Hydrate Stability Conditions of CO2 + TBPB + Cyclopentane plus Water System: Experimental Measurements and Thermodynamic Modeling

The current study provides experimental data and thermodynamic modeling of hydrate stability conditions of CO2 + tetra n-butylphosphonium bromide (TBPB) + cyclopentane (CP) + water system. The experimental results are presented from (288 to 292) K and (1.45 to 3.30) MPa at 0.10, 0.15, 0.17, and 0.20 mass fractions of TBPB in aqueous solutions. To measure the experimental data, an isochoric step-heating pressure-search method was applied. The validity of the method used in the current study was evaluated by regenerating previously reported experimental data on hydrate dissociation conditions of the carbon dioxide + TBPB + water system. The experimental results reveal that mixed promoters of TBPB + CP have the promotion effect compared to carbon dioxide + water and carbon dioxide + TBPB + water systems and the inhibition impact compared to the carbon dioxide + CP + water system; however, the inhibition effect is not considerable. Furthermore, a thermodynamic model was developed to predict/represent hydrate stability conditions. Nonrandom two-liquid (NRTL) activity coefficient model, e-NRTL activity coefficient model, and Peng-Robinson equation of state together with van der Waals-Platteeuw theory are applied to model the organic-rich phase and nonideality in the aqueous/liquid phase, vapor/gas phase, and hydrate phase, respectively. Experimental hydrate stability conditions at 0.10, 0.15, and 0.20 TBPB mass fractions were used to obtain the parameters of the thermodynamic model. Using the obtained parameters, the hydrate dissociation conditions at 0.17 mass fraction of TBPB were predicted. It is indicated that the thermodynamic model could predict the experimental data satisfactorily. The average relative deviation (ARD %) of the model results for hydrate dissociation pressures is about 5.4%.