Formation of Ice, Tetrahydrofuran Hydrate, and Methane/Propane Mixed Gas Hydrates in Strong Monovalent Salt Solutions

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Abstract

Electrolytes can thermodynamically inhibit clathrate hydrate formation by lowering the activity of water in the surrounding liquid phase, causing the hydrates to form at lower temperatures and higher pressures compared to their formation in pure water. However, it has been reported that some thermodynamic hydrate inhibitors (THIs), when doped at low concentrations, could enhance the rate of gas hydrate formation. We here report a systematic study of model natural gas (a mixture of 90% methane and 10% propane) hydrate formation in strong monovalent salt solutions in a broad range of concentrations, using a high pressure automated lag time apparatus (HP-ALTA). HP-ALTA can apply a large number (>100) of cooling ramps to a sample and construct probability distributions of gas hydrate formation for each sample. The probabilistic interpretation of data enables us to mitigate the stochastic variation inherent in the nucleation probability distributions and facilitates meaningful comparison among different samples. The electrolytes used in this work are lithium chloride (LiCl), lithium bromide (LiBr), lithium iodide (LiI), sodium chloride (NaCl), sodium bromide (NaBr), sodium iodide (NaI), potassium chloride (KCl), potassium bromide (KBr), and potassium iodide (KI). We found that (1) some salts may act as kinetic hydrate promoters at low concentrations; (2) the width of the probability distributions (stochasticity) of natural gas hydrate formation in these salt solutions was significantly narrower than that in pure water. To gain further insight, we extended the study of the solutions of the same nine salts to the formation of ice and model tetrahydrofuran (THF) hydrate for comparison.

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