Theoretical and experimental study of phase equilibrium of semi-clathrate hydrates of methane + tetra-n-butyl-ammonium bromide aqueous solution

In this study, the phase stability conditions of semi-clathrate hydrate of methane + tetra-n-butyl-ammonium bromide (TBAB) + water system is investigated. New experimental data on the hydrate-liquid-vapor equilibrium conditions of this system is measured. The measured data is for the pressure range of 2.88-14.1 MPa, temperature range of 285.6-295.9 K and TBAB mass fractions of 0.05, 0.15 and 0.3. A thermodynamic model is also developed to predict the phase stability conditions of hydrate for this system. In the developed model, the phase equilibrium of the hydrate of pure TBAB in water is predicted based on the Gibbs free energy minimization technique. This model is then combined with the statistical thermodynamic model of van der Waals-Platteeuw (vdW-P) to predict the phase stability conditions of semi-clathrate hydrate of methane with TBAB in aqueous solution. Binding mean spherical approximation (BiMSA) electrolyte model is used for aqueous phase properties prediction and modified Peng-Robinson equation of state (PR-EoS) is used for calculation of the gaseous phase properties. The results show that the developed model satisfactorily predicts the experimental data with Average Absolute Relative Deviation (AARD) of 12%. Moreover, the model is able to predict the different types of pure TBAB semi-clathrate hydrates and also the inhibition and promotion effects of this salt. The results also show that considering the association effect in the electrolyte model, can improve the predictions of the developed thermodynamic model for hydrate phase equilibrium of methane in the presence of TBAB aqueous solution.