Experimental Measurements and Thermodynamic Modeling of Hydrate Dissociation Conditions for Methane plus TBAB plus NaCl, MgCl2, or NaCl-MgCl2 + Water Systems

Much attention has been made to solidified natural gas (SNG) technology via clathrate hydrates in recent years. Tetra-n-butyl ammonium bromide (TBAB) is known as a promising promoter to tackle hydrate technology limitations. This research focuses on investigating the effect of NaCl, MgCl2, and the mixture of NaCl + MgCl2 (two major soluble salts in naturally occurring water are NaCl and MgCl2) on hydrate stability conditions of methane in the presence of TBAB aqueous solution. An isochoric pressure search method was employed to generate the dissociation/ equilibrium data in the temperature, pressure, and TBAB composition ranges of 275-291 K, 0.5-5.5 MPa, and 5-20 wt %, respectively. The experimental results reveal that in the case of WTBAB = 5%, NaCl and MgCl2 with the low concentration of 5% have a promotion effect for the systems of CH4 + TBAB + NaCl + H2O, CH4 + TBAB + MgCl2 + H2O, and CH4 + TBAB + NaCl + MgCl2 + H2O and shift the dissociation curve toward milder region (higher temperature and lower pressure). However, in the case of W-TBAB = 20%, NaCl and MgCl2 play an inhibition role in all of the aforementioned systems. A thermodynamic model was developed based on the van der Waals-Platteeuw (vdWP) solid solution theory, to predict the behavior of methane in the presence of the promoter in saline water. The Peng-Robinson equation of the state (PR EOS) is used to describe the thermodynamic properties of the gas phase, and the electrolyte non-random two-liquid (e-NRTL) activity coefficient model is employed to determine the activity coefficient of water and promoter in the electrolyte solution. The presented model results are in satisfactory agreement with the experimental data generated in this work. The discrepancy of the model results with experimental is 10.78%.