Effect of Carbon Chain Length of Organic Salts on the Thermodynamic Stability of Methane Hydrate

This study presents the phase equilibrium conditions for methane hydrate with one of the following organic ammonium salts differing in carbon chain length: tetramethylammonium bromide (TMAB), tetraethylammonium bromide (TEAB), tetrapropylammonium bromide (TPrAB), tetrabutylammonium bromide (TBAB), and tetrapentylammonium bromide (TPeAB). The hydrate phase equilibrium measurements were conducted for a temperature range of 278.94-291.85 K and pressure range of 4.79-14.32 MPa using the step -heating pressure search method. The addition of TBAB or TPeAB shifts the phase equilibria of the semiclathrate hydrates (SCHs) of CH4 to a lower pressure and higher temperature zone. At a given temperature, increasing the mole fraction of TBAB and TPeAB from 0.294 mol % to 0.620 mol % made the shift in phase equilibrium conditions greater. At a given dosage, TBAB consistently outperformed TPeAB in thermodynamically promoting methane hydrate formation. TMAB, TEAB, or TPrAB slightly shifts the phase equilibrium conditions to a higher pressure and lower temperature region. We analyzed the hydrate phase equilibrium data for TMAB, TEAB, and TPrAB using the colligative property equation and compared them with the phase equilibrium data of a CH4 and salt water system. The results suggest that these three organic salts have a small hydrate inhibiting effect that is comparable to NaCl. Promotion of the formation of CH4 hydrate by TBAB and TPeAB indicates that these additives provide a means to store CH4 at moderate pressure conditions, which could lower the cost of pressure reduction in hydrate formation. In contrast, TMAB, TEAB, and TPrAB could be used for prevention of formation of hydrates in systems where the use of NaCl is unsuitable.