Methane hydrate phase equilibria for systems containing NaCl, KCl, and NH4Cl

In this study, experimental data on methane hydrate phase equilibria containing electrolytes, sodium chloride (NaCl), potassium chloride (KCl), and ammonium chloride (NH4Cl) were measured for concentrations up to about 10 wt%. The concentration of the aqueous salt solution in the system containing hydrate and salt solution is continuously changed during the hydrate formation- and dissociation processes, so applying the isochoric method with the continuously temperature ramping to measure hydrate phase equilibria in the presence of salts may be unsuitable for precise and accurate measurements. An isochoric method, using a step-wise increase of temperature with sufficient equilibration time at every step, was introduced for measuring the hydrate equilibrium conditions for these systems containing electrolytes. To compare the results from the isochoric method, measurements using a high-pressure differential scanning calorimetry (DSC) were also performed using the same step-wise increase of temperature method. The results from both isochoric and DSC methods showed good agreement. The effects of the cation in the electrolyte on the hydrate inhibition were identified through the measurements, showing that hydrate inhibition strength by the sodium cation was slightly stronger than that of potassium and ammonium cations. (C) 2015 Elsevier B.V. All rights reserved.