Methane hydrate-liquid-vapour-equilibrium phase condition measurements in the presence of natural amino acids

This work reports the thermodynamic effect of five amino acids on methane hydrate phase boundary. The studied amino acids are glycine, alanine, proline, serine and arginine. To effectively investigate the impact of selected amino acids on methane hydrates formation, the methane hydrate-liquid-vapour-equilibrium (HLwVE) curve is measured in amino acids aqueous solutions. Experiments are performed at concentration range of 5–20 wt by employing the isochoric T-cycle method in a sapphire hydrate cell reactor at pressures and temperatures range of 3.86–9.98 MPa and 276.50–286.00 K, respectively. Results suggests that, all studied amino acid inhibits methane hydrate formation. Glycine showed the highest inhibition effect with an average depression temperature of 1.78 K at 10 wt. The impact of inhibition is due to amino acids hydrogen bonding energies, confirmed via COSMO-RS predictions and side group alkyl chain. The inhibition impact of glycine is found to be in the range of some ionic liquid (OH-EMIM-Cl) and slightly higher than ethylene glycol (a conventional thermodynamic hydrate inhibitor) at 10 wt. The methane hydrate dissociation enthalpies in the presence of amino acids are calculated using Clausius–Clapeyron equation, which suggests that, amino acids do not take part in methane hydrate cage occupation during hydrate formation. © 2016 Elsevier B.V.