Phase Equilibria of Carbon Dioxide and Methane Gas Hydrates in the Presence of Tetramethylammonium Chloride and Tetrapropylammonium Bromide-Based Deep Eutectic Solvents

The effects of four deep eutectic solvents (DESs) on the phase equilibrium conditions of CO₂ and CH₄ gas hydrates were investigated. The mixtures of tetramethylammonium chloride (TMAC) and tetrapropylammonium bromide (TPAB) as hydrogen-bond acceptors with glycerol and ethylene glycol (EG) as hydrogen-bond donors were used to formulate the DESs. The combinations of TMAC/glycerol, TMAC/EG, TPAB/glycerol, and TPAB/EG were all made at a 1:3 molar ratio. The concentrations of DESs in the aqueous solutions were 2 and 4 wt %. The hydrate dissociation conditions for CO₂ and CH₄ systems were measured using an isochoric pressure-search method in the temperature ranges of (277.85 to 282.97) K and (276.03 to 285.15) K, respectively, and pressure ranges of (2.03 to 5.28) MPa and (3.71 to 10.39) MPa, respectively. All the studied DESs demonstrated the thermodynamic gas hydrate inhibition effect on CO₂ and CH₄ hydrate formation. The gas hydrate inhibition ability ranking from the highest to the lowest was found to be in the order of: TMAC/glycerol > TMAC/EG > TPAB/glycerol > TPAB/EG. This result proved that DESs have great potential to be effective green thermodynamic hydrate inhibitors.