Phase equilibria, solubility and modeling study of CO2/CH4 + tetra-n-butylammonium bromide aqueous semi-clathrate systems

Interest has grown in tetra-n-butylammonium bromide (TBAB) semi-clathrates due to their formation at lower pressures and higher temperatures than conventional gas hydrates. This study focuses on TBAB semi-clathrates formed with carbon dioxide and methane and is to our knowledge the first study to report and model semi-clathrate former solubility under hydrate–liquid–vapor equilibrium conditions. A thermodynamic model was developed using the Trebble–Bishnoi equation of state to calculate vapor and liquid phase fugacity. Liquid activity was determined using the electrolyte non-random two liquid model (eNRTL). The solid semi-clathrate phase was modeled using a modified van der Waals–Platteeuw model (vdW–P), with parameters re-optimized based on the data obtained in this study. Equilibrium conditions were successfully modeled over the temperature range of 281–294 K, pressure range of 377–11,000 kPa and TBAB composition range of 5–40 wt%. The model functioned well for carbon dioxide with an average absolute relative error (AARE) of 4.7% for pressure prediction and an AARE of 17.5% for solubility. Methane was more difficult to model and yielded AAREs of 21.6% for pressure and 32.5% for solubility. Further understanding of semi-clathrate crystal structure and interactions between methane and TBAB would aid in improving the model.