Thermodynamic inhibition effects of an ionic liquid (choline chloride), a naturally derived substance (urea), and their mixture (deep eutectic solvent) on CH4 hydrates

Abstract

In this study, the thermodynamic inhibition effects of choline chloride (ChCl, a hydrogen bond acceptor), urea (a hydrogen bond donor), and their mixture (deep eutectic solvent, DES) on CH4 hydrates were investigated with both experimental and computational approaches. The synthesis of DES from the mixture of ChCl and urea was confirmed by measuring its melting temperature through a high-pressure micro-differential scanning calorimeter. Sigma (s)-profiles of ChCl, urea, and DES obtained by the COSMO-RS software indicated that these substances have great potential to be used as thermodynamic hydrate inhibitors (THIs). The three-phase (gas hydrate (H) - liquid water (L-w) - vapor (V)) equilibria of CH4 hydrates in the presence of ChCl (1.0, 3.0, and 5.0 mol%), urea (1.0, 3.0, and 5.0 mol%), and DES (3.0 mol%) demonstrated that ChCl, urea, and DES contributed to the significant shift of hydrate equilibrium temperature to inhibition regions at a specified pressure and thus can function as effective THIs. Moreover, quantum theory of atoms in molecules (QTAIM) analysis also demonstrated that the strength of hydrogen bonding between inhibitor molecules and hydrate cages (small 512 and large 5(12)6(2) cages) of CH4 hydrates was in the order of ChCl > DES > urea, which was consistent with experimental results. The experimental and computational results from this study would be helpful for a better understanding of the inhibition mechanism of ChCl, urea, and DES and for their possible application to flow assurance in oil and gas pipelines.