Phase Behavior and Raman Spectroscopic Analysis for CH₄ and CH₄/C₃H₈ Hydrates Formed from NaCl Brine and Monoethylene Glycol Mixtures

Abstract

We present pure CH4 and CH4/C3H8 mixed hydrate phase equilibria formed from a mixture of NaCl (10 wt %) and monoethylene glycol (MEG, 10 and 30 wt %) solutions. As expected for thermodynamic inhibitors, the mixture of salt and glycol causes the hydrate phase equilibrium boundary to shift to lower temperatures and higher pressures, and on increasing the MEG concentration, the hydrate stable region shifted more. The measured experimental data are also compared with a thermodynamic model recently developed, named the Hu–Lee–Sum correlation, showing that the data match well with the predictions. The experimental data were used to calculate the enthalpy of hydrate dissociation. The enthalpies of CH4 hydrates in the mixture of 10 wt % NaCl brine and 10 or 30 wt % MEG were found to be ∼58.7 and 54.63 kJ/mol, respectively, corresponding to structure I hydrates, whereas for the CH4/C3H8 (91.98:8.02 mol %) mixed gas system, the enthalpies of dissociation were found to be ∼101.10 kJ/mol (10 wt % NaCl + 10 wt % MEG) and 95.34 kJ/mol (10 wt % NaCl + 30 wt % MEG), confirming the mixed hydrates formed structure II. We also performed Raman analysis for CH4 hydrates and CH4/C3H8 mixed hydrates in the NaCl and MEG system and investigated their spectroscopic behavior and hydrate structure.

In Situ Raman Study of the Formation and Dissociation Kinetics of Methane and Methane/Propane Hydrates

“…18 Information about the experimental and calculation procedure can be found in previously published studies. [8][9][10]13,18 In this study, a hydration number of 6 was used for pure CH 4 hydrate, which is widely accepted in the literature. 8,10,13 The hydration number for CH 4 /C 3 H 8 hydrate was calculated using eq 1:…”

“…Pitzer’s correlation was used to determine the compressibility factor needed to analyze hydrate formation and dissociation kinetics . Information about the experimental and calculation procedure can be found in previously published studies. − ,, …”

“…To date, X-ray diffraction (XRD), nuclear magnetic resonance (NMR), and Raman spectroscopy have been used to understand the fundamental properties of gas hydrates. For example, these methods have investigated the host structure, degree of cavity filling, , and kinetic and phase behaviors of clathrate hydrates. − In most spectroscopic analyses, small specimens are often chosen in the bulk phase and then analyzed under specific conditions. , Using small specimens from original samples may be insufficient to cover the real conditions of hydrate formation and decomposition . Hence, we used in situ Raman spectrometers under dynamic conditions of a pressurized stirred reactor .…”

Morphology Study on the Effect of Thermodynamic Inhibitors during Methane Hydrate Formation in the Presence of NaCl

“…The produced water resulting from subsurface processes contains salts such as NaCl, which also thermodynamically inhibit the hydrate formation. , As the amount of salts in produced water is not sufficient to completely prevent hydrate formation, additional MEG or MeOH is required, resulting in MEG/MeOH solutions containing NaCl. , Therefore, it is important to understand the effect of salts on hydrate formation when they coexist with MEG or MeOH. Studies on understanding the effect of MEG or MeOH in the presence of salts have mainly focused on hydrate phase equilibrium conditions. − Lee and Kang studied hydrate phase equilibrium conditions of natural gas in the presence of MeOH (10–30 wt %) and MEG (10–50 wt %) with 3.5 wt % NaCl. The thermodynamic inhibition effect of MeOH was found to be better than that of MEG, and adding NaCl to MEG or MeOH solutions increased the thermodynamic inhibition effect.…”

Experimental Study of the Effect of the Combined Monoethylene Glycol with NaCl/CaCl2 Salts on Sour Gas Hydrate Inhibition with Low-Concentration Hydrogen Sulfide

“…Kwak et al evaluated the phase equilibrium conditions of pure CH 4 hydrate and a mixture of CH 4 and C 3 H 8 hydrate in the presence of sodium chloride (10 wt %) and monoethylene glycol (10 and 30 wt %) using the isochoric method . The experimental results showed that the hydrate phase equilibrium boundaries shifted to the lower temperatures and higher pressures with increasing the concentration of monoethylene glycol in a constant amount of sodium chloride.…”