Urea as a green thermodynamic inhibitor of sII gas hydrates

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 P421m phase I urea. The determined parameter of sII hydrate unit cell (17.17 angstrom) indicates the noninclusion of urea into the hydrate framework. An empirical correlation was proposed to describe the hydrate equilibrium temperature depression Delta Th depending on pressure and urea concentration. A linear relationship between Delta Th and a decrease in the ice freezing point Delta Tice 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 costeffective 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.