Promotion mechanism of carbon dioxide hydrate formation by l-Methionine and its competitive effects with NaCl.

The growth kinetics and macroscopic morphology evolution of CO 2 hydrate promoted by l -Methionine (L -Met) in NaCl aqueous solutions in a wide range of mass fractions were systematically investigated. 0.05 wt% was the threshold mass fraction of L -Met to have prominent promotion effects in a batch mode. The addition of NaCl could suppress its promotion performance, while it could be regained by further addition of L -Met to 1.0 wt%. Porous feature and wall-climbing phenomena of CO 2 hydrate were obvious in the presence of L -Met while it could be suppressed by NaCl. The tangential (lateral) growth rate of the hydrate film on planar pure water surface was the highest while both L -Met and NaCl could decrease it. For L -Met, it was the highest at 0.05 wt%. Special ball-like, mosaic-like and sword-like appearances of CO 2 hydrates were found. Both the adsorption-capillary effects of L -Met and the hydration effects of L -Met and NaCl were thought to account for all the phenomena and a detailed scenario was proposed and discussed. A macroscopic growth model based on the tangential growth rates of hydrate films was proposed and evaluated. • Excellent kinetic promotion effects of l -Methionine at 0.05 wt% were found. • NaCl suppressed the promotion effects by competing for water molecules. • CO 2 hydrate films first formed with different tangential growth rates. • Both the adsorption-capillary effects and hydration effects played a role. • Growth model was proposed and the control factor was evaluated. [ABSTRACT FROM AUTHOR]