Methane Hydrate-in-Oil Systems in the Presence of Natural Amino Acid-Equilibrium Phase Condition Measurements

This experimental study reports the thermodynamic influence of three different amino acids on methane hydrate in oil-dominated systems, namely, glycine, proline, and alanine. To thoroughly examine the effect of selected amino acids on methane (CH4) hydrate formation compared to the commercial inhibitor monoethylene glycol (MEG) in the presence of oil, the hydrate liquid-vapor equilibrium (H-Lw-Lo-V) curve is used to measure amino acid aqueous solutions. All experiments are performed at a concentration of 10 wt % by using the isochoric T-cycle technique in a high-pressure reactor cell at the selected range of pressures with temperatures of 4.0-9.0 MPa and 276.5-286.0 K, respectively. Results show that all studied amino acids inhibit hydrate formation of methane; the inhibition trend shows as glycine > alanine > proline in both systems; in the brine water system, the inhibition performance was higher than in the pure water system due to the presence of NaCl. Glycine showed the highest inhibition strength in both systems with an average reduced temperature in pure and brine water of 0.92 and 1.75 K, respectively, at 10 wt %, making the inhibition performance of glycine comparable to the commercial inhibitor MEG. The inhibition effect is attributed to the amino acid's hydrogen bonding energies and side group alkyl chain. Calculating the dissociation enthalpies of methane hydrates in the presence of amino acids using the Clausius-Clapeyron equation implies that the amino acids do not occupy the cage structures during methane hydrate formation.