Urea as a green thermodynamic inhibitor of sII gas hydrates

Abstract

This work is devoted to a systematic study of urea CO(NH2)2 as a promising and green gas hydrate inhibitor. The thermodynamic stability of sII gas hydrates forming from a binary gas mixture 4.34% C3H8 + 95.66% CH4 (mol%) in the presence of urea was analyzed. The hydrate equilibrium conditions were measured in a wide range of temperatures (267 – 294 K), pressures (0.9 – 9.4 MPa), and urea concentrations (0 – 50 mass%). The urea decreases the equilibrium temperature of sII hydrates formation, i.e., CO(NH2)2 acts as a thermodynamic hydrate inhibitor (THI). Powder X-ray diffraction at 173 K revealed the phases of sII gas hydrate, hexagonal ice, and tetragonal P4¯21m phase I urea. The determined parameter of sII hydrate unit cell (17.17 Å) indicates the non-inclusion of urea into the hydrate framework. An empirical correlation was proposed to describe the hydrate equilibrium temperature depression ΔTh depending on pressure and urea concentration. A linear relationship between ΔTh and a decrease in the ice freezing point ΔTice in urea aqueous solutions confirms the thermodynamic consistency of our data. Inhibition power of urea at 20 mass% and 6 MPa is 2 times less than that of methanol and is close to that for monoethylene glycol (MEG). In the range, up to 45 mass% urea is more effective THI than diethylene glycol (DEG). Urea is a less toxic compound compared to methanol and glycols, and also more cost-effective than MEG and DEG. Despite the medium anti-hydrate activity, urea can be considered as a safer for the environment hydrate inhibitor. Its application is justified if a shift of the hydrate equilibrium curve by no more than 10 – 12 K is required.