Thermodynamic Modeling of Phase Equilibrium of CO2 + TBPB + THF + NaCl + Water System

Abstract

In this work, the effect of adding thermodynamic promoters tetra-n-butyl phosphonium bromide (TBPB) and tetrahydrofuran (THF) on the conditions of CO₂ gas hydrate formation is investigated. Considering that seawater is used for the formation of hydrate in large volume, the effect of NaCl as the dominant salt in saline water on the phase equilibrium of the CO₂ + TBPB system in the presence of THF is studied. Also, in order to predict the conditions of hydrate formation, the van der Waals–Plateau model has been used to model the hydrate phase equilibrium. The SRK equation of state has been used to check the gas phase. The e-NRTL model has been used to estimate the activity coefficient, and the model parameters for the system are presented under study. The results show that addition of NaCl (3% mass fraction) to the (TBPB + THF) aqueous solution changes the equilibrium conditions of CO₂ hydrate formation. It is observed that the hydrate pressure predicted by the provided model is in good agreement with the available experimental data on the CO₂ hydrate phase equilibrium in the presence of TBPB, THF, and NaCl. The model parameters are also estimated by using experimental data and the optimization method. The average absolute deviation for hydrate dissociation pressure is 3.23%.