there is a lack of knowledge on the effect of salt on single crystal hydrate growth. Single crystals of structure II tetrahydrofuran (THF) hydrate, a model hydrate system which is stable at atmospheric pressure and temperatures above the ice point, were grown using a novel seed/template crystal method in a stoichiometric THF solution, and with the addition of 3.5 wt% NaCl. Growth of THF hydrate single crystals on seed crystals occurred mainly in the form of octahedral shapes, with the growth rates increasing with higher subcooling temperatures. The crystal morphology exhibits more structural defects at higher subcooling that take the form of a skeletal octahedron. The presence of salt as an impurity in the solution leads to inhibition effects to the single crystal growth due to the changes in thermodynamic subcooling effects, as well as modifying the single crystal growth habit, which introduces the formation of parallel intergrowths of octahedral hydrate crystals. In addition, the dramatic retardation by an average of ∼83% in single crystal growth rates with the addition of 3.5 wt% NaCl at the same subcooling temperatures as single crystals grown from freshwater is an unexpected phenomenon. These new results of single crystal growth in saline solutions provide important new insights for the different energy and technological applications of clathrate hydrates.