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.
This experimental study reports the kinetic and thermodynamic inhibition influence of sodium chloride (NaCl) on methane (CH4) hydrate in an oil-dominated system. To thoroughly examine the inhibition effect of NaCl on CH4 hydrate formation, kinetically by the induction time and relative inhibition performance and thermodynamically by the hydrate liquid-vapor equilibrium (HLwVE) curve, enthalpy (ΔHdiss) and suppression temperature are used to measure the NaCl inhibition performance through this experimental study. All kinetic experiments are performed at a concentration of 1 wt % under a pressure and temperature of 8 MPa and 274.15K, respectively, whereby for the thermodynamic study, the concentration was 3 wt % by using the isochoric T-cycle technique at the selected range of pressures and temperatures of 4.0-9.0 MPa and 276.5-286.0K, respectively; both studies were conducted using a high-pressure reactor cell. Results show that kinetically, NaCl offers slightly to no inhibition in both systems with/without oil; however, the presence of drilling oil contributes positively by increasing the induction time; thermodynamically, NaCl contributes significantly in shifting the equilibrium curve to higher pressures and lower temperatures in both systems. In the oil system, the contribution of the THI to the equilibrium curve increases the pressure with a range of 0.04-0.15 MPa and reduces the temperature with a range of 1-3 K, which is due to the NaCl presence in the systems that reduces the activity of water molecules by increasing the ionic strength of the solution. At a high pressure of 9 MPa, the NaCl inhibition performance was greater than that at lower pressures <5.5 MPa because, at the high pressure, NaCl increases the activity of water, which means that more water molecules are available to form hydrate cages around gas molecules.
This experimental study evaluates the inhibition performance of kinetic hydrates inhibitors (KHIs) of three amino acids, namely: glycine, proline, and alanine. It includes the performance comparison with the conventional inhibitor i.e., polyvinyl pyrrolidine (PVP) on methane (CH4) hydrate in oil systems in two different systems, i.e., deionized and brine water systems. The experiments were conducted in a high-pressure hydrate reactor replicating subsea pipeline conditions, i.e., the temperature of 274 K, pressure 8 MPa, and concentration of 1 wt%, by applying the isochoric cooling technique. The formation kinetics results suggest that all the studied amino acids effectively worked as kinetic inhibitors by potentially delaying CH4 hydrate formations due to their steric hindrance abilities. The interesting phenomenon was observed that the different studied amino acids behave differently in the brine-oil and deionized water-oil systems due to their side chain interaction. In a deionized water-oil system, glycine gives the highest inhibition performance by reducing the hydrate formation risk. On the contrary, in the brine-oil system, proline showed a significant inhibition effect. It should be noted that both glycine and proline were giving almost similar inhibition performance compared to the conventional hydrate inhibitor PVP, however glycine and proline significantly reduced CH4 consumption into hydrate due to their high surface active under CH4 conditions, which strengths the surface tension of the liquid/CH4 interface. Furthermore, according to the findings, it shows that increased side alkyl chain lengths of amino acids increase the efficacy of their kinetic hydration inhibition performance due to better surface adsorption abilities. The amino acids' ability to suppress growth is also linked strongly with hydrophobicity and alkyl side chain length. The findings of this study contribute significantly to current efforts to limit gas hydrate formation in offshore pipelines, particularly in oil-dominant pipelines.