Inhibiting effects of transition metal salts on methane hydrate stability

Abstract

The behavior of clathrate hydrates in the presence of transition metal salts was investigated using a Differential Scanning Calorimeter (DSC). Specifically, a DSC was employed to determine the onset temperature for methane hydrate decomposition in the presence of ferric chloride hexahydrate, [FeCl2(H2O)4]Cl·2H2O, anhydrous ferric chloride, FeCl3, MnSO4, FeSO4, CuSO4, and AgNO3, and to compare the inhibiting properties of these transition metal salts with NaCl and CaCl2, two well-known salt inhibitors. The degree of methane hydrate inhibition induced by the salts that were studied (as indicated by the reduction in dissociation temperature at a given pressure), when compared between mixtures with the same mole percentages of the salt, increases in the following order: FeSO4≈CuSO4<MnSO4≈AgNO3≈CaCl2<NaCl<FeCl3. A smaller decrease in the dissociation temperature was observed with salts that contained the larger sulfate anion when compared to salts that contained the smaller chloride anion. Smaller decreases in the dissociation temperature were observed with salts that contained smaller cations like Fe2+ when compared to salts that contained larger cations such as Ag+ and Mn2+. It is posited that the interaction between water with salt ions results in hydrate formation inhibition and the strength of the salt ion-dipole bond between the metal ion and water molecules correlates with the degree of inhibition. Consideration of the charge and size characteristics of the anion and cation components of the tested salts appears to explain this behavior.